A comparative study of the magnetic properties and phase separation behavior of the rare earth cobaltates, Ln0.5Sr0.5CoO3 (Ln=rare earth)

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Abstract

A comparative study of the magnetic properties of a few members of the Ln0.5Sr0.5CoO3 family with different radii of the A-site cations, 〈rA〉, in the range 1.19–1.40 Å has been carried out. The apparent Tc (where the magnetization undergoes an abrupt increase) decreases markedly with 〈rA〉 as well as the size-disorder arising from the mismatch in the size of the A-site cations. The value of the magnetization at low temperatures decreases markedly with decrease in 〈rA〉 or increase in size-disorder, suggesting that the relative proportion of the ferromagnetic (FM) species decreases relative to that of the paramagnetic (PM) species. Such a variation of the FM/PM ratio with composition and temperature is evidenced from the Mössbauer spectra of La0.5Sr0.5CoO3 as well. The variation of the FM/PM ratio with 〈rA〉 and size-disorder, as well as a local-probe study using 59Co Nuclear magnetic resonance spectroscopy suggest that electronic phase separation is an inherent feature of the Ln0.5Sr0.5CoO3 type cobaltates, with the nature of the different magnetic species in the phase-separated system varying with 〈rA〉 and size disorder.

Graphical abstract

Variation of (a) Tc and (b) FC magnetization at 1000 Oe with 〈rA〉 at 120 K in Ln0.5Sr0.5CoO3 and Dy0.34Nd0.16Sr0.40Ca0.10CoO3.

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Introduction

Lanthanum cobaltates of the general formula La1−xAxCoO3 (A=divalent alkaline earth) have been known for some time to show compositionally controlled insulator–metal transitions, with a gradual increase in the ferromagnetic (FM) metallic behavior with increase in x, at least up to x=0.5 [1], [2], [3]. It was realized a few years ago that La0.5Sr0.5CoO3, which was considered to be a good FM metal, was actually a magnetic cluster-glass with some long-range magnetic ordering wherein frustration arose from inter-cluster interactions at low temperatures [3], [4]. The coexistence of FM and glassy behavior in La0.5Sr0.5CoO3 has been examined in detail by magnetic relaxation and other measurements [4], the glassy behavior being indicated by a frequency-dependent maximum in the AC susceptibility data. Glassy ferromagnetism in La1−xSrxCoO3 was investigated in great detail by Wu and Leighton [5], who found that the compositions with x<0.18 consist of mixed phases with the characteristics of both FM and spin-glass behavior. The magnetic behavior of these systems has generally been interpreted in terms of short-range magnetic ordering. Cluster-glass behavior occurs at a high concentration of Sr (x>0.3), where the coalescence of short-range FM clusters is proposed to occur [5]. 139La nuclear magnetic resonance (NMR) studies confirm the coexistence of FM, paramagnetic (PM), and cluster-glass phases in La1−xSrxCoO3 [6] and the electronic phase separation in this system has been associated with the formation of isolated nanoscopic FM clusters [6], [7]. The insulating side of La1−xSrxCoO3 (for small x) generally undergoes phase separation having FM metallic clusters embedded in a non-FM matrix, giving rise to giant magnetoresistance [8]. The glassy behavior in La1−xSrxCoO3 is considered to arise from contributions due to a spin-glass-like phase and intercluster interactions [9]. Thus, the cobaltates comprise FM clusters, PM matrices and spin-glass-like phases, all contributing to the glassy magnetic behavior. The spin-glass region could act as the interface between the FM regions and the PM matrix.

In the rare earth cobaltates of the general formula Ln1−xAxCoO3 (Ln=rare earth other than La), analogous to the lanthanum derivatives previously discussed, the magnetic and electronic properties are affected by the relative size or radius of the A-site cations 〈rA〉, as well as the disorder arising from the mismatch between the A-site cations [1], [10], [11], [12]. The disorder is generally expressed in terms of the parameter σ2, which is defined as the variance of the A-cation radius distribution [13]. Size-disorder is indeed known to favour electronic phase separation in rare earth manganates. The rare earth cobaltates provide an interesting system to examine the effects of both 〈rA〉 and σ2. Some of the members of the Ln0.5Sr0.5CoO3 family such as those with Ln=Pr and Nd appear to exhibit apparent FM transitions accompanied by glassy behavior [12], [14]. A double magnetic transition in Pr0.5Sr0.5CoO3 has been reported [15], but the nature of the transition is difficult to understand. Nd1−xSrxCoO3 compositions (0.0<x<0.6) seem to show a magnetic behavior similar to that of La1−xSrxCoO3, including electronic phase separation [16]. Thus, 59Co NMR studies indicate the existence of phase separation in Nd1−xSrxCoO3 (0.0<x<0.5) [17].

In view of the interesting magnetic and electrical properties of the Ln0.5Sr0.5CoO3 family of cobaltates, we have carried out a comparative study of a few members of Ln0.5Sr0.5CoO3 associated with different 〈rA〉 values in the range 1.196–1.400 Å. Since we could not prepare Dy0.5Sr0.5CoO3 with 〈rA〉 of 1.196 Å in pure form, we have examined a cobaltate with the composition Dy0.34Nd0.16Sr0.40Ca0.10CoO3 with a comparable 〈rA〉. We considered it important to compare the magnetic and related properties of these materials, not only as a function of the A-site cation radius, but also of the size disorder, σ2, arising from the size mismatch between A-site cations. In doing so, we have investigated the properties of well-characterized materials, with special attention to glassy behavior and electronic phase separation. For this purpose, we have re-examined the magnetic properties as well as the literature data based on the use of local probes such as Mössbauer spectroscopy and NMR spectroscopy [18], [19]. In the case of Dy0.34Nd0.16Sr0.40Ca0.1CoO3 with the lowest 〈rA〉, we have carried out a 59Co NMR spectroscopic study to examine the nature of phase separation.

Section snippets

Experimental procedure

Polycrystalline samples of Ln0.5Sr0.5CoO3 were prepared by the conventional solid-state reaction method. Stoichiometric mixtures of the starting oxide materials, Ln2O3, SrCO3, CaCO3 and Co3O4, were weighed in the desired proportions and milled with propanol. After drying, they were calcined in air at 1223 K and after few intermediate grindings, the powders sintered at 1373 K for 24 h in air. The samples were then reground and the monophasic polycrystalline powders were hydrostatically pressed in

Results and discussion

Magnetic properties of La0.5Sr0.5CoO3 are fairly well understood. Although it shows a sharp transition in magnetization around 240 K (Tc), it exhibits a significant divergence between the FC and ZFC magnetization data (Fig. 1a) and shows a frequency-dependent AC susceptibility maximum around 165 K, suggesting a glassy nature [4]. More importantly, glassy ferromagnetism is accompanied by phase separation wherein FM clusters exist within a PM matrix. Evidence for the occurrence of phase separation

Conclusions

Evidence for electronic phase separation in La0.5Sr0.5CoO3 is provided by Mössbauer spectroscopy besides magnetization and NMR studies. In the Ln0.5Sr0.5CoO3 (Ln=rare earth) series, the value of the “apparent” ferromagnetic (FM) Tc, where the magnetization shows an abrupt increase, decreases with the decrease in the average radius of the A-site cations, 〈rA〉. This is accompanied by a marked decrease in the value of the magnetization at low temperatures (T<Tc), indicating a decrease in the

Acknowledgments

The authors are thankful to DRDO (India) for support of this research and to Anne Poduska for her assistance.

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