Particle Size Effects on Charge and Spin Correlations in Nd0.5

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Particle Size Effects on Charge and Spin Correlations in Nd0.5Ca0.5MnO3 Nanoparticles Shiming Zhou,* Yuqiao Guo, Jiyin Zhao, Laifa He, Cailin Wang, and Lei Shi* Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China, Hefei 230026, People's Republic of China ABSTRACT: The particle size effects on the charge and spin correlations in half-doped manganite Nd0.5Ca0.5MnO3, which exhibits a charge-ordered (CO) transition at 250 K in the bulk counterpart, have been investigated by magnetometry and electron spin resonance (ESR). Magnetic measurements show that reducing the particle size weakens the long-range CO transition, which completely disappears when the particle size is reduced down to 40 nm. Meanwhile, a weak ferromagnetic (FM) behavior appears at low temperatures and is gradually enhanced with the decrease of the particle size. The ESR intensities of the nanoparticles reproduce well these features. However, the temperature dependences of the ESR g-factor and line width exhibit the typical characteristics of the CO states in all the nanoparticles, even in the 40 nm sample, which suggests that, even though the long-range CO transition is completely suppressed by the size reduction, the CO state is still present in the short-range ordering form. Moreover, it is found that the onset temperatures of the CO states in all the nanoparticles are almost the same as that of the bulk, which strongly indicates that the strength of the CO correlations in this compound is not influenced much by the particle size. A detailed analysis on the magnetic susceptibilities and the ESR line width further reveals that both the antiferromagnetic (AFM) and the FM spin interactions are weakened by the size reduction, which suggests that the enhanced FM behavior in the nanoparticles is not due to the enhancement of the double-exchange FM interactions. We propose that, although the FM interactions are weakened, they gradually dominate over the AFM ones at low temperatures with the decrease of the particle size due to the more significant weakening of the latter by the size reduction, which, hence, gives rise to the development of the FM behaviors in the nanoparticles.

1. INTRODUCTION Doped perovskite manganite R1
xAxMnO3 (R, trivalent rareearth ion; A, bivalent alkaline ion) have been extensively studied for their complex phase diagrams with a variety of electronic and magnetic ground states and unusual properties, such as colossal magnetoresistance (CMR) and phase inhomogeneity.1
3 The rich variety of different ground states exemplifies the strong interactions between charge, spin, lattice, and orbital degrees of freedom. Both experiments and theory have revealed that the phase competition between two ordered states, that is, the ferromagnetic (FM) metal and the charge ordered (CO) and antiferromagneitc (AFM) insulator, is key to understanding the CMR effects.4,5 In doped manganites, the FM metallic state is induced by the double-exchange (DE) interactions between the Mn3þ and Mn4þ ions, whereas the charge ordering is one of the representative phenomena as a result of the predominant Coulomb interaction over the kinetic energy of the charge carriers. The charge ordering is characterized by a real space ordering of the Mn3þ and Mn4þ ions and is mostly observed when the concentration of charge carriers takes a rational value of the periodicity of the crystal lattice. Moreover, it has been demonstrated that the CO insulating state can be melted to the FM metallic state by a magnetic field, an electric field, and pressure.3,4,6 r 2011 American Chemical Society

Recently, the studies on nanosized near half-doped manganites have received significant interest due to the observation of some exotic phenomena associated with size effects.7
21 Rao et al. reported that, in nanowires of Pr0.5Ca0.5MnO3, the charge ordering is weakened.7 Zhang et al. demonstrated that, in La0.25Ca0.75MnO3, the CO temperature (TCO) decreases with the decrease of the particle size8 and that, in Pr0.5Ca0.5MnO3, the CO state is completely suppressed when the particle size is smaller than 40 nm.9 Similar suppression of CO transition is also found in nanosized Pr0.5Sr0.5MnO3,10,11 Nd0.5Sr0.5MnO3,12 La0.5Ca0.5MnO3,13,14 La0.4Ca0.6MnO3,15 and Nd0.5Ca0.5MnO3.16,17 Moreover, as a consequence, size-induced FM behaviors in place of the AFM phases appear at low temperature. For the narrow bandwidth manganites, such as Pr0.5Ca0.5MnO3,7,9,14 Nd0.5Ca0.5MnO3,16,17 and Sm0.5Ca0.5MnO3,18 an onset of FM clusters without long-range magnetic ordering is observed at ∼100 K, whereas for the relatively wider bandwidth manganites, such as Pr0.5Sr0.5MnO3,10,11 Nd0.5Sr0.5MnO3,12 and La0.5Ca0.5MnO3,13,14 the long-range FM ordering phase already forming at high T in Received: March 31, 2011 Revised: May 16, 2011 Published: May 16, 2011 11500

