Abstract
We report zero-field low-temperature cycling of saturation remanence (SIRM) produced at 300 or 10 K for crushed natural magnetites in nine size fractions from 0.6 to 135 µm, one set annealed to reduce stress, the other unannealed. Coercivities of isothermal remanence increase tenfold between 300 and 10 K, possibly explaining an apparent transition near 50 K. 300-K SIRM decreases continuously on cooling, losing 60–80% by TV = 120 K (Verwey transition), is constant from 120 to 10 K, then recovers a small memory in warming through TV to 300 K. A dip and recovery of remanence near TV for larger (> 15 µm) annealed grains is probably due to memory of cubic domain structures by monoclinic magnetite below TV, permitting partial recovery of initial remanence. In warming, 10-K SIRM is little affected until lost catastrophically near TV. A small memory is recovered in cooling to 10 K. The contrasting behaviors of 300-K and 10-K SIRMs result from the contrasting anisotropies and domain structures of cubic and monoclinic magnetite. Memories of initial remanences after full temperature cycles are attributed to monoclinic magnetite providing a template for partially regenerating initial cubic domain structures on the second passage through TV. Memory ratios as a function of grain size for our magnetites are too scattered to be granulometrically useful.
Highlights
Magnetite (Fe3O4) is cubic at room temperature with inverse spinel structure
The spectrum of coercivities measured at 10 K extends at least as high as 5 T, while at 300 K a field of 0.2 T will saturate the isothermal remanent magnetization (IRM)
We have made a systematic study of remanence cycling as a function of grain size in magnetites ranging from small PSD (≈ 1 μm) to moderate MD (100–150 μm) sizes and a less thorough investigation of the effect of internal stress, through the use of matching unannealed and annealed sample sets
Summary
Magnetite (Fe3O4) is cubic at room temperature with inverse spinel structure. Its first magnetocrystalline anisotropy constant K1 is negative and the easy axes for spontaneous magnetization Ms are the four 〈111〉 body diagonals. At the isotropic point near 130 K, K1 changes sign and the easy axes switch to 〈001〉. Domain structures must change as a result but there is usually little sign of this in the measured sample magnetization. The cubic lattice deforms only slightly and exchange interaction is scarcely affected, as evidenced by the near-continuity of Ms across the transition (e.g., Özdemir 2000; Kosterov 2001), but the magnetocrystalline anisotropy constants of the new monoclinic lattice increase more than tenfold.
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