Micromagnetic analysis of the hardening mechanisms of nanocrystalline MnBi and nanopatterned FePt intermetallic compounds.

The uniaxial intermetallic compounds of L10-FePt and the low temperature NiAs structure of MnBi are suitable alloys for application as high-density recording materials or as high-coercivity permanent magnets. Single domain particles of these materials are characterized by coercive fields above 1 T over a large temperature range. In particular MnBi shows a coercive field of 2 T at 450 K. Its extraordinary magnetic properties in the temperature range up to 600 K are due to an increase of the magnetocrystalline anisotropy constant from 1.2 MJ m(-3) at 300 K to 2.4 MJ m(-3) at 450 K. In spite of the large coercivities obtained for both type of materials their experimental values deviate considerably from the theoretical values Hc = 2K1/Js valid for a homogeneous rotation process in spherical particles. As is well known these discrepancies are due to the deteriorating effects of the microstructure. For an analysis of the coercive fields the Stoner-Wohlfarth theory has to be expanded with respect to higher anisotropy constants and to microstructural effects such as misaligned grains and grain surfaces with reduced anisotropy constants. It is shown that the temperature dependence and the angular dependence of Hc for FePt as well as MnBi can be quantitatively interpreted by taking into account the above mentioned intrinsic and microstructural effects.

Point-defect interactions in electron-irradiated titanomagnetites--as analysed by magnetic after-effect spectroscopy on annealing within 80 K < Ta < 1200 K.

During high-temperature growing of titanomagnetite single crystals (Fe(2.8-Δ)Ti(0.2)O(4), Δ < 0.005) in oxygen enriched atmospheres, specific Ti(4+)- and vacancy-based defect configurations are induced, giving rise to magnetic after-effect (MAE) spectra with peaks near 450, 200 and 65 K. The atomistic mechanisms of these relaxations are checked by exposing the crystals to low-temperature (80 K) electron (e(-)) irradiation and subsequent analysis of the interactions between radiation-induced and lattice-inherent defects on annealing over the range 80 K ≤ T(a) < or equal 1200 K. Within this interval, three characteristic temperature ranges are distinguished: (a) 80 K < T(a) < 500 K, revealing vigorous interactions between radiation-induced and inherent defect configurations, thus demonstrating their common point-defect nature; (b) 500 K < T(a) < 900 K wherein the MAE spectra re-assume, qualitatively, their initial structure with, however, mutually modified amplitude ratios; (c) 900 K < T(a) < 1200 K, being characterized by the complete annihilation of all MAEs but, interestingly, also the thermally induced re-appearance of vacancies and related defect configurations. The recovery kinetics of all prominent processes are numerically analysed and discussed with respect to their underlying atomistic mechanisms.