Magnetic fatigue: Effect of seismic-related loading on magnetic and
structural behavior of magnetite
Abstract
Powders of multidomain magnetite were subjected to fatigue, cyclic
loading at ambient conditions between 30 and 500°C using a dynamical
mechanical analysis system. The uniaxial static force of 6 MPa was
modulated with 3 MPa using 1, 10 and 100 Hz frequencies. Microstructural
analysis was performed by a combination of X-ray diffraction analysis
with optical microscopy. Magnetic properties were studied by
low-temperature magnetic susceptibility across the Verwey transition and
hysteresis behavior measured at room temperature. Irreversible
microstructures and magnetic signatures indicate the onset of magnetic
fatigue in magnetite. The increasing mechanical losses, are proportional
to the growing loading frequency and correlate with decreasing coherent
crystallite size and lattice parameters in magnetite as well as with its
lattice distortion. Hematite at grain boundaries of crashed magnetite is
documented by microscopy, however, magnetic analysis indicates that
magnetite is still the main magnetic carrier in the fatigue-loaded
samples and therefore, the changes in magnetic properties are dominantly
related to magnetic fatigue. The decay of magnetic susceptibility above
is independent of the occurrence of hematite but is more sensitive to
lattice strain and reduction of magnetic domains. The refinement of
magnetic domains is also demonstrated by the evolution of hysteresis
parameters and FORC diagrams, both indicating multidomain to
pseudo-single domain transition. As the applied loading frequencies are
similar to those observed during earthquake propagation or volcano
eruption, it is therefore discussed how the acquired signatures of
magnetic fatigue can be used as proxies of natural seismic-related
loadings.