Journal of Physics: Condensed Matter

You may also like

- Structural and magnetic characterization of
Magnetic properties of the mixed-valence Mn/NiFe bilayers with ion-beam-assisted
deposition
manganese oxide Pb3Mn7O15 Chun-Hsien Wu, Chao Zheng, Chun-
Cheng Chiu et al.

- Graphitic Mesoporous Carbon/Mn7C3 as

Polysulfide Host for High Rate Li-S
To cite this article: N V Volkov et al 2008 J. Phys.: Condens. Matter 20 055217
Batteries
Xiaoqiang Liang, Wenbin Guo, Panpan
Zhang et al.

- Multiple Auger cycle photoionisation of

View the article online for updates and enhancements. manganese atoms by short soft x-ray
pulses
S Klumpp, N Gerken, K Mertens et al.

This content was downloaded from IP address 14.139.128.12 on 26/12/2023 at 10:11

IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER
J. Phys.: Condens. Matter 20 (2008) 055217 (6pp) doi:10.1088/0953-8984/20/5/055217

Magnetic properties of the mixed-valence

manganese oxide Pb3Mn7O15
N V Volkov1,2 , K A Sablina1 , O A Bayukov1 , E V Eremin1 ,
G A Petrakovskii1,2, D A Velikanov1 , A D Balaev1 , A F Bovina1 ,
P Böni3 and E Clementyev4
1
Kirensky Institute of Physics SB RAS, 660036 Krasnoyarsk, Russia
2
Department of Physics, Siberian Federal University, 660041 Krasnoyarsk, Russia
3
Physics-Department E21, Technical University of Munich, D-85747, Garching, Germany
4
RFNC-Institute of Technical Physics, 456770 Snezhinsk, Russia

E-mail: volk@iph.krasn.ru

Received 22 June 2007, in final form 19 December 2007

Published 17 January 2008
Online at stacks.iop.org/JPhysCM/20/055217

Abstract
A Pb3 Mn7 O15 single crystal has been grown by the flux method and studied using x-ray
diffraction and magnetization measurements. The crystal is hexagonal ( P 63 /mcm space group,
Z = 4) and exhibits a pronounced layered nature. Along the [001] direction (c axis), the
structure consists of layers of edge-sharing MnO6 octahedra. Pairs of Mn atoms occupy the
octahedral sites located between layers forming ‘bridges’ along the c axis, which link
neighboring Mn layers. The magnetic properties of the crystal have been investigated using ac
and dc magnetization measurements in the temperature range 2–900 K at magnetic fields up to
90 kOe. The experimental data obtained suggest that in the temperature region under study
several different magnetic phases can be distinguished. Down to ∼250 K, the crystal is in the
paramagnetic state. Below this temperature, short-range antiferromagnetic ordering apparently
starts forming within Mn layers, although a transition to long-range magnetic order occurs at
70 K. The magnetization data obtained leads us to conclude that this state is canted
antiferromagnetic with moments lying in the basal plane of the crystal. In addition, below 20 K
the crystal undergoes one more magnetic transition that corresponds to spin reorientation.

1. Introduction considerable changes of their physical properties. A classical

example is the La1−x Cax MnO3 system, which graphically
Currently, novel materials on the basis of manganese oxides demonstrates how strong a level of doping and, consequently,
with mixed-valence manganese ions Mn3+ /Mn4+ are being the Mn3+ /Mn4+ ratio can influence magnetic and electronic
intensively studied due to their unusual, sometimes intriguing, states. The (x, H, T )-phase diagram of La1−x Cax MnO3
magnetic and electric properties. These include charge involves ferromagnetic metal, ferromagnetic insulator, canted
and orbital ordering, a metal/insulator transition, a colossal antiferromagnetic, inhomogeneous magnetic state, charge and
magnetoresistive (CMR) effect, existence of magnetic phases orbital ordering states [1–5]. In many respects, richness of the
with competing FM and AFM interactions, and magnetic magnetic phase diagram of the crystal is related to coexistence
phase separation. Until recently, attention of researchers of both antiferromagnetic (AFM) superexchange interactions
has been attracted mainly to doped manganese oxides of between Mn3+ ions and ferromagnetic (FM) double exchange
the R1−x Ex MnO3 family (R = La, Nd, Pr, Sm, etc, and (DE) interactions between Mn3+ and Mn4+ ions. The DE
E = Ca, Sr, Ba, Pb, etc). A perovskite-like structure of interaction results from transfer of an eg electron between
these compounds provides high chemical flexibility that allows neighboring Mn3+ and Mn4+ ions through the Mn3+ –O–Mn4+
changing the Mn3+ /Mn4+ ratio by doping within a wide path. The specific feature of the DE mechanism is determined
range with no significant changes of a crystal structure. At by the fact that motion of an itinerant eg electron favors FM
the same time, variations of the ratio between Mn ions with ordering of t2g local spins and, vice versa, the established
different valence in perovskite-like manganites may result in FM order facilitates motions of itinerant electrons. Thus,

