Field-Induced Spin-State Transition in LaCo1−xMxO3 (M = Al, Ga, Rh, and Ir)

We have investigated the high-field magnetization of lightly doped LaCo1−xMxO3 with M = Al, Ga, Rh, and Ir up to 67 T. The transition field for the field-induced spin-state transition at 4.2 K, which is μ0Hc = 60 T in LaCoO3, increases slightly for M = Al but does not change for M = Ga. On the other hand, the transition field decreases remarkably for M = Rh and Ir. The substitution effect on μ0Hc has been interpreted as arising from the lattice-volume dependence of the excitation energy ΔI of CoI from the low-spin (LS) to high-spin (HS) state. The fraction of CoI is 15% of the total number of Co ions. The remaining Co ions, CoII, are excited from the LS to intermediate-spin (IS) state and contribute to the magnetization at high temperatures above 100 K. We propose the coexistence of the HS and IS states in LaCo1−xMxO3. For M = Rh and Ir, Co species with the magnetic ground state, CoIII, are populated by their substitution. The formation mechanism and spin states of CoIII are discussed in comparison with those of the magnetic Co species in La1−xSrxCoO3.