M. Škrinjar

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

Properties of semi-infinite (S=1) Heisenberg ferromagnet with biquadratic exchange were studied in terms of surface exchange (e=IS/I) and biquadratic coupling (a). It was shown that a strict correlation exists, depending on e, between the type of surface spin waves (acoustic or optical) and the mean-field (MF) critical temperature, bulk (Tc) and surface TcS>Tc (for e>54). 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 e⩾54, when the ordering temperature TcS is approached from above (from paramagnetic phase). In the opposite case, e<54, 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.

On the mean-field theory of magnetic multilayers with bilinear and biquadratic Heisenberg exchange

A system consisting of several layers of magnetic ions interacting by both bilinear and biquadratic Heisenberg exchange is studied within the framework of the mean-field approximation. It is shown that for S=1 there exist two types of ordering: ferromagnetic and ferroquadrupolar. The stability of phases as the function of temperature, biquadratic exchange and surface exchange is discussed analytically and numerically and it was shown that similar to bulk samples there appear first- and second-order transitions and a tricritical point may appear depending on system parameters.

New approach to surface magnon states in the semi-infinite antiferromagnetic Heisenberg model with biquadratic exchange and single-ion anisotropy

Abstract We present a new approach to the study of excitations in a semi-infinite antiferromagnet described by the Heisenberg Hamiltonian with biquadratic exchange and single-ion anisotropy.

INFLUENCE OF TRANSLATIONAL SYMMETRY OF FERROMAGNETIC SUPERLATTICES IN THE STUDY OF ELEMENTARY EXCITATIONS

Elementary excitations in ferromagnetic superlattices are analyzed theoretically using the transfer matrix method. The method is extended to enable the study of the cases when the boundaries are not consisting of a single plane, but of a complete cell. In this way, semi-infinite superlattice, the interface between two superlattices and the film built of a superlattice are treated. However, it is shown that the choice of the transfer matrix strongly depends on the translational symmetry of the remaining part of the system. Energies of localized excitations are derived and compared both with bulk and the results of standard transfer matrix treatment proposed by Barnas. Also, a special Ansatz for spin amplitudes is proposed leading to identical results with the transfer matrix method and introducing large simplifications.

THE INFLUENCE OF BIQUADRATIC COUPLING AND UNIAXIAL ANISOTROPY ON THE SURFACE MODES IN A SIMPLE CUBIC ANTIFERROMAGNET

Abstract We study the surface spin excitations in a simple cubic two-sublattice Heisenberg antiferromagnet with both biquadratic coupling (a) and uniaxial anisotropy (D, DS). The combined effect is a lowering or narrowing of the energies, and changes of the range of the system parameters for which these modes can exist. For certain values of the system parameters, the surface antiferromagnetic resonance (SAFMR) mode at the Brillouin zone center can be well resolved from the bulk AFMR mode.

Magnetic excitations in the semi-infinite Heisenberg model with biquadratic exchange

Abstract We study the excitations in the semi-infinite ferromagnet described by the Heisenberg Hamiltonian with biquadratic exchange and single-ion anisotropy. The spectrum of excitations is shown to be more complex than the one previously discussed.

DEPENDENCE OF THE DAVYDOV'S SOLITON BEHAVIOR IN THE LINEAR POLYMER CHAIN ON THE EXCITON{PHONON COUPLING

The Davydovs soliton propagation in the linear polymer chain is analyzed numerically by solving the discrete equations of motion for exciton and phonon amplitudes. The main difference with respect to the results of continual approximation is that two exciton–phonon coupling constants influence separately the soliton behavior. Their influence is studied both in the ideal chain and the chain with single impurity.

TRANSFER-MATRIX APPROACH TO ELEMENTARY EXCITATIONS IN SEMI-INFINITE FERROMAGNETIC SUPERLATTICE WITH IMPURITY CELL

A transfer matrix formalism is employed to study the occurrence of the localized modes associated with an impurity cell embedded within a semi-infinite Heisenberg ferromagnetic superlattice with nearest-neighbor exchange interactions. The cell is at arbitrary distance from the deformed surface cell. We find that, in addition to bulk spin waves, there may arise localized modes associated both with the impurity cell and with the surface. Two situations were analyzed numerically: impurity is close to the surface and the impurity is practically in the bulk region. The modes influence each other when the impurity cell is near the surface, modifying spin wave energies.

The analytic solution for the spectrum of surface spin waves for two-sublattice Heisenberg ferrimagnet

Using a simplified procedure we find the general analytic solution for the spectrum of the surface spin excitations in two-sublattice Heisenberg ferrimagnets in the closed form. We obtain simultaneously two bulk energy bands and also one surface excitation branch associated with each bulk band. These surface excitation branches could be either optical or acoustic, depending on the ratio of surface to bulk exchange integral η = JS /J . Our results include as a special case the results for the purely geometrical surface effect (η = 1), previously known from the literature.  2001 Elsevier Science B.V. All rights reserved.

On the relevance of self-trapping as the mechanism for charge and energy transfer in biological systems

Abstract The concept that polarons and solitons may play a crucial role as the underlying transport agents in biological structures has been critically re-examined on the basis of the general theory of self-trapping (ST) phenomena. The conditions determining the existence and stability of polarons and solitons, and their character as a function of the values of the basic physical parameters of the system, were formulated. It was found that Davydovs soliton model cannot explain the intramolecular energy transfer in the α-helix and acetanilide, and an alternative formulation in terms of the so-called Takeno model is more realistic. However, Davydovs theory is sufficiently flexible to describe electron transfer in the α-helix and acetanilide.