Microscopic theory of non local pair correlations in metallic F/S/F trilayers

We consider a microscopic theory of F/S/F trilayers with metallic or insulating ferromagnets. The trilayer with metallic ferromagnets is controlled by the formation of non local pair correlations among the two ferromagnets which do not exist with insulating ferromagnets. The difference between the insulating and ferromagnetic models can be understood from lowest order diagrams. Metallic ferromagnets are controlled by non local pair correlations and the superconducting gap is larger if the ferromagnetic electrodes have a parallel spin orientation. Insulating ferromagnets are controlled by pair breaking and the superconducting gap is smaller if the ferromagnetic electrodes have a parallel spin orientation. The same behavior is found in the presence of disorder in the microscopic phase variables and also in the presence of a partial spin polarization of the ferromagnets. The different behaviors of the metallic and insulating trilayers may be probed in experiments. Received 4 July 2001 and Received in final form 8 November 2001     

Critical temperature oscillations and reentrant superconductivity due to the FFLO like state in F/S/F trilayers

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde‐Ferrell Larkin‐Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu₄₁Ni₅₉/Nb/Cu₄₁Ni₅₉ trilayers investigated in the present work.     

Effect of the variation of the exchange energy on the superconducting critical temperature of S/F/S trilayers

The effect of the exchange energy variation in weakly ferromagnetic alloys on the superconducting resistive transition of superconductor/ferromagnet/superconductor (S/F/S) trilayers is studied. Critical temperature, T _c , and resistive transitions versus the F-layer thickness, d _F , have been analyzed in Nb/Cu_0.41Ni_0.59/Nb and Nb/Pd_0.81Ni_0.19/Nb trilayers. We show that T _c (d _F ) dependence is sensitive to magnetic inhomogeneities in the F-layer for values of d _F corresponding to thickness range where the π-superconducting state is established.     

Magnetic properties of Fe/ZnSe/Fe trilayers

The magnetic properties of Fe/Zn/Fe trilayers have been studied by ferromagnetic resonance and magnetization measurements. These measurements have been used to investigate the magnetic anisotropy of the iron layers and the magnetic coupling across the semiconductor spacer. The angular dependence of the resonance spectra has been measured in-plane and out-of-plane in order to deduce magnetic anisotropy constants of the samples. Experimental data were fitted by using an energy-density expression that includes bulk cubic anisotropy, growth-induced uniaxial in-plane anisotropy and perpendicular-surface anisotropy terms. A small ferromagnetic coupling is observed in the trilayers with spacer thickness up to .     

Magnetic properties of Co-P-based trilayers

The magnetic properties of a film trilayer consisting of hard magnetic and soft magnetic Co-P layers separated by a nonmagnetic Ni-P spacer have been studied. The features of the hysteresis shift relative to the zero exchange magnetic field and of the coercivity of the soft magnetic layer in the dependence on thicknesses of the hard magnetic layer and spacer have been considered. The dynamic changes in the shift of the hysteresis of the soft magnetic layer versus the magnetization reversal time after saturation of the hard magnetic layer have been found and investigated.     

Dynamics of domain walls in weak ferromagnets with quadratic magnetoelectric interaction

The dynamics of domain walls of the ab type in weak ferromagnets with a quadratic magnetoelectric interaction has been studied in ac magnetic and electric fields. The specific features of the vibrational and drift motions of the domain walls as functions of parameters of the external fields and characteristics of the material have been discussed. The possibility of drift occuring the domain walls in a purely electric field has been predicted.     

Microscopic theory of optical properties of crystal surfaces

Light-propagation equations at crystal surfaces are solved in the long-wavelength limit, fully accounting for surface anisotropy, inhomogeneity and non-locality of the dielectric susceptibility tensor. Local-field and excitonic effects are also accounted in principle. Equations allowing calculation of internal and external reflectivity coefficients are given and used to analyze experimental data.     

Formation of inhomogeneous magnetic structures in weak ferromagnets with rhombohedral symmetry

A model of formation of inhomogeneous magnetic structures in weak ferromagnets with rhombohedral symmetry is proposed. This model is based on the general theory of ferromagnetism in these compounds. The quantitative calculations of the dependence of the period of magnetic inhomogeneities on the parameters of the samples are presented and compared with experimental data.     

Modulated magnetic phase of structurally heterogeneous easy-plane weak ferromagnets

The modulated magnetic phase of a structurally heterogeneous easy-plane weak ferromagnet is considered in terms of the thermodynamic Landau theory of phase transitions. The temperature and field dependences of the main magnetic order modulation parameters are determined. The results obtained are compared with the experimental data obtained for the orientational phase transition into a modulated magnetic state that occurs in hematite and iron borate crystals doped with diamagnetic ions to create structural heterogeneity. The proposed theoretical model is shown to describe the entire set of experimental results consistently with some exceptions.     

