Magnetic Vortex States in Small Octahedral Particles of Intermediate Titanomagnetite

Abstract

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.

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

“…The titanomagnetite sample was synthesized using the glass‐ceramic method (Worm & Markert, ) and described in detail in Khakhalova et al (). The sample contains titanomagnetite grains dispersed in a nonmagnetic glassy silicate matrix.…”

“…To gain further insights into TRM theory and provide observational data for validating micromagnetic models, we conducted high‐temperature domain observations of vortex states in two grains of intermediate titanomagnetite (Fe 3‐x Ti x O 4 with x = 0.54, or TM54), with known crystallographic orientations and diameters of ~1.5 μm. From a previous study on the same sample at room temperature (Khakhalova et al, ), it was found that these grains display SV states as the most common LEM state (global energy minimum or GEM state) and several multivortex (MV) states presumed to be higher‐energy LEM states. Here we use variable‐temperature magnetic force microscopy (MFM) to image vortex state evolution during in situ thermal demagnetization and TRM acquisition in a weak magnetic field of ~0.1 mT.…”

“…Single‐vortex (SV) states have been predicted to form just above the SD size range by numerical micromagnetic models (e.g., Williams & Dunlop, ), and more recently, been observed experimentally in particles of magnetite (~200 nm), titanomagnetite (~1,500 nm), and Fe‐Ni inclusions (<~100 nm) in dusty olivine grains (Almeida et al, ; Almeida, Muxworthy, Kovács, Williams, Brown, ; Almeida, Muxworthy, Kovács, Williams, Conbhuí, et al, ; Khakhalova et al, ; Kimura et al, ; Lappe et al, ). High thermal stability of SV states up to the Curie temperature has also been observed during high‐temperature observations in ~200‐nm magnetite grains with electron holography (Almeida, Muxworthy, Kovács, Williams, Brown, ; Almeida, Muxworthy, Kovács, Williams, Conbhuí, et al, ; Almeida et al, ).…”

Separating Geometry‐ From Stress‐Induced Remanent Magnetization in Magnetite With Ilmenite Lamellae From the Stardalur Basalts, Iceland

“…At larger sizes these grains form either (1) PSD magnetization structures with inhomogeneous structures and no clear domains or (2) MD magnetization structures, with distinct uniformly magnetized regions with domain walls. Previous studies using micromagnetic calculations commonly model geometries of idealized shapes (Fabian et al, 1996;Fukuma & Dunlop, 2006;Khakhalova et al, 2018;Newell & Merrill, 1999;Yu & Tauxe, 2008;Witt et al, 2005, Williams & Dunlop, 1995. Natural iron oxide geometries are usually more complicated (e.g., Larson et al, 1969;Shaar & Feinberg, 2013).…”

“…To acquire 3-D morphologies of such true particle geometries, we used nanotomography by Focused Ion Beam Scanning Electron Microscopy (FIB-SEM). Examples of other studies using MERRILL on magnetic oxides include submicron grains of iron (Einsle et al, 2016;Nagy, Williams, Tauxe, & Muxworthy, 2019a;Nagy, Williams, Tauxe, Muxworthy, & Ferreira, 2019b;Nichols et al, 2019;Shah et al, 2018); magnetite (Lascu et al, 2018) and titanomagnetite (Khakhalova et al, 2018;Khakhalova & Moskowitz, 2019).…”