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The Journal of Physical Chemistry C the bulk counterparts remains down to the lowest temperature. To understand these exotic phenomena, several possible mechanisms were proposed in the literatures. Dong et al. demonstrated that the relaxation of superexchange interaction at the surface of nanowires or nanoparticles destabilizes the CO phase and allows formation of an FM shell.22 Zhang et al. proposed that the uncompensated spins at the surface destroy the collinear AFM configuration and give rise to enhanced FM interactions.8 Recent Monte Carlo studies revealed that an increase of charge density due to the unscreened Coulomb interactions drives the surface layer from an AFM/CO to an FM cluster and canted AFM phase separated state and provides the FM tendency at the surface.23 Sarkar et al. suggested that the enhanced surface pressure due to the size reduction prevents the formation of the CO phase and stabilizes the FM state in the nanosized samples.13 Those quite different explanations mean that the mechanism of the weakening or suppression of the CO state and the enhancement of FM behavior by the size reduction are still the subject of intense debate. Therefore, more deeper and systemic investigations are needed. Specially, for a more clear understanding of the size effects on the charge ordering and the magnetic properties in the CO manganites, an important and fundamental issue should be addressed: How does the size reduction influence the strength of the charge and spin correlations? Does the weakening or suppression of the CO state and the enhancement of FM behavior observed in the nanosized CO manganites mean that the strength of the CO and FM correlations are weakened and enhanced with the decrease of the sample size, respectively? In this work, a detailed study on the magnetic properties of Nd0.5Ca0.5MnO3 (NCMO) nanoparticles by magnetometry and electronic spin resonance (ESR) spectroscopy is carried out, and the nanometer size effects on the charge and spin correlations are focused on. It is found that, although the long-range CO state is suppressed and the FM behavior is enhanced by the size reduction, the strength of CO correlations is not affected much, whereas that of FM ones is significantly weakened. This unexpected finding provides a deep insight into the role of the size reduction on the physics of the CO manganites.

2. EXPERIMENTAL SECTION The nanosized NCMO were prepared by a sol
gel method.24 The stoichiometric amounts of high-purity Nd2O3, CaCO3, and metal Mn were used as starting materials. They were first converted into nitrates by adding dilute nitric acid. An amount of citric acid and ethylene diamine was then added to the solution, which was slowly evaporated to get a gel and decomposed at about 400 °C for 4 h to result in a dark brown powder. The precursory powder was annealed at 650, 800, 900, and 1000 °C for 4 h to produce the nanosized samples with different particle sizes. The phase purity and crystal structure of the nanoparticles were determined by an 18 kW rotating anode X-ray diffractometer (type MXP18AHF, MAC Science) with graphite monochromatized Cu KR (λ = 1.5418 Å) radiation in the Bragg
Brentano geometry at room temperature. The particle sizes and morphology were determined by field emission scanning electron microscopy (FE-SEM, JEOL-6700F). The magnetic measurements were carried out with a superconducting quantum interference device magnetometer (Quantum Design MPMS XL-7). ESR spectra were collected with a Bruker ER200D

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Figure 1. X-ray diffraction patterns for the nanosized Nd0.5Ca0.5MnO3 annealed at different temperatures.

Figure 2. SEM images for the nanosized Nd0.5Ca0.5MnO3 annealed at 650 (a), 800 (b), 900 (c), and 1000 °C (d).

spectrometer at the X band (9.06 GHz) in the temperature range of 110
370 K.