0953-8984/08/055217+06$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK

J. Phys.: Condens. Matter 20 (2008) 055217 N V Volkov et al

coexistence of Mn ions in different oxidation states is one of compounds and allowing to avoid incorporation of foreign ions
the possible reasons for direct correlation between the FM state into a lattice. Synthesis of the Pb3 Mn7 O15 crystals started with
and conductivity in perovskite-like manganites. heating of a mixture of appropriate amounts of high purity
The variety of physical properties (with clear under- PbO and Mn2 O3 in a platinum crucible at 1000 ◦ C for 4 h.
standing of many phenomena being still lacking) observed Then the crucible was slowly cooled to 900 ◦ C with a rate
in doped perovskite-like manganites excites active search and V = 2–5 ◦ C h−1 and, finally, a furnace was cooled to room
study of other families of mixed-valence Mn oxides, which temperature. The single crystals of a plate–hexagonal form
do not possess of a perovskite structure. In particular, re- with black shiny surfaces were found at a level of a solidified
cently layered manganites of the Ruddlesden–Popper series liquid surface. The plates were up to 40 mm in ‘diameter’.
(E1−y R y )n+1 Mnn O3n+1 and RMn2 O5 compounds (R is the rare The grown crystals were extracted mechanically from the flux.
earth ion, either Y or Bi) have been intensively studied. The All the measurements reported here were performed on well-
former includes phases revealing the CMR phenomenon [6], polished plate-like samples with a required dimension cut from
whereas the latter exhibits strong correlation between the mag- the resulting single-crystal plates. The samples were oriented
netic and dielectric properties [7]. We paid attention to the Pb– by the back-Laue method.
Mn–O system, which includes a number of phases with com-
positions suggesting the mixed-valence state of Mn cations. 2.2. Experimental measurements
Some compounds belonging to this system were already de-
scribed [8], yet not thoroughly characterized and studied. Up Single-crystal x-ray patterns were collected using a SMART
to date, the available information on the electronic and mag- APEX autodiffractometer (Bruker AXS). These data were
netic properties of the mixed-valence Pb3 Mn7 O15 is insuffi- obtained with a purpose of refining a crystallographic structure
cient. Besides, the results of x-ray study presented by dif- at room temperature.
ferent authors are contradictory. Some authors described the The magnetic properties of the crystal were investigated
Pb3 Mn7 O15 structure in terms of an orthorhombic space group; using ac and dc magnetization measurements performed with
others indicated that the crystal belongs to the hexagonal space a physical property measurement system (PPMS, Quantum
group [9–12]. We should also notice a number of works de- Design) in the temperature range from 2 to 350 K at magnetic
voted to characterization of a crystal with the chemical formula fields up to 90 kOe. High-temperature (up to 900 K)
referred to as Pb3 Mn6 O13 [13]. Similarity of x-ray and mag- measurements of magnetic susceptibility were performed with
netic measurement data obtained by different authors suggests a vibrating sample magnetometer of our original construction.
that the material under study does belong to the Pb3 Mn7 O15
composition. However, the mentioned investigations had rather 3. Results
an incomplete character, so the authors could hardly draw more
or less definite conclusion about a crystal structure and mag- 3.1. Crystal structure
netic state. Nevertheless, we paid attention to study of the di-
electric properties, which suggested arising of the ferroelectric Structure of the Pb3 Mn7 O15 crystal was refined using the
(FE) or anti-FE state. This fact appeared not so surprising, single-crystal x-ray data collected at room temperature. All
considering the so-called stereoactivity of Pb2+ ions [14] that the reflections were indexed in the hexagonal space group
facilitates the creation of local dipoles and, thus, the forma- P 63 /mcm with lattice parameters a = 10.0287(4) Å and
tion of FE or anti-FE fashion. Possible correlation between c = 13.6137(6) Å. The atomic positions, selected bond
the magnetic and dielectric properties has been another reason distances, and bond angles are listed in tables 1–3, respectively.
stimulated the detailed study of Pb3 Mn7 O15 . There are four formula units per unit cell. It should be
In this paper, we present and discuss the results noted that the crystal structure of Pb3 Mn7 O15 we found
of thorough investigations of the unique structural and coincides with that of mineral Pb3 (Fe, Mn)4 Mn3 O15 known
intriguing magnetic properties of the Pb3 Mn7 O15 single as zenzenite [12], with just minor distinctions in lattice
crystal. Crystallographic structure of the compound has been parameters, bonding distances and angles, related apparently
refined using the single-crystal x-ray diffraction data. Possible to the presence of Fe ions in the mineral.
magnetic structure has been clarified through analyzing the In figure 1, a crystallographic structure of the compound
data of ac/dc magnetization measurements and calculations of is schematically presented. Mn cations occupy four
Mn–O–Mn interactions made in the framework of an indirect crystallographically nonequivalent positions (Mn1, Mn2, Mn3,
coupling model. We hope that our results and conclusions and Mn4), each of them being coordinated by six oxygen
would be of interest for researchers who study the compounds atoms in octahedral configuration. A unit cell includes
exhibiting mixed valence of manganese ions and develop twelve Mn1 sites (12i) located within slightly compressed
models of magnetic interactions in manganese oxides. oxygen octahedra, eight Mn2 sites (8h) located within trigonal
distorted octahedra, two Mn3 sites (2b) coordinated by oxygen
atoms in a regular octahedral configuration, and six Mn4
2. Experimental details sites (6f) having tetragonal oblate octahedra in environment.
Crystallographic structure of Pb3 Mn7 O15 has a pronounced
2.1. Sample preparation
layered nature. Along the [001] direction (c axis), the structure
Single crystals were grown by the flux method. As a flux, consists of layers of edge-sharing MnO6 octahedra (Mn1,
PbO was chosen, known as an effective solvent for many oxide Mn3, and Mn4 positions). Pairs of Mn2 atoms occupy the

