Ref 23 La0.9Ca0.1MnO3
Ref 23 La0.9Ca0.1MnO3
Ref 23 La0.9Ca0.1MnO3
chemical and electronic homogeneity on the nanoscale range ordering we have used the electron magnetic resonance
are the necessary prerequisites for understanding the internal 共EMR兲 technique, comprising the electron paramagnetic
regularities governing the properties of doped manganites. resonance 共EPR兲 and the ferromagnetic resonance. High sen-
On the other hand, almost all practical samples contain un- sitive EMR method allows one to detect small changes of the
avoidable structural imperfections, chemical disorder, and magnetic homogeneity induced by the local variations of the
associated strains.13 Thus, a basic question arises about the oxygen stoichiometry and chemical composition.25 Indepen-
extent to which the experimentally observed complex multi- dent ac and dc magnetic data complemented the EMR re-
phase behavior can be described by the simplified DE mod- sults. It was clearly shown that the stoichiometry of low-
els neglecting the chemical and magnetic disorder, as well as doped La1−xCaxMnO3 共x 艋 0.20兲 manganites determines their
the appearance of impurity bands.2–4
magnetic ordering.17,26 Therefore, special attention was paid
Let us consider La0.9Ca0.1MnO3 共LCMO兲 manganite com-
to the chemical composition and stoichiometry of our
pound as an example. The literature data concerning its mag-
netic ordering are strongly contradictive. The neutron dif- samples. We have assured that not only the chemical compo-
fraction and magnetic measurements on bulk crystals of sition, but also the oxygen stoichiometry of the investigated
LCMO reveal modulated canted AFM structure composed of bulk and nanosized LCMO were the same. We believe that
A-type canted AFM matrix and nanometer-scale FM our approach is much more appropriate than the one reported
clusters.14,15 In a marked contrast, low temperature coexist- in Refs. 22 and 23 where only the composition and stoichi-
ence of major FM and minor canted AFM insulating phases ometry of nanomanganites were concerned.
was reported for bulk LCMO crystal in Ref. 16. The neutron To draw conclusions about the magnetic correlations and
and high-resolution x-ray powder diffraction, as well as spin dynamics in the investigated samples the EMR param-
ac/dc magnetic measurements show only the homogeneously eters at the paramagnetic 共PM兲 temperature range were ana-
canted AFM phase in the stoichiometric LCMO ceramic.17 lyzed using the existing models.27–29 This allowed us to evi-
On the other hand, it was suggested in Refs. 18 and 19 that dence a notable difference in the features of Jahn-Teller
the nanosized FM metalliclike clusters are embedded into the transition and in the magnetic ordering in nanosized
FM insulatinglike matrix in LCMO bulk ceramics. It seems, La0.9Ca0.1MnO3 and in its bulk counterpart. We believe that a
therefore, that the basic question whether the extrinsic or higher chemical homogeneity of the nanometer-sized crystals
intrinsic reasons are responsible for appearance of coexisting leads to their basically FM ordering in a marked contrast to
magnetic/electronic phases in LCMO, as well as the question the mixed AFM+ FM order in the bulk crystals. We attribute
on the applicability of the simplified models2–4 may be ad- the difference in the magnetic order to a transition from an
dressed to this very case. inhomogeneous confined state of charge carriers in the
It is well known that finite-size effects induce a plethora chemically/magnetically disordered bulk crystal to a more
of new phenomena in the solid state magnetism.20,21 In par- mobile one within impuritylike band5 in homogeneous nano-
ticular, it is believed that the reduction of the sample size crystals. The change in the electronic states and in the mag-
down to the nanometer size scale is capable of influencing netic ordering seems to be an intrinsic one and not induced
the magnetic ordering in doped manganites via the coupling by differences in the chemical composition or oxygen sto-
between the spin subsystem 共spins of both Mn ions and car- ichiometry between the bulk and nanosized LCMO. This fact
riers兲 and the lattice. Recent experiments showing FM like motivated us to propose a model describing the influence of
ordering in crystalline Pr0.5Ca0.5MnO3 nanowires22 and the chemical/magnetic disorder suppression associated with
Nd0.5Ca0.5MnO3 nanoparticles23 in a marked difference to the size reduction upon magnetic order in doped manganites.
the observations of the AFM charge ordered ground state in The proposed model explains not only our experimental re-
the bulk crystalline form of these manganites, were inter- sults but also those reported previously for Pr0.5Ca0.5MnO3
preted as an evidence of the suppression of the AFM/CO and Nd0.5Ca0.5MnO3 compounds.22,23 Our results may there-
state in nanosized samples. The actual mechanism of this fore help to fill the gap between the idealized models2–4 and
suppression is currently a subject of a discussion.22,23 Addi- numerous experiments evidencing the phase coexistence in
tionally, a progressive increase of low temperature, low field doped manganites.