3. RESULTS Figure 1 shows the room-temperature X-ray diffraction patterns of the NCMO nanoparticles annealed at different temperatures. The sharp diffraction peaks indicate good crystalline forms for all the samples. All peaks of each specimen can be indexed by a single phase with an orthorhombic crystal structure (space group Pnma) without any impurities. With decreasing the annealing temperature, the diffraction peaks become increasing wide, which reveals the gradual reduction of the particle size. Figure 2 shows the SEM micrographs of the samples annealed at different temperatures. One can see that the particle size is homogeneous for all samples and the average diameter is about 40, 80, 150, and 350 nm for the samples annealed at 650, 800, 900, and 1000 °C, respectively. Figure 3 shows the temperature dependences of the magnetization measured at H = 50 kOe after field-cooling (FC) procedures for all the samples. For the 350 and 150 nm samples, 11501

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The Journal of Physical Chemistry C the magnetizations at high temperatures have almost the same values and exhibit pronounced peaks at about 250 K, corresponding to the robust CO transitions. Both CO transition temperatures (TCO) are well in agreement with that of the bulk reported in the literature,25,26 which suggests that the long-range CO transition in this system is not affected much as the particle size is bigger than 150 nm. However, at low temperatures, several variations associated with the size reduction are easier to find. First, unlike the bulk, where an AFM transition appears as a shoulder in the magnetization at 155 K, both nanosized samples show no visible AFM transitions. Instead, the magnetizations for both samples increase gradually below 150 K, which implies the appearance of the weak FM behaviors.9,16 Second, it is clear that the magnetization at low temperature for the 150 nm sample is larger than that of the 350 nm sample, which indicates that the FM behavior is enhanced by the size reduction. When the particle size decreases to 80 nm, in addition to the magnetization presenting a further enhancement at low temperatures, the CO transition peak broadens and decreases in intensity, which reveals that the robust charge ordering is weakened in this sample. With the particle size further reduced down to 40 nm, the CO transition can no longer be found and the magnetization rises

Figure 3. Temperature dependence of the FC magnetization measured under H = 50 kOe for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes.

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monotonically within the whole temperature range, which means that the long-range CO state is completely suppressed and the FM behavior is strongly enhanced. Figure 4 displays the ESR spectra at some representative temperatures for the NCMO nanoparticles with different particle sizes. One can see that, for all the samples, the spectra at high temperatures show a single symmetric resonance signal. When the temperature is lowered below about 200 K, the signals become distorted and asymmetric. The temperature-dependent intensity of the ESR spectra for the nanoparticles, derived by double integration of the first derivative spectra and normalized by the sample mass, is shown in Figure 5. For the 350 and 150 nm samples, the ESR intensities show a peak at TCO, whereas for the 80 and 40 nm samples, the intensity reveals that the CO transition is weakened and suppressed, respectively. Moreover, the intensity shows an increase below about 150 K with decreasing the particle size, which confirms the development of the FM behavior in the nanoparticles. The ESR intensities of these nanoparticles correlate well with their magnetization curves. Figure 6 shows the g-factors obtained from the resonance fields by hν = gμBHr between 370 and 110 K for the NCMO nanoparticles. One can see that there is a similar temperature dependence of the g-factor for each sample. With lowering the

Figure 5. Temperature dependence of the ESR intensity for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes.

Figure 4. ESR spectra at some representative temperatures for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes. 11502

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Figure 6. Temperature dependence of the ESR g-factor for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes.

Figure 7. Temperature dependence of the ESR line width for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes.

temperature from 370 K, the g-factor shows a slight increase and then drops slightly below about 250 K. Upon further cooling, it exhibits a sharp rise below around 180 K. These features are well consistent with those reported in the single-crystal NCMO.26 Figure 7 shows the ESR peak-to-peak line widths ΔHPP for the NCMO nanoparticles. Like the g-factor, the line widths of the nanoparticles with different particle sizes display similar dependences on the temperature. Upon cooling, the line widths decrease, first, to reach a minimum value at Tmin ∼ 250 K and then increase sharply below this temperature. This behavior agrees well with those reports on the CO manganites.26
28 As the temperature is further lowered, however, unlike those CO manganites, the NCMO nanoparticles exhibit a more complex behavior in the line widths, where a rapid decrease emerges below about 180 K again and then is followed by a rise below about 130 K. On the other hand, when the particle size ranges from 350 to 40 nm, some obvious variations are also found. Above 250 K, the line width significantly increases with the particle size reduced below 150 nm. Below 250 K, the magnitude of the rise in the line width decreases with the decrease of the particle size.

