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Fundamentals of
Low Dimensional Magnets
A low-dimensional magnet is a key to the next generation of electronic devices. In some respects,
low-dimensional magnets refer to nanomagnets (nanostructured magnets) or single-molecule magnets
(molecular nanomagnets). They also include the group of magnetic nanoparticles, which have been widely
used in biomedicine, technology, industries, and environmental remediation.
Low-dimensional magnetic materials can be used effectively in the future in powerful computers
(hard drives, magnetic random- access memory, ultra- low power consumption switches, etc.). The
properties of these materials largely depend on the doping level, phase, defects, and morphology. This book
covers various nanomagnets and magnetic materials. The basic concepts, various synthetic approaches,
characterizations, and mathematical understanding of nanomaterials are provided. Some fundamental
applications of 1D, 2D, and 3D materials are covered.
This book provides the fundamentals of low-dimensional magnets along with synthesis, theories,
structure-property relations, and applications of ferromagnetic nanomaterials. This book broadens our
fundamental understanding of ferromagnetism and mechanisms for realization and advancement in
devices with improved energy efficiency and high storage capacity.
Series in Materials Science and Engineering
The series publishes cutting-edge monographs and foundational textbooks for interdisciplinary materials
science and engineering. It is aimed at undergraduate and graduate-level students, as well as practicing
scientists and engineers. Its goal is to investigate the relationships between material qualities, structure,
synthesis, processing, characterization, and performance.
The series publishes cutting-edge monographs and foundational textbooks for interdisciplinary materials
science and engineering.
Its purpose is to address the connections between properties, structure, synthesis, processing, char-
acterization, and performance of materials. The subject matter of individual volumes spans fundamen-
tal theory, computational modeling, and experimental methods used for design, modeling, and practical
applications. The series encompasses thin films, surfaces, and interfaces, and the full spectrum of mate-
rial types, including biomaterials, energy materials, metals, semiconductors, optoelectronic materials,
ceramics, magnetic materials, superconductors, nanomaterials, composites, and polymers.
This book in the series is aimed at undergraduate and graduate-level students, as well as practicing
scientists and engineers in the field of nanomaterials.
Proposals for new volumes in the series may be directed to Carolina Antunes, Commissioning Editor
at CRC Press, Taylor & Francis Group (Carolina.Antunes@tandf.co.uk).
Fundamentals of
Low Dimensional Magnets
Edited by
Ram K. Gupta
Sanjay R. Mishra
Tuan Anh Nguyen
First edition published 2023
by CRC Press
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and by CRC Press
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CRC Press is an imprint of Taylor & Francis Group, LLC
© 2023 selection and editorial matter Ram K. Gupta, Sanjay R. Mishra and Tuan Anh Nguyen,
individual chapters, the contributors
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Fundamentals of Low Dimensional Magnets
LCCN
2022011779
Publisher Account
CRC Press
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ISBN: 978-1-032-04872-7 (hbk)
ISBN: 978-1-032-05421-6 (pbk)
ISBN: 978-1-003-19749-2 (ebk)
DOI: 10.1201/9781003197492
Typeset in Times
by Apex CoVantage, LLC
Contents
Editors ix
vii
viii Contents
Index 367
Editors
Dr. Ram K. Gupta is Associate Professor at Pittsburg State University. Dr. Gupta’s research focuses on
conducting polymers and composites, green energy production and storage using biowastes and nano-
materials, optoelectronics and photovoltaics devices, organic-inorganic hetero-junctions for sensors, bio-
based polymers, flame-retardant polymers, bio-compatible nanofibers for tissue regeneration, scaffold and
antibacterial applications, corrosion inhibiting coatings, and bio-degradable metallic implants. Dr. Gupta
has published over 250 peer-reviewed articles, made over 300 national, international, and regional pre-
sentations, chaired many sessions at national/international meetings, edited many books, and written
several book chapters. He has received several million dollars for research and educational activities from
many funding agencies. He is serving as Editor-in-Chief, Associate Editor, and editorial board member
of numerous journals.
Dr. Sanjay Mishra joined the Department of Physics at the University of Memphis in 1999. He has been
consistently productive in research, instruction, and service to the University of Memphis (UoM) since
1999. Dr. Mishra initiated an active multidisciplinary Materials Research program at the UoM. Before
receiving postdoctoral experience from the Lawrence Berkeley National Laboratory, the University of
California-Berkeley at the Advanced Light Source Synchrotron Facility, he received his PhD in Physics
from the Missouri University of Science and Technology—Rolla; his MS from Pittsburg State University,
Pittsburg, KS; his MSc from the South Gujarat University, Surat, India; and his postgraduate diploma
in Space Sciences from Gujarat University, Ahmedabad, India. Dr. Mishra’s research work focuses on
magnetic nanomaterials and nanocomposites for energy applications, including alloys, ferrites, and mag-
netocaloric materials. Dr. Mishra has published more than 300 peer-reviewed journal articles and has
given numerous presentations at national and international conferences. He has been successful in secur-
ing federal grants of more than two and a half million dollars over the years and has received numerous
awards from the UoM for his outstanding research accomplishments.
Dr. Tuan Anh Nguyen completed his BSc in Physics from Hanoi University in 1992 and his PhD in
Chemistry from Paris Diderot University (France) in 2003. He was Visiting Scientist at Seoul National
University (South Korea, 2004) and the University of Wollongong (Australia, 2005). He then worked as
Postdoctoral Research Associate and Research Scientist at Montana State University (U.S.), 2006–2009.
In 2012, he was appointed Head of the Microanalysis Department at the Institute for Tropical Technology
(Vietnam Academy of Science and Technology). He has managed four PhD theses as thesis director, and
three are in progress. He is Editor-In-Chief of Kenkyu Journal of Nanotechnology & Nanoscience and
Founding Co-Editor-In-Chief of Current Nanotoxicity & Prevention. He is the author of 4 Vietnamese
books and the editor of 32 Elsevier books in the Micro & Nano Technologies Series.
ix
Nanomagnets
Basics, Applications, and
New Prospectives
1
Biswanath Bhoi1,2 and Mangesh Diware3
1 National Creative Research Initiative Center for Spin Dynamics and Spin-Wave Devices,
Nanospinics Laboratory, Department of Materials. Science and Engineering, Seoul
National University, Seoul, South Korea
2 Department of Physics, Indian Institute of Technology (Banaras Hindu University)
Varanasi, Varanasi, India
3 CeNSCMR and Institute of Applied Physics, Department of Physics and Astronomy,
Seoul National University, Seoul, South Korea
Contents
1.1 Introduction 2
1.2 Theoretical Background 3
1.2.1 Magnetism at Nanoscale 3
1.2.2 Magnetization Dynamics 5
1.3 Magnetic Nanostructures 7
1.4 Synthesis of MNMs 8
1.4.1 Chemical Methods 8
1.4.2 Physical Methods 10
1.4.3 Biological Methods 11
1.4.4 Advanced Synthesis Method 11
1.5 Characterization of MNMs 13
1.6 Applications of MNMs 13
1.6.1 High-Density Data Storage 13
1.6.2 Nano-Electronics and Spintronic Applications 14
1.6.3 Biomedical Applications 15
1.6.4 Biosensing Applications 16
1.6.5 Environmental and Agricultural Applications 16
1.6.6 Other Applications 17
1.7 Emerging Research Areas in MNMs 17
1.8 Summary 19
1.9 Acknowledgments 19
References19
DOI: 10.1201/9781003197492-1 1
2 Fundamentals of Low Dimensional Magnets
1.1 INTRODUCTION
Nano-scale magnetism has been a subject of intense research within the last few decades, not only for tech-
nology development but also for understanding material science and fundamental physics. With the advance-
ments in nanoscience and nanotechnology, studies have been centered on designing and manipulating the
properties of magnetic nanomaterials (MNMs) by controlling their size, morphology, and composition. As
MNM research has progressed, the prime focus has shifted from the chemistry of the materials (synthesis
routes, morphology control, and characterization) to the investigation of functionality and integration with
the physical, electrical, and biomedical fields. The notable potential applications of MNMs are in a range of
multidisciplinary fields like magnetic memory, magnetic resonance imaging, biomedicine and health science,
spintronics, and other areas [1–3], as shown in Figure 1.1. Recently, MNMs have increasingly gained atten-
tion in emerging research fields such as spin-torque nano-oscillators (STNOs), spin logic, two-dimensional
(2D) ferromagnetism, and quantum magnonic-based on magnetization dynamics in nanomagnets [4–6].
