Monte Carlo simulations are used to investigate the effect of surface anisotropy on the spin configurations and hysteresis loops of ferromagnetic nanoparticles. Spherical particles of radius a are composed of N atoms located on a simple cubic lattice with interatomic spacing a. The particles have 2 ഛ ഛ 13. A classical Heisenberg model is assumed, with surface and bulk anisotropy. When surface anisotropy is positive there are two types of ground states separated by a large energy barrier: a "throttled" configuration with reduced magnetization for intermediate values of surface anisotropy and a "hedgehog" configuration with zero magnetization in the strong surface anisotropy limit. Beyond a threshold, surface anisotropy of either sign induces ͗111͘ easy axes for the net magnetization. Easy-axis hysteresis loops are then square, with a continuous approach to saturation, and the effective anisotropy is deduced either from the switching field or from the initial slope of the perpendicular magnetization curve. The hedgehog state shows a stepwise magnetization curve involving discrete configurations, and it passes to a throttled configuration before saturating. The hysteresis loop has the unusual feature that it involves a state in the first quadrant, which lies on the reversible initial magnetization curve; it is possible to recover the zero-field cooled state after saturation. A survey of the exchange and anisotropy parameters for a range of ferromagnetic materials indicates that the effects of surface anisotropy on the spin configuration should be most evident in nanoparticles of ferromagnetic actinide compounds such as US, and rare-earth metals and alloys with Curie points below room temperature; the effects in nanoparticles of 3d ferromagnets and their alloys are usually insignificant, with the possible exception of FePt.
Nanostructured iron fluoride powders were prepared using the grinding route for different times and different intensities. Their structural, microstructural and magnetic properties are investigated by means of both transmission Mössbauer spectrometry as a function of temperature and in-field 57 Fe Mössbauer spectrometry. We report a fitting procedure which successfully describes the zero-field Mössbauer spectra recorded at different temperatures. It allows us to describe the powders as crystalline grains and grain boundaries which behave as antiferromagnets and speromagnets, respectively. Such arrangements are confirmed by in-field Mössbauer spectrometry. According to x-ray diffraction data, the size of grains and the thickness of grain boundaries are found to be strongly dependent on the grinding conditions. The occurrence of superparamagnetic effects at high temperature gives clear evidence for the role of grain boundaries in the magnetic coupling of crystalline grains.
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