2
J. Phys.: Condens. Matter 20 (2008) 055217 N V Volkov et al

Table 1. Structural parameters of Pb3 Mn7 O15 . For all Pb

2 2
Uiso = 0.009(1) Å , for all Mn Uiso = 0.006(1) Å , for all O 3
2
Uiso = 0.018(7) Å . Agreement indices: Rwp = 7.66%, 4 1 1 4
1 3
R p = 9.95%. 3 1
4 1 4
3

Atom Site x y z 2 2

Pb1 6g 0.6115(1) 0.6115(1) 3/4

Pb2 6g 0.2652(1) 0.2652(1) 3/4 2

Mn1 12i 0.8314(1) 0.1685(1) 1/2

Mn2 8h 1/3 2/3 0.147 16(2)
Mn3 2b 0 0 0
Mn4 6f 1/2 1/2 1/2
O1 24l 0.490 63(6) 0.332 20(6) 0.079 66(4)
O2 12j 0.521 3(8) 0.175 2(7) 1/4
O3 12k 0.836 9(8) 0.836 99(6) 0.927 4(5)
O4 12k 0.666 16(8) 0.666 16(6) 0.072 77(5)

Table 2. Selected Mn–O bond lengths for Pb3 Mn7 O15 (the left
column) and zenzenite [12] (the right column). c

