LAUSR

We have prepared Zn1−xCoxFe2O4 nanoparticles (NPs) by using a hydrothermal method, and then studied their structural and magnetic properties. The analyses of x-ray diffraction (XRD) patterns and Raman scattering spectra reveal that the samples crystallized mainly in a cubic-spinel structure with the lattice parameter a ≈ 8.4 Å. Averaged crystallite sizes determined from the XRD linewidth are about 16–22 nm, close to the particle sizes of 19–28 nm determined from scanning electron microscopy images. Magnetization measurements versus temperature, M(T), in the field H = 100 Oe indicate that the ferromagnetic–paramagnetic (FM-PM) phase transition temperature (TC) of Zn1−xCoxFe2O4 NPs increases from 606 K for x = 0 to ~823 K for x = 1. The features of the M(T) curves also indicate magnetic inhomogeneity in the samples, and their magnetic property is unstable versus temperature. This is ascribed to the changes in the structural characterization and/or concentration of magnetic ions situated at the A and B sites in the spinel lattice. At room temperature, we found that both the saturation magnetization (Ms) and coercivity (Hc) increase with increasing Co content, with Ms = 59–70 emu/g and Hc = 100–1100 Oe. These results reflect that the Co doping into ZnFe2O4 NPs greatly improves their magnetic property, making them more useful for practical applications. Additionally, we also assess magnetic interactions and the magnetocaloric effect in the samples based on analyzing initial magnetization data, M(H), recorded at temperatures around TC.