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

Properties of semi-infinite (S = 1) Heisenberg ferromagnet with biquadratic exchange were studied in terms of surface exchange (epsilon = I-S/I) and b iquadratic coupling (a). It was shown that a strict correlation exists, dep ending on epsilon, between the type of surface spin waves (acoustic or opti cal) 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-Ginsbur g theory for semi-infinite simple cubic ferromagnet, a detailed study is pr esented of the critical behaviour of the system, in particular in the vicin ity of the tricritical point which is the consequence of the biquadratic in teraction. It is shown that tricritical exponents satisfy exactly the scali ng relations for d = 3. The analysis of the spin-spin correlation function within the framework of the same theory, shows that there occurs the critic al magnetic scattering of low-energy electrons (LEED) from the surface in t he 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 a lso shown that in the vicinity of the tricritical point, the biquadratic in teraction increases the range of validity of the MF approximation. (C) 2000 Elsevier Science B.V. All rights reserved.