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The anisotropy of magnetic susceptibility of uniaxial superparamagnetic particles: Consequences for its interpretation in magnetite and maghemite bearing rocks
Author(s) -
Lanci Luca,
Zanella Elena
Publication year - 2016
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2015jb012255
Subject(s) - superparamagnetism , maghemite , anisotropy , condensed matter physics , single domain , materials science , magnetic susceptibility , magnetic anisotropy , magnetite , particle (ecology) , magnetic nanoparticles , nuclear magnetic resonance , nanotechnology , physics , nanoparticle , magnetization , magnetic field , geology , optics , metallurgy , quantum mechanics , oceanography
A simple model that provides a quantitative description of the magnetic susceptibility of superparamagnetic to stable single‐domain uniaxial magnetic particles can be built in the framework of the theory of stochastic resonance. This model expands that of Mullins and Tile (1973) for superparamagnetic grains by considering the dependence of superparamagnetic susceptibility on the particle orientation and thus describes the anisotropy of magnetic susceptibility (AMS) of ensembles of superparamagnetic as well as single‐domain particles. The theory predicts that on the contrary of stable single domain, the maximum anisotropy of superparamagnetic particles is parallel to their easy axis and shows that the AMS of ensembles of uniaxial particle is strongly dependent on the distribution of particle grain size, coercivity, measurement temperature, and frequency. It also explains why the inverse AMS pattern expected for stable single‐domain particles is rarely observed in natural samples. We use examples of well‐characterized obsidian specimens to show that, as predicted by the theory, in the presence of significant superparamagnetic contributions, the maximum susceptibility axis of AMS is directed along the preferential direction of particles easy axis.