JOURNAL OF APPLIED PHYSICS
VOLUME 92, NUMBER 12
15 DECEMBER 2002
Direct observation of phase separation in La0.45Sr0.55MnO3À ␦ Run-Wei Li,a) Zhen-Rong Zhang, Qing-An Li, Ji-Rong Sun, Guang-Jun Wang, Zhao-Hua Cheng, Zhi-Hong Wang, Shao-Ying Zhang, Bao-Shan Han, and Bao-Gen Shen State Key Laboratory of Magnetism, Institute of Physics and Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China
共Received 26 November 2001; accepted 6 October 2002兲 We provide evidence of phase separation in La0.45Sr0.55MnO3⫺ ␦ using electron spin resonance, magnetic force microscopy 共MFM兲, x-ray diffraction, and magnetic and transport measurements. The results reveal that ferromagnetic and antiferromagnetic phase coexist at low temperature and that ferromagnetic and paramagnetic phases coexist in the temperature range between the Ne´el and the Curie temperature. Moreover, the size and shape of ferromagnetic phase 共the minority phase兲 in the sample were observed directly by MFM. From these results, we infer an electroneutral type phase separation, possibly resulting from a nonuniform distribution of oxygen vacancies, as opposed to charge segregation. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1524308兴
I. INTRODUCTION
phases, coexist in different temperature regions in LSMO. Moreover, the size and shape of the FM regions 共the minority phase兲 were directly measured by MFM. It was suggested that the PS in the LSMO is not due to charge segregation, but due to a nonuniform distribution of oxygen vacancies.
Hole-doped Ln1⫺x Ax MnO3 共Ln⫽rare earth, A⫽alkaline earth兲 manganites with a distorted perovskite structure demonstrate a rich variety of physical phenomena such as colossal magnetoresistance,1–3 charge and orbital ordering,4 – 6 and phase separation 共PS兲.7–16 Recently, PS has envoked considerable interest. Theoretical study of the PS revealed that ferromagnetic 共FM兲 and paramagnetic 共PM兲 phases,17 antiferromagnetic 共AFM兲 and PM phases, as well as FM and AFM phases7,18,19 can coexist. PS phenomena were confirmed experimentally in half-hole-doped La0.5Ca0.5MnO3, 9,12 which is located at the boundary between competing FM and AFM phases. Subsequent electron spin resonance 共ESR兲,11 nuclear magnetic resonance 共NMR兲14,15 and Mo¨ssbauer20 investigations indicated that PS occurs not only in half-doped samples, but also in other compositions. However, the detailed scenario of PS including the size and shape of the minority phase, which is important for understanding the nature of PS, has not been directly measured up to now, although PS as an intrinsic feature of perovskite manganites has been acknowledged gradually. La1⫺x Srx MnO3 compounds show similar magnetic and transport properties to La1⫺x Cax MnO3 in the low-doped regimes. Urushibara et al.21 reported a phase diagram for the La1⫺x Srx MnO3 system 共0⬉x⬉0.6兲 in 1995 and predicted that the compounds will be FM with a Curie temperature (T C ) of about 350 K in a doping range of 0.4⬍x⬍0.6. However, subsequent investigations showed that AFM charge ordering also takes place in La1⫺x Srx MnO3 for 0.46⬉x⬉0.53.22,23 This discrepancy may be due to a difference in sample preparation procedures. In this article, we report a detailed investigation of the La0.45Sr0.55MnO3⫺ ␦ 共LSMO兲 through x-ray diffraction, ESR, magnetic force microscopy 共MFM兲, and magnetic and transport measurements. We found that FM and PM phases, as well as FM and AFM
II. EXPERIMENT
A polycrystalline sample of LSMO was prepared by the solid state reaction technique. La0.67Sr0.33MnO3 , which exhibits optimal ferromagnetism,21 was also prepared for comparison. A well-mixed stoichiometric mixture of La2O3, SrCO3 , and MnCO3 was calcined at 1000 °C in air for 24 h. The resulting powder was reground by high-energy ball milling in air for 4 h, then pressured into pellets and sintered at 1300 °C in air for 48 h. The phase purity and lattice parameters of the synthesized samples were examined by x-ray diffraction, performed on a D/Max 2500 x-ray diffractometer with a rotating anode and Cu K ␣ radiation. Rietveld refinement results indicate that there is no secondary phase to the resolution limit of x-ray diffraction 共Fig. 1兲. The LSMO was slightly oxygen poor 共␦⬇0.01兲 as determined by redox titration. Magnetization measurements were performed in a commercial superconducting quantum interference device 共SQUID兲 magnetometer. The micromagnetic properties were investigated by ESR measurements carried out at 9.50 GHz using a Bruker-200D spectrometer. Magnetic force images were obtained by a Nanoscope IIIa MFM operated at room temperature. III. RESULTS AND DISCUSSION
Figure 2共a兲 shows the temperature dependence of the magnetization (M – T) of the LSMO measured in zero-fieldcooling mode under a field of 0.02 T. A peak at about 240 K and a PM–FM transition at about 375 K can be observed clearly in the M–T curve. These observations are different from those of Urushibara et al.21 and of Fujishiro et al.23 We know the samples were prepared in different atmosphere in
a兲
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FIG. 1. Rietveld refinement results for the La0.45Sr0.55MnO3⫺ ␦ sample at room temperature.
