Fine particles of titanomagnetites (Fe3‐xTixO4, x > 0.5) in the pseudo‐single‐domain (PSD) size (0.5–20 μm) are important carriers of natural remanent magnetization in basalts. Understanding the mechanism of magnetic recording in these grains has important implications for paleomagnetic studies. This study reports first observations of magnetic vortex states in intermediate titanomagnetite. We imaged magnetic structures of 109 synthetic titanomagnetite grains with x = 0.54 (TM54) and 1–4‐μm size using magnetic force microscopy. For six grains, we explored local energy minimum states after alternating field demagnetization and saturation isothermal remanent magnetization. According to the magnetic force microscopy results, 80% of TM54 grains display in‐plane magnetization with one to four domains, vortex‐like or flux‐closure structures, and Néel‐like domain walls. Electron backscatter diffraction data on six grains showed that their surface orientations are cutting planes of octahedral crystals and those with approximately square cross sections are within 15° of a (100) crystallographic plane. Magnetic force microscopy observations of magnetic structures in ~1.5‐μm grains agree well with numerical micromagnetic modeling of a pyramidal shaped grain with a (100) square base and displayed four discrete local energy minimum states: a single vortex as a ground state and three multivortex states with higher energy. Our observations show that vortex states in titanomagnetite grains (1–5 μm) occur at the lower end of the PSD size range in this mineral and corresponding to a size range known to carry stable and reliable remanence in natural titanomagnetites.

Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo‐single‐domain (titano)magnetite. Our experiments reveal shock‐induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum‐heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First‐order reversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters.

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 (Fe2.46Ti0.54O4) 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.