Bond length (Å) Bond length (Å) a b

Atoms Pb3 Mn7 O15 zenzenite
Figure 1. Schematic representation of a crystallographic structure of
Pb1–O1 2.706(8) × 4 2.718(10) × 4 Pb3 Mn7 O15 . Mn cations are located in the center of oxygen
Pb1–O2 2.344(8) × 2 2.301(11) × 2 octahedra; Pb ions are shown as light (Pb1 sites) and dark (Pb2 sites)
Pb2–O2 2.275(9) × 3 2.276(10) × 3 circles.
Pb2–O4 2.509(10) × 3 2.492(11) × 3 (This figure is in colour only in the electronic version)
Mn1–O1 1.927(9) × 2 1.904(10) × 2
Mn1–O3 1.982(9) × 2 1.974(12) × 2
Mn1–O4 1.945(8) × 2 1.930(12) × 2
Mn2–O1 1.985(9) × 3 1.974(10) × 3 two degenerate orbital states of a Mn3+ ion with electronic
Mn2–O2 2.070(9) × 3 2.080(11) × 3 configuration t2g 3 eg 1 in the regular octahedral crystal field [15].
Mn3–O3 1.910(8) × 6 1.926(12) × 6 The second factor can be associated with stereoactivity of an
Mn4–O1 1.964(8) × 4 2.002(10) × 4 electron lone pair on 6s2 Pb2+ ion [14]. Interactions between
Mn4–O4 1.939(9) × 2 1.983(11) × 2 the lone pair electrons and adjacent bond pairs between cation
and ligands give rise to repulsion of the lone pair with the Pb–
O bonds. This leads to an asymmetric distribution of the bonds
Table 3. Selected Mn–O–Mn bond angles for Pb3 Mn7 O15 . around Pb and, as a result, to significant shifts of some oxygen
Atoms Bond angle (deg) positions. The assumption that distortions of the coordination
octahedra arise due to stereoactivity of Pb2+ ions is consistent
Mn1–O3–Mn1 95.2(5)
Mn1–O3–Mn3 97.5(5) with a fact that distorted MnO6 octahedra consist of oxygen
Mn1–O1–Mn4 95.4(5) × 2 ions located in the nearest environment of Pb1 and Pb2 ions.
Mn2–O1–Mn1 126.5(5) × 3 At the same time, the Pb ions occupying both crystallographic
Mn2–O2–Mn2 85.1(5) × 3 sites are not bonded with oxygen atoms of the regular Mn3O6
Mn2–O1–Mn4 126.1(5) × 3 octahedron.
Now let us consider the question concerning the oxidation
states of manganese ions in Pb3 Mn7 O15 . Authors of all the
sites located between the layers that form sort of ‘bridges’ reports we are familiar [9–12] with suggest that this crystal
along the c axis, which link neighboring Mn layers. Mn2O6 contains Mn ions in two oxidation states, namely Mn3+ and
octahedra share corners with adjacent manganese octahedra Mn4+ , in a ratio 4:3. At the same time, the chemical
in the layers, and share faces with each other. Lead atoms formula Pb3 Mn7 O15 is also consistent with the presence of
are located at half a distance between layers and occupy two Mn2+ and Mn4+ ions in the crystal in a ratio 2:5. Each
crystallographically nonequivalent positions, Pb1 and Pb2. of the above variants certainly suggests a different magnetic
They can be described as coordinated by four and six nearest state with own characteristic peculiarities. An analysis of
oxygen atoms, respectively, although both Pb1 and Pb2 are off- ion sizes for different valence states of Mn and the Mn–
center in coordination polyhedra. O bond lengths make us prefer the variant with the mixed-
Local polyhedral distortions observed in Pb3 Mn7 O15 may valence state Mn3+ /Mn4+ . As for as, valence states and
be caused by one of the two following factors. The first ion sizes are closely related, which allows using a Mn–O
corresponds to the possible presence of Jahn–Teller Mn3+ bond length to calculate Mn valence via the bond-valence-sum
(3d4 ) cations on some Mn sites (a question concerning analysis [12]. This method using a tabulated [16] empirical
oxidation states of manganese ions in the crystal will be parameters for a cation–anion pairs enables also to extract
discussed below). The Jahn–Teller distortions originate from a reasonable distribution of the trivalent and tetravalent ions

3
J. Phys.: Condens. Matter 20 (2008) 055217 N V Volkov et al

2.8
T2
8 T3
T1
80000

χ'ac (10 emu/g)

2.6

-4
60000
χ'ac (10 emu/g)

6 2.4
θp=-520 K

1/χ
0 100 200 300
-4

Temperature (K)
40000

4
T3 T1
20000 T~250 K

2 0
-500 -250 0 250 500 750
Temperature (K) Temperature (K)
Figure 2. Real part of ac magnetic susceptibility χac as a function of Figure 3. Reciprocal magnetic susceptibility versus temperature
temperature measured at a frequency f = 1 kHz and ac field ( H = 0.5 kOe). The solid line is a fit to the Curie–Weiss law.
amplitude of 10 Oe without dc bias field. Inset: the same curve in an
extended scale.

