https://doi.org/10.1140/epjb/e2011-11015-5
Spin-spin correlations in ferromagnetic nanosystems
1
University of Hamburg, Institute for Applied Physics, Jungiusstr. 11a, 20355 Hamburg, Germany
2
Instituto Madrileño de Estudios Avanzados en Nanociencia, IMDEA-Nanociencia,
Campus Universidad Autónoma de Madrid, 28049 Madrid, Spain
3
University of Hamburg, I. Institute for Theoretical Physics, Jungiusstr. 9, 20355 Hamburg, Germany
Corresponding author: a vedmeden@physnet.uni-hamburg.de
Received:
27
December
2010
Revised:
13
February
2011
Published online:
16
March
2011
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 nearest-neighbor 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.
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2011
