Particle Size-Dependent Charge Ordering and Magnetic Properties in

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J. Phys. Chem. C 2009, 113, 15817–15823

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Particle Size-Dependent Charge Ordering and Magnetic Properties in Pr0.55Ca0.45MnO3 Ping Chai, Xueyu Wang, Shuang Hu, Xiaojuan Liu, Yao Liu, Minfeng Lv, Guangshe Li, and Jian Meng* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China, State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350000, People’s Republic of China, Changchun UniVersity of Science and Technology, Changchun, 130022, People’s Republic of China, and Graduate School, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: February 22, 2009; ReVised Manuscript ReceiVed: July 22, 2009

A series of Pr0.55Ca0.45MnO3 compounds with average particle size ranging from 2000 to 30 nm have been synthesized by the sol-gel method and their charge ordering (CO) and magnetic properties are investigated. It is observed that with particle size decreasing, the CO transition is gradually suppressed and finally disappears upon particle size down to 35 nm, while the ferromagnetism (FM) emerges and exhibits a nonmonotonous variation with a maximum at 45 nm samples. The FM components in all samples never reach long-range ordering but rather only show short-range clusters. A new explanation considering the coupling between lattice, charge, and spin in the system is raised to understand the suppression of the CO state. Both the competition between the CO/AFM and FM states and the core-shell model are employed to explain the variation of the FM phase. These results may provide a deeper insight into the physics of particle size effect on the charge ordering manganite. Introduction Doped perovskite manganite RxA1-xMnO3 with orthorhombic structures (where R and A represent rare earth elements and divalent alkaline earth elements, respectively) have attracted much attention mainly due to the fruitful physical properties they exhibit.1-3 Charge ordering (CO), characterized by a real space ordering of the Mn3+ and Mn4+ ions, is one of the representative phenomena as a result of the predominant Coulomb interaction over the kinetic energy of the charge carrier.4 The charge ordering phenomenon has been observed mostly when the concentration of charge carriers takes a rational value of the periodicity of the crystal lattice.5,6 It has been demonstrated that in a crystal, the CO state, which favors an antiferromagnetic (AF) insulating phase, often competes with a ferromagnetic (FM) metallic state and can be melted to the FM metallic state by the introduction of chemical substitution or the application of some external forces, such as a magnetic field, a higher pressure, an exposure to laser radiation, or even an electrical field.7-11 Nanoparticles of magnetic materials have recently been the subject of intense research because of their unique properties differing from those of the bulk counterparts and their great potential technological application, including magnetic-recording media, sensors, permanent magnets, ferrofluids, etc.12,13 As for doped perovskite manganite, it has been reported that the reduction of particle size of the systems has a strong influence on their properties, especially on the CO state. For example, experimental evidence of the suppression of the AFM/CO state in the Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 nanoparticles was obtained;14,15 the CO and AFM phase transition observed in the * To whom correspondence should be addressed at the Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. E-mail: [email protected].

bulk Nd0.5Ca0.5MnO3 disappeared in the nanoparticles and made way for an emergent FM metallic phase;16 the particular robust CO state in bulk Pr0.5Ca0.5MnO3, where a magnetic field of 27 T is necessary to cause a transition to the FM metallic phase, has also been reported to be suppressed in both Pr0.5Ca0.5MnO3 nanoparticles and nanowires.17,18 It is clear that the reduction of particle size can suppress the formation of the CO state. However, this conclusion is not applicable to all the CO systems. Biswas et al. observed charge ordering in nanocrystalline Pr0.65Ca0.35MnO3,19 which was obviously contradictory to the situation of Pr0.5Ca0.5MnO3 nanoparticles though the two compounds in bulk form show the same generic behavior. This point is reminiscent of the situation of La0.4Ca0.6MnO3, where the disappearance of CO in nanoparticles was reported by Lu et al.,20 but later an opposing result of the enhanced stability of the AFM/CO ground state in La0.4Ca0.6MnO3 with a grain size of 17 nm was published.21 It seems that whether the reduction of the particle size can suppress the CO or not is still disputed and needs to be further investigated. In this paper, we studied the particle size effect on the CO state and the related magnetic behavior of Pr0.55Ca0.45MnO3 compounds. There are two reasons to choose Pr0.55Ca0.45MnO3 to investigate. First, based on the previous results of Pr0.5Ca0.5MnO3 and Pr0.65Ca0.35MnO3, it is necessary to know how the nanosamples affect the intermediate carrier concentration. Second, previous investigations have primarily concentrated on the comparison between nanocompounds and bulk compounds, very little attention has been paid to the study of the evolution of CO and magnetic behaviors with a gradual reduction of particle size. Our studies indicate that the electronic and magnetic states of Pr0.55Ca0.45MnO3 experience complicated variations as the average particle size is reduced. We proposed a mechanism and provide a detailed analysis.

