Mössbauer study of ${\mathrm{Ni}}_{0.25}$${\mathrm{Zn}}_{0.75}$${\mathrm{Fe}}_2$${\mathrm{O}}_4$. I. Spin fluctuations

A Mössbauer study of ${\mathrm{Ni}}_{0.25}$ ${\mathrm{Zn}}_{0.75}$ ${\mathrm{Fe}}_2$ ${\mathrm{O}}_4$ has been made in the temperature range from 4.2 to 300 K, in the presence of longitudinal fields up to 80 kG. It is possible to distinguish between the effects of ionic spin relaxation and superparamagnetism on the shape of a Mössbauer spectrum using an external magnetic field, and effects of ionic spin relaxation are found to dominate the spectra of the ferrite. The Néel temperature is found to be higher than 350 K, although the collapse of the magnetic splitting in the absence of an external field occurs at ∼250 K. Ionic spin relaxation cannot give rise to the collapse of the magnetic splitting below $T_N$. This is concluded to be due to superparamagnetism.