Memory of the magnetic field applied during cooling in the low‐temperature phase of magnetite: Grain size dependence

Abstract

Low‐temperature magnetic hysteresis properties of polycrystalline magnetite samples were studied as a function of the magnetic field (HFC) applied during cooling from 300 K to 10 K. The samples ranged in mean grain size from 0.04 to 100 μm, representing mostly single‐domain (SD), pseudosingle‐domain (PSD), and multidomain (MD) magnetic states. The low‐temperature field memory effect, a striking ability of magnetite to memorize the field HFC, is well expressed in PSD magnetite samples (mean grain size ranging from 0.15 to 5 μm). The field memory effect manifests itself as an inflection point of a magnetic hysteresis loop, located in the vicinity of HFC. The effect is greatly reduced in the samples containing larger than 5 μm magnetite grains and is absent in the sample containing large (40 to 200 μm) MD grains. Little or no distortion of hysteresis loops is observed in the samples dominated by SD magnetic grains. The experimental results confirm that the low‐temperature field memory effect is a generic property of PSD magnetite and give further support to a phenomenological model in which the field memory originates from an interplay between the magnetic and twin domains in monoclinic magnetite. The observed grain size dependence of the effect implies that the PSD state is a physically distinct magnetic state, rather than simply a manifestation of SD and MD mixing. The distinction is determined by the relative effect of twinning‐related crystalline defects on the remagnetization process.