The MNMs are an interdisciplinary subject, with researchers from both basic sciences and engineer-
ing equally interested in developing novel materials with controlled morphology and properties able to
perform multiple functions. Further, magnetic properties like saturation magnetization (MS), coercivity
(HC), and anisotropy vary significantly with the morphology and composition of the MNMs within the
nano-scale regime. There are several kinds of MNMs available based on iron (Fe), cobalt (Co), nickel (Ni),
and their alloys and oxides (e.g., ferro-and antiferromagnets), rare-earth metals (e.g., gadolinium [Gd],
terbium [Tb], dysprosium [Dy]), and multicomponent compounds for various applications [7–11]. More
efforts are needed to uncover the intrinsic relationship in MNMs between size/morphology/structure and
magnetic properties. Apart from superparamagnetism and spin-glass phenomena, MNMs offer an excit-
ing platform for investigating the interplay among various competing magnetic and electronic order and
magnetic phenomena involving the quantum confinement effect. Recent studies on magnetization dynam-
ics of MNMs have been extremely vigorous owing to the accelerating miniaturization of magnetic units
in spintronic-based devices and other spin logic devices.
Moreover, the new technologies require structuring magnetic materials in all three dimensions (3D)
at various length scales and the utilization of novel phenomena ranging from magnetic vortex to spin-wave
propagation. This introduces multiple magnetic structures at different length scales, such as nanodots,
-wires, -stripes, -discs, and 2D materials. We attempt to summarize the underlying mechanism and issues
that have yet to be realized in the various emerging nanomagnetism research fields.
1
AK 2
Ds 18 2
(1)
0 M S
where A is the exchange stiffness constant in J/m, K, is the magnetocrystalline anisotropy constant in J/m3,
µ0 is the permeability of free space (4 π × 10 −7 H/m), and MS is the saturation magnetization. Depending
on the size and material composition, the magnetic moments of single-domain particles can be 103–105 µB.
Coercivity (HC) is extremely sensitive to particle size, unlike saturation magnetization, which is size-
independent in principle, as shown in Figure 1.2a. In the multi-domain (MD) region, the HC gradually
increases with decreasing particle diameter (subdivided into domains). In contrast, it rapidly decreases to
zero in the SD region with decreasing particle size. At the limiting condition d DS , the SD particle mag-
netized uniformly along the anisotropic easy axes, leading to a substantial HC enhancement. Below DS, HC
value decreases with the particle size due to the decrease of the magnetic anisotropy energy (Ea KV , V
is the particle volume). As the size is reduced further, the anisotropy energy value decreases and becomes
comparable or even lower than the thermal energy (kBT , kB is Boltzmann constant). As a result, thermal
4 Fundamentals of Low Dimensional Magnets
FIGURE 1.2 (a) Schematic illustration of the coercivity-size (HC – d) relations of small ferromagnetic particles.
As the size of ferromagnetic particles decreases, the HC initially increases and reaches a maximum value at
the critical single domain size (DS ). Furthermore, in the single domain regime (d < DS ), the HC decreases as the
particle size decreases until it reaches zero at (d = DSP ), known as a superparamagnetic regime. (b) Variation of
critical size for single domain DS and superparamagnetic limit DSP for common ferro and ferri materials.
energy dominates the energy barrier for magnetization reversal, which spontaneously randomizes the
magnetization of a particle from one easy axis to other directions even in the absence of a magnetic field.
In the most extreme case, when the particle size is reduced further, the HC becomes zero, resulting in a
superparamagnetic state. Figure 1.2 depicts how the critical size for superparamagnetic transition (DSP)
and from a single domain to a multi-domain state (Ds) for several common magnetic materials depends
on the type of material. Unlike the ferro-to paramagnetic transition in bulk magnets, the ferro-to super-
paramagnetic in nanoparticles is entirely from the size effect.
In the limit, kB T KV the superparamagnetic particle can be considered freely fluctuating with a fre-
1
quency f or a relaxation time, 2 f , which can be modeled by the Néel-Brown theory, expressed
as [13]
KV
0 exp (2)
k BT
where 0 is a material-specific relaxation time in the range of 10 –9 to 10 –13 s. The increases as the samples
are cooled to lower temperatures, which means fluctuations slow down. When >> m (experimental measur-
ing time), the system becomes static. Therefore, magnetization is measurable only if m. However, m
the particle is said to be blocked and the magnetic properties are characterized as “blocking” temperature,
TB, below which the particle moments appear to be frozen for m . The TB can be obtained from Eq. 2 as [13]
KV
TB (3)
kB ln m 0
It is clear from Eq. (3) that for the same size non-interacting particles, TB depends on the magnetocrystal-
line anisotropy and the volume of magnetic nanoparticles. However, interparticle interaction modifies the
energy barrier and produces collective properties in the case of multicore nanoparticles. The Vogel-Fulcher
model describes the magnetic properties of interacting MNMs with reasonable accuracy, given by [13]
Ea
0 exp (4)
kB T T0
1 • Nanomagnets 5
where T0 is the Vogel-Fulcher temperature (a measure of the interaction strength) and activation energy
(Ea kB ) is required to overcome the energy barrier for magnetization reversal. With increased inter-
particle interaction, a spin-glass-like collective state, called super-spin glass, is formed. Also, in the
super-ferromagnet state, long-range ferromagnetic order is occurred due to strong enough interparticle
interaction. Recently, super-spin glass and the super-ferromagnetic state have been realized in Fe3O4 and
Co nanoparticles. The inter-/intraparticle interactions can be controlled to further manipulate the mag-
netic properties, apart from size, shape, composition, and morphology.
dm dm
m H eff m (5)
dt dt
where m M Ms, Ms , γ, α, H eff are the saturation magnetization, gyromagnetic ratio, Gilbert damping,
and effective magnetic field, respectively. The first term on the right of Eq. (5) corresponds to the preces-
sional motion of the magnetization vector m about the effective magnetic field direction. The second
term is the damping term responsible for the magnetization vector’s alignment in the direction of H eff .
Schematic representations of magnetization precession with and without damping are shown in Figure 1.3.
Under the macro-spin model and for a uniform ellipsoidal particle with demagnetizing tensor axes
Nx, Ny, and Nz, where Nx + Ny + Nz = 4π, Kittel [14] derived the frequency for the uniform precessional
mode, also known as ferromagnetic resonance (FMR) mode as
r H 0 Ms N x Nz H 0 Ms N y Nz (6)
FIGURE 1.3 Sketch of magnetization precession around an applied magnetic field (H) (a) without damping
and (b) with Gilbert damping.
6 Fundamentals of Low Dimensional Magnets
For a thin film magnetized in the film plane (N x 1, N y N z 0 in SI unit), this formula reduces to
[15] r H H 0 M s , while for magnetizations oriented in the axis normal to the film plane (N x Ny 0
, N z 1), it turns out to be r H 0 M s . For a spherical particle (N x N y N z ), the FMR can be
written as r equations are valid for the case of a vanishing wave vector, k sw 0.
H . These
For the cases of k sw 0, the phase of the precessions of the neighboring spins differs, and thus
the spins are no longer parallel with each other. Thus, the exchange interaction cannot be neglected.
