Magnetic enhancement of Co_0.2Zn_0.8Fe₂O₄ spinel oxide by mechanical milling

Abstract

The mechanical milled Co_0.2Zn_0.8Fe₂O₄ spinel oxide with average particle size ≈ 20nm shows an enhancement in both magnetization (i.e., more ferromagnetic) and ordering temperature comparing the bulk sample. The annealing of the as milled sample at different temperatures starts the grain (particle) growth process and magnetically recovering the bulk properties after heat treatment. As for example, the as milled sample after annealing at ~1273K for 6 h exhibits two coexisting magnetic states, i.e., cluster spin-glass state and ferrimagnetic state, similar to the as prepared bulk sample. The room-temperature Mossbauer spectra of the as milled sample, annealed at 573 K for different duration (up to 575 h), indicate that the ferromagnetic behavior above room temperature is strongly related to site exchange of cations among tetrahedral (A) and octahedral (O) sites in the spinel structure. The experimental data also suggest that the lowering of B site spin canting and increase of strain induced unidirectional anisotropy by mechanical milling play an important role in controlling the magnetic order of the as milled sample. Importantly, the grain boundary spins of the as milled sample do not show to be magnetically inactive, rather they give rise to a preferential orientation due to strain induced anisotropy.