Refs. 21 共in an O2 ) and 23 共in an Ar mixture兲23 共in O2 ) than here 共in air兲. This discrepancy may be due to the difference in sample preparation procedure, which will affect the Mn3⫹/Mn4⫹ ratio. Near 240 K, the lattice parameter shows a large change 关Fig. 2共b兲兴, and the coercivity drops to nearly zero 关Fig. 2共c兲兴. The magnetic transition at 240 K coincides with the large change in lattice constant, which may be due to strong magnetostructural coupling. The magnetic moment per formula unit as a function of the field applied (M – H), measured at 5 and 300 K, respectively, is shown in the inset of Fig. 2共a兲. The magnetization exhibits a steep jump in the low field region, indicating the existence of FM phase, and then a linear increase with field applied. Below 0.5 T, the (M – H) curves obtained at temperatures of T⫽5 and 300 K are consistent with each other. When the field applied is above 0.5 T, the M – H curve at 300 K increases more steeply with the field compared with that at 5 K. A rough estimation shows that the magnetic moment per formula unit ( exp⫽0.23 B at 5 K and exp⫽0.51 B at 300 K兲 is far
FIG. 2. Temperature dependence of the magnetization measured under a field of 0.02 T from 5 to 400 K by a warming process in zero-field-cooled mode. The inset shows the magnetic moment per formula unit as a function of the field applied at 5 and 300 K, respectively. Temperature dependence of the lattice parameters and the coercivity.
FIG. 3. ESR spectra measured from 120 to 400 K in field-sweeping mode from 0 to 8000 G.
from the saturation value cal⫽3.45 B ) even under a field of 5 T. The large difference between exp and cal suggests that 共i兲 the sample is of a canted spin structure or 共ii兲 there may exist a weak magnetic phase, i.e., AFM or PM phase, in addition to FM phase. The latter implies that a PS appears in the sample. The ESR spectra were measured from 120 to 400 K in field-sweeping mode 共0-8000G兲. As shown in Fig. 3, above T C , the ESR signals consist of a single peak of Lorentzian line shape with a Lande factor of g⫽2.0, essentially independent of the temperature. This signal has been ascribed to PM Mn ions.25 Below T C , there is a clear extra resonance peak at low fields, a sign of the appearance of FM phase. By further decreasing the temperature below 240 K 共the magnetization-peak temperature in the M – T curve兲, the original PM resonance peak disappears, but the FM resonance peak remains. As we know, the AFM resonance peak cannot be observed in our measurement conditions. Therefore, the magnetization and ESR data suggest that an AFM–PM transition occurs near 240 K. Different from La1⫺x Srx MnO3 with 0.48⬉x⬉0.52,23,24 La0.5Ca0.5MnO3 共Ref. 26兲 and Nd0.5Sr0.5MnO3 , 27 in which an AFM–FM transition appears followed by a FM–PM transition with an increase in temperature, in LSMO, the intermediate FM state does not appear. So we may define the peak temperature as the Ne´el temperature (T N ), while the AFM phase transforms into PM phase. As a consequence, the magnetization and ESR measurements proved that PS occurs in LSMO, FM and AFM phases coexist below T N , and the FM and PM phases coexist when the temperature is between T N and T C . It is worth noting that 共i兲 the PS phenomena also appear in hightemperature superconductors such as La2CuO4 with excess cations or oxygen. It was found that the superconducting phase formed in the AFM background exhibits an optimal superconducting transition temperature. In LSMO, the T C of FM phase is 375 K 关see Fig. 2共a兲兴, which is in excellent accord with that of the optimal FM La1⫺x Srx MnO3 with x⫽0.33.21 In other words, similar to the high temperature superconductor, the PS facilitates the FM phase in LSMO by creating optimum conditions of their appearance. The PS may be an intrinsic feature of the system with strongly correlated electrons. 共ii兲 At low temperature, there are interac-
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FIG. 4. Magnetic force images taken at room temperature for 共a兲 La0.67Sr0.33MnO3 , 共b兲 La0.45Sr0.55MnO3⫺ ␦ , and 共c兲 La0.45Sr0.55MnO3⫺ ␦ annealed.
