JOURNAL OF APPLIED PHYSICS 97, 10A509 共2005兲
Spin dynamics and magnetic frustration effects in La1−xSrxCoO3 „0 < x Ï 0.5… compounds Nguyen Van Khiema兲 and Nguyen Xuan Phuc Institute of Materials Science, Vietnamese Academy of Science and Technology (VAST), Caugiay, Hanoi, Vietnam
The-Long Phan and Seong-Cho Yub兲 Department of Physics, Chungbuk National University, 361-763, Cheongju, Korea
Manh-Huong Phan Department of Aerospace Engineering, Bristol University, Queens Building, University Walk, Bristol BS8 1TR, United Kingdom
共Presented on 10 November 2004; published online 28 April 2005兲 La1−xSrxCoO3 共0 ⬍ x 艋 0.5兲 have been thoroughly studied by means of dc magnetization, ac susceptibilities, and electron-spin-resonance 共ESR兲 spectra. Spin-glass behavior and its transition process for x ⬍ 0.2 compositions appeared to occur at temperature Tg ranging from 14.6 to 68 K, whereas for further strontium substitution the system was characterized by growing ferromagnetic clusters. The frequency dependence of the spin-glass freezing temperature T f has been analyzed using the conventional critical slowing-down scaling law. The temperature dependence of Ta with n respect to the external magnetic field Hex obeyed an exponential function of a = Ta / TC ⬀ −Hex .A complete magnetic phase diagram of La1−xSrxCoO3 is drawn up. Besides, the internal dynamics in the samples are elucidated by the ESR spectra. The results obtained provide more insights into the nature of spin dynamics and so-called magnetic frustration phenomena in such a cobaltate system. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1855199兴 Recently, there has been much interest in the close interplay between magnetism and transport properties in La1−xSrxCoO3 compounds.1–8 Herein, the substitution of Sr2+ for La3+ ions converted an adapted number of the Co3+ ions into the Co4+ ones introducing a predominantly ferromagnetic 共FM兲 order due to the double-exchange 共DE兲 interactions between the Co4+ and CoIII or Coiii ions and antiferromagnetic 共AF兲 superexchange 共SE兲 interactions between the Co ions of the same valency state. A magnetic diagram for La1−xSrxCoO3 was also depicted by several author groups,1–4,8 where this system was found to show a spinglass 共SG兲 phase for 0 ⬍ x 艋 0.18 and a cluster-glass 共CG兲 phase for 0.18⬍ x 艋 0.5 at low temperatures, and the paramagnetic 共PM兲 phase at high temperatures. Nevertheless, there are still several controversies regarding to the realistic origin of the anomalies in the temperature-dependent magnetization curves as well as to the threshold for cluster formation.1,6,9,10 In this context, a thorough study of the magnetic properties of La1−xSrxCoO3 共0 ⬍ x 艋 0.5兲 compounds has been made by means of dc magnetization, ac susceptibilities, and electron-spin-resonance spectra. La1−xSrxCoO3 samples with x = 0.05, 0.07, 0.1, 0.14, 0.15, 0.2, 0.3, 0.4, and 0.5 were prepared by a solid-state reaction method with the use of the same conditions. The a兲
Permanent address: Department of Natural Science, Hongduc University, 307 Lelai Str., Thanhhoa City, Vietnam; Electronic address:
[email protected] b兲 Author to whom correspondence should be addressed; FAX: ⫹82-432956416; Electronic address:
[email protected] 0021-8979/2005/97共10兲/10A509/3/$22.50
x-ray diffraction data confirmed the quality of the samples. The temperature dependences of the zero-field-cooled 共ZFC兲 and field-cooled 共FC兲 magnetizations M ZFC共T兲 and M FC共T兲, respectively, were measured using a Quantum Design MPMS5 superconducting quantum interference device 共SQUID兲 magnetometer in the temperature range of 5 – 300 K. A Lakeshore 7225 susceptometer was used for ac susceptibility measurements with an applied field of 10 Oe at different frequencies. The electron-spin-resonance 共ESR兲 measurements were performed at 9.2 GHz 共X band兲 with a JEOL-JES-TE300 ESR spectrometer, in the temperature range of 77– 500 K. Figure 1 shows the plots of M ZFC共T兲 and M FC共T兲 for the representative samples measured in an applied field of 20 G. The PM to FM phase transition for the x = 0.2, 0.3, 0.4, and 0.5 samples occurred at TC = 196, 228, 235, and 249 K, respectively. As can be seen clearly from Fig. 1, for all the samples investigated, there was a prominent deviation of M FC共T兲 and M ZFC共T兲. M ZFC共T兲 showed a cusp at temperature Ta which shifts to a lower temperature as the Hex is increasingly applied 共see Fig. 2兲, and the M ZFC共T兲 cusp was broadened with increasing Hex. Our measurements of the real ⬘共T兲 共in phase兲 and imaginary ⬙共T兲 共out of phase兲6,10 of the ac susceptibility ac共T兲 showed that, for the 0.18⬍ x 艋 0.5 samples, there was only a peak in ⬘共T兲 followed by a monotonic decrease, as T → 0 K. Given the proximity to TC, the frequencyindependent nature of the peak can be understood as the onset of the FM ordering. As expected, the temperature at which ⬘ reaches a maximum decreases as x is decreased towards a critical doping content xc ⬇ 0.18, where the signal
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FIG. 1. M ZFC共T兲 and M FC共T兲 at Hex = 20 G: 共a兲 x = 0.05, 0.07, 0.12, and 0.14; 共b兲 x = 0.2, 0.3, 0.4, and 0.5 samples. The arrow marks the specific temperatures.
