MS-13-IN-1719 Nanoscale magnetism of magnetite induced by phase transition and oxidation/reduction reactions
Magnetite (Fe3O4) is the dominant carrier of paleomagnetic and paleoclimatic information in rocks and sediments on the Earth and on other planets. It is important to understand the formation and recording mechanisms, as well as fidelity, of the magnetic minerals within natural systems. Below ~125 K, magnetite undergoes a first-order phase transition, known as the Verwey transition, from a cubic structure to a closely-related monoclinic structure. The transition has a profound impact on its magnetic properties. Kasama et al. [1] carried out the first TEM measurements of the magnetic microstructure of magnetite below the Verwey transition, showing strong interactions between magnetic domain walls and twin domain walls. In addition, the ability of magnetite to preserve the remanence of the Earth's and other planets’ magnetic fields is greatly influenced by progressive oxidation or reduction to different magnetic minerals. Here, we use various TEM techniques including off-axis electron holography and environmental TEM (ETEM) to investigate fundamental magnetic properties of magnetite (1) below the Verwey transition and (2) under oxidizing/reducing environments.
(1) The low temperature monoclinic phase has closely-spaced magnetic domains separated by 90˚ or 180˚ magnetic domain walls, which are defined strictly by underlying monoclinic twin domains with a monoclinic [001] easy axis resulting from a large magnetocrystalline anisotropy (Fig. 1). Direct imaging after the application of an external magnetic field during cooling showed that the magnetic field affects the choice of monoclinic c-axis and monoclinic twin formation with some complication due to specimen geometry. Furthermore, magnetic domains in magnetite above or below the Verwey transition can inherit a part of magnetization from the prior phase during zero field cooling or zero field warming through the Verwey transition.
(2) Magnetite nanoparticles were reduced in-situ under 2 mbar H2 atmosphere at 460˚C in a microscope column of the ETEM (Fig. 2). After 5 hours, the particles become rounded and smaller as a result of reduction of magnetite to metallic Fe. The reduced Fe particles have single domain states with dipolar interactions with their neighbors, while the initial magnetite particles have more collective behavior with vortex states. Similar experiments were performed using ~200-nm-sized magnetite particles under oxidizing conditions, showing that the hematitization of the magnetite particles changes their magnetic microstructures dramatically. These results suggest that the magnetic remanence is altered by redox reactions and should be used with an understanding of geological history in a site of interest.
[1] T. Kasama et al., Earth Planet. Sci. Lett. 165 (2011), 229.
We thank R.E. Dunin-Borkowski, J. Jinschek, T.W. Hansen, R.J. Harrison, A.R. Muxworthy, Z.-A. Li and S. Yazdi for discussions.
Fig. 1: (a) Experimental magnetic induction map (1x phase amplification) of monoclinic magnetite obtained using off-axis electron holography. (b) Simulated induction map overlaid onto a Lorentz image. The symbols ‘C0˚’ and ‘C45˚’ refer to c-axes, oriented in-plane and at 45˚ to the plane of the specimen. GB: grain boundary, TWB: twin boundary. |
Fig. 2: (a) TEM image of initial cube-shaped magnetite nanoparticles. (b) Electron hologram of the nanoparticles that were reduced under 2 mbar H2 atmosphere at 460˚C in an ETEM column. (c) Magnetic contour map of the reduced nanoparticles shown in (b). The contour spacing is 0.042 radians. |