High-Temperature Observation of Transdomain Transitions in Vortex States in Intermediate Titanomagnetite

Natural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single-domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5-μm grains of titanomagnetite (Fe_2.46Ti_0.54O₄) using variable-temperature magnetic force microscopy. At room temperature, grains displayed single-vortex and multivortex states. Upon heating, the single-vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single-vortex states. During cooling in a weak field (~0.1 mT), single-vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single-vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single-vortex states occur in titanomagnetite and, like single-domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.