E. F. Sarmento

Spin waves in a magnetic superlattice

Abstract A superlattice consisting of alternating layers of two simple-cubic Heisenberg ferromagnets is considered. The dispersion equation for spin waves propagating in a general direction in the superlattice is derived by the transfer-matrix method. The result is illustrated numerically; the curves are analogous to those found for phonons in a diatomic superlattice.

Phase transitions of ferromagnetic Ising films in a transverse field

Abstract The critical behaviour of an Ising thin film is studied by use of an effective field theory that includes correlations. Surface exchange enhancement within the surface and between the surface and the nearest neighbouring layer is included. The effect of transverse fields at the surface and in the bulk of the film on the phase diagrams is investigated. An exact expression for the critical exchange coupling leading to a Curie temperature higher than that for the infinite bulk material is obtained. The thickness dependence of the critical temperature is examined, and the reason for the conflicting results that already exist in the literature for the zero-field case resolved. The transverse field dependence of the critical temperature is studied, and contact with previous results obtained for a semi-infinite slab is made in the limit that the film thickness L → §.

Ferromagnetic Ising films with surface dilution in a transverse field

Abstract The effect of surface dilution on the critical behaviour of a ferromagnetic Ising film in transverse surface and bulk fields is studied within the framework of an effective field theory based on a single-site cluster method. The method preserves the single-site kinematic relations for the spin operators, and the configurational averaging includes correlations between the spatial disorder and the local configurational-dependent thermal averages of the spin variables. Surface exchange enhancement (or reduction) is considered. The dependence of the critical temperature on film thickness, and phase diagrams in the fields, exchange interaction strength ratio, and dilution parameter space, are presented.

Phase diagrams of the spin-one transverse Ising model

Abstract An effective field theory is proposed and used to derive the phase diagrams of the spin-1 Ising model in the presence of an applied transverse field. The lines of critical points in the T−Ω plane are analyzed for several lattice structures and compared with the standard mean field (MFA) predictions. The spin-1 phase boundary line is also compared with that of spin- 1 2 for the honeycomb lattice. In spite of its simplicity, the present formalism yields results which represent a remarkable improvement on the usual MFA treatment.

Thermodynamical properties of the spin-one transverse Ising model

A new type of effective-field theory that has recently been used with success for many applications concerning the two-state transverse Ising model (spin-12 TIM), is herein extended to the spin-1 TIM. The method, which can explicitly and systematically include correlation effects, is illustrated by employing its simplest approximation version, in which multispin correlations are neglected. The lines of critical points in the T-Ω plane as well as the thermal behavior of all relevant statistical-mechanical quantities are analysed for several lattice structures and compared with the standard mean-field predictions. It is shown that the present formalism, in spite of its simplicity, yields results quite superior to those currently obtained within the molecular-field approximation.

Ferromagnetic phase breakdown of a quenched bond-mixed Ising model

Abstract Within the framework of an effective field theory beyond Mean Field Approximation, we discuss the ferromagnetic phase stability limit in the temperature-concentration space of a quenched bond-mixed spin- 1 2 Ising model in square lattice for both competing and noncompeting interactions J1 and J2. Quite reasonable results are obtained in both situations. In particular for the case of competing interactions, numerical estimates of the vanishing temperature critical bond concentrations are predicted for particular values of the ratio J 1 J 2 .

Quenched bond-dilute Ising ferromagnet in square lattice: Thermodynamical properties

Within an effective field framework which substantially improves the Molecular Field Approximation, we calculate the phase diagram, magnetization, specific heat and susceptibility associated with the quenched bond-dilute Ising ferromagnet in square lattice. The results are qualitatively (and within certain extent quantitatively) satisfactory; in particular the effects, on the specific heat and susceptibility, of the (eventually) coexisting finite and infinite clusters are exhibited.

Surface ferromagnetism induced by surface amorphization in a layered antiferromagnetic system

Abstract A model layered antiferromagnetic system is proposed as a good candidate for finding the surface ferromagnetism induced by a surface amorphization. The transition temper a tures of the surface, second and bulk layers are obtained by the use of a new-type effective field theory.

Magnetic properties in the Ising mixed spin‐1/2–spin‐1 superlattice

An effective‐field theory that accounts for the self‐spin correlation function is used to derive the phase diagrams and magnetization curves of a magnetic superlattice, consisting of a mixed spin‐ 1/2–spin‐1 (with a single‐ion anisotropy D) alternate monolayers of ferromagnetic material. It is found that the phase diagrams exhibit a number of interesting phenomena resulting from the dependence of the crystal‐field interaction. Both limit cases for very large positive and very large negative D reproduce respectively the T(3D)=5.07 and the T(2D)=3.08 Ising critical temperatures which are in agreement with earlier results. On the other hand, in particular, no tricritical point could be found. The magnetization curves are presented for a range of interaction strengths and D, respectively.

A quenched bond-mixed Ising model: Ferromagnetic phase stability limit

Abstract Within the framework of an effective field theory we discuss the ferromagnetic phase stability limit of a quenched bond-mixed spin fr12 Ising model in a simple cubic lattice. Whenever comparison is possible a satisfactory qualitative and to a certain extent quantitative agreement is observed.