Properties of Magnetic Nano-Particles |
| P.A. Lindgård Department of Condensed Matter Physics, Risø National Laboratory, 4000 Roskilde, Denmark |
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| The intrinsic thermodynamic magnetic properties of clusters are discussed using spin wave theory for a Heisenberg model, with a fixed magnitude of the spins Si=S and site independent nearest neighbor exchange interaction. The consequences of the more realistic Hubbard model is considered in which we allow for a magnetization profile at T=0 and a structural relaxation, which in turn will give rise to a site dependent exchange interaction. It is concluded that correlation effects among the electrons play a very important role in small clusters, albeit not modifying the thermodynamic properties drastically. The finite cluster size gives foremost rise to a discrete excitation spectrum with a large energy gap to the ground state. The relaxation of the magnetization during the reversal of the external magnetic field is discussed. A first step towards a quantitative understanding of the nonequilibrium statistical mechanics in single-domain ferromagnetic particles is a systematic study of the kinetic Ising model. Results from Monte Carlo simulation and droplet theory are reviewed with particular attention to the effects of various boundary conditions, including a decrease in the number of surface bonds and an addition of surface anisotropy. A new dynamic "outside-in" flip mode is proposed. |
| DOI: 10.12693/APhysPolA.91.121 PACS numbers: 75.30.Ds, 75.40.Gb, 76.20.+q |