HIGH-TEMPERATURE MAGNETOSTRICTION OF MAGNETITE AND TITANOMAGNETITES

Magnetostriction of synthetic polycrystalline magnetite and titanium-rich titanomagnetites has been measured from 25-degrees-C to their Curie temperatures. These are the first reported measurements of magnetostriction above room temperature for titanomagnetites. The magnetostriction constants are measured with a recording rotating field magnetostrictometer, which uses a strain gage bridge technique and two rotating SmCo5 permanent magnets. Magnetostriction is recorded continuously as the sample is slowly heated and cooled in a fixed magnetic field. Measurements were made on sintered polycrystalline samples of magnetite, titanomagnetite (Fe3-xTixO4, x=0.0, 0.4, and 0.6), and an aluminum-magnesium substituted titanium-rich titanomagnetite (AMTM60). The thermal dependence of the polycrystalline magnetostriction constant lambda(s)(T) varies as (1-T/T(c))1.3 for titanium-rich titanomagnetites and as (1-T/T(c))0.9 for pure magnetite. The polycrystalline magnetostriction constant is well approximated by a power law equation in terms of the saturation magnetization, lambda(S)(T) is-proportional-to M(s)(T)n, where n=2.31 (TMO), 3.47 (TM40), 2.75 (TM60), and 2.80 (AMTM60). The magnetostriction results for AMTM60 are in excellent agreement with previous predictions based on domain observations.