Furthermore, the dynamic part of the magnetization is a function of the position. Herrings and Kittel
derived the dispersion relation of dipole-exchange spin waves in a ferromagnetic material of infinite size
as given by the formula:
2 2
ksw H eff Dksw H eff Dksw 0 M s sin 2 (7)
where is the angle between the direction of the spin-wave vector and the static magnetization; Heff
denotes effective magnetic field, which is as defined by the vector sum of externally applied magnetic
field and the demagnetization field in the magnetic film, and D 2 gL B A M s is the exchange stiffness
constant, where gL is the Landé factor and B is the Bohr magneton.
However, the dispersion relation cannot be explained by Eq. (7) for ultrathin films. R. W. Damon
and J. R. Eshbach solved the Landau-Lifshitz-Gilbert (LLG) equation by considering Maxwell’s equa-
tions in the magnetostatic limit for an in-plane magnetized thin film [14]. They discovered two types of
solutions: the surface (Damon-Eshbach or DE) and the volume mode. In general, for an in-plane mag-
netized film, the surface mode called magnetostatic surface wave (MSSW) propagates perpendicularly
to the magnetization, while the volume modes, called backward volume magnetostatic wave (BVMSW),
propagate along the magnetization direction. Considering negligible anisotropy, the dispersion relations
of MSSW and BVMSW modes are given by [15]
2 2
Ms
0 0Ms 2 ksw d
fMSSW H eff e (8a)
2 2 2
1 e ksw d
fBVMSW H eff H eff 0 Ms (8b)
2 ksw d
The negative slope of the dispersion of BVMSW implies that the group and phase velocity are in opposite
directions. On the other hand, a forward volume magnetostatic wave (FVMSW) mode can be excited in
normally magnetized films, whose dispersion is described as
1 e ksw d
fFVMSW H eff H eff 0 Ms 1 (8c)
2 ksw d
The amplitude of the magnetization precession has a cosinusoidal distribution across the film thick-
ness for both volume modes (FVMSWs and BVMSWs). On the other hand, MSSWs are localized to
one film surface on which they propagate. The distribution of precessional amplitude across the film
thickness is exponential, with a maximum at one surface of the film, and can be switched to the other
surface by reversal (i.e., 180° rotation) of either the field or the propagation direction. The SWs can
be quantized in the plane of the film of confined magnetic structures. For a nanostructure of width
w, the quantized SW vectors can be written as k n = 2π λn = nπ/w. However, solving the LLG equation
(5) within the framework of micromagnetism is a convenient alternative for calculating the quantized
SWs [16].
1 • Nanomagnets 7
FIGURE 1.4 Nanostructure geometries: (a) nanoparticles of different shapes, (b) core-shell spherical nanopar-
ticle, (c) chain of fine particles, (d) striped nanowire, (e) cylindrical nanowire, (f) nanodots, (g) antidots, and
(h) self-assembly of nanorods.
8 Fundamentals of Low Dimensional Magnets
or with nonmagnetic metal and insulating materials. Recently, there has been an increasing demand for
fabricating epitaxial thin films in the nanostructured form to miniaturize integrated devices and extend
the potential of establishing full nano-scale architectures crucial for modern and future spin-electronic-
based devices.
The research on MNM progressed smoothly from elongated dots or thin-film patches to nanowires
form, which is scientifically exciting due to their future applications in advanced nanotechnology [20].
Initially, the research on magnetic nanowires focused on exploratory issues, including the synthesis pro-
cess to control or establish magnetic easy axis and magnetostatic interactions between the wire and the
significance of shape anisotropy over magnetocrystalline. More recently, attention has shifted towards
understanding the magnetization dynamic in nanowires or patterned magnetic media for magneto-optical,
microwave nano-electronics applications [6].
Finally, the MNM in granular and composites form has significant importance in nanotechnology
and science. The structural correlation lengths of typical nanocomposite materials range from 1 nm in
amorphous X-ray structures to several 100 nm in submicron structures. Some well-known nanocomposite
materials have been used in devices including Nd–Fe–B alloy as permanent magnets, Fe–Cu–Nb–Si–B
amorphous alloy as soft magnets, and Co–Ag granular composites as magnetoresistive materials.
FIGURE 1.5 Different approaches and methods for synthesizing magnetic nanomaterials.
Hydrothermal synthesis, which employs the liquid-solid-solution (LSS) reaction, is another important
chemical synthesis method that provides excellent control over the shape and size of the MNMs. The
general strategy is based on the phase separation occurring at the interface of LSS phases present in the
reaction at different temperature conditions. For example, metal nanomaterials were produced by reduc-
ing metal ions with ethanol at the interfaces of solid metal linoleate, the liquid phase of ethanol-linoleic
acid, and water-ethanol solutions at different temperatures under hydrothermal conditions.
Another method to synthesize MNMs is the microemulsion approach, where two immiscible liquids
are mixed to form a thermodynamically stable isotropic dispersion, and the interfacial film of surfactant
molecules stabilizes the microdomain of one or both liquids. The microemulsion technique has been used
to prepare a variety of MNMs, including metals and oxide, as detailed in several recent articles [3, 8].
Co-precipitation is a facile and convenient method for synthesizing oxides MNMs from aqueous
metal salt solutions by adding a base in an inert atmosphere at room or elevated temperature. On the
other hand, the shape, size, and composition of the MNMs are determined by the type of salts used (e.g.,
chlorides, nitrates, or sulfates), the reaction temperature, the pH value, and the ionic strength of the media.
Significant progress has been made in preparing monodisperse MNMs using organic additives as a stabi-
lizer or reducing agent. For example, Fe3O4 nanoparticles (4–10 nm) can be stabilized in 1 wt% polyvinyl
alcohol (PVA) aqueous solution, but it forms chainlike clusters when PVA with 0.1 mol% carboxyl groups
is used as the stabilizer. This finding suggests that selecting a suitable surfactant is critical for the stabili-
zation of nanoparticles [3, 19].
The sol-gel is a proper wet chemical method that involves three steps. The first is to initiate a sol of
nm-sized particles by hydroxylation and condensation of molecular precursors. Second, further condensa-
tion and inorganic polymerization result in forming a 3D metal oxide network known as a wet gel. Finally,
heat treatment of gels results in fine crystalline MNMs. The shape, size, and composition of the MNMs
are determined by the type of solvent, precursors, additives and catalysts, pH value, reaction temperature,
10 Fundamentals of Low Dimensional Magnets
and mechanical agitation that affects the reaction rate, hydrolysis, and condensation process during the
reaction. Compared with other methods, the sol-gel process offers several advantages for metal oxides
compared to other methods. This includes excellent homogeneity, low cost, and high purity [2, 22].
Table 1.1 summarizes the benefits and drawbacks of the aforementioned synthesis methods. Asides
from the methods listed prior, electrochemical reactions, solvothermal synthesis, atomic or molecular
condensation, plasma or flame spraying synthesis, sputtering and thermal evaporation, chemical vapor
deposition, and bio-assisted synthesis are all used to prepare MNMs [1, 3, 20, 23–25].
as separating NMs from solution or retreatment of byproducts. Although this method is environmentally
friendly and requires minimal energy to produce less contaminated nanopowders, the produced MNMs
are not monodispersed [3].
The wire explosion technique is a new physiochemical technique that is a safe and clean process for
synthesizing MNMs. This method is a one-step, highly productive process that requires no additional
steps like separating NMs from solution and retreatment of byproducts. This method was previously used
to prepare iron oxide MNMs for the removal of arsenic from water. It is environmentally safe and requires
minimum energy for making less contaminated nanopowders. However, the NMs produced through this
method are not monodispersed [3].
