High‐temperature magnetostriction of magnetite and titanomagnetites

Magnetostriction of synthetic polycrystalline magnetite and titanium‐rich titanomagnetites has been measured from 25°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 SmCo 5 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 (Fe 3‐ x Ti x O 4 , 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 λ 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, λ s ( T )αM s ( T ) n , where n =2.31 (TM0), 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.