High ferromagnetic resonance and thermal stability spinel Ni0.7Mn0.3−xCoxFe2O4 ferrite for ultra high frequency devices

Magnetic properties and ferromagnetic resonance (FMR) frequency of Ni0.7Mn0.3−xCoxFe2O4 ferrites (NiMnCo: x = 0.00, 0.04, 0.06, and 0.10) were controllable with the concentration of cobalt (Co). The measured saturation magnetization remains almost constant at 57 emu/g in the studied range of the Co content. However, the coercivity increased to 72 Oe (x = 0.10) from 12 Oe (x = 0.00). Ferromagnetic resonance frequency increased from 129 MHz to the GHz frequency with increasing the Co content from x = 0.00 to x = 0.10, but the real part of permeability decreased to 1.6 from 16.7. This is because FMR frequency increases with the magnetocrystalline anisotropy (Hk) of the Co according to fr = (γ/2π)Hk. The permeability decreased with the Co content by µ = 1 + (4πMs/Hk). Temperature coefficient, in the range of 20–100 °C, was 0.002 for Ni0.7Mn0.3Fe2O4 (x = 0.00) and 0.008 for Ni0.7Mn0.2Co0.1Fe2O4 (x = 0.10). The results suggest that Ni0.7Mn0.3−xCoxFe2O4 ferrites have excellent temperature stability, and also are applicable to very high frequency: 30–300 MHz) and ultra high frequency: 300 MHz–3 GHz) devices.