Ferro and antiferromagnetism of ultrafine-grained hematite

Aluminum-substituted hematite (here referred to as Al-Hm) is an important magnetic mineral for paleo and environmental magnetism. However, the magnetic properties of nanosized Al-Hm are poorly known. In this study, a series of Al-Hm samples (fourteen samples) and their nonsubstituted counterparts (referred to as pure-Hm) (seven samples) were synthesized with particle sizes ranging from 14 to 124 nm. With decreasing particle size, coercive force (Bc), magnetic remanence (Mr) acquired in a 5 T field at room temperature, and the maximum blocking temperature (Tb) of Al-Hm descend gradually; Tb drops abruptly from 800 to 200 K at 17.2 ± 3.7 nm. This size defines the superparamagnetic (SP)/single-domain (SD) threshold of Al-Hm which is significantly lower than that of pure-Hm (27.5 ± 1.5 nm). The antiferromagnetic high-field susceptibility (χanti) and the saturation magnetization (Ms) for the weak ferromagnetic component in hematite are both correlated negatively with temperature between 20 and 300 K, and grain size in the vicinity of the SP threshold. For very small grains (below 16–17 nm), Tb increases with increasing particle size. However, beyond this size, it descends with increasing particle size up to 19 nm before increasing with grain size again. These trends are attributed to the competition of surface anisotropy and bulk anisotropy (elastic anisotropy). This new study substantially improves our understanding of the complex magnetic properties of fine-grained Al-Hm and pure-Hm in natural sample, which is significant for the paleoenvironmental and climatic studies of natural samples, e.g., red beds/soils.