Spin dynamics and unconventional magnetism in insulating La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$

Abstract

We study the structural, magnetic, transport and electronic properties of LaCoO3 with Sr/Nb co-substitution, i.e., La (1−2x) Sr2xCo (1−x) NbxO3 using x-ray and neutron diffraction, dc and acmagnetization, neutron depolarization, dc-resistivity and photoemission measurements. The powder x-ray and neutron diffraction data were fitted well with the rhombohedral crystal symmetry (space group R 3c) in Rietveld refinement analysis. The calculated effective magnetic moment (≈3.85 µB) and average spin (≈1.5) of Co ions from the analysis of magnetic susceptibility data are consistent with 3+ state of Co ions in intermediate-spin (IS) and high-spin (HS) states in the ratio of ≈50:50, i.e., spin-state of Co 3+ is preserved at least up to x = 0.1 sample. Interestingly, the magnetization values were significantly increased with respect to the x = 0 sample, and the M-H curves show nonsaturated behavior up to an applied maximum magnetic field of ±70 kOe. The ac-susceptibility data show a shift in the freezing temperature with excitation frequency and the detailed analysis confirm the slower dynamics and a non-zero value of the Vogel-Fulcher temperature T0, which suggests for the cluster spin glass. The unusual magnetic behavior indicates the presence of complex magnetic interactions at low temperatures. The dc-resistivity measurements show the insulating nature in all the samples. However, relatively large density of states ≈10 22 eV −1 cm −3 and low activation energy ≈130 meV are found in x = 0.05 sample. Using x-ray photoemission spectroscopy, we study the core-level spectra of La 3d, Co 2p, Sr 3d, and Nb 3d to confirm the valence state.