(ii) lift-off. A film is first deposited on a substrate in the case of the etching process. The film is then spin-
coated with a photoresist to form the desired resist pattern for lithographical patterning. Finally, the film
is etched through the resist mask to create the desired pattern. However, in the case of the lift-off process,
a resist is first spin-coated onto a substrate. After patterning the resist, a film is deposited on the substrate
1 • Nanomagnets 13
containing the patterned resist. After removing the resist from the substrate, the thin film deposited on the
resist-free regions remains. Many reports are on the static and dynamic magnetic properties of nanostruc-
tured MNMs, including NiFe, Fe, Co, and Co/Pt [5, 16].
Another important approach for nanostructures fabrication is template-assisted techniques, with two
significant advantages over other methods [26]. The first is the template determines the shape and the size
of the nanostructures formed, and the second is the ease of fabrication of complex nanostructures with
precise control of the composition along different branch lengths. On the other hand, this method has the
inherent disadvantage of being a two-step process involving the production of high-quality templates fol-
lowed by the deposition of magnetic material in the template. Furthermore, although several templates are
commercially available, the options for pore size, thickness, and uniformity necessitate in-house template
fabrication. Therefore, the method has usually employed the fabrication of highly oriented metal (Ni, Co)
and metal oxide nanowires [1].
progress has been made in HAMR, and companies in the HDD industry are looking to bring HAMR
technology to products in the coming years. Seagate expects to have 36 TB HDDs by 2022, 48 TB drives
before 2024, and 100 TB units in 2025. According to Seagate, HAMR can double the areal density every
2.5 years. The current PMR technology may be phased out in the future, and it appears that HDDs will
remain a valuable asset for those seeking the lowest price per GB.
Researchers are interested in patterned magnetic media such as 2D dot arrays because of their poten-
tial applications in information storage or nonvolatile magnetic random access memory (MRAM) [5–6].
Microwave-assisted magnetic recording (MAMR) was also proposed as another energy-assisted record-
ing approach [29]. MAMR reduces the switching field by an order of magnitude by using microwaves gen-
erated by a spin-transfer oscillator patterned in the write gap of the write head. Significant progress has
been made lately on this technology. As a huge amount of data is generated and needs to be stored every
day, advancement and progress for HDD, the leading candidate for next-generation storage, are required,
while flash-based solid-state drives (SSDs) are proving to be a fierce competitor to conventional HDDs.
spin-wave excitations, which form the basis of a new spintronics concept called magnonics. The details
are discussed in the section on emerging research in nanomagnetism. Furthermore, the discovery of spin-
transfer torque (STT) and spin-orbit torque (SOT) of giant TMR in MgO-based magnetic tunnel junc-
tions (MTJs) and large interfacial magnetic anisotropy at magnetic metal/oxide interfaces has led to the
development of scalable nonvolatile MRAMs [5, 32]. The commercial STT-MRAMs are now used as a
replacement for embedded flash (eFlash) memory or static RAM (SRAM) in embedded cache memories
due to their easy integration with CMOS technology, low power consumption, and ultrafast switching
with superior endurance [6].
T Msol
SAR Cs (9)
t M MNM
where Cs is the specific heat capacity of the solvent, Msol is the mass of the solvent, MMNM is the mass of
the MNM, and ΔT/Δt is the temperature-time-dependent slope.
An important application of magnetic nanoparticles in biomedicine is MRI contrast agents, where
superparamagnetic nanoparticles shorten the spin-spin relaxation time (T2) of surrounding water protons
in the presence of an external magnetic field, enabling these regions to darken in the T2 weighted images
16 Fundamentals of Low Dimensional Magnets
[8, 18, 37–39]. To ensure effective deployment as a contrast agent, MNM needs to target the desired tissue
specifically. The specific targeting requires surface functionalization of MNM using various targeting
agents. Iron oxide nanoparticles are used as the contrast agent with and without specific targeting [38].
Many other actively targeted magnetic nanostructures, including anti-carcinoembryonic antigen conju-
gated iron oxide and chlorotoxin conjugated PEG-coated iron oxide nanoparticles, have improved con-
trast in magnetic imaging [40]. In addition, spinel ferrites MFe2O4 (where M is +2 cation of Mn, Fe, Co,
or Ni) were also investigated for in vivo MR targeted imaging [18, 40]. Herceptin conjugated Mn-doped
iron oxide nanoparticles showed highly sensitive targeted in vivo mice imaging among all the targeting
agents. Nevertheless, there are various reports of multifunctional nanoparticles consisting of magnetic
and plasmonic materials, which can provide multifunctionality and will remain an active research area
for biomedical applications.
Another successful application of MNMs in the environment includes plant protection, seed germi-
nation, and soil quality improvement. For example, iron is an essential element that is required for many
physiological activities in plants, including respiration, chlorophyll, biosynthesis, and redox reactions.
Therefore, functionalized iron oxide MNMs were used as soil nutrients to increase production with mini-
mal negative impacts [44]. The use of MNMs possesses great potential with unprecedented opportunities
in improving water, soil, and environmental quality [45]
where information is encoded in the presence or absence of magnetic domain walls, which can be revers-
ibly shifted using electric charge currents. Furthermore, the chirality of the domain walls can also pro-
duce more complex spin textures known as skyrmions, which can be used in information carriers [49].
Indeed, it has already been demonstrated that 2D patterns can be coherently moved in magnetic-field-
driven skyrmion bubbles paving the way for massively parallel racetrack memories [49].
The SHE discussed prior has been instrumental in ushering in a new field of research, spin-
caloritronics [32], wherein, in addition to purely charge current–driven effects, the interaction of spin and
charge degrees of freedom with heat currents are also investigated. The key phenomenon that started this
field is the spin Seebeck effect (SSE), where the generation of a spin current in a bilayer system made up
of a ferromagnetic material (FM) and a normal metal (NM) via the magnetization dynamics induced by
the application of a thermal gradient. The nontrivial connections between heat and spin transport have
opened several fundamental questions and new application possibilities.
As discussed in Section 1.2.2, “Magnetization Dynamics”, spin waves are the fundamental quasipar-
ticle excitations of magnetically ordered systems and are also referred to as magnons. Spin waves have
been proposed as information carriers for low-power data storage and processing, which has given rise
to the field of magnonics [15, 50]. The utilization of magnonic approaches in spintronics gave birth to the
field of magnon spintronics. Furthermore, magnonic crystals are critical components for magnon spin-
tronic applications as they enable access to novel multifunctional magnonic devices. These devices can be
used as spin-wave conduits and fitters, sensors, delay lines, phase shifters, auto-oscillators’ components,
frequency and time inverters, data-buffering elements, power limiters, nonlinear enhancers in a magnon
transistor, and components of logic gates [4, 51].
The ongoing development of devices and experimental techniques that utilize the quantum nature of
the system for the storage, transfer, and processing of quantum information is important to achieve the
ambitious goals of workable quantum computation. In the last few years, an exciting strategy has been
explored to access, through hybrid quantum systems, novel capabilities in existing quantum technologies
by combining different physical systems (e.g., photons and magnons) possessing distinct characteristics
[52]. Combining the superconducting resonator and nano-scale FM material opens a promising platform
for investigating on-chip quantum magnonics and spintronics. It brings new potential for coherent manip-
ulation and long-distance propagation of spin information.
Cleavable two-dimensional van der Waals (vdW) nanomaterials, which can thin down to monolayer,
showed the multiple functionalities due to quantum confinement of the electrons in one plane, revolu-
tionizing the research world. As noted earlier, that size can significantly modify the spin configuration.