values calculated for the mixed-valence state Mn3+ /Mn4+

 manganese ions in the parent chemical formula μeff =
th
of
among nonequivalent Mn sites in the crystal. In our case, the
calculated valence for Mn2 and Mn3 are within 0.1 of 3 and 4(g 2 · 2 · (2 + 1)) + 3(g 2 · 3/2 · (3/2 + 1)) ≈ 11.9 μB on
4, respectively, whereas for Mn1 and Mn4 sites the calculation the assumption that the g -factor equals to 2.
gives intermediate values between 3 and 4. Thus, the bond- Temperature dependences of dc magnetization M for
valence-sum analysis suggests that the Mn2 site contains only Pb3 Mn7 O15 under the zero-field-cooled and (ZFC) and field-
trivalent ions and Mn3 site does only tetravalent ions, whereas cooled (FC) conditions are given in figure 4. They are
the Mn1 and Mn4 sites are occupied by both trivalent and presented for two characteristic directions of magnetic field in
tetravalent ions in common proportion 4:5. In the framework of the crystal: along the sixfold axis ([001] direction) and in the
the model used, it is difficult to make a final conclusion about basal plane ([100] direction). Just as in the ac susceptibility
the distribution of Mn3+ and Mn4+ among each of the Mn1 measurements, a clear maximum is observed at a temperature
and Mn4 sites. To clarify this point, more detailed study is T1 ∼ 160 K that indicates the onset of the cooperative effect in
required. a magnetic subsystem of the crystal. Generally, the behavior
of M in this temperature range is typical for a transition
from the paramagnetic to antiferromagnetic state. There are
3.2. Magnetic measurements
two circumstances, however, worthy of being noted: rather a
Figure 2 and the inset their show the temperature dependence large width of the maximum and the absence of anisotropy

of a real part of ac magnetic susceptibility χac for Pb3 Mn7 O15 in M below the transition temperature for longitudinal and
at a frequency f = 1 kHz and ac field of 10 Oe without transverse directions of magnetic field relative to a sixfold axis

superimposed dc field. A sharp anomaly χac at T2 = 70 K of the crystal.

evidently indicates a magnetic phase transition. Two weakly Below the temperature T2 = 70 K, where the sharp χac
pronounced peaks, which can also be related to the change of anomaly takes place, M undergoes an abrupt increase for the
a magnetic state, are observed at T1 ∼ 160 K and T2 ∼ 20 K. transverse field direction. Upon further cooling, difference
 
In general, ac susceptibilities χac and χac were measured for between the FC and ZFC magnetization appears at about
different frequencies in the range from 10 Hz to 10 kHz for 45 K and increases with a decrease in temperature. In ZFC

ac field amplitudes from 1 to 10 Oe. Plots of the real χac measurements, M reaches a maximal value at 45 K and

and imaginary χac parts of susceptibility versus T show no decreases at lower temperatures; at the same time, in the FC
frequency dependence over the measured temperature range, measurements, it increases up to 16 K, remaining practically
except for the region of low-temperature maximum, where invariable below this temperature. Such a behavior of M
 
slight dependence of χac and χac versus measuring frequency clearly indicates the presence of a weak ferromagnetic effect.
is observed. Taking into consideration the dependence of M obtained for
Measurement of dc susceptibility was performed at the longitudinal field direction, we may conclude that the
high temperatures up to 900 K. Figure 3 shows reciprocal magnetic moment lies in the basal plane of the crystal.

susceptibility that has a linear dependence above ∼250 K. This In the region of the low-temperature χac anomaly, M
indicates that susceptibility follows the Curie–Weiss behavior also has peculiarities observed during both the FC and
exp
characterized by a paramagnetic temperature θp = −520 K ZFC measurements when a magnetic field direction is in
exp
and an effective paramagnetic moment μeff = 13.3 μB the basal plane. For a longitudinal field direction, these
per formula unit. The latter value is close to spin-only peculiarities become apparent only in high magnetic fields.

4
J. Phys.: Condens. Matter 20 (2008) 055217 N V Volkov et al

Magnetization (10 emu/g)

1.3 T1
T3
a T=4.2 K

-2
Magnetization (emu/g)

65

Magnetization (emu/g)
1.2
75

T2 1.1
100 150 200 250
Temperature (K)

[100] ZFC
[100] FC H II [100]
T1 [001] FC
H=500 Oe

b 65
Temperature (K)
75

Magnetization (emu/g)
Figure 4. Dc magnetization versus temperature for different
directions of magnetic field relative to a crystallographic axis in 4.2
Pb3 Mn7 O15 under the zero-field-cooled (ZFC) and field-cooled (FC)
conditions ( H = 0.5 kOe). Inset: the same curves in an extended
scale.

