N. Sugiura

Magnetic Properties of Hydrothermally Recrystallized Magnetite Crystals

The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution (<0.3 micrometer) and of crushed sifted grains (>0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 1010 meter -2) and natural (1 x 1010 meter-2) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.

ARM, TRM and magnetic interactions: Concentration dependence

Abstract ARM (anhysteretic remanence)/SIRM (saturation isothermal remanence) and TRM (thermoremanence)/SIRM were measured as a function of the concentration (volume fraction) of single-domain magnetite ( 3 × 10 −6 ≲ C ≲ 2 × 10 −2 ), ARM/SIRM increases with decreasing concentration, showing that there is magnetic interaction between fine particles. The role of magnetic interaction in TRM acquisition is also important at higher concentrations of magnetite ( C ≳ 0.1% ), where the value of TRM/SIRM increases with decreasing concentration. It is only for concentrations of C ≲ 0.1% that the value of TRM/SIRM is fairly constant and interactions among magnetite grains seem to be ignored. The ratio TRM/ARM decreases from seven to almost unity as the concentration of magnetite decreases.

The effects of annealing and concentration on the hysteresis properties of magnetite around the PSD-MD transition

Abstract Hysteresis parameters H cr , H c , J rs , J s , and their ratios H cr /H c , J rs /J s have been measured for a large number of accurately prepared grain size fractions of magnetite in the range between 5 and 150 μm. For several grain size fractions three different concentrations of magnetite are used: 100, 0.1, and 0.002 vol.%. Most of the measurements were repeated after annealing the specimens to 600°C. For some specimens in the pseudo-single (PSD) and multidomain (MD) range H c and H cr have been measured as functions of temperature. Plots of the results from H c , H cr /H c and J rs /J s versus the grain size reveal curves with a convex and a concave part. Concentration and annealing affects the values of the hysteresis parameters, especially for grains coarser than 25 μm but the shape of the curves remains the same. The inflection point from convex to concave for all curves occurs at 25 μm and it appears to be independent of concentration and annealing. It is therefore proposed to define the transition from PSD to MD as the inflection point of these curves.

A paleomagnetic conglomerate test using the Abee E4 meteorite

Abstract The meteorite Abee is a type 4 enstatite chondrite with many centimeter-size clasts. The paleomagnetic conglomerate test was applied to these clasts, to study the thermal and magnetic history of the meteorite. The directions of magnetization in mutually oriented clasts are significantly different, suggesting the meteorite was not reheated to temperatures much above 100°C during or after accretion. Paleointensity estimates were made using Thelliers method. Interior samples which were probably not reheated during entry into the earths atmosphere show paleointensities of several oersteds. The fusion crust is also strongly magnetized, showing paleointensities up to 60 Oe.

Field dependence of blocking temperature of single-domain magnetite

Abstract Blocking phenomenon of single-domain magnetite was studied in detail by measuring magnetization in a field as a function of temperature. Blocking temperature decreases with the increase of applied field. In addition, blocking temperature spectrum becomes very broad with increasing applied field. These tendencies are in agreement with Neels single-domain theory. In fact, semi-quantitative agreement between experimental and theoretical blocking temperature spectrum is satisfactory.