magnetization with decreasing mean grain size in LaMnO3
nanoparticles was reported in Ref. 24. These experimental II. EXPERIMENT
findings strongly support a claim that the reduction of sample
size influences magnetic ordering in doped manganites. Bulk LCMO crystals were grown by a radiative heating
In this paper, we report on a comprehensive study of the floating-zone method.30 Nanocrystals were prepared by the
crystalline structure and magnetic ordering in bulk and nano- sonication-assisted coprecipitation and subsequent low tem-
sized La0.9Ca0.1MnO3 manganite single crystals. The studies perature crystallization at 900 K,31 i.e., below the tempera-
were performed in order to obtain additional experimental ture of its structural rhombohedral to orthorhombic
insight into the problem of complex multiphase behavior of transition.32 Such nanocrystals fabrication procedure effec-
LCMO by benefiting from a special advantage of using the tively eliminates twin defects, reduces stoichiometry fluctua-
homogeneous nanometer sized samples as the reference tions, and renders the samples almost free from extended
ones. The notable difference in chemical and structural ho- defects and associated local strains, leaving the crystallite
mogeneity between the bulk and nanometer sized crystals of surface as the only dominant defect. The structure of the
the same La0.9Ca0.1MnO3 composition arises from different nanocrystals was checked using JEM 2010 high resolution
fabrication protocols employed. For probing the magnetic transmission electron microscope 共TEM兲 with the linear
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FIG. 5. 共Color online兲 EMR spectra of 共a兲, 共b兲 bulk and 共c兲, 共d兲 nanocrystalline samples recorded at different temperatures above and
below magnetic transition points.
nonlinear, nonhysteretic, and nonsaturated M共H兲 curves are nanosample starts to increase already at T = 250 K and grows
recorded for nanosamples between 150 K and 240 K; see monotonically towards a weak maximum at T ⬃ 20 K.
Fig. 4共a兲. The coercive field HC decreases with increasing Temperature dependences of the resonant linewidth
temperature and vanishes around 150 K in the nanocrystal, 共⌬Hpp兲 are shown in Fig. 7共b兲. The ⌬Hpp共T兲 of the bulk goes
and around 130 K in the bulk—Fig. 4共b兲. through a broad minimum at T = 420 K, well pronounced
The results of the magnetization and susceptibility mea- minimum at T = 175 K, and a subsequent increase until the
surements 共Figs. 3 and 4兲 of the LCMO bulk crystal are EMR signal becomes unobservable around 115 K. In the
consistent with the existing literature data.14,15 The magnetic nanosample the ⌬Hpp共T兲 sharply decreases in the vicinity of
ground state is a modulated canted AFM structure composed 500 K which is close to the temperature of the LCMO Jahn-
of A-type AFM matrix and nanometer-scale FM clusters. Teller 共JT兲 transition; see Ref. 36, and references therein.
There are two subsequent magnetic transitions in the bulk: A The narrowing of the EMR linewidth in the vicinity of JT
FM transition 共long range ordering of the FM clusters兲 at transition was reported, e.g., in Ref. 37. Upon further cooling
TC = 130± 2 K and an AFM transition of the matrix at Néel the ⌬Hpp共T兲 goes through a broad minimum at T ⬃ 250 K
temperature TCA = 112± 1 K. The magnetic ordering in
LCMO nanocrystals is clearly different from the one ob-
served in its bulk counterpart.
The EMR data confirm a marked difference in magnetic
orderings. EMR detects the power P absorbed by the sample
from the transverse magnetic microwave field as a function
of the static field H. The signal-to-noise ratio of the spectra is
improved by recording the derivative dP / dH. EMR spectra
共dP / dH兲 of the bulk show a single resonance line within the
entire temperature range of measurements; see Figs. 5共a兲 and
5共b兲, while a complex line appears in the spectra of the
nanosample between 115 and 240 K; see Fig. 5共d兲. Figure 6
shows that this complex line may be simulated/fitted as being
composed of a broad Gaussian and a narrow Lorentzian line.
The doubly integrated EMR intensity 共DIN兲 of the bulk
sample starts to increase with decreasing temperature at T
⬃ 175 K, reaches a maximum at Tmax = 125 K and practically FIG. 6. 共Color online兲 EMR spectra of nanocrystalline LCMO
vanishes below 90 K; see Fig. 7共a兲. The vanishing of DIN recorded at the temperature interval of two magnetic phase coexist-
reflects directly the zeroing of the resonance signal in bulk ence and its simulation with a broad Gaussian and a narrow Lorent-
LCMO, as shown in Fig. 5共a兲. In contrast, DIN of the zian contribution.
