Charge-order breaking and ferromagnetism in La0.4Ca0.

6MnO3
nanoparticles
C. L. Lu, S. Dong, K. F. Wang, F. Gao, P. L. Li et al.

Citation: Appl. Phys. Lett. 91, 032502 (2007); doi: 10.1063/1.2753749

View online: http://dx.doi.org/10.1063/1.2753749
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Published by the American Institute of Physics.

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 APPLIED PHYSICS LETTERS 91, 032502 共2007兲

Charge-order breaking and ferromagnetism in La0.4Ca0.6MnO3 nanoparticles

C. L. Lu, S. Dong, K. F. Wang, F. Gao, P. L. Li, L. Y. Lv, and J.-M. Liua兲
Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China and
International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, China
共Received 3 May 2007; accepted 10 June 2007; published online 17 July 2007兲
La0.4Ca0.6MnO3 nanoparticles of grain size as small as ⬃20 nm are prepared and their magnetic
behaviors are investigated in order to understand the size effect of the charge ordering in
manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction
of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic
phase separation due to the competition between ferromagnetic state and charge-ordered state as
well as the surface spin disordering is responsible for the spin-glass-like state at low temperature.
© 2007 American Institute of Physics. 关DOI: 10.1063/1.2753749兴

Perovskite manganites with a general formula that of the narrow bandwidth manganites.15,16
R1−xAxMnO3 共where R is rare earth and A is alkaline earth兲 The LCMO nanoparticles at x = 0.6 were prepared by the
have attracted attention not only for their potential applica- facile sol-gel method.8 At the end of the process, the gel was
tions but also for the intriguing fundamental problems asso- heated in a furnace at 250 ° C for about 10 h and porous
ciated with colossal magnetoresistance 共CMR兲 effect and materials were synthesized. In order to obtain samples of
charge-ordered 共CO兲 state.1–3 The CO state, a real-space or- different average particle sizes, the powder was calcined at
dering of Mn3+ and Mn4+ ions and usually with antiferro- 900 ° C for 2 h and 700 ° C for 1 h, respectively, producing
magnetic 共AFM兲 order, remains to be an essential ingredient the final particles of ⬃60 and ⬃20 nm in average size, la-
of the physics of manganites.4 It may be melted into ferro- beled as LCMO-60 and LCMO-20. For the comparison pur-
magnetic 共FM兲 metallic state by external magnetic field, giv- pose, bulk LCMO 共designated as LCMO-bulk兲 was prepared
ing rise to the CMR effect,5 although this CMR effect is by the conventional solid-state reaction in air and the average
somehow different from the conventional CMR originating grain size is about several microns and above. The samples
from the double-exchange mechanism. However, the melting were then characterized by x-ray diffraction 共XRD兲, trans-
of the CO state usually requires high magnetic field, mission electron microscopy 共TEM兲. The magnetization M
making practical applications of the CMR effect inacces- under conditions of zero-field cooling 共ZFC兲 and field cool-
sible. Thus, an alternative approach to distabilize the CO ing 共FC兲 was measured as a function of temperature T and
state is appealed. magnetic field H using Quantum Design superconducting
Recently, quite a few experimental and theoretical stud- quantum interference device magnetometer. The magnetic
ies focusing on the size effect of perovskite manganites were loops for the LCMO-bulk were recorded at H = 0 – 3 T and
reported,6–14 in which some interesting effects associated T = 3 K and H = 0 – 6.5 T and T = 50 K, respectively, and the
with the downsizing of the materials to tens of nanometers loops for LCMO-60 and LCMO-20 were measured at H
were revealed. One of these effects is the distabilization of = 0 – 5 T and T = 3 K.
the robust CO state as the ground state of bulk manganites. The XRD patterns with clean background for all the
In consequence, a transition of the AFM order to the weak samples, as shown in Fig. 1, can be indexed to a single
FM state was observed in both nanowires7 and orthorhombic crystal structure with the Pnma symmetry.
nanoparticles,8 where the key role of the surface effect was Prechecking utilizing induction-coupled plasma analysis was
argued. Similar prediction was made by considering the sur- made in order to exclude impurity in the samples. X-ray
face phase separation 共PS兲 sequence,14 although more rel- photoelectron spectroscopy was made and no Mn2+ ions
evant experimental evidence is required. Therefore, it would were identified. Figures 2共a兲 and 2共b兲 present the typical
be of interest to investigate whether those manganites of the TEM images for the LCMO-20 and the LCMO-60, confirm-
high CO stability have such significant size effect or not. ing the particle sizes of ⬃20 and ⬃60 nm respectively. The
In this work, we studied the stability of the CO state and HRTEM image obtained from a portion of an individual
related magnetic behaviors of electron-doped La1−xCaxMnO3 LCMO-60 nanoparticle 关Fig. 2共c兲兴 shows the clear lattice
共LCMO兲 nanoparticles at x ⬃ 0.6, noting that earlier work planes 共the layer spacing is 0.271 nm, corresponding to the
was mainly on LCMO at x ⬃ 0.5 where the CO state is asso- 共200兲 planes or/and 共121兲 planes兲 and thus indicates the well-
ciated with the charge exchange 共CE兲 phase.13 A choice of crystallized structure inside the particle. The SAED image
LCMO at x = 0.6 was based on the fact that LCMO is of taken from individual particle of LCMO-60, as shown in Fig.
intermediate bandwidth and shows tremendous CMR effect. 2共d兲, also reveals sharp diffraction spots.
For x ⬎ 0.5, LCMO is electron doped and the stability of the Figure 3 presents the measured M as a function of T for
CO ground state increases with x and the maximal CO tran- the ZFC case and FC case with a magnetic field of 100 Oe.
sition point is at x ⬃ 0.6. Thus, it is believed that the energy At a first glance, the detected M over the whole T range is
difference between the CO state and FM state is smaller than very different from one and another for these samples no
matter how the magnetic ordering is. When the LCMO-bulk
a兲
Author to whom correspondence should be addressed; electronic mail: sample shows its M ⬃ 10−2 emu/ g, the value of M for the
liujm@nju.edu.cn LCMO-60 and LCMO-20 is roughly two orders of magni-