10.1021/jp901722h CCC: $40.75  2009 American Chemical Society Published on Web 08/13/2009

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Figure 1. XRD patterns at room temperature for all samples annealed at different temperatures.

Experimental Section Preparation. Polycrystalline powders of Pr0.55Ca0.45MnO3 were prepared by the sol-gel method. In a typical experimental process, high-purity Pr(NO3)3 · 6H2O, Ca(NO3)3 · 4H2O, and Mn(CH3COO)2 · 4H2O in stoichiometric proportions were dissolved in distilled water to obtain a clear solution, the pH value of which was then adjusted to 3-4 by adding HNO3 and NH3 · H2O. An appropriate amount of citric acid, in a 1:1 molar ratio with respect to the metal nitrates, was added to the solution with constant stirring. Subsequently, the mixture was kept at 50 °C for about an hour until the sol formed. Next, the sol was dried overnight at about 100 °C to form dried gel, followed by firing at about 250 °C to obtain black precursor powder. Finally, the precursor powder was separated into several parts and annealed at different temperatures from 550 to 1350 °C for 3 h to gain samples with different average particle sizes. Note that the sample remains amorphous when the annealing temperature is below 550 °C. Charaterization. The phase purities and crystal structures of the obtained samples were characterized by X-ray diffraction (XRD) at room temperature on a Rigaku D/Max 2500 powder diffractometer with Cu KR radiation (λ ) 1.5406 Å). The average particle sizes and microstructure were determined with a field emission scanning electron microscope (FESEM, Hitachi S4800) and a transmission electron microscope (TEM, JEOL 2010). Element stoichiometry was checked by using an energy dispersive X-ray (EDX) analysis technique and inductively coupled plasma atomic emission spectroscopy (ICPAES). The valence ratio of Mn3+/Mn4+ was studied by X-ray photoelectron spectroscopy (XPS), using an ESCALAB MKII spectrometer with an Al KR X-ray excitation source. The magnetic characterization was performed with a Quantum design superconducting quantum interference device (SQUID) magnetometer.

Figure 2. SEM micrographs of Pr0.55Ca0.45MnO3 samples annealed at different temperatures: (a) 550, (b) 600, (c) 650, (d) 800, (e) 900, and (f) 1350 °C.

TABLE 1: The Annealing Temperatures (T) and Corresponding Average Particle Sizes (D) T (°C) D (nm)

1350 2000

900 150

800 80

650 45

600 35

550 30

samples and the average diameter ranges from 30 to 2000 nm with the annealing temperature increasing from 550 to 1350 °C. The annealing temperatures and the corresponding average particle sizes are shown in Table 1. The representative TEM images for samples annealed at 600 and 650 °C are shown in Figure 3. There is good agreement between the average particle sizes determined by SEM and TEM. To make sure that the as-prepared samples have proper stoichiometry and hence the proper carrier concentration, various methods were used to check the stoichiometry. ICP characterization was first employed to obtain an average ratio of Pr:Ca: Mn:O close to 0.55:0.45:1:3 for all samples, which indicates that all samples have the proper stoichiometry. The EDX measurement done on every sample at various points was also used. Typical EDX spectra for samples annealed at 600 and 650 °C at several locations are shown in Figure 4. It is evident

Results and Discussion The crystal structure and micromorphology of samples were characterized first. Figure 1 shows the measured XRD patterns for all samples annealing at different temperatures. The sharp diffraction peaks indicate the single phase nature and good crystalline form for each sample. All samples crystallize in orthorhombic perovskite structure with space group Pnma, well matched with the structure of bulk Pr0.55Ca0.45MnO3.22 The diffraction peaks become increasingly wide with decreasing annealing temperature, revealing the gradual reduction of the particle size. Figure 2 shows the FESEM micrographs of samples subjected to different thermal treatment, from which it can be found that the particle size is homogeneous for all

Figure 3. The representative TEM images for samples annealed at 600 and 650 °C.

