Unexpected Magnetic Behavior of Natural Hematite-Bearing Rocks at Low Temperatures

Abstract

Hematite is a commonly occurring magnetic mineral in nature that has numerous scientific and technological applications. A characteristic property of hematite is a low-temperature spin-flop transition called the Morin transition. Above the transition temperature, hematite is a canted antiferromagnet that can carry a remanent magnetization. Below this transition, spin canting disappears and hematite becomes a true antiferromagnet although a small defect moment is usually preserved. We observe Morin transition behavior in natural samples that has not been reported before for hematite. During repeated thermal cycling of a remanent magnetization acquired at room temperature, the remanence intensity at the end of the cycle oscillates between a high remanence state at the end of odd-numbered cycles and a low remanence state (LRS) at the end of even-numbered cycles. Alternation of the high and LRSs during repeated thermal cycling points to hysteretic behavior of the spin-flop process, likely due to sublattice magnetization alignment switches along different easy magnetization axes in samples with preferred crystallographic orientations of hematite particles. We report these observations to seek to expand explanations of the magnetism of hematite.