0003-6951/2007/91共3兲/032502/3/$23.00 91, 032502-1 © 2007 American Institute of Physics

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032502-2 Lu et al. Appl. Phys. Lett. 91, 032502 共2007兲

FIG. 1. XRD ␪-2␪ spectra at room temperature for 共a兲 the LCMO-bulk, 共b兲
the LCMO-60, and 共c兲 the LCMO-20, respectively.

tude larger. This tremendous difference refers to the fact that

the dominant magnetic order in the LCMO-bulk sample is
AFM type, while it is not the case for the LCMO-60 and FIG. 3. 共Color online兲 Measured M as a function of T under the conditions
LCMO-20. For details, the LCMO-bulk sample exhibits the of ZFC and FC for 共a兲 the LCMO-bulk, 共b兲 the LCMO-60, and 共c兲 the
LCMO-20. The inset in 共b兲 and 共c兲 is the first derivative of M with respect
typical CO peak at TCO ⬃ 260 K, consistent with earlier to T.
report.16 The AFM transition is normally indicated by a
much weaker feature with the Neel point TN ⬃ 140 K, al-
though it is not clearly visible on the present scale. The point TC for both the LCMO-60 and LCMO-20 is ⬃220 K,
nearly overlapped ZFC and FC M-T curves over 50– 300 K determined by the inflection point defined by the minimum
also confirm the AFM order. As T ⬍ 50 K, the magnetization of dM / dT, as shown in the insets in Figs. 3共b兲 and 3共c兲,
of the FC case is slightly larger than that of the ZFC case, respectively. Furthermore, for both the LCMO-60 and
indicating a canted AFM ground state.17 LCMO-20, the tremendous difference between the ZFC
For the present nanoparticle samples, no clear CO tran- curve and FC curve over a broad T range indicates the nature
sition is identified from the M-T curves. Instead, a clear of spin-glass-like behaviors, although we have no intention
paramagnetic-FM transition can be identified and the Curie to identify it as spin glass in the strict sense. In particular, a
cusplike peak in the ZFC curve for the LCMO-20 sample
can be observed, and the spin-glass-like transition tempera-
ture T = T f ⬃ 120 K, indicated by the arrow in the inset of
Fig. 3共c兲.18 A general argument is that the nanoparticles or
nanowires have the surface spin-disordered magnetic state
composed of noncollinear spin arrangement due to the re-
duced coordination and broken exchange bonds between sur-
face spins.19 Due to this magnetic frustration behavior, the
spin-glass-like surface layer would contribute to the large
difference between the ZFC and FC curves as well as the
cusplike peak for the ZFC case.
We also measured the M-H hysteresis for the LCMO-
bulk at H = 0 – 3 T and T = 3 K, and that at H = 0 – 6.5 T and
T = 50 K, respectively. The hysteresis for the LCMO-60 and
LCMO-20 was obtained at H = 0 – 5 T and T = 3 K, as plotted
in Fig. 4. For the LCMO-bulk sample, the hysteresis is very
thin and does not show any saturation as H is as large as 3 T.
FIG. 2. Typical TEM images for 共a兲 the LCMO-20 and 共b兲 the LCMO-60,
respectively. 共c兲 The HRTEM pattern and 共d兲 the SAED pattern of the
From the inset in Fig. 4共a兲, one notes that the CO state does
LCMO-60 nanoparticles. The scale bar is 20 nm in 共a兲, 50 nm in 共b兲, and not show any melting feature until H = 6.5 T at T = 50 K
5 nm in 共c兲, respectively. Ⰶ TCO. However, typical FM hysteresis can be observed for
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032502-3 Lu et al. Appl. Phys. Lett. 91, 032502 共2007兲

mation of the FM amount in the nanoparticle system.14

Given that this model was applied to the present LCMO
system 共x ⬃ 0.6兲, if the critical magnetic field needed to melt
the CO state is taken to be ⬃10 T 共our results show that the
CO state remains stable against a field of 6.5 T at 50 K兲, the
calculated FM fraction is ⬃25% when the grain size is
⬃60 nm and ⬃40% when the size is ⬃20 nm. The FM frac-
tion estimated from the measured M-H hysteresis at T
= 3 K and H = 3 T is ⬃30% for the LCMO-20 and LCMO-
60, which has the same magnitude order as the value pre-
dicted by the surface PS model.14 Nevertheless, it should be
pointed out that this surface PS model is oversimplified and
the calculated FM fraction may not be reliable in the quan-
titative sense, while the essential physics already lies in the
standard models as mentioned above.
In addition, a clear T f can be defined for the LCMO-20.
The spin-glass-like behavior below T f may originate from
the phase separation. The decreasing of the size may affect
significantly the magnetic properties of LCMO because of
the broken exchange bonds between the surface Mn cations.
However, while the size is small enough 共e.g., ⬃20 nm兲, the
effect of surface spin disordering would become more evi-
dent, and thus results in the spin-glass-like state at low T,
although this spin-glass-like behavior is not very significant.
This work was supported by the Natural Science Foun-
dation of China 共50601013, 50528203兲, the National Key
Projects for Basic Research of China 共2006CB921802兲, and
Nanjing University 共2006CL1兲.
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