Synthesis of Pr0.55Ca0.45MnO3 Compounds

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Figure 4. The typical EDX spectra for samples annealed at (a) 600 and (b) 650 °C at various locations of each sample.

Figure 5. XPS spectra of the Mn 2p3/2 level of Pr0.55Ca0.45MnO3 samples with various average particle sizes. The solid line indicates measured data and the dash lines correspond to fitting curves by Mn3+and Mn4+ ions, respectively.

that for every sample the spectra at different locations are nearly the same, which suggests that the samples have quantitatively good homogeneity. In addition, all samples exhibit nearly identical EDX spectra with the corresponding element ratio agreeing well with the expected one within experimental error, further confirming the proper stoichiometric composition for all samples. The ratio value of Mn3+/Mn4+, which is dependent on the oxygen stoichiometry, is also examined by XPS measurement. Figure 5 shows the XPS spectra of the Mn 2p3/2 level of the samples and the corresponding fitting curves by Mn3+and Mn4+ species with binding energy values of 641.5 and 642.8 eV, respectively.23,24 It is clear that the spectra for each sample are very similar, revealing that all samples should have a close Mn valence state. The fitting results are quite good, and from the calculation of the internal peak areas of the two fitting curves, we can see that all the samples have almost the same Mn3+/Mn4+ value close to the expected one. This result further indicates that all the samples have better oxygen stoichiometry. The proper stoichiometry for all samples with different average particle sizes allows us to be conclusive that any changes of physical properties arise only from a reduction in the particle size. The electronic and magnetic properties of the as-prepared Pr0.55Ca0.45MnO3 samples were investigated. Figure 6 shows the temperature (T) dependence of the zero-field cooled (ZFC) and field cooled (FC) magnetization (M) measured under a magnetic field of 1 T for the samples with different particle sizes. For the 2000 nm sample, the M vs. T curves are in good agreement

with the available data for bulk Pr0.55Ca0.45MnO3 showing several features related to the electronic and magnetic transitions in this compound.25 A maximum at about 243 K indicates the charge ordering transition, which leads to a strong carrier localization; with decreasing temperature, a second maximum peak corresponds to the onset of antiferromagnetism, while a gradual rise below 100 K coincides with the transition to canted spin order. As the particle size is reduced to 150 nm, at first glance, it can be seen that the M vs. T curves have almost the same transition peaks and transition temperature as that of the 2000 nm sample, revealing that the 150 nm sample keeps an electronic and magnetic structure similar to that of the 2000 nm one. However, some variations are also easier to find. It is clear that the magnetization at low temperature becomes larger in the 150 nm sample, implying the appearance of FM fluctuations. For the 80 nm sample, in addition to the CO transition peak broadening and decreasing in intensity, the magnetization below the CO transition temperature presents an obvious rise as the temperature is gradually lowered, suggesting that lager FM components appear in the present sample. When the particle size further decreases to 45 nm, the CO peak can no longer be found and the magnetization rises monotonically within the whole temperature range, in particular, the magnetization at low temperature displays significant enhancement. These features reveal a clear transition from paramagnetic to ferromagnetic (FM) state as the temperature decreases. However, it should be noted that the M-T curves are not exactly a ferromagnet with a proper long-range magnetic order where a sharp transition at the ferromagnetic transition should be observed.17 Furthermore, note that there is strong thermomagnetic irreversibility between the FC and ZFC curves as indicated by the appearance of large bifurcation between them at low temperature. The ZFC curve exhibits a maximum at a temperature Tf and the FC curve continues to increase with decreasing temperature. These behaviors agree well with the observations in many similar systems such as Nd0.5Sr0.5MnO3, La0.5Sr0.5CoO3, and La0.75Ca0.25MnO3 nanoparticles,15,26,27 which indicate the nature of the cluster-glass (CG) state at low temperature. Moreover, although the CO transition is invisible from the M-T curves for the 45 nm sample, the existence of the CO cannot be completely ruled out because the transition peak is easily covered up due to the existence of the FM components. Considering that the change of magnetization with temperature can provide more precise information about the phase transition, we carry out the dM/dT vs. T curve corresponding to the ZFC in order to confirm whether the CO transition exists or not. A part of the dM/dT Vs. T curve in expanded scale is shown in inset I of Figure 6. It is obvious that there is a broad peak with a center at 210 K responding to the minimum of the absolute

