A Study of RS-LMTO-ASA on Ni and the CurieTemperature of Ni Thin Films

A convergence study was made on the exchange constants of Ni with respect to computational parameters in RS-LMTO-ASA. Specifically the convergence was examined with respect to structure constant cutoff distance, system size and recursion depth. It was found that the exchange parameters converge for all three. The system converges quickly for the cutoff distance, in accordance with the use of tight binding parameters. For the system size fluctuations are apparent up to a threshold value. Interestingly while the exchange parameters converge for recursion depth study of the magnon dispersion show that there exists a sweetspot before convergence where the calculated properties match experiments best. However examination of the density of states showed that this is likely coincidental. Using the exchange parameters calculated from RS-LMTO-ASA a metropolis Monte carlo study was made of thin film Ni examining the change of Curie temperature with respect to thickness and magnetic interaction length. It is found in accordance with earlier experimental work that the Curie temperature decreases with thickness in accordance with finite size scaling theory and that this is independent of the stacking. It is also found that including longer magnetic interactions gives a stronger decrease witch mathces the experimental results better, as found in earlier theoretical work on a model system. With regard to the shift exponent it is however found that it is not appropriate to calculate it using a least squares regression to the reduced Curie temperature data. This is due to a high dependence on where one places the thick film limit. As no theoretical predictions exists for this limit the shift exponents extracted through this method are arbitrary.