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TABLE I. The temperatures of magnetic transitions 共TCA is the Néel temperature of the A-type AFM
matrix in bulk兲 and parameters of the DIN−1 fit. The parameters are the following: T0 is the estimated
temperature of cooperative/electron Jahn-Teller transition 共in the bulk crystal it corresponds to an average Ca
content兲; T is the smearing interval of the above transition 共applies to the bulk crystal only兲; ⌰1 and ⌰2 are
the Curie-Weiss temperatures below and above the transition; C2 / C1 is the ratio of the Curie constants above
and below the transition.
Transition
Sample temp. 共K兲 T0 共K兲 T 共K兲 ⌰1 共K兲 ⌰2 共K兲 C2 / C1
The DIN−1 versus T curve for the bulk, shown in Fig. 8, is dence of the linewidth in the PM range, which was indeed
different from the one recorded with the nanocrystals. Al- observed in the ceramic La1−xCaxMnO3.27 The saturation
though two CW-like regimes are also observed, the deviation value, ⌬H共⬁兲 was estimated27 to be proportional to the
from the piecewise straight line is noticeable and the transi- Dzialoshinsky-Morya interaction44,45 and the crystal field46
tion between two regimes has a smeared steplike form. To strengths squared divided by an effective exchange interac-
explain such a behavior we refer to the evidence nonhomo- tion modulus.
geneous Ca distribution in La1−xCaxMnO3 bulk crystals.43 In our samples, in particular in the nano LCMO, the tem-
Let us assume that the Ca concentration is Gaussian distrib- perature dependence of ⌬Hpp is not fully consistent with the
uted with a mean x and dispersion x. Then, for small constant L concept. To account for the ⌬Hpp at the PM state
enough x, the distribution of the structural transition tem- we assume, on the base of the Hr data from Fig. 7共c兲, that eg
peratures will also be Gaussian with a mean T0 = TO⬘-O*共x兲 electrons, even if localized, do not occupy the manganese
and the dispersion T = 兩dTO⬘-O*共x兲 / dx兩x. For the spatially sites. Following this assumption, the kinetic coefficient may
averaged susceptibility, adopting the CW law with different be naturally decomposed into two parts L = LMn4+ + Leg. For
parameters C1, ⌰1 and C2, ⌰2 in the O⬘ and O* phases, the first term, which is due to spin-spin relaxation of Mn4+
respectively, we obtain ions, we still retain the assumption of Ref. 27, viz.
共T兲 =
1
2
冋 冉 冊册
1 + erf
T0 − T
T
C1
T − ⌰1
LMn4+共T兲 = ⌬H⬁i . 共4兲
冋 冉 冊册
The second term is due to the spin relaxation of eg electrons
1 T − T0 C2 mediated by various interactions with the lattice imperfec-
+ 1 + erf , 共2兲 tions and can be estimated using DE model with potential
2 T T − ⌰2
disorder5 and a simplest electron-impurity interaction with
where erf共z兲 is the error function. Note that Eq. 共2兲 provides the spin reversal, which is a reason for broadening of the
an excellent fit for the DIN versus T data for the bulk EPR line in semiconductors.28 The result for the PM state is
LCMO, as shown in Fig. 8. The fitting parameters are shown
Leg共T兲 = BTe−EA/kT , 共5兲
in Table I. T0 of the bulk is lower than TO⬘-O* for the corre-
sponding ceramic LCMO36 even taking into account the where B is a constant proportional to the spin-orbit coupling
smearing range ⬃T. Let us emphasize that the ratios of the squared divided by the eg band width. The activation energy
Curie constants above and below the electronic/structural EA is the difference between the Fermi energy and the
transition in the nano and bulk LCMO are practically the eg-band top. Usually EA equals full or half the gap between
same, see Table I. the eg-band and an acceptorlike impurity band, which
The analysis of the PM spin dynamics, closely related to emerges due to crystal imperfections and doping.5 Remark-
the analysis of the DIN in the paramagnetic range, is based ably, Leg is proportional to the conductivity estimated within
on the formula employed previously27 for La1−xCaxMnO3 ce- the same model.29
ramics For the nanocrystals, ⌬Hpp at PM state is an increasing
⌬Hpp共T兲 = L共T兲0共T兲/共T兲. 共3兲 function of T and shows a jumplike behavior when approach-
ing T0. But contrary to the abrupt change of , the steplike
Here 0共T兲 = CT−1 is the Curie susceptibility, C is the highest variation of ⌬Hpp extends over ⬃50 K around T0; cf. Figs. 8
temperature Curie constant, and L共T兲 is a kinetic coefficient. and 9. We have fitted Eqs. 共3兲–共5兲 to the ⌬Hpp data in the
The relation of L共T兲 to a time correlation function of quan- temperature range 300 K 艋 T 艋 460 K ⬍ T0. We could not use
tum torques which cause total spin components to relax was the entire range of the experimental data, up to 600 K, be-
discussed in Ref. 27. It was shown that L共T兲 should saturate cause of very few data points available above T0 and the
very fast at T Ⰷ ⌰ if the manganite can be described as a absence of the formula for L in the transition range. The fit
mixture of rigid Mn3+ and Mn4+ ions. For this case the sus- shown in Fig. 9 yielded EA = 0, indicating the bandlike nature
ceptibility factor in Eq. 共3兲 dominates the temperature depen- of carriers in the nano LCMO. The ⌰1 values obtained from
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TABLE II. The parameters of fits for the EPR linewidth. Spin dynamics parameters are the following:
⌬H⬁i is a high temperature asymptote of Mn4+ spin-spin relaxation contribution; B is a parameter of inter-
action between eg electrons and impurities with spin reversal and EA is the activation energy as described in
the text.