Retardation of a moving domain wall in weak ferromagnets

The retardation of a single domain wall in its motion at subsonic and supersonic velocities in YFeO₃ and FeBO₃ plates is investigated using the magneto-optical Faraday effect. The experimental results are discussed with due regard for the interaction of the magnetic and elastic subsystems of the crystal.     

Domain wall structure of weak ferromagnets according to Raman

Visualizing the domain structure and the fine structure of domain walls in orthoferrites based on Raman was proposed. The Raman mapping imaging was obtained for the straight and curved domain wall at line 221 cm⁻¹. The parameters of the domain structure and wall obtained by Raman are consistent with magnetooptical measurements.     

Perturbation theory for equilibrium properties of molecular fluids

Perturbation theory for the angular pair correlation function g(r₁₂ ω ₁ ω ₂), using a fluid with isotropic intermolecular forces as the reference system, is applied to the calculation of a variety of macroscopic properties. Comparisons with experiment are made for methane, oxygen and nitrogen (and carbon monoxide for infra-red and Raman band moments) in the dense fluid and liquid states. Theoretical expressions are given and calculations made for thermodynamic properties (isothermal compressibility, pressure, configurational energy, entropy and specific heat) both along and away from the vapour-liquid co-existence curve, for infra-red and Raman band moments, and for neutron scattering cross sections. Excellent agreement with experiment is obtained for all properties, except for the infra-red and Raman band moments; this latter comparison is inconclusive because of large experimental uncertainties. The anisotropic intermolecular forces are found to have very little effect on the liquid isothermal compressibility, in agreement with the first-order theory. Molecular anisotropy has a relatively small effect on the configurational energy and on the Helmholtz free energy, but the effect is large for pressure and specific heat. The pressure is more sensitive to short-range anisotropic forces than the other properties, whereas the specific heat is particularly sensitive to the long-range anisotropic forces. Mean squared torques (derived from infra-red and Raman band moments) are very sensitive to the strengths of the anisotropic forces, and are more sensitive to higher terms in the multipole series than are the other properties. The structure factors for oxygen and nitrogen are found to be little affected by the anisotropic forces.     

Microscopic theory of scattering of weak electromagnetic radiation by a dense ensemble of ultracold atoms

Based on the developed quantum microscopic theory, the interaction of weak electromagnetic radiation with dense ultracold atomic clouds is described in detail. The differential and total cooperative scattering cross sections are calculated for monochromatic radiation as particular examples of application of the general theory. The angular, spectral, and polarization properties of scattered light are determined. The dependence of these quantities on the sample size and concentration of atoms is studied and the influence of collective effects is analyzed.     

Microscopic phonon theory of Si/Ge nanocrystals

Microscopic phonon theory of semiconductor nanocrystals (NCs) is reviewed in this paper. Phonon modes of Si and Ge NCs with various sizes of up to 7 nm are investigated by valence force field theory. Phonon modes in spherical SiGe alloy NCs approximately 3.6 nm (containing 1147 atoms) in size have been investigated as a function of the Si concentration. Phonon density-of-states, quantum confinement effects, as well as Raman intensities are discussed.      

Crystal-field model of invar effects in weak and strong ferromagnets

This paper is principally concerned with the role of crystal fields in Invar-type behaviour. It is divided into two sections. The first seeks to examine the connection between Invar effects and weak ferromagnetism by the use of a simple crystal field model with volume-dependent exchange. By varrying the ratio of the exchange energy to the field splitting it is possible to reproduce the characteristic behaviour of weak and strong ferromagnets and exchange-enhanced paramagnets. The second part discusses some recent experimental results which are closely related to the theoretical model.     

Spin wave theory of double exchange ferromagnets

We construct the 1/S spin wave expansion for double exchange ferromagnets at T = 0. It is assumed that the value of Hund's rule coupling, J(H), is sufficiently large, resulting in a fully saturated, ferromagnetic half-metallic ground state. We evaluate corrections to the magnon dispersion law, and we also find that, in contrast to earlier statements in the literature, magnon-electron scattering does give rise to spin wave damping. We analyze the momentum dependence of these quantities and discuss the experimental implications for colossal magnetoresistance compounds.     

Thermodynamic properties of ferromagnets with uniaxial and biaxial anisotropy

The thermodynamic properties of spin-one Heisenberg ferromagnets with uniaxial and biaxial anisotropy are investigated in the molecular field approximation (MFA) and random phase approximation (RPA). In MFA a full phase diagram comprising three lines of bicritical points, is obtained.Using the standard-basis operator method, the collective excitation spectrum is studied in detail; moreover the softening of the excitations at the phase boundaries is discussed. A self-consistent version of RPA with emphasis on the role of kinematic restrictions with regard to the standard-basis operators is analyzed.Moreover, the phase transitions are considered in the Ising model, where a line of tricritical points occurs, and the planar model, where a line of bicritical points, two lines of tricritical points and a line of triple points, occur.