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Figure 7. Particle size dependence of the magnetization at 2 K obtained from the ZFC curves.

Figure 8. Magnetic field dependence of magnetization for samples with various average particle sizes. The inset shows the particle size dependence of the coercivity (HC).

Figure 6. Temperature dependence of ZFC (solid symbol) and FC (open symbol) magnetization in an applied field of 1 T for samples with various average particle sizes. The insets I and II show the corresponding dM/dT vs. T curves with respect to ZFC.

value of dM/dT, which should be related to the CO transition. These features imply that the CO state, albeit still present, is already greatly suppressed when the particle size is down to 45 nm, where, however, the FM is enhanced significantly. For both 35 and 30 nm samples, the M-T curves are similar to that of the 45 nm sample exhibiting enhanced FM with decreasing

temperature and cluster-glass (CG) state at low temperature. However, it can be found that in these two smaller samples the CO state is completely suppressed as indicated by the absence of any peak even in the dM/dT Vs. T (shown in inset II of Figure 6) curves. Moreover, the magnetization starts to reduce with the particle size decreasing below 45 nm. As indicated in Figure 7, the magnetization at 2 K obtained from the ZFC curves decreases from 8.25 emu/g for the 45 nm sample to 7.5 emu/g for the 30 nm sample. To further confirm the magnetic state of the samples, we performed the magnetic field H dependence of magnetization M for all samples at 5 K with H varying from -5 to 5 T, as shown in Figure 8. For the 2000 nm sample, the curve presents typical AFM property with a little hysteresis, which should arise from the canted spin order at low temperature.25 With particle size decreasing, the curves gradually exhibit a FM characteristic as indicated by the rapid increase in magnetization at low magnetic field, which together with that the magnetization does not saturate even at a field of 5 T indicates that the FM and AFM coexist in most regions of the samples. The magnetization exhibits a rise as the particle size decreases until 45 nm and then starts to diminish with particle size further decreasing, which is consistent with the results shown in Figure 7. We have also examined the FM components, using the value of magnetization at the highest field of 5 T. We obtained 0.56 µB/Mn for the 150 nm sample, 0.73 µB/Mn for the 80 nm sample, 0.94 µB/Mn for the 45 nm sample, 0.86 µB/Mn for the 35 nm sample, and 0.85 µB/Mn for the 30 nm sample. Compared with the

