Nuclear Magnetic Resonance of Paramagnetic Co₃O₄. I. Experimental Study

Abstract

Nuclear magnetic resonance of Co⁵⁹ in polycrystalline Co₃O₄ and ZnCo₂O₄, and Al²⁷ in CoAl₂O₄ in paramagnetic state was observed. The cobalt signal was identified with the Co³⁺ ions at the octahedral sites of the spinel structure. The line shift of Co⁵⁹ in Co₃O₄ was separated into two terms: temperature-independent and temperature-dependent ones. The temperature-independent shift (1.5%) is interpreted as the second order chemical shift due to the mixing of the low lying excited state. The temperature-dependent shift is attributed to the hyperfine coupling between the cobalt nuclear spin and the electron spins on the Co²⁺ ions at the tetrahedral sites. The isotropic hyperfine coupling constant A is estimated at 13×10⁻⁴ cm⁻¹. The results on ZnCo₂O₄ and CoAl₂O₄ give supports to the above interpretations. The line shapes were studied and the thermal relaxation time T₁ was estimated to be 1.5×10⁻⁵ sec.