of the onset of FM ordering disappeared. Whereas, below xc ⬇ 0.18, the samples showed a frequency-dependent peak in ⬘共T兲 which occurred very closely to the point at which M ZFC共T兲 reached a maximum. The observed frequency dependence of ⬘共T兲 is a direct consequence of slow spin dynamics and this allows us to associate the noted peak with the freezing temperature T f of the spins or the FM clusters. In such observed glassy behaviors, T f was found to increase monotonically with increasing frequency f. This frequency dependence of T f can be well described by the conventional critical “slowing down” of the spin dynamics11 as
/o ⬀ 关共T f − Tg兲/Tg兴−z ,
共1兲
where ⬀ f 1, Tg is the critical temperature for the glassy order 共this is equivalent to the f → 0 value of T f 兲, z is a constant exponent, and o is the characteristic time scale for spin dynamics. As accepted in Eq. 共1兲, the best fit can be obtained by choosing the value of Tg, which minimizes the leastsquare deviation from a straight-line fit. The values of o and z are then extracted from the intercept and slope, respec-
FIG. 2. M ZFC共T兲 and M FC共T兲 at different magnetic fields for the x = 0.05 and 0.5 samples.
FIG. 3. Magnetic phase diagram of La1−xSrxCoO3 共0 ⬍ x 艋 0.5兲. SG, CG, and PM represent spin-glass, cluster-glass, and paramagnetic phases, respectively. The exponential parameter n, the spin-glass freezing temperature T f , the noted temperature Ta, and the Curie temperature TC is displayed as a function of the Sr-doped content x.
tively. At small doping levels, x 艋 0.18 共the SG phase兲, ⬙ was considerably reduced in magnitude and simply displayed a peak at T f . The existence of a frequency-dependent peak in ⬙共T兲 occurring well below TC even in the ‘FM phase’ means that the x 艋 0.18 samples did not exhibit a conventional longrange FM order. In such these cases, one can also connect the glassy behavior of La1−xSrxCoO3 to the blocking of superparamagnetic single-domain particles, where the freezing process is related to a distribution of particle sizes and relaxation times.12 If we consider the system as an assembly of clusters, for a certain field and frequency, with decreasing temperature, the larger size clusters will be frozen at higher temperature when their relaxation time becomes longer than the characteristic time of the measurement m = 2 / . Unlike in the case of the conventional superparamagnetic single-domain particles, the clusters in La1−xSrxCoO3 would also change their sizes with temperature, together with the presence of frustration, thereby resulting in a long-time relaxation and an age-dependent effect. Moreover, the frustration may exist in the FM region even at temperatures close to TC so the entire system is actually not in a true FM state frustration on the surface of the clusters. In an attempt to compare to ordinary spin glass in validity of the De Almeida–Thouless 共AT兲 line,11 we plotted the reduced temperature a = Ta / TC vs Hex and have found that for the x = 0.05, 0.15, 0.2, 0.3, 0.4, and 0.5 samples a scales n , where CAT and n are with Hex as a ⬀ 1 − 共CAT / TC兲 ⫻ Hex constants. The parameter values 1 / n obtained by best fit with respect to the Sr-doped content, x 共Fig. 3兲. Clearly, the n vs x behavior is totally different for the two regions of the Srdoped content. In the region of 0.2艋 x 艋 0.5, n increases monotonically with increasing x 共its value increases from 0.23 to 0.58 for x changing from 0.2 to 0.5兲. This two-region behavior of n coincides with the magnetic diagram proposed initially by Itoh and co-workers.1,8 Moreover, the obtained values of n for x = 0.05 and 0.15 compositions are consistent with the theoretical value n = 2 / 3 predicted by de Almeida and Thouless for canonical spin glasses.11 In such these ordinary SG systems, the M ZFC共T兲 cusp was governed by SG dynamic transition and was usually observed in the temperature region where the SG transition took place. Whereas, in