Synthesis of Pr0.55Ca0.45MnO3 Compounds expected value of 3.55 µB/Mn, the FM components only occupy 15.8%, 20.6%, 26.5%, 24.2%, and 23.9%, respectively. This implies that the FM components in all samples should be shortrange clusters, consistent with the conclusion obtained from the M-T measurements. The inset of Figure 8 shows the particle size dependence of the coercivity (HC). It can be seen that HC increases as particle size decreases, reaches a maximum around 45 nm, and then decreases. This variation is reasonable because the coercivity is closely related to the FM ordering.28 On the basis of the above magnetic measurements and analysis, we can find that the electronic and magnetic state of Pr0.55Ca0.45MnO3 experience complicated variations with the particle size decreasing. There exists strong competition between CO/AFM and FM phases. First, the CO is gradually suppressed as the particle size is lowered. When the particle size is reduced to 35 nm, the CO disappears completely. Second, the FM, in contrast to the situation of the CO, emerges and enhances gradually until the particle size is reduced to 45 nm. According to this situation, it should be speculated that the FM should reach the expected value of 3.55 µB/Mn when the particle size decreases to 35 nm due to the complete disappearance of CO. However, this is not the case. In fact, with the particle size further decreasing from 45 nm, the FM starts to decrease slowly, the trend of which is retained down to our lowest particle size of 30 nm. The FM components in all samples never reach longrange ordering. These complicated evolutions can be understood as follows. The CO suppression in nanoparticles has been found in many compounds, and many possible mechanisms have been raised to attempt to explain it. Anis et al. employed the similarity of CO transition to the Martensitic transformation, considering that it was impossible for the nanoparticles of Nd0.5Sr0.5MnO3 to accommodate the Martensitic strain during charge ordering transition and thus CO was hindered.15 Tapati et al. ascribed the CO suppression in La0.5Ca0.5MnO3 nanoparticles to an effective hydrostatic pressure created by surface pressure, which caused the room temperature structure to freeze and hence suppressed the CO transition.29 Rao et al. considered that it is the surface disorder in nanoparticles of Nd0.5Ca0.5MnO3 that prevented the formation of the long-range CO state.16 Gao et al. attributed a suppression of the CO state in La1/3Sr2/3FeO3 to the a and c axis expansion in nanoparticles.30 Recently, Zhang et al. suggested that the destruction of collinear AFM configuration and the enhanced surface energy were the reasons of the CO suppression in nanoparticles of La0.25Ca0.75MnO3.27 It is clear that these explanations are inconsistent and the real mechanism of the CO suppression in nanoparticles still needs to be investigated. On the basis of this situation, here, we give an explanation for the CO suppression in the present Pr0.55Ca0.45MnO3 nanoparticles from another new consideration. It is well recognized that the nanomaterial properties depend not only on the size and shape of particles but also on the interparticle interactions. Here the shape of Pr0.55Ca0.45MnO3 nanoparticles is nearly spherical, so it should be the size of the particles and the interparticle interactions that determines the properties. We first investigate the influence of the interparticle interactions on the properties. Due to the immeasurability of the exact value of the interparticle interactions, we employ the property comparison between the as-prepared Pr0.55Ca0.45MnO3 nanoparticles and the corresponding highly separated nanoparticles to estimate the strength of the interparticle interactions. The highly separated Pr0.55Ca0.45MnO3 nanoparticles were obtained by carrying out the grinding of the mixture of Pr0.55Ca0.45MnO3 nanoparticles and Al2O3 nanoparticles by low-

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Figure 9. Temperature dependence of ZFC magnetization in an applied field of 1 T for (a) the highly separated 80 nm sample and (b) the highly separated 30 nm sample. For comparison, the ZFC curves for the as-prepared 80 nm sample and 30 nm sample are also shown.

energy ball milling, which can physically separate the particles without inducing changes in the particle size and thus can only lead to a drastic decrease in the strength of the interparticle interactions.31 Figure 9a shows the ZFC curves for the highly separated 80 nm sample. As compared with the corresponding as-prepared sample, it can be seen that the separation of the particles cannot modify the electronic and magnetic transition but only shift the transition toward low temperature and lead to a slight decrease in magnetization, which should result from the reduction of the exchange interaction between the particles. For the 30 nm sample, as indicated in Figure 9b, the separation of the particles has an even smaller effect on their properties. These results indicate that the interparticle interactions in the as-prepared Pr0.55Ca0.45MnO3 nanoparticles can be considered to be negligible. This is not unexpected because the interparticle interactions mainly have influence on the superparamagnetic relaxation, which is not found in the asprepared Pr0.55Ca0.45MnO3 nanoparticles. Due to the negligible interparticle interactions, we can conclude that it is only the particle size that determines the properties of Pr0.55Ca0.45MnO3 nanoparticles. Thus our explanation for the CO suppression in the present Pr0.55Ca0.45MnO3 nanoparticles is mainly based on two points. First, the core-shell model is employed, where a nanoparticle is considered to be composed of two parts, i.e., the core denoting the inner part of the particle and the shell corresponding to the grain boundary or surface layer in the outer part.32,33 Second, the long-range and short-range Coulomb interaction among electrons, as a main driving force of the CO, is considered.34 The formation of the CO can be considered as a balance state reached by the Coulomb interactions among electrons at the background lattice potential.35 As for Pr0.55Ca0.45MnO3 nanoparticles, the shell should exhibit a disordered state as well as the defects, vacancies, stress, and broken bonds, which can inevitably influence the local crystal structure and then change the corresponding lattice potential. The change of the local lattice potential can subsequently disturb the balance state formed by the Coulomb interactions among electrons, which will in turn lead to the change of the electronelectron Coulomb interaction. As a result, the effective potential energy due to electron-electron interaction can be changed. With the decrease in particle size, due to the increase of the shell ratio, the change of the electron effective potential will grow more serious, which then can alter the charge distribution so that the CO is weakened gradually. More important, in manganite, the degrees of the charge and magnetic configurations are closely coupled to each other. The effective AF

