A systematic study of the magnetic properties of ultra-fine particles of Mn_0.5Fe_0.5Fe₂O₄ spinel system has been undertaken. The effect of temperature on the magnetic properties of particles and the ferrofluid has been studied. Analysis of the data yields information on the anisotropy constant, particle size distribution and superparamagnetic behaviour. The results are explained on the basis of existing theories.

ESR spectra of a laboratory synthesized kerosene base magnetic fluid containing ultrafine magnetic particles (average diameter of 100A) of Zn_0.1 Fe_0.9Fe₂O₄ are recorded at different temperatures. A narrow signal was observed above the melting point of the carrier liquid (200 K) which can be attributed to a very small volume fraction of superparamagnetic particles in the system. The peak-to-peak line width for both low and high field cooled configurations show an increase with decreasing temperature. This observed behaviour has been explained by considering various energy terms which contribute to the line width.