Spin wave impurity states in linear chain ferromagnets and antiferromagnets

A simple linear chain model is used to find the perturbed states in Heisenberg ferromagnets and antiferromagnets with a magnetic impurity. The defect is characterized by a different nearest neighbor exchange coupling, spin, anisotropy field and gyromagnetic ratio from the corresponding parameters for the host lattice. A semiclassical calculation demonstrates that for some values of the defect parameters, resonant states in the spin wave band can arise near the bottom of the band, while for other values, localized states will occur. The various physical conditions for the occurrence of those states are given. When an applied magnetic field is included in the model calculation, a gap mode instability can occur for some values of the gyromagnetic ratio of the impurity spin. For some combinations of defect parameters, a local mode can be transformed into a resonant mode and vice versa by the magnetic field. The impurity-induced absorption is also calculated.     

Spin wave excitations in amorphous itinerant ferromagnets II. Dynamical susceptibility

The Izuyama, Kim, and Kubo theory of dynamical susceptibility in crystalline itinerant ferromagnets is extended to the amorphous itinerant ferromagnets. Effects of amorphousness in spin wave spectrum, given by the poles of the susceptibility, is discussed.     

Spin wave theory of strongly anisotropic magnets

A strong anisotropy gives rise to a non-spherical precession of the spins with different amplitudes in the x and y directions. The highly anharmonic exchange interaction thereby becomes effectively anisotropic. The possibility of detecting a genuine two-ion anisotropy is discussed, and comments are given on the interpretation of the spin wave data in Tb and Er.     

Spin waves in quantum ferromagnets

Low-temperature properties of Heisenberg quantum ferromagnets (spin waves) are derived within aconfiguration space formalism. Most of the work is done without explicitly assuming translational invariance. We provide a general criterion, classical domination, to decide about the nature and uniqueness of ground states for a large class of quantum ferromagnets. We also analyze and clarify the Dyson formalism and indicate why an energy gap between the physical ground state and the improper (unphysical) states does not exist. This is of particular relevance to the kinematical interaction. Using reflection positivity we provide upper and lower bounds to the contribution of the dynamical interaction to the free energy. In a certain approximation, these bounds imply that the dynamical interaction may be dropped if the inverse temperature and the spin quantum numberS are large enough.Suported in part by FAPESP.Supported in part by CAPES.Partial support by FAPESP and CNPq.     

Rotationally invariant theory of magnetoacoustic wave attenuation in cubic ferromagnets

Rotationally invariant theory is used to study the effect of magnetization relaxation on the spectrum of magnetoacoustic waves propagating in a cubic ferromagnet with an induced uniaxial anisotropy along the [011] direction. It is found that the inclusion of rotationally invariant terms leads to certain contributions to the propagation velocity and rate of attenuation of magnetoelastic waves, thus increasing the degree of anisotropy of these characteristics. Among different types of coupled waves, only quasi-sound modes exhibit a relaxation nature in the region of magnetic phase stability loss. The introduction of dissipation and rotational invariance affects the acoustic birefringence, while the latter gives rise to an additional term in the expression for the phase shift.     

Spin waves in the easy-plane ferromagnets

The modified version of the spin operator diagram technique is presented with regard to the dynamical properties of the easy-plane ferromagnets. The non-interacting spin wave (SW) spectrum and scattering amplitudes are obtained with the help of the spin Hamiltonian diagonalization procedure⁸). The SW relaxation frequencies due to the processes of the SW scattering on each other and on the longitudinal spin components thermal fluctuations are calculated.     

Spin tunneling in junctions with disordered ferromagnets

We provide compelling evidence to establish that, contrary to one's elementary guess, the tunneling spin polarization (TSP) of amorphous CoFeB is larger than that of fcc CoFeB. First-principles atomic and electronic structure calculations reveal striking agreement between the measured TSP and the predicted s-electron spin polarization. Given the disordered structure of the ternary alloy, not only do these results strongly endorse our communal understanding of tunneling through AlO(x), but they also portray the key concepts that demand primary consideration in such complex systems.     