Most likely, the low-temperature anomaly is related to the H II [001]

change of magnetic anisotropic interactions leading to a spin-
reorientation transition.
Isothermal magnetization curves for Pb3 Mn7 O15 are
presented in figure 5. The dependences of M obtained at
H  [100] and H  [001] below T2 confirm the presence of Magnetic field (kOe)
a spontaneous weak ferromagnetic moment lying in the basal
Figure 5. Isothermal magnetization curves for Pb3 Mn7 O15 : (a) for H
plane of the crystal. With a decrease in temperature, the value in the basal plane ([100] direction); (b) for H along the sixfold axis
of spontaneous magnetization increases reaching 0.13 μB f.u., ([001] direction).
which is very small as compared to the theoretical saturation
value 25 μB (the spin-only one). The magnetization curves exp
temperature θp = −520 K, suggesting that interatomic
for H  [100] show hysteresis for the fields from −20 to
magnetic interactions in the crystal are significant and have
20 kOe at 4.2 K, which abruptly decreases with an increase
an AFM nature over the measured temperature range. The
in temperature and vanishes at T2 . In the case of H  [001], presence of strong exchange interactions is also confirmed
hysteresis phenomena are not observed in the magnetization by the fact that deviation of magnetic susceptibility from the
curves over the measured temperature range. Curie–Weiss behavior starts at sufficiently high temperatures
(∼250 K). Hence, it would be quite reasonable to interpret
4. Discussion the maximum in ac and dc magnetizations near 160 K as
a transition from the paramagnetic to AFM ordered state.
The main topic to discuss in this paper is the intriguing mag- However, this maximum appeared to be too broad for the
netic properties of the Pb3 Mn7 O15 compound. Crystallo- conventional Néel temperature. This circumstance and,
graphic structure of the compound has a pronounced layer na- in addition, the absence of anisotropy in the behavior of
ture that should inevitably cause characteristic peculiarities of dc magnetization below the transition temperature expected
the magnetic properties. However, determination of a mag- for the AFM state suggest that the observed specific
netic structure seems to be fairly a difficult problem to solve features of magnetization do not coincide with the onset of
because of the obvious complication of Pb3 Mn7 O15 structural long-range magnetic ordering. Possibly, only short-range
chemistry. Indeed, although only Mn ions possess of a mag- antiferromagnetic ordering starts forming at this temperature
netic moment, the total number of them is 28 per unit cell of and the clusters may be low-dimensional because of the
the crystal. Besides, they are distributed over the four indepen- pronounced layer structure of the crystal. Anyway, at present
dent crystal sites, and different sites can be occupied by Mn the origin of the feature in magnetization at 160 K remains
cations in different oxidation states, which, accordingly, differ a matter of speculation. Magnetization measurements show
in electronic configurations. the onset of the magnetic ordered state in Pb3 Mn7 O15 below
Now let us consider a problem concerning magnetic 70 K. The behavior of magnetization is compatible with a
states of Pb3 Mn7 O15 realized in different temperature ranges. supposition that this state is the canted antiferromagnetic with
Approximation of high-temperature susceptibility by the a weak spontaneous ferromagnetic moment lying in the basal
Curie–Weiss law gives unexpectedly high paramagnetic plane of the crystal.