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superexchange is favored by the charge-ordered configuration, meanwhile the effective repulsion interaction responsible for the CO is also favored by the AF configuration, so that the reduction of the CO will weaken the AF interaction, resulting in a change of the magnetic configuration from AF to FM; such a change can further suppress the CO. So when the particle size decreases to some extent the surface effect will become strong enough to suppress the CO completely. In a word, here we suggest that maybe the disordered surface in nanoparticles changes the local crystal structure and then breaks the balance of the electron state in the system, which subsequently results in the suppression of CO and further changes the magnetic configuration. Our explanation describes a better picture of the coupling between lattice, charge, and spin in the present system. It should be noted that in the present system, the CO disappears completely when the particle size decreases to 35 nm. The CO suppression in the present compound is similar to that in Pr0.5Ca0.5MnO3 but in contrast to that in Pr0.65Ca0.35MnO3.17-19 Noting that the Pr1-xCaxMnO3 compounds with doping concentration 0.3 e x e 0.5 in bulk form show the same generic behavior in their phase diagram,4 it is interesting that the nanoparticles with doping concentrations 0.35, 0.45, and 0.5 exhibit different properties, which need to be investigated further. As for the evolution of FM, it can be understood that ferromagnetism in different particle sizes has different origins. When the particle size is 150 nm, considering that the CO is slightly influenced, it can be concluded that the core of this sample keeps consistent properties with the bulk sample and the slight increase in magnetization at low temperature compared with 2000 nm sample should only arise from the surface disordered spins. This point is easily understood by the fact that for the AFM system, the antiparallel spin arrangement gives the lowest magnetic contribution; with surface disordered spins appearing, the deviation of the shell spins from a collinear AFM arrangement of the core spins is expected and then uncompensated surface spins will lead to increased magnetizm.31 For the 80 nm sample, due to the increase of the disordered shell ratio, the CO in the core is obviously suppressed but still maintains the majority indicated by the obvious CO peak in M-T curves and thus only a small fraction phase lose the CO and transform to FM; meanwhile, the weak FM component due to uncompensated surface spin still exists. Therefore, it is the minor FM phase in the core together with the uncompensated surface spin that makes a contribution to the enhanced magnetizm for the 80 nm sample at low temperature. With the particle size decreasing to 45 nm, the core of particles are severely disturbed so that the CO is greatly suppressed and consequently the core turn to a system consisting of a majority FM phase and a small amount of CO phase. In this situation, the disordered spin in the shell plays the opposite role compared with that in an AFM system and starts to weaken the FM in the core due to their disorder spin compared with the major FM of the core.32 So the FM in the 45 nm sample mainly originates from the FM phase in the core of the particles. When the particle size is 35 nm, the CO phase is completely suppressed and thus the core exhibits greater FM. Meanwhile, as compared with the 45 nm sample, the disordered shell ratio increases greatly so that the disorder spin weakens the FM more significantly. As a result, the magnetization in the 35 nm sample is smaller than that in the 45 nm sample. With the particle size further decreasing, the core maintains FM but the disordered shell ratio further increase and consequently the magnetizm further decreases for the 30 nm sample. So for the samples of particle size below 45