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the samples of the CG region, the M ZFC共T兲 cusp might be assumed to be governed by a local anisotropy field acting on the spins inside each cluster.6,8 It is therefore proposed to the broadening cusp at Ta marked as a crossover region where the average anisotropy energy, Ea共T兲 = Ha共T兲 ⫻ M共T兲, 关Ha共T兲 and M共T兲 are the temperature dependences of the average anisotropy field and the magnetization vectors, respectively, and the energy Eex共T兲 = Hex共T兲 ⫻ M共T兲 caused by the external field 共Hex兲 are comparable with each other. Accordingly, it is possible to deduce Ea ⬀ Ha ⬀ −T1/n a and this means that the local anisotropy increases with decreasing temperature via a power law. As one can see clearly from Fig. 3, the obtained values of 1 / n in the CG region decreases strongly with increasing x, from 1 / n ⬇ 4.34 共x = 0.2兲 to 1 / n ⬇ 1.75 共x = 0.5兲. The distinct change in 1 / n at the boundary between SG and CG phases 共x ⬇ 0.18兲 is probably related to some change in the structure of the cluster surface occurring at this Sr-doping level. In view of our magnetic phase diagram of La1−xSrxCoO3 共Fig. 3兲 in comparison with other works,1–4 we propose that 共i兲 for the 0.18⬍ x 艋 0.5 samples, the magnetic behavior in the temperature region close to TC reflects the predominant intracluster ferromagnetism, but this is not a true conventional FM state because of the presence of magnetic frustration and 共ii兲 for the x ⬍ 0.18 samples, a dynamic scaling analysis using the conventional critical slowing down indicates a finite transition to the spin-glass phase at Tg = 14.6 and 28.3 K for x = 0.05 and 0.15 compositions, respectively. The existence of the SG phase evidences disorder and frustration or a random distribution of FM and AF interactions in the system. The Ta decreases with increasing magnetic field via a power law. A good agreement between the values of the exponential parameter 共n兲 obtained in the present case and that of the AT theory evidently supports the assumption of SG behavior for x 艋 0.18 compounds. The increase of n for x, increasing from 0.2 to 0.5, indicates a decrease of the temperature dependence of the local anisotropy, which is probably originated from the increase in the size of FM clusters in this CG region. The abrupt change of n at xc ⬇ 0.18 is related to an overall change in cluster structure appearing at this doping concentration. In order to further elucidate the internal dynamics of La1−xSrxCoO3 共0 ⬍ x 艋 0.5兲 compounds, ESR spectra have been recorded above their Curie temperature. As shown in Fig. 4, a symmetrical absorption with g ⬃ 2.0 appeared to occur at T = 343 K for x = 0.05 composition and T = 388 K for x = 0.5 composition, due to the magnetic uncoupling of the spins in the PM phase. As T ⬍ 343 K for x = 0.05 composition, or T ⬍ 388 K for x = 0.5 composition, the asymmetric and complex signals in ESR spectra were observed. There are two kinds of ESR signals one is the low-field absorption with a g value much greater than 2.0 and the other is the high-field absorption with a g value very close to 2.0 at temperatures above TC 共=249 K, for x = 0.5 composition兲 and Tg 共=14.6 K, for x = 0.05 composition兲. This is somewhat contrary to that of the pseudocubic manganites, where the only ESR line with g = 2.0 is presented in the PM regime.13 A shift of the low-field line to a lower field with a further decrease of temperature reflects enhancement of the FM coupling in
FIG. 4. ESR spectra recorded at various temperatures for x = 0.05 and 0.5 samples.
clusters. As the temperature was lowered to TC, the ESR lines of the samples began shifting to a lower field. It is therefore concluded that a coexistence of short-range FM ordering and PM states appeared to occur in the PM region, which in turn caused magnetic inhomogeneities in the system. ACKNOWLEDGMENTS
The work in Korea was supported by the Korea Research Foundation Grant No. KRF-2003-005-C00018, and in Vietnam was supported by the National Program on Basic Research of Vietnam and the Research and Training Cooperation Project between the Institute of Materials Science 共IMS兲 and Hongduc University 共HDU兲. 1
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