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4. DISCUSSION The magnetic measurements on the nanosized NCMO reveal that the long-range CO transition is not affected much as the particle size is bigger than 150 nm, whereas it is weakened in the 80 nm sample and is completely suppressed as the particle size is reduced down to 40 nm. This great change in the macromagnetic behavior is well reproduced by the ESR intensities of those nanoparticles because, in the manganites, the ESR signals are ascribed to originate from the collective motion of Mn3þ and Mn4þ ions and the ESR intensity is approximately proportional to the static magnetic susceptibility.26
30 This result is in good agreement with the recent reports in various nanosized CO mangaites.7
9,14,18,20 However, unlike the magnetization and the ESR intensity where quite different temperature dependences are found as the particle size is reduced from 350 to 40 nm, both the ESR g-factor and the line width show very similar temperature dependences for the nanosized NCMO with the different particle sizes. Specially, at 250 K, corresponding to the CO transition in the bulk NCMO, both of all the samples exhibit clear anomalies, that is, the appearance of the shoulder and the minimum, respectively. Previous ESR studies on the CO manganites, such as Pr0.6Ca0.4MnO327 and YBaMn2O6,29 reported similar anomalies in the g-factor and the line width at the CO transition temperatures. In the manganites, the charge ordering is always accompanied by the orbital ordering. The gradual buildup of the orbital ordering can change the spin
orbit coupling constant and as well as the crystal-field splitting and hence the g-factor. Thus, the shoulder in the g-factor at TCO would be associated with the formation of the CO phases. Moreover, the sharp rise in the line width below TCO was also proposed to characterize the onset of the CO state and the development of the AFM correlations.27 Therefore, the shoulder in the g-factor and the minimum in the line width at 250 K for our NCMO nanoparticles reflect the formation of the CO states. For the three nanosized NCMO with larger particle sizes, these features are well consistent with the results from the magnetizations and the ESR intensities where the CO transitions are clearly detected. However, for the 40 nm sample, it is unexpected to find this scenario because no visible long-range CO transition is present in the magnetization and the ESR intensity. Taking into accounting that the ESR g-factor and the line width are very sensitive to the micromagnetic structures and the changes in the local symmetry of the spins,29 and have been successfully used to probe the shortrange spin and charge orderings in the manganites,31,32 we suggest that a short-range CO state should be present in the 40 nm sample. Most importantly, it is clearly seen that, as the particle size ranges from 350 to 40 nm, the onset temperature of the CO states derived from the g-factor and the line width remains almost unchanged, which strongly indicates that the strength of the CO correlations in the nanosized NCMO is not affected much by the size reduction. This result is also unexpected because the long-range CO phases were generally found to be weakened or suppressed in the various nanosized halfdoped CO manganties. At low temperatures, from the magnetic measurement and the ESR intensity, it is clear that the FM behavior in the NCMO nanoparticles is enhanced with the decrease of the particle size. This enhancement is frequently reported in various nanosized CO manganites.7
9,14,18,20 However, its origin is still under debate. Actually, in the bulk half-doped CO manganites, it is well established that the high-temperature paramagnetic (PM) 11503

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Figure 8. Temperature dependence of the inverse susceptibilities under H = 50 kOe for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes. The lines are the best-fitted results by the CW laws.

