Spin State of Co Ions in Lightly Doped Strontium LaCoO3: Via Study of High-Field Induced Spin State Transition

Abstract

We have investigated the high field magnetization of La 1- x Sr x CoO 3 for x ≤0.05 up to µ 0 H ext = 67 T. In a dilute limit, about 30 nonmagnetic Co ions of two species (Co I and Co II in LaCoO 3 ) are converted to magnetic Co ions (Co III ) by introducing one Sr 2+ , leaving the remaining Co I and Co II little affected by the Sr doping. This finding shows that a new spin-phase separation with three Co species appears by the Sr doping. The Co III ions form a magnetic cluster with a net magnetization several times smaller than that expected from the ferromagnetic aligned full moment. A spin-glass freezing appears below T g = 10 and 18 K for x = 0.03 and 0.05, respectively, in a weak magnetic field of 2 mT. In µ 0 H ext ≥10 T, however, the magnetization of Co III for 0.01 ≤ x ≤0.05, as a function of temperature and H ext , is almost independent of x after normalization with the Co III population. It means that the magnetic correlation among Co III within the cluster is robust against H ext , but the magnetic interactions among the clusters, which cause the spin glass transition, are decoupled by H ext . The origin of the strong magnetic interaction among Co III in the cluster is considered to arise from the mobile hole within the cluster as proposed in “spin-polaron” model.