Therefore, for the completeness of the topic, we briefly review the advances in vdW MNMs. The 2D
magnetism research field is rapidly growing since the long-range magnetic order recently added function-
ality in the vdW material’s category, which can survive down to the monolayer limit. A wide variety of
materials with different spin ordering were realized with a short period of the last five years, and many
more are theoretically predicted. Namely, insulating antiferromagnets (FePS3, MnPS3, NiPS3, MnPSe3),
ferromagnetic semiconductors (CrI3, CrGeTe3), and ferromagnetic metals (MnSe2, VSe2). The magnetic
anisotropy, spin direction, interlayer magnetic ordering, exchange gap, and magneto-optical behavior can
be easily tuned via just varying the halogen composition within the same material family. Different types
of magnetic ordering behavior are observed in the same materials (CrI3) depending on the odd number of
layers used. The magnetic ordering temperature of the vdW MNMs ranges from room temperature down
to liquid helium. The current synthesis status of the vdW MNMs is laboratory-based single-crystal growth
using chemical vapor transport and self-flux methods, and the monolayer is exfoliated using mechanical
means. There is a long way to go to achieve batch processing for practical devices. The biggest hurdle
is the stability of vdW MNMs in the air, which severely affects performance. Investigating vdW MNMs
aims to discover a stable semiconducting material with long-range FM order above room temperature
with gate tenability, which means the magnetic order can be controlled by an applied electric field. This
allows realizing low-power data storage and transport application that can synchronize with current
CMOS technology.
1 • Nanomagnets 19
1.8 SUMMARY
This introductory chapter discusses broad aspects of the research on magnetic nanomaterial MNMs,
from their fundamental nanomagnetism to synthesis to essential applications. The synthesis of MNMs
has made significant progress focusing on the geometry of nanoparticles and uniformity yield, particu-
larly with chemical routes and advanced techniques such as nanolithography and laser ablation. Details
of some of the important synthesis routes and different shapes of nanostructures are presented based on
the available literature. Next, diverse applications of MNMs are summarized, ranging from biomedical
to information technology. Biomedical is the most benefited field where MNMs are applied to diagnose
the infected region through nanomedicine imaging or treatment. Various magnetic nanoparticle modali-
ties are being studied in clinical trials for cancer cell imaging and therapy. The information technology
sector is constantly searching for low-power, efficient, and high-density data storage and transfer devices
to tackle energy crises. Some of the recent developments in nanomagnetism with new computational
paradigms for information processing technologies have been proposed. The vdW MNMs are promising
candidates for low-power devices; a short description is given here. This chapter provides a helpful intro-
duction and different perspectives on nanomagnetism and its emerging research field for the researcher
working on physics or chemistry or the beginner material scientist or biologist.
1.9 ACKNOWLEDGMENTS
One of the authors, B. Bhoi, acknowledges BK21 PLUS SNU Materials Education/Research Division for
Creative Global Leaders for providing financial support while preparing this book chapter.
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Nanostructured
Magnetic
Semiconductors
2
Alessandra S. Silva1, Éder V. Guimarães1,
Tasso O. Sales2, Wesley S. Silva2, Elisson A.
Batista3, Carlos Jacinto2, Anielle C. A. Silva4,5,
Noelio O. Dantas4, and Ricardo S. Silva1
Contents
2.1 Introduction 24
2.2 Nanostructured Materials and Quantum Dots 24
2.3 Diluted Magnetic Semiconductor (DMS) Nanocrystals 25
2.3.1 Exchange Interactions in DMS Nanocrystals 26
2.3.2 Crystal Field Theory (CFT) 26
2.4 Experimental Procedures 26
2.4.1 Growth of NCs in Glassy Matrices 26
2.4.2 Growth of NCs by the Precipitation Method 27
2.5 Nanostructured Magnetic Semiconductors 27
DOI: 10.1201/9781003197492-2 23
24 Fundamentals of Low Dimensional Magnets
2.1 INTRODUCTION
Nanoscience and nanotechnology are present in the world market in the form of technologically sophisti-
cated products, such as state-of-the-art microprocessors, digital TVs, cell phones, and broadband Internet
with optical fibers, among others. Advances in nanomaterials have allowed the development of new types
of lasers, as well as an increase in densities and digital data storage capacities [1]. There is also the diag-
nosis of diseases using nanosensors and in the release and control of drug concentration in the body [2],
more diversified and efficient nanometric catalysts [3], advanced materials for prostheses [4], and destroy-
ing viruses or cancer cells, where they are located in the body [5].
Due to the worldwide relevance of nanoscience and nanotechnology, several research groups are engaged
in studying a variety of nanostructured compounds. The main motivation for the study of nanostructures/
nanocrystals (NCs) is that, at nanometer size scale, the materials have unique and often surprising proper-
ties caused by effects predicted by quantum mechanics. That is, due to size reduction or shape modification,
these NCs can present quantum confinement effects of their charge carriers, thus being called quantum dots
(QDs). On the other hand, when these QDs are doped with magnetic impurities, their properties can be mod-
ified by interaction dependent on the size and concentration of the magnetic ions in the host environment.
Possible applications for materials with these properties include: dramatically increasing the data stor-
age and processing capacity of computers; creating new mechanisms for drug delivery that are safer and less
harmful to the patient; creating materials for buildings, cars, and airplanes that are lighter and more resistant
than metals and plastics; and many more innovations aimed at saving energy, protecting the environment, and
using less scarce raw materials. In the field of nanotechnology, these are very current and concrete possibilities.
technological applications of photonic devices operating in the blue and ultraviolet region and fabrication
of nanodevices electronics, among others [8]. Lead sulfide (PbS) is a crystalline material that belongs to
the narrow gap semiconductor class (bandgap of 0.41 eV). PbS has been used in optoelectronics, sensors,
solar cells, diodes, and laser technologies. However, the structural, electronic, optical, and thermal prop-
erties of this semiconductor can be enhanced with changes in the gap energy. The relatively large exciton
Bohr radius (20 nm) [9] provides the PbS quantum confinement regimes that keep the crystal structure
(rock-salt) undisturbed. Different quantum confinement regimes change the bandgap to 5.2 eV in PbS
quantum dots [10]. The breaking of translational symmetry, large surface area, structural defects, and
anisotropic surface [11] are phenomena provided by quantum confinement that differ from such proper-
ties in PbS. PbS NCs are efficient sensitizers: that is, materials that can be applied as photodetectors and
photovoltaic cells in the near-infrared region [12]. At room temperature, ZnTe NCs have very interesting
physical properties, such as cubic zincblende-type structure, exciton Bohr radius aB = 5.2 nm, and an
energy gap around Eg = 2.26 eV, [13]. Depending on their size and shape, these quantum dots can absorb
and emit light in the visible and near-ultraviolet (UV) electromagnetic spectrum. In this context, ZnTe
QDs have been synthesized using several methods, such as: molecular beam epitaxy (MBE), in which
ZnTe NCs are usually self-assembled in layers of another semiconductor, such as ZnSe, and grown on
a GaAs substrate [14]; colloidal, in which the ZnTe QDs are obtained from organic solutions and it is
possible to control the size and shape through the synthesis temperature, growth time, concentration of
precursor reagents, and chemical nature of the ligands [15]; and mechanical alloying, in which the precur-
sors in powder form are subjected to mechanical grinding, resulting in the formation of NCs with desired
properties, after suitable thermal treatment of the nanopowders [13].
Although QDs have been synthesized by the aforementioned methods, some possible applications
require these NCs to be incorporated into robust and transparent host materials. Thus, a way to produce
high-quality QDs is to grow them in a host glassy system synthesized by the fusion method. Thus, from
post-fusion heat treatments, precursor ions diffuse and form NCs. Two parameters are fundamental at
this stage: the time and temperature of the heat treatment, which allow for the control of the size of the
NCs. Another effective and viable way to produce these nanostructures is by the chemical precipitation
method via aqueous solution. Among the characteristics of this method, it can be mentioned that it is a
relatively simple, efficient, reproducible process, applicable on a large scale, with relatively low cost, envi-
ronmentally friendly, and allows direct control over synthesis parameters, enabling greater control over
the composition, shape, and size of NCs [16].