5
J. Phys.: Condens. Matter 20 (2008) 055217 N V Volkov et al

The appearance of hysteresis loops characterized by high 4:3. Data of magnetic measurements suggest that several dif-
coercive fields up to 20 kOe at a temperature of 4.2 K ferent magnetic phases can be distinguished over the studied
is actually unexpected. It is well known that the high temperature region. The paramagnetic behavior is observed at
coercivity may arise due to relatively large concentration temperatures down to ∼250 K. On further cooling, short-range
of defects that hinder motion of magnetic walls or very correlations occur in the system and extensive antiferromag-
high magnetocrystalline anisotropy of a sample in the single- netic clusters start forming at ∼160 K. At 70 K the long-range
domain state. High intrinsic magnetic anisotropy seems magnetic order is established, and low spontaneous magnetiza-
fairly a probable reason for the observed high coercivity tion is observed in all the ordered regions. At ∼20 K one more
in Pb3 Mn7 O15 . Indeed, large magnetocrystalline anisotropy magnetic transition occurs in the crystals, which can be related
with an easy axis occurring in a uniaxial crystal system to reorientation of a magnetic moment.
(Pb3 Mn7 O15 belongs to this type of symmetry) may lead
to the high coercivity because the sample cannot become Acknowledgments
demagnetized without rotating magnetization towards a hard
direction. Meanwhile, for the crystal under investigation We are grateful to Professor Victor Zinenko (Kirensky Institute
hysteresis loops are observed only at magnetization lying in of Physics SB RAS) for useful discussion. This study
the basal plane, although, relative to rotation of the magnetic was supported by the INTAS (Project No. 06-1000013-
moments in this plane, magnetic anisotropy should not be 9002) and the Program ‘Spin-dependent Effects in Solids and
high for a hexagonal antiferromagnetic in the easy plane state. Spintronics’ of the Division of Physical Sciences of RAS
Actually, in terms of the thermodynamic approach, the in- (Project No. 2.4.2 SB RAS). NV also thanks the Foundation
plane anisotropy is determined by the sixth order invariants on for Support of Russian Science.
components of the antiferromagnetism vector.
We can expect arising of domains with noncollinear References
antiferromagnetism vectors in the basal plane of the crystal,
since there are several equivalent antiferromagnetic axes [1] Kim K H, Uehara M, Kiryukhin V and Cheong S W 2004
in this plane. When magnetic field is applied along the Colossal Magnetoresistive Manganites
ed T Chatterji (Dordrecht: Kluwer–Academic)
sixfold axis, domains have equal energies, and the field
[2] Rodriguez-Martinez L M and Attfield J P 1996 Phys. Rev. B
does not cause a displacement of interdomain walls. In 54 R15622
this case, the magnetization process takes place, as if an [3] Dagotto E, Burgy J and Moreo A 2003 Solid State Commun.
antiferromagnetic is in the single-domain state. Indeed, the 126 9
experimental dependences of magnetization do not reveal [4] Dagotto E, Hotta T and Moreo A 2001 Phys. Rep. 344 1
[5] Argyriou D N and Ling C D 2004 Colossal Magnetoresistive
any peculiarities that would indicate rearrangement of the
Manganites ed T Chatterji (Dordrecht: Kluwer–Academic)
domain structure. However, when magnetic field is applied [6] Chatterji T, Jackeli G and Shannon N 2004 Colossal
in the basal plane, domains are energetically nonequivalent, Magnetoresistive Manganites ed T Chatterji (Dordrecht:
and the field exerts a pressure on interdomain walls causing Kluwer–Academic)
their displacement. Irreversible displacement gives rise to [7] Higashiyama D, Miyasaka S, Kida N, Arima T and
Tokura Y 2004 Phys. Rev. B 70 174405
magnetic hysteresis. Now we can make no conclusion
[8] Latourrette B, Devalette M, Guillen F and Fouassier C 1978
about the nature of possible anisotropic centers that determine Mater. Res. Bull. 13 567
the displacement of interdomain walls, since unambiguous [9] Darriet B, Devalette M and Latourrette B 1978 Acta
explanation of the observed phenomena certainly requires a Crystallogr. B 34 3528
detailed micromagnetic study. [10] Marsh R E and Herbstein F H 1983 Acta Crystallogr. B 39 280
[11] Le Page Y and Calvert L D 1984 Acta Crystallogr. C 40 1787
[12] Holtstam D, Lindqvist B, Johnsson M and Norrestam R 1991
5. Conclusion Can. Mineral. 29 347
[13] Bush A A, Titov A V, Al’shin B I and Venevtsev Yu N 1977
The Pb3 Mn7 O15 single crystal was synthesized; its crystal Russ. J. Inorg. Chem. 22 1211
structure and magnetic properties were investigated. Crystallo- [14] Moore P B, Sen Gupta P K and Le Page Y 1989
Am. Mineral. 74 1186
graphic structure of Pb3 Mn7 O15 possesses of hexagonal sym- [15] Levy L P 2000 Magnetism and Superconductivity
metry and has pronounced layered nature. The crystal contains (Berlin: Springer) chapter 2, p 56
Mn ions in two oxidation states Mn3+ and Mn4+ in a ratio [16] Brown I D and Altermatt D 1985 Acta Crystallogr. B 41 244