1984 ::: Influence of unequal magnetization direction probabilities on the Mössbauer spectra of superparamagnetic particles

D. G. Rancourt and J. M. Daniels

Phys. Rev. B  29, 2410 – Published 1 March 1984
https://doi.org/10.1103/PhysRevB.29.2410

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 Mössbauer measurement frequency (fm ∼ 0.5 × 10^8 Hz). In this frequency range, two features which are commonly associated with the Mössbauer 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 fm 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.

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