Magnetic hysteresis in the F.M.R. spectra of fine-grained spherical iron: Possible evidence for a new carrier of hard remanence in lunar soils and rocks

Abstract

Magnetic hysteresis effects have been observed in ferromagnetic resonance (FMR) spectra obtained at 9 and 16 GHz for certain simulated lunar glasses which were reduced by H 2 in the melt and rapidly quenched. Transmission electron microscopy has revealed that these samples contained spherical particles in the size range ∼0.01–0.5 μm. FMR spectra obtained at 35 GHz (applied field ∼ 12.5 kOe) exhibited a line shape characteristic of spherical, single-domain (SD) iron particles with no hysteresis. Computer simulations of the latter spectra confirmed that the average deviation from sphericity must be ≲3% and that (2 K 1/ M s) ≈ + 600 Oe for the precipitated magnetic phases. The principal features of the spectra obtained at all three frequencies have been explained on the basis of a simple theoretical treatment for spherical iron particles which have 2 domains in applied fields ≲7 kOe, but become saturated at higher fields. Isothermal remanent magnetization (IRM) of these samples has been studied by both FMR and standard static techniques; the mean coercive force measured by the former (∼4 kOe) contrasts with the mean value determined by the latter (∼550 Oe). Apparently, FMR singles out and even amplifies the contributions of two-domain particles (which are magnetically hard), while the static measurement is more sensitive to the average of all particles present. The intensity of the FMR hysteresis of typical lunar soils is found to be ∼1% of the total FMR intensity. In spite of this seemingly small value, two-domain iron particles may be important carriers of natural remanent magnetization (NMR) in certain lunar rocks.