H H0 H int H sp d Hd d (1)
in which H0 describes the kinetic and potential energies of the exciton in a perfect crystal, Hint describes the
intrinsic interaction of the exciton with the external magnetic field, Hsp-d describes the magnetic exchange inter-
actions (sp-d) [electrons (e) and holes (h)] and the magnetic dopants, and Hd-d refers to the interaction between
the neighboring TM ions, which interact through the so-called d-d double exchange interaction. Depending on
the magnetic dopant concentration (low concentrations), this d-d interaction is weaker than the sp-d interaction,
since, in this case, it can be disregarded. The term Hint is independent of the magnetic dopant concentration, and
it can be considered as an intrinsic contribution to the total Zeeman splitting observed in doped semiconduc-
tors. Already the term Hsp-d is directly dependent on the concentration of the magnetic dopants.
The compounds in powder form were properly weighed, following the appropriate stoichiometry,
and mixed and homogenized using sterilized alumina crucibles. Afterward, the composition of the glassy
matrix PZABP underwent a fusion process at 1300oC for 30 minutes. Already the SNABP glassy matrix
was subjected to 1200oC for 30 minutes. Soon after the fusion, the resulting melt was poured onto a metal
plate, at a temperature of approximately 0oC, becoming glass.
The doping of the glassy matrices, synthesized by the fusion method, was carried out, remelting them
already pulverized, with the addition of dopants and also at the same melting temperature and time. The
compounds were homogenized and then subjected to fusion. Thus, adopting the same synthesis procedure
of the glassy matrices, the melt of the doped glassy matrix was poured onto a metallic plate at 0oC, obtain-
ing, in this way, glass sheets doped with precursor ions. Soon after fusion, followed by rapid cooling, the
glassy samples were subjected to appropriate heat treatments to favor the nucleation and growth of NCs
from different dopings.
FIGURE 2.1 (a) TEM and AFM/MFM images, XRD diffractograms, Raman spectra, and EPR spectra of the
samples containing Zn1-x MnxTe NCs, with Mn concentrations of x = 0.00, x = 0.05, and x = 0.10. Reproduced
with permission from [29]. Copyright (2012) Elsevier. (b) OA and EPR spectra and TEM images of the samples
containing Zn1-x CuxTe NCs, with Mn concentrations of x = 0.00, x = 0.05, and x = 0.10; Incorporation of Mn2+
or Cu2+ ions into the ZB crystal structure of ZnTe. Reproduced with permission from [30]. Copyright (2018)
Elsevier.
2 • Nanostructured Magnetic Semiconductors 29
the Zn1-xMnxTe spherical NCs (with x = 0.05) and an interplanar spacing d ~ 0.346 nm corresponds to
the densest (111) plane of ZnTe (zincblende) [25]. AFM and MFM images refer to ZnTe NCs (x = 0.00)
and Zn0.95Mn0.05Te NCs (x = 0.05) embedded in the glass. Each panel shows a topographic image and a
corresponding magnetic phase image, as well as the size distribution of NCs. The images corresponding
to Zn0.95Mn0.05Te NC are enlarged to better visualize the formation of NCs and magnetic contrasts. The
mean radius range is from 2 to 10 nm, assigned to two groups of NCs with different sizes: a group that
presents quantum confinement (QDs), with an average radius around ~ 2 nm, and another group without
quantum confinement, with an average radius around ~ 10 nm, also called bulk NCs. This result has
already been well discussed in our previous work [26].
XRD patterns were obtained to evaluate the crystallographic characteristics of the as-synthesized
Zn1-xMnxTe NCs in terms of Mn x-content (0.00, 0.05, and 0.10), as well as of the pure PZABP sample
(Figure 2.1a). The XRD pattern of the PZABP glassy matrix shows an amorphous band at around 20°
< 2θ° < 30°, confirming the glassy characteristics [27]. Zn1-xMnxTe NC samples present the typical (111),
(220), and (311) diffraction peaks of ZnTe with a cubic zincblende (ZB) structure (JCPDS: 15–0746) [27].
It is noted that the typical bulk ZnTe zincblende crystal structure is preserved for the Zn1-xMnxTe NC
samples having an Mn-concentration of x = 0.05. Nevertheless, the characteristic XRD peaks are shifted
towards higher diffraction angle values as the Mn2+ incorporation in the host ZnTe increases. This is a
clear indication of decrease in the lattice constant with doping [28]. This decrease in the lattice constant
is related to the replacement of Zn2+ ions in the zincblende ZnTe crystal structure by Mn2+ ions. At lower
concentrations, the zincblende crystalline structure remains unchanged. On the other hand, for high con-
centrations of Mn2+, a decrease in the crystallographic quality of the samples is expected.
Raman spectra revealing transitions at approximately 215 cm-1 (1LO), 323 cm-1 (TO/LO + LA), and
428 cm-1 (2LO) correspond to the normal phonon modes characteristic of the ZnTe phase (zincblende)
[29]. It is observed that Raman scattering, refer to the 1LO phonon, presents a blue shift with increasing
Mn concentration. This blue shift is related to the different atomic mass of ZnMnTe NCs where a heavier
Zn2+ ion is replaced by a lighter Mn2+ ion: m Mn (55) < m Zn (65) [29]. This causes an increase in phonon
mode frequencies with increasing Mn concentration.
In Figure 2.1a, the six absorption lines in the EPR spectrum result from the hyperfine interaction
between the electron spin (S = 5/2) and the nuclear spin (I = 5/2) of Mn2+ ions and are due to transitions
between electronic states MS = ±1/2 obeying the selection rules ΔMS= ±1 and ΔMI = 0 [31], as shown by
the energy diagram. This result indicates that the paramagnetic Mn2+ ions are well incorporated into the
Zn1-xMnxTe NCs.
Figure 2.1b presents OA and EPR spectra and TEM images of the samples containing Zn1-xCuxTe
NCs, as a function of Mn concentration. When x = 0.00, are observed absorption bands centered at 3.10
eV (400 nm) and 2.33 eV (535 nm) that are attributed to ZnTe QDs and bulk NCs, respectively [26]. This
result confirms the evidence observed from the AFM images in Figure 2.1a. An increase in Cu concentra-
tion (from 0.00 to 0.05) causes a redshift in the OA bands assigned to the QDs, from 3.10 eV (400 nm) to
2.95 eV (420 nm). This decrease is the strong evidence of Cu2+ being incorporated into ZnTe QDs, since
the energy gap of ZnTe bulk (2.26 eV) tends to CuTe gap bulk (1.5 eV) [30]. This interesting behavior
shows that the sp-d exchange interaction in the QDs is stronger than in the bulk-like NCs, when x = 0.05.
However, when x = 0.10, a large redshift of the absorption bands of both QDs and bulk ZnTe NCs is
observed. This result is related to the fact that at high concentrations of Cu2+, it is not possible to distin-
guish between Cu2+ ions that are incorporated into the NCs and those that are dispersed in the PZABP
glass matrix. The increase in the band centered at 535 nm (2.32 eV) with increasing copper concentra-
tion is due to two overlapping absorption bands: bulk NCs (around 535 nm) and a band around 531 nm
(2.33 eV), which has been attributed to 2B1g → 2Eg transition copper ions with monovalent nature (Cu1+)
[30], dispersed in the glass matrix. In addition is observed one band centered at 890 nm (1.39 eV), which
becomes more intense with increasing Cu concentration. This band is attributed to the 2B1g → 2B2g transi-
tion of Cu2+ ions in the distorted octahedral sites [30]. In the octahedral crystal field, the free term for Cu2+
(d9) ion is 2D, which is divided into 2Eg and 2T2g, with 2Eg being the state of lowest energy. Due to the Jahn-
Teller effect, which causes distortions in octahedral symmetry, the 2Eg ground state can be divided into
30 Fundamentals of Low Dimensional Magnets
2B1g and 2A1g and the 2T2g state into 2B2g and 2Eg, with 2B1g being the ground state [30]. The energy diagram,
containing these transitions, is represented in Figure 2.1b. It is well known that in a crystal structure of
the zincblende type, e.g., the ZnTe lattice, a cation Zn2+ is bound to four Te2– anions (i.e., the coordination
number CN = 4), with tetrahedral coordination (Td). Thus, according to the results obtained from the OA
spectra, one can infer that part of the Cu2+ ions are dispersed in the PZABP glass matrix, since, probably,
these ions may have octahedral (Oh) symmetry with tetragonal distortion, and the other part of these are
incorporated into ZnTe NCs, which further increases these tetragonal distortions in Oh symmetry.
