Low‐temperature magnetic properties of magnetite using first‐order reversal curve analysis: Implications for the pseudo‐single‐domain state

Abstract

Low‐temperature magnetic hysteresis properties of nearly stoichiometric magnetite were studied using the first‐order reversal curve (FORC) technique. FORC distributions were measured at 20 K after cooling in zero magnetic field or cooling in the presence of a strong (1.5 T) magnetic field. Five polycrystalline samples ranged in mean grain size and shape, representing mostly single‐domain (SD), pseudo‐single‐domain (PSD), and multidomain (MD) magnetic hysteresis behavior. The FORC distributions measured from PSD magnetite after zero field cooling (ZFC) manifest an unusual behavior, characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field (Hu = 0) axis on a FORC diagram. In contrast, the low‐temperature FORC distributions for SD and MD samples have a single maximum located at the Hu = 0 axis. The appearance of two maxima on low‐temperature FORC distributions is interpreted as a manifestation of a perminvar‐like behavior due to stabilization of the magnetic domain structure by irregularities of crystalline lattice induced by twinning in monoclinic magnetite. The experimental data suggest that the effect of monoclinic twinning on remagnetization is only pronounced within a certain grain size range. The double‐maxima ZFC FORC distribution appears to be a distinctive property of PSD hysteresis behavior, which cannot be reproduced by mixing SD and MD magnetites.