Hill, Adrian H.8; Jacobsen, Henrik Skåret3; Stewart, J. Ross4; Jiao, Feng5; Jensen, Niels P.6; Holm, Sonja L.3; Mutka, Hannu9; Seydel, Tilo9; Harrison, Andrew9; Lefmann, Kim3
1 Department of Energy Conversion and Storage, Technical University of Denmark2 European Synchrotron Radiation Facility3 University of Copenhagen4 Rutherford Appleton Laboratory5 University of Delaware6 Technical University of Denmark7 Institut Laue-Langevin8 European Synchrotron Radiation Facility9 Institut Laue-Langevin
Samples of nanoscale hematite, α-Fe2O3, with different surface geometries and properties have been studied with inelastic time-of-flight neutron scattering. The 15 nm diameter nanoparticles previously shown to have two collective magnetic excitation modes in separate triple-axis neutron scattering studies have been studied in further detail using the advantage of a large detector area, high resolution, and large energy transfer range of the IN5 TOF spectrometer. A mesoporous hematite sample has also been studied, showing similarities to that of the nanoparticle sample and bulk α-Fe2O3. Analysis of these modes provides temperature dependence of the magnetic anisotropy coefficient along the c-axis, κ1. This is shown to remain negative throughout the temperature range studied in both samples, providing an explanation for the previously observed suppression of the Morin transition in the mesoporous material. The values of this anisotropy coefficient are found to lie between those of bulk and nano-particulate samples, showing the hybrid nature of the mesoporous 3-dimensional structure.
Journal of Chemical Physics, 2014, Vol 140, Issue 4