phases are dominated by the short-range FM spin correlations originating from the dynamic DE interactions between Mn3þ and Mn4þ ions.33
35 The existence of the FM correlations is generally supported by a positive Curie
Weiss (CW) temperature θ from fitting the susceptibility in the high-temperature PM regime by the CW law, χ = C/(T
θ), where C is the Curie constant and θ is the CW temperature, and the strength of the FM interactions is usually characterized by the value of θ. As the temperature is lowered, the AFM spin correlations from the superexchange couplings between Mn ions progressively build below TCO and result in the long-range AFM transition at TN. The peak in the magnetization as well as the ESR intensity at TCO reflects the competition of the FM and AFM spin correlations. As the system enters into the CO state, the AFM correlations dominate over the FM ones, which is generally supported by a negative θ obtained by fitting the susceptibility between TCO and TN by the CW law.34,35 Moreover, the fitting on the susceptibility below TCO by the CW law produces a much larger Curie constant than that for an effective Mn ion (the weighted average SMn4þ = 3/2 and SMn3þ = 2).34,35 It is proposed that the magnetic clusters formed by Mn dimers34 or a large group35 exist in this temperature range. For the NCMO nanoparticles, the susceptibilities derived from the magnetizations are fitted by the CW laws for the temperature regions above and below TCO, respectively, as shown in Figure 8. The fitted parameters against the particle size are plotted in Figure 9, where CHT and CLT are the Curie constants and θHT and θLT are the CW temperatures, for the high- and low-temperature PM phases, respectively. In the hightemperature PM region, it is found that the values of CHT are around 0.017 emu 3 K/g 3 Oe for all the nanoparticles, which are very close to the theoretical value of the bulk NCMO (0.0167 emu 3 K/g 3 Oe). Because the Curie constants of the

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Figure 9. Particle size dependence of CHT, CLT, θHT, and θLT for the nanosized Nd0.5Ca0.5MnO3.

manganites are very sensitive to the stoichiometry, this result suggests that all the NCMO nanoparticles have good stoichiometry in the cation composition and oxygen content, which allows us to conclude that any change of the physical properties for our NCMO nanoparticles arises only from the size reduction. However, the value of θHT exhibits a significant decrease with the particle size, which indicates that the FM interactions are weakened by the size reduction. Previous studies on the FM manganites, such as La0.3Sr0.3MnO3,36 La0.67Ca0.33MnO3,37,38 and La0.6Pb0.4MnO3,39 had reported that the strength of the FM interactions can be weakened as the particle size is reduced to nanoscale. It was proposed that, in the nanoparticles, the lattice and/or magnetic structure are disordered at the surface, resulting in the weaker FM couplings.38,39 Recently, similar weakening of the FM interactions is also found in the nanosized half-doped manganites Pr0.5Sr0.5MnO3,10 Pr0.5Ca0.5MnO3,14 and Sm0.5Sr0.5MnO3.19 On the basis of the structural and magnetic studies, Jirak et al. concluded that, not the lattice distortion, but surface spin disorders are responsible for the weakening.14 In the lowtemperature PM region, for each nanosized sample, the value of CLT is found to be much larger than that of CHT, as reported in the bulk half-doped manganites,34 which indicates that the magnetic clusters are formed by Mn dimers34 or a large group35 below TCO in our nanoparticles. However, unlike CHT, CLT shows a monotonic decrease as the particle size decreases, suggesting that the size of the magnetic clusters is reduced, possibly due to the increase of the surface spin disorders with the decrease of particle size. On the other hand, contrary to θHT, θLT displays a great increase with the decrease of the particle size. More importantly, the sign of θLT changes from negative to positive as the particle size is reduced below 150 nm, which indicates that the domination of the low-temperature PM states 11504

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Figure 11. Particle size dependence of Θ and ΔH0 for the nanosized Nd0.5Ca0.5MnO3. Figure 10. Temperature dependence of ΔHPP*T for the nanosized Nd0.5Ca0.5MnO3 with different particle sizes. The lines are the fitted results by Huber et al.’s approach (see text).

