Size dependence of hysteresis properties of small pseudo-single-domain grains

The Day plot (M-rs/M-s versus H-cr/H-c) is widely used by paleomagnetists t o estimate the size of ferromagnetic grains and classify them as single-dom ain (SD), pseudo-single-domain (PSD,) or multidomain (MD). How reliable is this plot? To find out, a numerical micromagnetic model is used to calculat e hysteresis loops as a function of grain size for two grain shapes (cube a nd cuboid with X = 1.5Y = 1.4Z). Magnetocrystalline anisotropy is ignored. The average M-rs/M-s is calculated for a collection of randomly oriented gr ains: In the elongated grain it drops from 0.4 to 0.06 over a negligible si ze range, almost missing the usual PSD range altogether. Other hysteresis p arameters, H-c, H-cr, and chi(0), can only be calculated for a grain at a t ime. This is done for two magnetic field directions (close to the longest a xis and close to the shortest axis). The single-grain values of H-cr/H-c de pend strongly on field direction, but it is clear that the average jumps ra pidly from SD to MD values. There are large, rapid fluctuations in H-cr and chi(0) associated with changes in rernancnt states. However, these fluctua tions may not be apparent when averaged over a broad size range. This slay explain why H-cr and chi(0) depend weakly on grain size in real samples. In the size range studied (L less than or equal to 0.25 mu m), hysteresis par ameters do not represent a typical grain size. Instead, they depend strongl y on the size distribution.