Nano 1318± 211 1.25± 0.38 0 214± 6 Not involved in this fit
Bulk 1785± 70 148± 6 375± 12 137± 2 141± 14 1.29± 0.05 363± 5 49± 5
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Mn4+ ions’ PM g factor; see, e.g., Ref. 47兲. It means that the electrons and Mn4+ ions are the same in both types of
site occupation picture does not apply to the majority of eg samples. The linearity of the Curie constant in the carrier
electrons. This conclusion agrees well with the idea of a concentration has been theoretically demonstrated in the
bond-centered occupation proposed for La0.86Sr0.14MnO3.48 frame of a DE model.52 Moreover, the closeness of the very
A similar conclusion was drawn from the refinement of the Curie-Weiss temperatures in the bulk, below and above JT
neutron diffraction data for Pr0.6Ca0.4MnO3 single crystal.49 transition, and in the nanosample above JT transition, Table
In addition, the domination of Mn4+ ions in magneto-optical I, also points out to the same general mechanisms as being
activity was reported for La0.925Sr0.075MnO3.50 On the base responsible for the magnetic ordering. These facts are the
of the above findings and our own results,47,51 we suggest clear hallmarks of the same oxygen stoichiometry in addition
that in nano LCMO eg electrons of former Mn3+ ions move to the same cation composition, which allowing us to claim
bandlike and experience strong phonon-polaron effect, which that the observed change in electronic/magnetic ordering is
results in shrinking of the eg band above T0. Therefore, no an inherent property of LCMO. The difference in the mag-
cooperative JT lattice distortion occurs in the nano LCMO netic ground state between the bulk and nanoform of LCMO
due to the pronounced electron itinerancy. In bulk LCMO eg crystals appears as a result of a size reduction down to na-
electrons are also separated from the manganese sites but nometer scale and improved chemical homogeneity and crys-
they are localized in clusters, which involve only few Mn4+ tallinity due to different fabrication method.
ions, due to concurrence of the extrinsic effects, such as dop- To conclude, we propose a realistic model of the
ing inhomogeneity and crystalline defects, and the DE inter- chemical/magnetic disorder suppression associated with the
actions. We suggest that the lifetime of Mn3+ ions in the nano size reduction in doped manganites using the representative
LCMO is set by an intersite tunneling time of a band elec- example of the bulk and nanocrystalline La09Ca0.1MnO3. The
tron, while Mn3+ lifetime in the bulk, although longer than in data presented in this work evidence that structural imperfec-
the nanocase, is still shorter than the characteristic time of an tions and chemical disorder play a dominant role in the co-
EPR experiment. The observation of the cooperative JT dis- existence of different magnetic/electronic phases and are ca-
tortions in the bulk, Fig. 1共b兲, may be seen as resulting from pable of modifying significantly the phase diagram of the
the confinement of eg electrons in clusters containing some mixed valence manganites.
adjacent MnO6 complexes in which intermediate valences of
ACKNOWLEDGMENTS
Mn ions occur.49 As discussed above, the extrinsic imperfec-
tions in the bulk lead to the essential smearing of this JT This research was supported by the Israeli Science Foun-
transition. dation administered by the Israel Academy of Sciences and
The closeness of the ratios of the Curie constants above Humanities 共Grant No. 845/05兲 and by the Israeli-Korean
and below JT transition in the bulk and nano LCMO crystals, bilateral Grant No. 3-2217. S.S.B. acknowledges support by
Table I, shows that the band width in both types of samples Grant No. AOARD-064054. The authors thank Yudith Grin-
changes in the same manner. Namely, the concentration of eg blat for her assistance in HRTEM measurements.
*Corresponding author. evgenyr@bgu.ac.il Shannon, and K. Samwer, Phys. Rev. Lett. 89, 237203 共2002兲.
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