Superparamagnetic behaviour of nanocrystalline Ni – Zn,Zn – Mn and Ni – Mn ferrites processed by reverse micelle method

The magnetic behaviour of nanocrystalline nickel–zinc, zinc–manganese and nickel–manganese ferrites processed by reverse micelle technique was studied by Superconducting Quantum Interference Device. The surface roughness of the nanoparticles as measured by atomic force microscopy was less than 3 nm and the size of the particles was in the range of 25–100 nm. The mixed (Ni–Zn, Zn–Mn and Ni–Mn) ferrites exhibit blocking temperatures of 11, 20 and 15 K, respectively. The small differences in the blocking temperatures are attributed to the magnetocrystalline anisotropy and L-S coupling. The saturation magnetisation of nanocrystalline Ni–Zn, Zn–Mn and Ni–Mn at 300 K was 4.5, 9 and 7 emu g^-1, respectively and at 2 K was 15, 19 and 14 emu g^-1, respectively. The lower saturation magnetisation of the nanocrystalline ferrites compared to their bulk counterparts is attributed to the core–shell morphology of the particles consisting of ferromagnetically aligned core spins and a spin-glass like surface layer.