Influence of unequal magnetization direction probabilities on the Mössbauer spectra of superparamagnetic particles

Abstract

The effect of unequal magnetization direction probabilities is most distinctly recognized when the superparamagnetic relaxation frequencies are between about 10 and ${10}^{3}$ times the M\"ossbauer measurement frequency (${f}_{m}\ensuremath{\sim}0.5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ Hz). In this frequency range, two features which are commonly associated with the M\"ossbauer spectra of superparamagnetic particles (a quadrupole-doublet contribution and a broad distribution of hyperfine fields) can be artifacts of an interpretation which assumes equal magnetization direction probabilities. When the frequency is larger than about ${10}^{3}{f}_{m}$ and if the probabilities are sufficiently different, a "superferromagnetic" state exists and represents a new mechanism for the observed reduced hyperfine splittings of particles below their blocking temperature.