Magnetic relaxation in nano-particles: a Mossbauer study

Ultra fine superparamagnetic particles of iron compounds with size 50 to 125 A probed through Mossbauer spectroscopy usually show magnetic relaxation effects at low temperatures. A magnetic sextet is observed for long relaxation times compared to nuclear lifetime whereas the magnetic interaction disappears for short relaxation times and this change usually occurs between 25 to 100 K. A theoretical model has been developed for the magnetic relaxation process considering that on microscopic level it involves an exchange of energy between magnon and phonon sub-systems via magnon-phonon scattering induced by dynamic exchange. It is seen that the relaxation rate increases with temperature in a complicated manner besides its dependence on the number of magnetic ions or particle-size. The calculated rates appear consistent with the observed Mossbauer results at low temperatures for some of the iron oxides.