tions between coexisting phases. When the temperature is below T N , the large coercivity should be attributed to the AFM matrix pinning the small FM phase. When the temperature approaches T N , the AFM ordering is destroyed, and the pinning interaction is seriously weakened. As a consequence, the coercivity tends toward nearly zero 关Fig. 2共c兲兴. At room temperature, we cannot get an intrinsic magnetic domain structure by MFM measurement due to the small coercivity of the FM phase. However, it is possible to obtain the size and shape of the FM phase. MFM images of LSMO and La0.67Sr0.33MnO3 共for comparison兲 were obtained under a zero applied field using a MFM with a hard Coalloy-coated silicon tip magnetized along the tip axis, perpendicular to the polished sample surface. The lift height was 30 nm. As shown in Fig. 4共a兲, for the x⫽0.33 sample, which
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is completely FM at room temperature, the bright and dark regions 共corresponding to magnetic domains with antiparallel magnetization perpendicular to the sample surface兲 are distributed homogeneously in the MFM image. The result for LSMO is very interesting 关Fig. 4共b兲兴. It is found that some regions with black circle of mesoscopic size are embedded in the background without the MFM signal. We believe that these local black regions are FM and the rest are PM. The magnetic tip magnetized these FM regions, producing attractive force between the tip and the FM regions. Therefore, only the black regions can be observed. In some sense, the brightness of the local black regions reflects the depth of the FM phase embedded in the LSMO sample. It is possible for two different types of PS to occur in manganites. One is charge segregation and the other is an electroneutral PS. For the former, the competition between the double exchange and Coulomb coupling determines the size and shape of minority phases. As a result, the size of the minority phase regions is expected to be very small 共about the order of a nanometer兲 due to extended Coulomb interaction. For the latter, the size can be much larger than that of the former due to the absence of Coulomb force. According to the MFM image of LSMO, the maximum size of the FM phase is about 0.7 m, which is too large to be explained by the charge segregation type PS. Therefore, we prefer to consider that the PS in LSMO is of electroneutral type. As is well known, Ln1⫺x Ax MnO3 is apt to be oxygen rich if x⬉0.2 and oxygen poor if x⬎0.5. In low-doped regions, for example, in La1⫺x Cax MnO3 with x⬉0.2, the excess oxygen is distributed inhomogeneously in the whole sample. The oxygen-rich regions have a large hole density and the holes establish local FM order.28,29 The PS is electroneutral because the densities of holes and excess oxygen are the same in a given region. However, for highly doped regions, we propose that oxygen vacancies have a tendency to become concentrated, which decreases the local Mn4⫹ content and enhances local FM order. As a consequence, the electroneutral PS occurs in the highly doped regions due to the nonuniform distribution of oxygen vacancies. To verify the above assumption, the LSMO sample was annealed in vacuum of ⬃1.5⫻10⫺3 Pa at 700 °C for 30 min to create more oxygen vacancies. The magnetic force image of the annealed LSMO 关shown in Fig. 4共c兲兴 shows that the size of FM clusters obviously increased compared to that of the original one. Therefore, it is evident that the PS in LSMO is closely related to oxygen vacancies. Moreover, Moreo et al.30 have proposed theoretically that giant clusters can coexist in doped manganites due to disorder related to, for example, some inhomogeneous strain, cationic disorder, or nonuniform oxygen distribution. As a consequence, our results provide strong support for their theory. Figure 5 and its inset show the temperature dependence of the resistance and magnetoresistance 共MR兲, respectively. The magnetoresistance was defined as MR⫽关 R(0)-R(H)/ R(0)]⫻100%, where R(0) and R(H) are the resistance in the absence and presence of a magnetic field, respectively. The sample is insulating over the whole temperature range studied. When the temperature approaches T N , an obvious change in slope can be observed in the R – T curves, which
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measurements. This work was supported by the National Natural Science Foundation of China and State Key Project of Fundamental Research of China.
1
FIG. 5. Temperature dependence of the resistance measured in the presence and absence of the magnetic field, respectively. The inset shows the temperature dependence of the magnetoresistance under a field of 5 T.
indicates different conduction mechanisms below and above T N . The MR is less than 7% in the low temperature range and approaches zero near 400 K. A striking feature is that a MR peak appears near T N . According to the magnetization and MFM measurements, the FM phase fraction is far below the percolative threshold although the FM phase fraction can be increased slightly by a field being applied. Therefore, the insulating behavior and the small MR can be readily understood. Moreover, the insulating matrix in LSMO is AFM below T N , but PM above T N . The different spin structure of the insulating matrix could be responsible for the different slope of the R – T curve below and above the T N . The MR peak near T N is possibly related to a first-order phase transition. IV. SUMMARY
Strong evidence of a PS in LSMO has been provided through ESR, MFM, x-ray diffraction, and magnetic measurements: the FM and AFM phases coexist at low temperature, while the FM and PM phases coexist between T N and T C . The MFM was used to observe the size and shape of FM regions. The large size of FM phase 共the minority phase兲 suggests that the PS in LSMO is not a charge segregation PS, but an electroneutral PS. We propose that oxygen vacancies have a tendency to condense and local FM order can result in the manganites. ACKNOWLEDGMENTS
Thanks go to J. Zhang, Y. Li, T. S. Ning, S. Y. Fan, G. H. Rao, and J. R. Chen for their help in sample preparation and
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