Magnetic Properties of 2D Nano-Islands Subject to Anisotropy and transverse fields: EFT Ising Model (original) (raw)
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Magnetic properties of 2D nano-islands II: Ising spin model with out-of-plane magnetic field
Journal of Magnetism and Magnetic Materials, 2011
An Ising effective field theory model is presented to calculate the magnetic properties of 2D nanoislands on a nonmagnetic substrate, subject to an externally out-of-plane applied magnetic field. The system Hamiltonian contains nearest neighbor exchange interactions, single-atom magnetic anisotropies, and the Zeeman term. The calculations yield, in particular, the single site spin correlations, the magnetizations, and the isothermal susceptibilities, for the core and periphery domains of the nanoisland. The choice of a spin S ¼ 1 for the atoms of the system permits the analysis of local spin fluctuations via the single site spin correlations. We investigate in this respect the effects due to the different magnetocrystalline anisotropies and reduced dimensionalities, for the core and periphery domains, and in particular the critical influence of the applied magnetic field. Detailed theoretical results are presented for the square and hexagonal lattice symmetries, with numerical applications for the 2D monolayer Co nano-islands on a Pt substrate. It is shown that the remarkable differences between the magnetic properties of the core and periphery domains in zero field are washed out when an out-of-plane field is applied. The applied field also provokes critical discontinuities for the spin correlations and magnetization reversals, for the core and periphery domains, which are especially evident for the hexagonal lattice nano-island in the range of fields of interest. The discontinuities and magnetization reversals occur over elementary temperature widths, and shift to lower temperatures with increasing field. The field-dependant isothermal susceptibilities show new features very different from those for the susceptibilities in zero field. The present Ising model does not show any blocking temperature transition to superparamagnetism.
Size and Shape Dependence of Thermal Spin Transitions in Nanoislands
Physical Review Letters, 2013
Theoretical calculations of thermal spin transitions in nanoscale clusters on a surface are presented. The mechanisms for magnetization reversal are identified and the activation energy and pre-exponential factor for the rate are evaluated using a recently developed harmonic transition state theory and a Heisenberg-type Hamiltonian. A maximum is found in the pre-exponential factor as a function of cluster size corresponding to a crossover from a uniform rotation mechanism to temporary domain wall formation. As the islands grow, the energy barrier increases up to a limit where the domain wall is fully established. For larger islands, the minimum energy path becomes flat resulting in a significant recrossing correction to the transition state theory estimate of the rate. The parameters of the Hamiltonian are chosen to mimic Fe clusters on a W(110) surface, a system that has previously been studied extensively in the laboratory and the calculated results are found to be in close agreement with the reported measurements.
Giant Spin-Polarization and Magnetic Anisotropy of Nanostructures at Surfaces
e-Journal of Surface Science and Nanotechnology
We present spin-polarized scanning tunneling microscopy measurements demonstrating spin-polarizations of up to 80 % for Co islands on a Pt(111) surface and a tunnel magneto resistance of 850 % between the islands and an anti-ferromagnetic Cr-coated W-tip. These values are stable up to ±0.7 V bias. We report on the magnetic moments and anisotropy energies of two-dimensional Co islands on Pt(111) comprising only a few atoms. Our results show the correlation between orbital moments and magneto-crystalline anisotropies and reveal that both properties strongly depend on the lateral atomic coordination. The anisotropy of single adatoms is found to be 200 times the Co hcp bulk value. We also present well ordered superlattices of Co islands self-assembled on Au(788). The particles have uniaxial out-of-plane magnetization and no dipolar interactions. They present a model system for ultra-high density storage media since they have the most uniform anisotropy energies and the highest density of non-interacting particles so far realized.
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Progress in Materials Science, 2007
We review some of our recent work on first principles calculations of the magnetic structure of surface and bulk nanostructures. The calculations are based on implementation of relativistic density functional theory within state of the art surface embedding and order-N multiple scattering Green's function methods. First principles spin-dynamics and the constrained local moment approximation are reviewed as they relate to optimization of moment configurations in highly inhomogeneous materials such as surface and bulk nanostructures. Results are present for three prototypical nanostructures-short Co-chains adjacent to a Pt{1 1 1}-surface step-edge, a Cr-trimer on the Au{1 1 1}-surface, and Fe-chains and impurities in Cu-that illustrate the need to treat the underlying electronic interactions on a fully self-consistent basis in which the very different energy scales appropriate to exchange coupling and magneto-crystalline anisotropy are treated on an equal footing.
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Science, 2010
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Unusual spin correlations in a nanomagnet
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Physical Review B, 2011
We analyzed the size effect of spin-crossover transition nanoparticles in a two-dimensional core-shell model, where the edge atoms are constrained to the high-spin (HS) state. Using Monte Carlo (MC) simulations, we showed that this specific edge effect lowers the equilibrium temperature and enhances the HS residual at low temperature; these results are in very good agreement with recent experimental data. Within a very simple working assumption, we obtained an analytical expression for the size dependence of the equilibrium temperature that is in excellent agreement with the MC results. The model leads to a nontrivial size dependence of the hysteresis width, which is similar to a-size-dependent-negative pressure effect induced by the HS edges. To reach the best agreement with experimental data, we accounted for the size distribution of the experimental samples.
Journal of Physics: Conference Series
Some six-coordinate iron (II) coordination compounds exhibit thermal-, optical-, electrical-, magnetic-and pressure-induced switching between the diamagnetic low-spin (LS, S=0) and the paramagnetic high-spin (HS; S=2) states [1]. This may lead to potential application of these complexes in molecular devices such as temperature and pressure sensors [2]. An Ising-like model has been proposed to explain the occurrence of the thermal hysteresis behaviour [3,4] of this switchable solids. In this contribution, the local mean field approximation is applied to solve the Hamiltonian modelling interactions pertaining to 2D nanoparticles embedded in a magnetically-inactive matrix.