Elsevier

Journal of Alloys and Compounds

Volume 628, 15 April 2015, Pages 251-256
Journal of Alloys and Compounds

Cr enhanced ferromagnetism in La0.5Ba0.5CoO3 due to possible double-exchange interaction

https://doi.org/10.1016/j.jallcom.2014.12.188Get rights and content

Highlights

  • The magnetic studies have been carried out on the R-doped La0.5Ba0.5Co0.9R0.1O3.

  • Cr3+ gives an extraordinary effect of enhancing TC a lot among all these doping ions.

  • The electron spin resonance spectra confirm the Co-O-Cr exchange interaction.

  • The magnetotransport implies a double-exchange property of this interaction.

Abstract

The magnetic studies have been carried out on the R-doped La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) cobaltites. X-ray absorption near-edge structure (XANES) proves these ions are in the valence state of Cu2+, Zn2+, Cr3+, Ni3+, Ti4+ and Ru4+. Magnetic moment calculation shows both Co3+ and Co4+ hold in the intermediate spin state. Based on the ratio of Co3+/Co4+, the average B-site size, the tolerance factor and electron itinerancy, the different influence of these ions on the magnetism were discussed. Among all these ions, Cr3+ gives an extraordinary effect of enhancing TC a lot. It is attributed to the Co-O-Cr exchange interaction, which is confirmed by the electron spin resonance spectra. The typical magnetotransport behavior implies a double-exchange property of this exchange interaction.

Introduction

Perovskite cobaltites with the general formula Ln1−xAxCoO3 (Ln = trivalent rare earth cation and A = divalent alkaline earth cation) have been explored by many authors due to their unusual magnetic and transport properties [1], [2], [3], [4], [5], [6]. Three alternative spin configurations, low-spin, intermediate-spin and high-spin are induced by the strong competition between the crystal-field splitting energy and the intra-atomic Hund coupling[7], [8], [9], [10], [11], [12], [13], [14], [15]. The La0.5Ba0.5CoO3, with a 1:1 mixture of Co3+ and Co4+, shows a paramagnetic–ferromagnetic transition at TC = 189 K and turns to a magnetic glassy state below 140 K [4], [15]. Such a cluster glass behavior occurs as a result of the competition between randomly distributed ferromagnetic and antiferromagnetic interactions[16]. Fauth et al. [4] observed a semi-metallic behavior with a metal–metal transition at TC in this compound, whereas Nakajima et al. [15] observed a metallic behavior down to 140 K, with an abrupt increase in resistivity below this temperature. Fauth et al. also observed a structural change from cubic to tetragonal which is compatible only with static Jahn–Teller (JT) distortion of CoO6 octathedra. The origin of the ferromagnetic state in metallic cobaltites has been a subject of discussion for a long time. One knows that cobaltites are extremely sensitive to order–disorder, doping, temperature, and crystal structure [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. The substitution of other ions for Co ions could directly modify the magnetic ordering and facilitate understanding the nature of the ferromagnetic state as well as clarifying the spin state of Co ions. Our previous work has researched the effect of the substitution of Ti ions for Co [22]. The magnetic measurements under external pressures suggest that both Co3+ and Co4+ ions are stabilized in the intermediate-spin (IS) state, and a detailed phase diagram has been constructed. In this paper, we performed a contrastive research on the effect of introducing foreign species Cu, Zn, Cr, Ni, Ti and Ru at the Co sites. All these ions stabilize the intermediate spin state of both Co3+ and Co4+. For Cu2+, Zn2+, Ni3+ and Ru4+, they decrease TC and induce a complex magnetism. Whereas Cr3+ increases TC a lot. Definitely, there is a Co-O-Cr exchange interaction existing in the system, which is confirmed by ESR experimental results as well. The magnetoresistance effect with a typical peak at TC implies a double-exchange property of this exchange interaction.

Section snippets

Experiment

La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) samples and the parent compound were prepared by solid state reaction. Dry pure La2O3, BaCO3, Co3O4, CuO, ZnO, Cr2O3, Ni2O3, TiO2 and RuO2 powders were well mixed in the stoichiometric amounts and initially decarbonated in the air at 1170 K for 20 h. After regrinding, the samples were fired in the air at 1470 K for 15 h and slowly cooled for several times [4], [23]. The cooling rate is 30 K/h. The same method as described in Ref. [32] to produce Pr

Results and discussion

XRD patterns at room temperature shown in Fig. 1 confirms that all La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) samples and the parent compound are in pure single phase. All the compositions can be refined in perovskite structure with cubic symmetry (space group Pm3¯m). For the parent one, the unit cell parameter (a = 3.8865 Å) is consistent with stoichiometric La0.5Ba0.5CoO3 compound (a = 3.885 Å) [4], [15]. The cell parameter are shown in Table 1.

Fig. 2(a-g) plots the magnetization as a

Conclusion

The magnetism of La0.5Ba0.5Co0.9R0.1O3 (R = Cu, Zn, Cr, Ni, Ti and Ru) cobaltites are studied. The intermediate spin state of Co3+ and Co4+ is more stabilized with ions doping. For Cu2+, Zu2+, Ni3+, Ti4+ and Ru4+ doping, TC decreases and a complex magnetism presents at low temperature. Increasing of rB, raising or lowering the ratio of Co3+/Co4+, electronic itinerant of doping ion would suppress the FM interaction. In contrast, doping with Cr3+ increases TC. We presume that the Co-O-Cr exchange

Acknowledgements

This work was supported by the State Key Project of Fundamental Research, China (2010CB923403), Natural Science Foundation of China (11304323/A0402) and (11174262/A0402) and Natural Science Foundation of Anhui (1208085MA10). Wei tong acknowledges the support from Youth Innovation Promotion Association of CAS and All authors would like to gratefully acknowledge the National Synchrotron Radiation Laboratory (NSRL).

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