Temperature-dependent magnetic behavior of Mn-Mg spinel ferrites with substituted Co, Ni & Zn, synthesized by hydrothermal method

Mn_0.1Mg_0.2(Co, Ni, Zn)_0.7Fe₂O₄ compounds with particles sizes ranging between 11 nm and 17 nm were successfully synthesised by hydrothermal process. These particles sizes are in agreement with TEM results. Bulk samples with particle sizes ranging between 69 nm and 72 nm were subsequently produced by annealing at 1100 °C under argon gas. SEM and EDX were used to study the morphology and confirm the elemental compositions. Temperature-dependent magnetization showed a strong dependence of coercive fields on measuring temperature and substituent. The coercive field variation followed Kneller's law with values varying between 0.5 kOe and 12 kOe from about 300 K to 2 K. An average moment of about ≈10⁴ was deduced from the magnetic measurements of the as-prepared samples. Temperature-dependent magnetic measurements in as-prepared samples showed superparamagnetic behavior with. The Langevin function fittings on M-H data at 300 K give particle size distributions with average diameters corresponding well with that estimated from XRD analysis. Field cooling (FC) and zero-field cooling (ZFC) measurements showed the dependence of bifurcation temperature on the applied magnetic field.