Spin-Crossover Cobaltite

The spin-crossover or spin-state transition and related phenomena in perovskite cobaltites have been attracting widespread interest among researchers for decades. One of the main issues has been the Co\(^{3+}\)-\((3d)^{6}\) spin-states in LaCoO\(_{3}\). Despite extensive efforts of researchers, it has still been controversial as to what the magnetic excited state induced around 100 K is: intermediate spin (IS) with S = 1 or high spin (HS) state with S = 2. The IS state, which has a degree of freedom on \(e_{g}\) orbitals, is strongly supported by the experiments showing the Jahn–Teller distortion and/or orbital fluctuation. Several spectroscopic studies afford strong evidence for the HS state. However, HS is incompatible with the magnitude of observed magnetization and the ferromagnetic correlation among Co-spins. Therefore, neither spin-state model can comprehensively explain all the experimental facts. A heterogeneous spin phase including IS and HS as magnetic states has been proposed, which requires the presence of interaction among the magnetic Co spin-states. This idea, along with several recent experiments clarifying the collective nature of the spin-crossover phenomena, indicates that consideration beyond the \(3d^{6}\) electronic state of single Co\(^{3+}\) is essential to understand the spin-crossover phenomena. The spin-crossover around 100 K is unique to LaCoO\(_{3}\), whereas the spin-crossover accompanied by the insulator-to-metal transition (IMT) at around 500 K occurs commonly in RECoO\(_{3}\) (RE: rare earth element). The fact also suggests a contribution of the nonlocal electronic state to the spin-crossover phenomena. In this review, we describe the history and current status of the research on the spin-crossover phenomena in perovskite cobaltites. We also review the anomalous magnetic and electronic properties such as the spin polaron and IMT at around 500 K, which are resulting from the electronic state responsible for the spin-crossover phenomena in RECoO\(_{3}\).

Experimental electronic structure of LaCoO\(_3\) and other RCoO\(_3\) (R = rare earth) in connection with the spin crossover phenomena in LaCoO\(_3\) is reviewed. As the experimental probes, we mostly deal with electron spectroscopic methods, namely photoemission spectroscopy and x-ray absorption spectroscopy. Experimental results are compared with ab initio band-structure calculations and configuration-interaction cluster-model calculations. First, we discuss how electronic-structure measurements can contribute to studying spin crossover phenomena, particularly of LaCoO\(_3\). Then in Sect. 2.2, we give a brief historical review of electron spectroscopic studies on LaCoO\(_3\) in order to highlight the conflict between the two major models, namely the low spin-high spin model and the low spin-intermediate spin-high spin model. Sections 2.3 to 2.5 are devoted to discussing this conflict and how we may be able to resolve it beyond this binarism. In Sect. 2.6, we remark the probing depth of the methods and the surface magnetism of LaCoO\(_3\). In Sect. 2.7, we briefly discuss a part of recent advances both in experiment and theory in this field. We run through the electronic structure of RCoO\(_3\) and other related Co oxides in Sect. 2.8, and finally we give the concluding remarks in Sect. 2.9.

Recent theoretical researches in correlated electron systems with the spin-state degree of freedom are reviewed. Novel electronic states which appear under the competition between the low-spin and high-spin states in perovskite cobaltites are focused on. Calculated results obtained from the two-orbital Hubbard model, an effective model for the low-energy electronic structure, and the five-orbital Hubbard model are introduced. In particular, we pay our attention to the possibilities of the following three exotic states: (i) The excitonic insulating state, in which the electronic wave function is represented by the linear combination of the low-spin and high-spin states, is introduced. The ground state and finite-temperature phase diagrams and the elementary excitations are shown. We propose the possible experimental methods to identify the excitonic insulating state in cobaltates. (ii) Possibility of the electronic phase separation by hole-carrier doping is introduced. The ground state energy as a function of carrier number indicates that a homogeneous electronic state is unstable, and the electronic state is separated into the low-spin band insulator and the high-spin ferromagnetic metallic state. A microscopic mechanism of this phase separation phenomena is discussed. (iii) A bound state between the high-spin state and the hole state induced by the photoirradiation is proposed by the complement theoretical calculations. This is termed the photoinduced high-spin polaron state. Implications of this characteristic state to the optical pump-probe experiments are discussed.

One of the most interesting topics regarding spin-crossover (SC) materials is optical control of the electronic state. In this chapter, recent trials on the photocontrol of some SC cobaltites are reviewed. In contrast to iron SC complexes, the time scale required for photonic change is on the sub-picosecond level for the cobaltites in general, so femtosecond laser spectroscopy is indispensable for revealing photoinduced SC phenomena. On the basis of an ultrafast linear and nonlinear spectroscopic technique using femtosecond laser pulses, we discuss the photoinduced change of the electronic structure as well as real space dynamics for some exotic SC cobaltites, Pr\(_{0.5}\)Ca\(_{0.5}\)CoO\(_3\) and BiCoO\(_3\).

In this chapter, we review the recent research on the spin-state ordering in epitaxial thin films of perovskite LaCoO₃ from the viewpoint of optical spectroscopy and X-ray scattering. A remarkable feature is that a variety of spin/orbital orderings, which have not been identified in bulk samples, can be induced by tuning the epitaxial strain in thin film samples. In the moderately tensile-strained films, a spin-state/orbital ordering accompanying the ordering of high-spin-state site and low-spin-state site occurs around 130 K, and subsequently, a ferromagnetic/ferrimagnetic ordering emerges around 90 K. In the weakly strained films, another spin-state/orbital ordered phase with longer modulation period emerges around 40 K, followed by a ferromagnetic ordering transition around 20 K. By analyzing the Co-3d orbital state by the resonant soft X-ray scattering technique, we identified that two kinds of high-spin state with different site symmetries emerges in this phase. Moreover, by means of grazing-incidence resonant soft X-ray scattering technique, it is demonstrated that the transition temperature of spin-state ordering is higher in the surface-layer of film than inside the film due to the strain relaxation. These results demonstrate that the strain engineering of epitaxial thin films offers a fertile playground to study the spin-state/orbital ordering in the correlated spin-crossover material.

Perovskite oxide BiCoO₃ belongs to the class of compounds that show the spin transition of the Co ion. The spin state of Co³⁺ in BiCoO₃ is closely correlated with its crystal structure. The application of high pressure induces structural transition accompanied by spin transition. This chapter describes the correlation between the large structural distortion and the spin state of Co³⁺ in BiCoO₃. In addition, negative thermal expansion, which is observed in the BiCoO₃-BaTiO₃ system, and polarization rotation in the BiCo_1−xFe_xO₃ solid solution system are introduced.

One of the most important topics of cobalt oxides is the large thermopower (absolute Seebeck coefficient), which has been attracted attention due to their potential application as thermoelectric conversion material. In this chapter, the basics of thermoelectrics and thermoelectric response in cobalt oxides and related transition metal oxides are introduced. Thermopower is corresponding to the carried entropy by the electric current. In the strongly correlated electron systems, the spin and orbital degrees of freedom contribute to the entropy flow. The role of spin and orbital degrees of freedom on the thermoelectric effect is theoretically discussed.