The effect of a surface on the dynamic and thermodynamic properties of S=1 Heisenberg ferromagnet with biquadratic exchange

摘要

Properties of semi-infinite (S = 1) Heisenberg ferromagnet with biquadratic exchange were studied in terms of surface exchange (epsilon = I-S/I) and biquadratic coupling (a). It was shown that a strict correlation exists, depending on epsilon, between the type of surface spin waves (acoustic or optical) and the mean-field (MF) critical temperature, bulk (T-c) and surface T-c(S) > T-c (for epsilon > 5/4). Within the Framework of the Landau-Ginsburg theory for semi-infinite simple cubic ferromagnet, a detailed study is presented of the critical behaviour of the system, in particular in the vicinity of the tricritical point which is the consequence of the biquadratic interaction. It is shown that tricritical exponents satisfy exactly the scaling relations for d = 3. The analysis of the spin-spin correlation function within the framework of the same theory, shows that there occurs the critical magnetic scattering of low-energy electrons (LEED) from the surface in the case epsilon greater than or equal to 5/4, when the ordering temperature T-c(S) is approached from above (from paramagnetic phase). In the opposite case, epsilon < 5/4, there occurs no surface critical scattering. It was also shown that in the vicinity of the tricritical point, the biquadratic interaction increases the range of validity of the MF approximation. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 17]