Abstract
The surface scaling theory previously presented by the authors is developed further, and derived heuristically from a cluster model. Monte Carlo calculations are carried out to obtain the spatial and temperature dependence of the magnetization in Ising and Heisenberg systems with free surfaces. The exponent of the (surface) layer magnetization is shown to agree with the scaling value () previously derived. In the Heisenberg system, the results at low temperature agree with a spin-wave calculation by Mills and Maradudin. Ising models with modified exchange on the surface are considered, both in mean-field theory and by means of high-temperature-series expansions. The critical value for surface ordering is found from the series to be 0.6, compared to the mean-field value of 0.25. For there is a temperature region in which the surface behaves like a bulk two-dimensional Ising model near its phase transition. The critical exponents experience a crossover at , which is reflected in poorly behaved series, and effective exponents differing from the true ones for . In the case of weakened surface exchange (), the layer magnetization is shown to fit a linear temperature dependence over a large temperature range below , thus providing a possible explanation for previous experiments. For sufficiently strong negative , mean-field theory predicts that the surface will order antiferromagnetically while the bulk is ferromagnetic.
- Received 10 August 1973
DOI:https://doi.org/10.1103/PhysRevB.9.2194
©1974 American Physical Society