Spin-spin correlations in ferromagnetic nanosystems

Abstract

Using exact diagonalization, Monte-Carlo, and mean-field techniques, characteristic temperature scales for ferromagnetic order are discussed for the Ising and the classical anisotropic Heisenberg model on finite lattices in one and two dimensions. The interplay between nearestneighbor exchange, anisotropy and the presence of surfaces leads, as a function of temperature, to a complex behavior of the distance-dependent spin-spin correlation function, which is very different from what is commonly expected. A finite experimental observation time is considered in addition, which is simulated within the Monte-Carlo approach by an incomplete statistical average. We find strong surface effects for small nanoparticles, which cannot be explained within a simple Landau or mean-field concept and which give rise to characteristic trends of the spin-correlation function in different temperature regimes. Unambiguous definitions of crossover temperatures for finite systems and an effective method to estimate the critical temperature of corresponding infinite systems are given.

Crossover from conventional to inverse indirect magnetic exchange in the depleted Anderson lattice

“…We nevertheless expect that the finite T C predicted by DMFT is physically significant even for D = 1 (and D = 2) and indicates the onset of ferromagnetic ordering of the magnetic moments on intermediate length scales. 44 This corresponds to a thermodynamically stable ferromagnet only if the SU(2) symmetry is broken explicitly, e.g., due to the presence of additional anisotropic terms in the Hamiltonian.…”

Laser spectroscopy of finite size and covering effects in magnetite nanoparticles

“…This approach enables to obtain rather accurate quantitative results and is widely used in nanophysics now. In particular, a number of calculations for magnetically uniform clusters have been carried out [27][28][29][30][31]. The investigation of the surface effects in the magnetic NPs FePt was presented by Labaye et al [32]; the authors considered the effect of the surface anisotropy on an isolated single-domain spherical NP using atomic Monte Carlo simulation of the low-temperature spin ordering.…”

“…This approach enables one to obtain rather accurate quantitative results and is widely used in nanophysics now. In particular, a number of calculations for magnetically uniform clusters have been carried out [27][28][29][30][31].…”