Micromagnetic Experiments on Small Single Particles: Defects, Nucleation, and Wall-Pinning

University Of Utah, Salt Lake City UT

Investigators

Abstract

Halgedahl EAR-0125993 The physical origins of pseudosingle-domain (PSD) behavior in small magnetic particles have been subjects of lively debate, because PSD grains are thought to carry much of the stable remanence in rocks. Two ingredients contribute most to PSD behavior: transitions in magnetic domain state and defects that pin domain walls. Despite the important role of defects having been recognized for decades, recent theories and experiments in rock magnetism underscore how little we understand about their impact on key problems such as domain wall nucleation, wall-pinning, the stabilities of remanences to alternating field (AF) demagnetization, acquisition of thermal remanent magnetization (TRM), and even the potential for extracting paleointensities from PSD grains of certain magnetic minerals. Single particles of judiciously-chosen magnetic minerals can provide "magnified" views of both nucleation events and wall-defect interactions. Owing mainly to its weak demagnetizing field and few domains, hematite is such a magnifier. This combination of properties opens up a window for studying the intrinsic impact of defects on magnetic behavior. We will use large, single platelets of hematite to better understand how defects control nucleation and domain wall pinning. Six sets of core experiments are proposed here. These would determine: (1) the thermal stability of SD-like (or near SD-like) states induced at room temperature; (2) domain states for weak-field TRM; (3) temperature dependences of nucleation fields; (4) temperature dependences of critical fields that unpin mature domain walls; (5) AF stabilities of truly multidomain hematite carrying strong-field remanence and weak-field anhysteretic magnetization, and (6) the frequency distribution of critical unpinning fields.