Relaxation process and ferromagnetic resonance investigation of ferrofluids with Mn–Zn and Mn–Fe mixed ferrite particles

Abstract The magnetic relaxation processes in two ferrofluids with Mn0.4Zn0.6Fe2O4 (sample F1) and Mn0.6Fe0.4Fe2O4 (sample F2) mixed ferrite particles, dispersed in n-decan and kerosene, respectively, are investigated through the determination of components χ′ and χ′′ of the complex magnetic susceptibility in the range of (2–30) MHz. The values of the saturation magnetization of the two ferrofluids are M∞=5.28 kA/m for sample F1 and M∞=10.99 kA/m for sample F2. A maximum of the imaginary component χ′′ was observed for both samples at frequencies of tens MHz. This maximum was assigned to relaxation processes of Neel type. The effective anisotropy constant K of the particles from the studied samples was evaluated, using both static and dynamic measurements and the values were found to be K1=6.12×103 J m−3 for the ferrofluid F1, and K2=5.60×103 J m−3 for the ferrofluid F2. From ferromagnetic resonance measurements, and based on the theoretical values computed for the Lande factor (g), the effective anisotropy constants for the mixed ferrite particles in the studied ferrofluids and the anisotropy field values were determined using a new method. The values obtained in this way for the anisotropy constants K1 and K2 are compared to the ones determined from magnetic relaxation measurements.