Ac magnetic susceptibility studies on dry milled zinc ferrites

Figure represents the ac magnetic susceptibility graphs for dry milled zinc ferrites in the temperature range of 800K to 3000K. The experiments were carried out at 2 Oe field and at a frequency of 133.33 Hz. A transition is observed for 30 minutes milled, 5 hours milled and 10 hours milled samples. A transition at lower temperatures is expected for unmilled and 5 minutes milled samples.
Since the milling decreases the particle size which is well established by our XRD and TEM studies, if the transition is due to the superparamagnetic blocking it should decrease with milling. The particle size from XRD studies and the approximate transition temperature derived from ac susceptibility studies. It is a well-established theory that the superparamagnetic blocking temperature is directly proportional to particle volume and can be represented as
TB = KV/25k

where TB is the superparamagnetic blocking temperature, K is the crystalline anisotropy constant, V the particle volume and k the Boltzmann constant. But our results are entirely opposite to this and hence the transition is not due to superparamagnetic blocking but is due to the freezing of spin clusters resulting in a spin glass like feature. Below the broad maximum spin clusters randomly freezes to decrease the magnetisation. A substantial amount of susceptibility at transition temperature is due to the short-range ferrimagnetic ordering as a result of cation redistribution. The susceptibility value with milling is increasing which is an indication of the fact that the high-energy ball milling facilitates the cation redistribution.