Critical properties of Heisenberg ferromagnets with lattice inhomogeneity

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Abstract
Two types of lattice inhomogeneity were studied by self-consistent Monte Carlo calculations. The critical isotherm of a system with a free surface can be described by a power law m1 approximately h1 delta 1/ for the surface-layer magnetization with a critical exponent delta 1=2.3+or-0.1, fulfilling the recently proposed scaling relations satisfactorily. A homogeneous system with a missing cluster of 23 spins was investigated in zero field. The critical behaviour of the local order parameter m1 approximately (-t)beta 1 close to this defect shows a changeover for beta 1:0.331<or approximately=0.41 near (-t) approximately=0.05. This may be important for the interpretation of Mossbauer and NMR experiments.