for the NCMO nanoparticles evolves from by the AFM interactions to by the FM ones. Because the FM interactions are not enhanced, but weakened, in the NCMO nanoparticles, as discussed above, this evolution strongly suggests that, similar to the FM interactions, the AFM ones in this system are also weakened by the size reduction. It is worthwhile to note that, although the values of θLT for the 80 and 40 nm samples are positive, they are much smaller than their values of θHT. Therefore, we can conclude that the AFM interactions are just strongly weakened, but not completely suppressed, in both the nanoparticles. This feature together with the results from the ESR g-factor and line width indicate that the AFM/CO states should remain in the short-range nature for the nanosized NCMO even where the long-range CO/AFM transitions disappear due to the size reduction. The particle size effects on the FM and AFM interactions in the NCMO can also be derived from the ESR line width because it is very sensitive to the changes of spin correlations in the manganites. From Figure 7, with the decrease of the particle size, two obvious changes in the line width are found above and below 250 K, respectively. Above 250 K, the line width greatly increases as the particle size is reduced below 150 nm. To understand this increase, we analyzed the temperature dependences of the line width for all the nanoparticles by Huber et al.’s approach.30 The approach assumes that strong exchange coupling provides a bottleneck regime of resonating spins and gives the following formula,28 ΔHpp(T) = ((T
Θ)/T)ΔH0, where Θ is the CW temperature and ΔH0 is the high-T asymptote due to the Mn ions spin
spin relaxation mechanism. As shown in Figure 10, the line widths of all the samples obey well the approach above 310 K. Figure 11 plots the fitted parameters Θ and ΔH0 against the particle size. The value of Θ is very close to that of θHT obtained from the high-temperature susceptibility for each sample and also shows a similar decrease with the particle size, which confirms that the FM interactions in this system are weakened by the size reduction. However, the value of ΔH0 for these nanoparticles exhibits a notable increase with decreasing the particle size. Recently, in La0.4Ca0.6MnO328 and La0.7Ca0.3MnO3,40 a similar increase in ΔH0 was also found as the particle size is reduced to nanometer scale. It is proposed that a size-enhanced surface anisotropy in the nanoparticles is a possible origin of this

increase. On the other hand, below 250 K, the magnitude of the rise in the line width decreases with the particle size. Because the rise is attributed to the development of AFM correlations,26 this decrease suggests that the AFM interactions in the NCMO nanoparticles are weakened by the size reduction, which also confirms the result from the susceptibility. Therefore, our detailed analysis on the susceptibilities and the ESR line width clearly reveals that both the FM and the AFM interactions in NCMO are weakened by the size reduction. This result indicates that the enhanced low-temperature magnetizations in the NCMO nanoparticles are not due to the enhancement of the DE FM interactions.

5. CONCLUSIONS In conclusion, we have investigated the magnetic properties of half-doped manganite NCMO nanoparticles with the average particle size ranging from about 350 to 40 nm by magnetometry and ESR in detail. The magnetization and the ESR intensity reveal that, with the decrease of the particle size, the long-range CO transition in NCMO is weakened and completely suppressed as the particle size is reduced down to 40 nm. Meanwhile, a weak FM behavior appears at low temperatures and is gradually enhanced. These results are in good agreement with the recent studies on various nanosized near half-doped manganites. However, the temperature dependences of the ESR g-factor and line width present the typical characteristics of the CO states in all the nanoparticles, even in the 40 nm sample, which suggests that, even though the long-range CO transition is completely suppressed by the size reduction, the CO state still exists with a short-range ordering. Moreover, we find that the formation temperatures of the CO states in all the nanoparticles are almost the same as that of the bulk, which strongly indicates that the strength of the CO correlations in this compound is not influenced much by the particle size. A detailed analysis on the magnetic susceptibilities and the ESR line width below and above TCO further reveals that both the FM and the AFM spin interactions in the nanoparticles are weakened by the size reduction. This result indicates that the enhanced low-temperature magnetizations in the NCMO nanoparticles are not due to the enhancement of the FM interactions. Because the magnetic behaviors of the half-doped CO manganites strongly depend on the competition of the FM and AFM interactions, we propose that, with the decrease of the particle size, the weakening of the AFM interactions gives rise to the gradual domination of the FM 11505

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.Z.), [email protected] (L.S.).