A theory of double exchange in infinite dimensions

This paper gives a simplified model of the double exchange which is a kind of indirect exchange interaction between localized magnetic moments. The presented model is solved exactly in the case of infinite - dimensional space. Equations for single-particle Green's function and magnetization of the localized spins subsystem are obtained. It is shown that our simple double exchange model reveals an instability to the ferromagnetic ordering of localized moments. Magnetic and electric properties of this system on Bethe lattice with are investigated in detail. Received: 24 January 1997 / Revised: 14 February 1997 / Received in final form: 18 August 1997 / Accepted: 25 August 1997     

Spin chirality fluctuations and anomalous Hall effect in itinerant ferromagnets

The anomalous Hall effect due to the spin chirality order and fluctuation is studied theoretically in a Kondo lattice model without the relativistic spin-orbit interaction. Even without the correlations of the localized spins, sigma(xy) can emerge depending on the lattice structure and the spin anisotropy. We reveal the condition for this chirality-fluctuation driven mechanism for sigma(xy). Our semiquantitative estimates for a pyrochlore oxide Nd2Mo2O7 give a finite sigma(xy) approximately equal 10 Omega(-1) cm(-1) together with a high resistivity rho(xx) approximately equal 10(-4)-10(-3) Omega cm, in agreement with experiments.     

Some new aspects of the theory of the double exchange mechanism

The energy spectrum of the Anderson-Hasegawa molecule has been considered. Analysis of the peculiarities of the Spectrum has allowed us to show that the Curie-Weiss law holds for the initial magnetic susceptibility in the range of both very and moderately high temperatures. At very high temperatures the Curie paramagnetic temperature is determined solely by the intratomic exchange integral of a conduction electron with localized electrons, whereas at moderate temperatures by the ratio of the square of the transfer to the intratomic exchange integral. A substantiation is given for the Heisenberg effective exchange operator whose eigenvalues approximate the molecule spectrum. The Heisenberg hamiltonian is well founded in two senses, viz, in the statistical and in the sense of eigenvalues. In both cases the effective exchange coupling of two atomic spins is of an antiferromagnetic nature. It is shown that, with the intratomic exchanges integral being infinite, the transitions of an electron from atom become impossible, the electron is localized in an accidental manner on one of the atoms, and any exchange coupling between the spins of different atoms completely disappers.     

Green's-function theory for disordered anisotropic Heisenberg ferromagnets

The Green's function theory for disordered Heisenberg Ferromagnets developed by Montgomery et al. has been extended for anisotropic systems. The densities of spin-wave states for such systems exhibit split-bands. This behavior differs qualitatively from that of the isotropic case and offers a partial explanation for the neutron scattering experiment of Buyers et al. in which they observe two well defined branches of magnetic excitations in disordered substitutional anti-ferromagnetic alloys.     

Spin waves in itinerant ferromagnets with hund's coupling

Spin waves in Zener-like model of ferromagnetism with the intra-atomic Hund's coupling are considered. Dispersion relations for both acoustical and optical branches are calculated. The spin-wave effective mass for the acoustical branch in the case of Ni and Fe is discussed and a qualitatively agreement with experimental data is found.     

Spin exchange in iodine-doped polydiacetylene

It is shown that the observed transformation of the ESR spectrum in polydiacetylene films upon doping with iodine agrees with an exchange interaction between the spin systems of localized paramagnetic centers (S) and polaron states (P) by means of which current-carrier transport occurs. The data obtained indicate carrier transport in the form of uncorrelated hops with probability determined by the lifetime of an exchange-coupled (S-P) pair. Fiz. Tverd. Tela (St. Petersburg) 41, 1879–1881 (October 1999)     

Relativistic multiple scattering theory of electrons by ferromagnets

In view of investigating for 3d- and 4f-ferromagnetic crystals the elastic scattering and the emission of polarized low-energy electrons as well as the bulk electronic structure, a relativistic multiple scattering theory has been developed, in which the exchange interaction with the magnetic ground state electrons is treated in a local density functional approximation. In a layer-KKR-type approach, the Dirac equation, which contains an effective magnetic field term, is first solved for a single crystal atom and subsequently for a monoatomic layer and for a semi-infinite crystal. Spin-orbit coupling and magnetic exchange interaction are thus simultaneously taken into account.     

Phenomenological theory of spin transport in molecular-field ferromagnets

The effects of electron-lattice collisions on the nonuniform transverse susceptibility of an itinerant ferromagnet are found from the quantum-statistical equations of motion in the molecularfield approximation. The effects of pure orbital collisions and the corresponding “diffusion” correction are found to be negligible, in agreement with the conclusions of P. Fulde and A. Luther. The effect of magnetic (spin-orbital) collisions is included in the Gilbert damping and does not contribute to the “diffusion” correction. The form of the susceptibility and of the corresponding macroscopic equation of motion, and also the contradictions between this and a number of previous studies are discussed in detail.