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Spin state is a characteristic degree of freedom in a multi-electron and multi-orbital system (e.g. magnetic ion under crystal field) where crystal field splitting (Δ) and Hund’s coupling (J ) are delicately competitive. Low spin [LS] (or high spin [HS]) state emerges when Δ (or J ) overwhelms the other. Recently, we found a low entropy phase in a spin crossover cobaltite LaCoO3 at ultrahigh magnetic fields of above 100 T by means of magnetization measurement up to 140 T [1]. Various exotic orders are claimed as its origin such as LS/HS order and excitonic order. They are considered to be stabilized by the correlation effects based on analysis of the two-orbital Hubbard model [2, 3]. The excitonic phase is characterized with the spontaneous and quantum-mechanical mixing of the wave functions of LS and HS states. Whereas, the LS/HS ordered phase is the spatial alternation of the LS and HS states.

In the talk we briefly review our experimental results on LaCoO3 [1] and Y-doped Pr0.7Ca0.3CoO3 [4]. Then we discuss the physical origin of the possible stabilization of exotic phases [2] and their field effects [3]. We also mention our future plan.

[1]A. Ikeda et al., Phys. Rev. B 93, 220401(R) (2016).
[2]J. Nasu et al., Phys. Rev. B 93, 205136 (2016)
[3]T. Tatsuno, et al., J. Phys. Soc. Jpn. 85, 083706 (2016)
[4]A. Ikeda et al., Phys. Rev. B 94, 115129 (2016).

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