Field-dependent low-field enhancement in effective paramagnetic moment with nanoscaled Co3O4

Paramagnetic (PM) properties of columnar cobaltosic oxide (Co3O4) nanoparticles, about 75 nm in diameter, have been investigated by magnetization measurements at T>TN=39 K. In zero or low applied field, the effective PM moment per formula unit (FU), μeff, enhances significantly from the bulk value of 4.14 μB/FU. It decreases asymptotically from 5.96 μB/FU at Happ=50 Oe down to 4.21 μB/FU as the applied field increases to Happ=10 kOe. The field-dependent PM properties are explained by a structural inversion, from the normal spinel (spin-only moment ∼3.9 μB/FU) to the inverse spinel structure (spin-only moment ∼8.8 μB/FU). The structural inversion is reversible with the variation in the applied field. The lattice structure becomes almost 100% normal spinel in the high field exceeding 10 kOe, as revealed by the magnitude of the effective PM moment. The reversible, field-dependent structure inversion is an important property with promising application potential. It is interesting for the future investigations.