Near-neighbour impurity effect on the spin-state transitions in LaCoO₃ at low temperature (12<T<300 K)

Abstract

The low-temperature magnetic susceptibility behaviour of LaCoO₃ prepared under different conditions as well as substituted samples such as LaCo_0.95M_0.05O₃ (M=Al, Ga, Cr, Fe, Mn, Ni) and La_0.98Sr_0.02CoO₃ have been investigated in the temperature range 12-300 K. Earlier interpretations of the magnetic susceptibility have been reexamined. In the case of LaCoO₃ samples containing Al, Ga, Cr and Fe impurities spin-state transitions involving a temperature independent activation energy (~0.01 eV) are observed atT<200 K. Analysis of the data indicates that either the excited state has an intermediate-spin (t⁵_2g e ¹_g) configuration or only half the Co ions are involved in the activated transition to the high-spin (t⁴_2g e²_g ) configuration. Al³⁺, and Cr³⁺ increases the activation energy considerably. Substitution of L ≠ 0 ions such as Mn³⁺ and Ni³⁺ or Co⁴⁺ (low-spin) seems to introduce ferromagnetic interactions and stabilizes the paramagnetic state. LaCoO₃, when Co is substituted by Mn (5%) or La is substituted by Sr (2%) show giant magnetic moments. When Co is substituted by Ni (5%) a ferromagnetic ground state is observed.