Chai et al. nm, due to the complete disappearance of the CO state, their FM all originate from the core of the particles. Here we can see that all samples are composed of both FM and AFM phase; because of the existence of the large interfacial strains on the boundaries between the FM and AFM regions, the FM components find it hard to reach long-range ordering but rather only exhibit short-range clusters. Moreover, for the smaller nanoparticles, the disordered surface layers strengthen, so magnetic frustration occurs and then gives rise to a cluster glass magnetic state at low temperature. Conclusions In summary, we have studied the effect of size reduction on charge ordering and magnetic properties in Pr0.55Ca0.45MnO3 compounds, which have been synthesized by using sol-gel method. All samples are single phases crystallizing in orthorhombic perovskite structure with space group Pnma. As the particle size is reduced, the CO transition gradually weakens until its complete disappearance at the 35 nm sample. This behavior can be understood by considering that the disordered surface in nanoparticles can change the local crystal structure and then break the balance of the electron state in the system, which in turn results in the suppression of CO. Meanwhile, the FM is observed and reaches a maximum at the 45 nm sample and then starts to reduce with particle size further decreasing. The FM for larger samples mainly arise from the uncompensated surface spins, while for the smaller samples the FM mainly results from the transformation of the CO in the core. Acknowledgment. We gratefully acknowledge financial support (Grants 20831004, 20671088, 20601026, and 20771100) from the National Natural Science Foundation of China (NSFC). References and Notes (1) Colossal MagnetoresistiVe Oxides; Tokura, Y., Ed.; Gordon and Breach Science: Amsterdam, The Netherlands, 2000). (2) Dagotto, E.; Hotta, T.; Moreo, A. Phys. Rep. 2001, 344, 1. (3) Salamon, M. B.; Jaime, M. ReV. Mod. Phys. 2001, 73, 583. (4) Tomioka, Y.; Asamitsu, A.; Kuwahara, H.; Moritomo, Y. Phys. ReV. B 1996, 53, R1689. (5) Moritomo, Y.; Tomioka, Y.; Asamitsu, A.; Tokura, Y. Phys. ReV. B 1995, 51, 3297. (6) Battle, P. D.; Gibb, T. C.; Lightfoot, P. J. Solid State Chem. 1990, 84, 271. (7) Tomioka, Y.; Asamitsu, A.; Moritomo, Y.; Kuwahara, H.; Tokura, Y. Phys. ReV. Lett. 1995, 74, 5108. (8) Kuwahara, H.; Tomioka, Y.; Asamitsu, A.; Moritomo, Y.; Tokura, Y. Science 1995, 270, 961. (9) Moritomo, Y.; Kuwahara, H.; Tomioka, Y.; Tokura, Y. Phys. ReV. B 1997, 55, 7549. (10) Kiryukhin, V.; Casa, D.; Hill, J. P.; Keimer, B.; Vigliante, A.; Tomika, Y.; Tokura, Y. Nature (London) 1997, 386, 813. (11) Rao, C. N. R.; Raju, A. R.; Ponnambalam, V.; Parashar, S.; Kumar, N. Phys. ReV. B 2000, 61, 594. (12) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; Won Molnar, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (13) Cowburn, P.; Welland, M. E. Science 2000, 287, 1466. (14) Biswas, A.; Das, I.; Majumdar, C. J. Appl. Phys. 2005, 98, 124310. (15) Biswas, A.; Das, I. J. Appl. Phys. 2007, 102, 064303. (16) Rao, S. S.; Tripathi, S.; Pandey, D.; Bhat, S. V. Phys. ReV. B 2006, 74, 144416. (17) Sarkar, T.; Mukhopadyay, P. K.; Raychaudhuri, A. K.; Banerjee, S. J. Appl. Phys. 2007, 101, 124307. (18) Rao, S. S.; Anuradha, K. N.; Sarangi, S.; Bhat, S. V. Appl. Phys. Lett. 2005, 87, 182503. (19) Biswas, A.; Das, I. Phys. ReV. B 2006, 74, 172405. (20) Lu, C. L.; Dong, S.; Wang, K. F.; Ga, F.; Li, P. L.; Lv, L. Y.; Liu, J. M. Appl. Phys. Lett. 2007, 91, 032502. (21) Rozenberg, E.; Auslender, M.; Shames, A. I.; Mogilyansky, D.; Felner, I.; Sominskii, E.; Gedanken, A.; Mukovskii, Y. M. Phys. ReV. B 2008, 78, 052405.

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