TEM image confirms the formation of the Zn1-xCuxTe spherical NCs (with x = 0.00 and x = 0.05)
in the PZABP glass matrix. The amplified region of the TEM image shows an interplanar spacing d ~
0.346 nm corresponds to the densest (111) plane of the zincblende ZnTe crystal system [30]. The invari-
ance of this interplanar spacing with the incorporation of Cu2+ is expected since there is a little difference
between the Zn2+ (0.68 Å) and Cu2+ (0.73 Å) ionic radii [30]. Thus, it is expected that the lattice parameter
of the NCs is not modified with the incorporation of Cu2+ ions in ZnTe NCs.
The EPR spectra of the PZABP glass samples and this containing Zn1-xCuxTe NCs with concentra-
tions of 0.00, 0.05, and 0.10 are shown in Figure 2.1b. In these spectra, one can observe four parallel weak
components in the lower magnetic field region and a corresponding intense resonance signal to the four
components perpendicular to the higher magnetic field region. The parallel components are due to the
hyperfine interaction between the electron spin (S = 1/2) and nuclear spin (I = 3/2) of 63Cu and/or 65Cu,
resulting from transitions between electronic states MS = ±1/2 and MI = ±3/2, ±1/2 [32], according to the
selection rules ΔMS = ±1 and ΔMI = 0, as shown in the inset of Figure 2.1b. This hyperfine interaction, due
to Cu2+ ions, results from the presence of a distorted axially octahedral crystal field [32]. Since the sym-
metry is smaller than octahedral, anisotropy occurs mainly in the values of g and A tensors. Thus, this
anisotropy observed in EPR spectra may be related to Cu2+ ions incorporated into sites with tetrahedral
symmetry (Td) of ZnTe NCs, replacing Zn2+ ions (following the redshift observed in the OA spectra), as
well as dispersed in PZABP glass matrix, especially in high Cu concentrations. The hyperfine interac-
tion, due to Cu2+ ions incorporated in the zincblende lattice of ZnTe NCs, results from the presence of a
crystalline field in these NCs, which favors the strong interactions of exchange between the d sublevel of
electrons of Cu2+ ions and electrons in the sp sublevel of host semiconductor (ZnTe).
Figure 2.1b shows the ZB crystal structure of ZnTe with the incorporation of A2+ ions (A = Mn or
Cu). As discussed in the introduction of this chapter, this incorporation enables an exchange interaction
between the host electronic subsystem and the electrons of the partially filled d or f levels of the magnetic
ions. This allows for the control of several physical and chemical properties, such as those mentioned in
this study. In turn, these can be explored for various technological applications, such as spintronic devices
and new lasers.
Mn2+ ions have often been used as sensitizers via energy transfer to RE ions such as Nd3+, Er3+, Pr3+,
and Eu3+. As is well known, the transition metal Mn2+ ion–doped luminescent materials, especially the
DMS NCs, are very interesting as they feature a wide range of emissions from green to infrared, depend-
ing on the crystalline environment of said hosts [35]. In this context, the motive of the present study is
to investigate the energy transfer (ET) process between Mn2+ and Eu3+ ions (Mn2+ → Eu3+) on a PZABP
phosphate glass system containing Zn1-xMnxTe NCs, with x ranging from 0.0 to 0.05, and doped with
Eu2O3 (1 wt.%). We use AO and PL at room temperature to study the effect of Mn2+ concentration on
ET processes. OA spectra were recorded with a UV-VIS-NIR spectrometer model UV-3600 Shimadzu
operating between 190–3300 nm, with a resolution of 1 nm. The emissions were obtained using the
Fluorimeter NanoLog™ (HORIBA) armed with a Xenon lamp (CW 450 W) as the excitation source and a
photomultiplier detector (model R928P). The emissions of the sample containing Zn 0.99Mn 0.01Te NCs
of Zn0.99Mn0.01Te were taken with a 410 nm (~3.02 eV) continuous-wave laser focused to a ~ 200 μm ray
with an excitation power of 13 mW [36].
Figure 2.2 presents OA spectra of PZABP glass matrix doped with 1.0 wt.% of Eu2O3, named
PZABP:1Eu, and PL of another PZABP glass matrix doped with 1.0 wt.% of Mn, using a 410 nm (3.02 eV)
excitation line; simplified energy levels diagrams with all the transitions relative to absorption and emis-
sion observed for Mn2+ ions and Eu3+ ions, as well as the energy levels likely involved in the ET processes
between Mn2+ and Eu3+ ions; PL spectra, obtained under excitation at 410 nm (3.02 eV), for glass samples
containing Zn0.99Mn0.01Te NCs (x = 0.01) and doped with 1.0 wt.% of Eu2O3 (named Zn0.99Mn0.01Te:1Eu);
and illustrative scheme of the energy transfer process from Mn2+ to Nd3+/Eu3+ ions.
The OA spectrum shows characteristic transitions of Eu3+ ions centered at 365, 380, 392, 415, 465, 530,
and 583 nm, assigned respectively to the following transitions: 5D0 → 7F0 (580 nm), 5D0 → 7F1 (592 nm),
5D → 7F (612 nm), 5D → 7F (653 nm), and 5D → 7F (700 nm) [37]. The PL spectrum shows the 4T (4G)
0 2 0 3 0 4 1
→ 6A1(6S) main emission of Mn2+ ions. The overlap between the broad emission band centered at around
605 nm, characteristic of Mn2+ ions, the 7F0 → 5D0,1 absorption transition, and characteristic of Eu3+ ions
suggest that an energy transfer can occur from Mn2+ to Eu3+ ions in Zn1-xMnxTe NCs, as shown in the
energy diagram beside. The PL spectra of Zn0.99Mn0.01Te and Zn0.99Mn0.01Te:1Eu samples show, in addition
to the Mn2+ ions characteristic emission, the Eu3+ characteristic emission transitions (5D0 → 7F0 (580 nm),
5D → 7F (592 nm), 5D → 7F (612 nm), 5D → 7F (653 nm), and 5D → 7F (700 nm)) [37]. Comparing
0 1 0 2 0 3 0 4
the PL spectra of both samples, there is a observed decrease in 4T1(4G) → 6A1(6S) emission intensity with
Eu2O3 doping. This decrease is due to the efficient energy transfer from Mn2+ to Eu3+ ions since this ET
efficiently competes with the spontaneous emission of Mn2+ ions. The illustration in Figure 2.2 shows that
Mn2+ ions are excited at 410 nm and emit at 605 nm transferring energy to Eu3+ ions, which emit at 612 nm.
These results indicate that the said glass system is a potential candidate for use in various technological
applications, as in the development of fiber-optic amplifiers used in communication devices and solid-state
laser systems.
FIGURE 2.2 OA spectrum of the PZABP:1Eu glassy sample, and PL spectrum of the PZABP:1Mn glassy sam-
ple, using a 410 nm excitation line; simplified energy levels diagrams with all the absorption and emission tran-
sitions observed for Mn2+ and Eu3+ ions; PL spectra of the Zn0.99Mn0.01Te and Zn0.99Mn0.01Te:1Eu glass samples;
and illustrative scheme of the energy transfer process from Mn2+ to Nd3+/Eu3+ ions.
2 • Nanostructured Magnetic Semiconductors 33
FIGURE 2.3 (a) TEM image of Pb1-xCoxS NCs (x = 0.10). In the inset of (a), the NC size distribution histogram.
