Remanence cycling of 0.6–135 µm magnetites across the Verwey transition

Abstract

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 T V = 120 K (Verwey transition), is constant from 120 to 10 K, then recovers a small memory in warming through T V to 300 K. A dip and recovery of remanence near T V for larger (> 15 µm) annealed grains is probably due to memory of cubic domain structures by monoclinic magnetite below T V, permitting partial recovery of initial remanence. In warming, 10-K SIRM is little affected until lost catastrophically near T V. 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 T V. Memory ratios as a function of grain size for our magnetites are too scattered to be granulometrically useful.

Keywords

• Magnetite
• Verwey transition
• Saturation remanence
• Low-temperature cycling