Low-temperature behavior and memory of iron-rich titanomagnetites (Mt. Haruna, Japan and Mt. Pinatubo, Philippines)

摘要

We report low-temperature remanence and memory of octahedral crystals of titanomagnetite from Mt. Haruna, Japan and Mt. Pinatubo, Philippines. The crystals have Curie temperatures of 460-490 ℃, indicating a low Ti content (0.11 ≤ x ≤ 0.16). Saturation isothermal remanence (SIRM) produced at 20 K decreased rapidly in warming through the isotropic temperature, 42 K ≤ T_i ≤ 55 K, where the first magnetocrystalline anisotropy constant K_1 changes sign and momentarily vanishes. 96-99% of the original SIRM was demagnetized at T_i and none was recovered in recooling from 300 K to 20 K. SIRM produced at 300 K decreased continuously in the course of zero-field cooling, leveling out around T_i. On rewarming to 300 K, 45-60% of the initial SIRM was recovered. This memory or recovered remanence is very large compared to that of magnetite and must be due to domain walls that are more strongly pinned, probably magnetostrictively by crystal defects, which are common in natural crystals. The present low-temperature experiments provide a quick, non-destructive method of identifying iron-rich titanomagnetites in soils, sediments and rocks.