On the right of (a), FCC unit cell and Pb1-xCoxS quantum dot with Co2+ dopant at tetrahedral sites. (b) EDS
spectrum of the yellow circle area in the MET images, suggesting the presence of the elements Co, Pb, and S.
(c) MFA topographic image and (d) MFM magnetic image for Pb1-xCoxS NCs (x = 0.10). (e) DRX diffractograms
show the peak (111), characteristic of PbS NCs. (f) Optical absorption spectra of PbS and Pb0,9Co0,1S NCs
incorporated in the SNABP glassy matrix annealed at 500°C for 2 h. The absorption spectrum of the SNABP
glassy matrix is represented in the black background line. (g) Power level diagram for PbS and Pb1-xCoxS NCs.
Reproduced with permission from [29]. Copyright (2021) Elsevier.
34 Fundamentals of Low Dimensional Magnets
strong quantum confinement regime and, consequently, the formation of quantum dots. Due to the similar
growth kinetics, NCs with approximate spherical symmetry have a uniform mean diameter (D = 4.2 nm).
The Gaussian fit performed on the size distribution histogram is shown in the inset of Figure 2.3a. In the
EDS analysis of the circle area (Figure 2.3a), a characteristic Co peak at 6.91 KeV suggests the doping of
PbS NCs with Co2+ ions. In Figure 2.3a the value of the interplanar space d111 = 0.353 nm of the crystal
plane (111) of the PbS rock-salt structure was measured.
The FCC structure of the PbS semiconductor is confirmed by the XRD (111) peak compatible with
the JCPDS card powder diffraction pattern No. 05–0592 (Figure 2.3e) [39]. The sharpness of the XRD
peak shows the presence of crystalline PbS NCs. The small shift of the peak (111) is evidenced by the Co2+
incorporation into PbS NCs. According to Bragg’s law, increasing the θ angle decreases the d111 distance
of the crystal plane (111) and the lattice parameter a = 5.936 Å of the FCC structure.
The AFM (Figure 2.3c) and MFM (Figures 2.3d) images represent the sp-d electronic exchange
interactions that contribute to the total magnetic spin moment in the NCs indicated by the red rectangles.
The light/dark contrast in MFM images is due to the repulsion/attraction of the magnetized tip towards
the NCs. MFM images are evidence of the presence of spin domain upon the incorporation of Co2+ in the
PbS NCs.
Figure 2.3f shows the optical absorption spectrum of PbS and Pb0.9Co0.1S NCs (nominal composition)
grown in the SNABP glassy matrix and thermally annealed at 500ºC for 2 h. The position of the PbS band
around 860 nm (1.43 eV) is evidence of carrier mobility in a strong confinement regime. The SNABP
matrix is transparent in the adsorption and photon emission spectral region of the PbS and Pb0.9Co0.1S NCs.
In the crystal environment of Pb0.9Co0.1S SMD NCs, the free Co2+ ion senses the presence of a ligand
field of S2- ions. The free Co2+ ion energy states represented by the fundamental terms 4F and excited
4P unfold into the spectral terms 4A + 4T + 4T e 4T . Such a phenomenon occurs under the disturbing
2 1 2 1
influence of the expansion of the electron cloud of the tetrahedral crystalline field (Td). These energy
states of the Co2+ ion at crystal sites of the PbS host were identified with the OA spectroscopy technique
UVVISNIR together with the crystal field theory (Figure 2.3f) [38].
The absorption bands in the VIS and NIR regions were analyzed and identified based on the
Tanabe-Sugano energy diagram (C/B = 4.5) 3d7 (Td). The transition energies, the parameters of Racah B
(B = 792 cm−1), and crystal field division (Δ = 3897 cm−1) confirm the high-spin state (weak field) of the
Co2+ ion at tetrahedral coordination sites [CoS4]6− in the crystal structure of the Pb1-xCoxS NCs [38]. The
covalent character of the Co−S bond in the [CoS4]6− complex can be described with the term “electron
cloud expansion” or the nephelauxetic effect. For the free Co2+ ion B = 1028 cm−1. The ratio β = B[CoS4]6−/
BCo2+ for the Pb1-xCoxS NCs (β = 0.73) evidences the reduction in the interelectronic repulsion in relation
to the free Co2+ ion and, consequently, the covalence of the Co−S bond [40]. The excitation energy of a
photoelectron in the NIR promotes the electron from the state e4t23 to e3t24. The transitions allowed by spin
are identified by a green dot. The transition allowed by spin 4A2(4F) → 4T1(4F) around 1477 nm (NIR) can
be explained by a strong spin-orbit coupling interaction. The Co2+ ions at Td sites divide the excited state
4T (4F) into three sub-energy states designated by points of symmetry Γ , Γ e Γ + Γ [40]. The blue dot
1 6 8 8 7
on the spectrum identifies the energy states of the spin-orbit coupling.
In the VIS region of the spectrum, the band around 593 nm is attributed to the transition allowed
by spin 4A2(4F) → 4T1(4P). Such excitation energy consists of the electron in the first excited state of
electron configuration e2t25. The absorption bands at 530 and 661 nm (red dot) are spin-prohibited transi-
tions 4A2(4F) → 2T2(2G) and 4A2(4F) → 2T1(2G), respectively [41]. The energy level diagram (Figure 2.2g)
depicts the PbS and Pb1-xCoxS NCs heat-treated at 500ºC for 2 h. The confinement energy (gap energy) of
exciton charge carriers in NCs with quantum dot properties is much higher than the narrow bandgap (0.41
eV) of the corresponding bulk material (PbS). Cobalt doping provides characteristic d-d transition ener-
gies of the Co2+ ion under the influence of the crystal field Td in terms of ∆ of S2− ([CoS4]6−) ligand ions.
Therefore, sp-d potential and kinetic exchange interactions arise from the hybridization between the 3d
orbitals of Co2+ and the sp orbitals of S2−. That is, from the spectroscopic states of the Co2+ 3d7 ion to the
PbS semiconductor conduction band [41]. Consequently, the broadband coming from the strong quantum
confinement energy intensifies the sp-d exchange interactions.
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Which when she saw, downe on the bloudy plaine
Her selfe she threw, and teares gan shed amaine;
Amongst her teares immixing prayers meeke,
And with her prayers reasons to restraine[64]
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FOOTNOTES:
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[47] vi 3 worth: 1596
[48] vii 4 skill] sill 1596
[49] viii 4 disaduaunce, 1596
[50] 8 avengement 1609
[51] ix 6 n’ote] not 1596
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[54] xix 5 bend, 1609
[55] 6 souse auoydes, it 1609
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[57] xxiii 7 teene, 1596
[58] xxv 6 strooke 1609 passim
[59] xxvi 1 blowes, 1596
[60] xxix 2 waste 1609
[61] xxxiii 6 sword 1609
[62] xl 8 haste 1609 passim
[63] xliii 5 quiet-age Morris
[64] xlvii 7 restraine, 1596
[65] l 3 To] Too 1596
Cant. IIII.
The last day came, when all those knightes againe xxxvii
Assembled were their deedes of armes to shew.
Full many deedes that day were shewed plaine:
But Satyrane boue all the other crew,
His wondrous worth declared in all mens view.
For from the first he to the last endured,
And though some while Fortune from him withdrew,
Yet euermore his honour he recured,
And with vnwearied powre his party still assured.
FOOTNOTES:
[66] lii 1 feasts 1609
[67] 9 elswere 1596
[68] i 4 depends. 1596
[69] ii 3 als] els 1596
[70] 4 Scudamour] Blandamour 1679 rightly.
[71] viii 2 Ferrat 1596
[72] ix 5 sight. 1596
[73] x 5 worst 1596
[74] xvii 4 satyr-headed conj. Church
[75] xix 7 an] a 1609
[76] xxi 5 Palimord 1609
[77] xxiii 5 glode. 1596
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[80] xxvii 3 behalue. 1596
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