’ ACKNOWLEDGMENT This project was financially supported by the National Science Foundation of China (Grant Nos. 10904135 and 10874161), the Foundation for the Excellent Youth Scholars of Anhui Province of China (No. 2010SQRL007ZD), and the National Basic Research Program of China (973 program, 2009CB939901). ’ REFERENCES (1) Dagotto, E.; Hotta, T.; Moreo, A. Phys. Rep. 2001, 344, 1. (2) Dagotto, E. New J. Phys. 2005, 7, 67. (3) Tokura, Y. Rep. Prog. Phys. 2006, 69, 797. (4) S-en, C.; Alvarez, G.; Dagotto, E. Phys. Rev. Lett. 2007, 98, 127202. (5) S-en, C.; Alvarez, G.; Dagotto, E. Phys. Rev. Lett. 2010, 105, 097203. (6) Tao, J.; Niebieskikwiat, D.; Varela, M.; Luo, W.; Schofield, M. A.; Zhu, Y.; Salamon, M. B.; Zuo, J. M.; Pantelides, S. T.; Pennycook, S. J. Phys. Rev. Lett. 2009, 103, 097202. (7) Rao, S. S.; Anuradha, K. N.; Sarangi, S.; Bhat, S. V. Appl. Phys. Lett. 2005, 87, 182503. (8) Zhang, T.; Zhou, T. F.; Qian, T.; Li, X. G. Phys. Rev. B 2007, 76, 174415. (9) Zhang, T.; Dressel, M. Phys. Rev. B 2009, 80, 014435. (10) Pramanik, A. K.; Banerjee, A. Phys. Rev. B 2010, 82, 094402. (11) Rao, S. S.; Bhat, S. V. J. Phys. D: Appl. Phys. 2009, 42, 075004. (12) Biswas, A.; Das, I. J. Appl. Phys. 2007, 102, 064303. (13) Sarkar, T.; Raychaudhuri, A. K.; Chatterji, T. Appl. Phys. Lett. 2008, 92, 123104. (14) Jirak, Z.; Hadova, E.; Kaman, O.; Knízek, K.; Marysko, M.; Pollert, E. Phys. Rev. B 2010, 81, 024403. (15) Lu, C. L.; Dong, S.; Wang, K. F.; Gao, F.; Li, P. L.; Lv, L. Y.; Liu, J.-M. Appl. Phys. Lett. 2007, 91, 032502. (16) Rao, S. S.; Tripathi, S.; Pandey, D.; Bhat, S. V. Phys. Rev. B 2006, 74, 144416. (17) Rao, S. S.; Bhat, S. V. J. Phys.: Condens. Matter 2009, 21, 196005. (18) Zhou, S. M.; Shi, L.; Yang, H. P.; Wang, Y.; He, L. F.; Zhao, J. Y. Appl. Phys. Lett. 2008, 93, 182509. (19) Zhou, S. M.; Guo, Y. Q.; Zhao, J. Y.; He, L. F.; Shi, L. J. Phys. Chem. C 2011, 115, 1535. (20) Chai, P.; Wang, X. Y.; Hu, S.; Liu, X. J.; Liu, Y.; Lv, M. F.; Li, G. S.; Meng, J. J. Phys. Chem. C 2009, 113, 15817. (21) Zhou, S. M.; Zhao, S. Y.; Guo, Y. Q.; Zhao, J. Y.; Shi, L. J. Appl. Phys. 2010, 107, 033906. (22) Dong, S.; Gao, F.; Wang, Z. Q.; Liu, J.-M.; Ren, Z. F. Appl. Phys. Lett. 2007, 90, 082508. (23) Dong, S.; Yu, R.; Yunoki, S.; Liu, J.-M.; Dagotto, E. Phys. Rev. B 2008, 78, 064414. (24) Zhou, S. M.; Shi, L.; Zhao, J. Y.; Yang, H. P.; Chen, L. Solid State Commun. 2007, 142, 634. (25) Millange, F.; de Brion, S.; Chouteau, G. Phys. Rev. B 2000, 62, 5619. (26) Joshi, J. P.; Gupta, R.; Sood, A. K.; Bhat, S. V.; Raju, A. R.; Rao, C. N. R. Phys. Rev. B 2001, 65, 024410. (27) Gupta, R.; Joshi, J. P.; Bhat, S. V.; Sood, A. K.; Rao, C. N. R. J. Phys.: Condens. Matter 2000, 12, 6919.

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