Ordered phase of nuclear spins in uniaxial ferromagnets at ultralow temperatures

Paper investigates the onset of nuclear magnetic ordering caused by the indirect Suhl-Nakamura interaction in ferromagnets. The necessary condition for nuclear spin ordering with definite ordering vector is obtained. Particularly, it is shown that ferromagnetically ordered phase of nuclear spins could be observed only in case of disk shaped samples. The spectrum of the nuclear spin excitations is also found. Received 25 January 1999 and Received in final form 5 May 1999     

On evaluation of transverse spin fluctuations in quantum magnets

A numerical method is described for evaluating transverse spin correlations in the random phase approximation. Quantum spin-fluctuation corrections to sublattice magnetization are evaluated for the antiferromagnetic ground state of the half-filled Hubbard model in two and three dimensions in the whole U/t range. Extension to the case of defects in the AF is also discussed for spin vacancies and low-U impurities. In the limit, the vacancy-induced enhancement in the spin fluctuation correction is obtained for the spin-vacancy problem in two dimensions, for vacancy concentration up to the percolation threshold. For low-U impurities, the overall spin fluctuation correction is found to be strongly suppressed, although surprisingly spin fluctuations are locally enhanced at the low-U sites. Received 27 April 1998 and Received in final form 13 August 1998     

Spin-wave contributions to nuclear magnetic relaxation in magnetic metals

The longitudinal and transverse nuclear magnetic relaxation rates 1/T ₁(T) and 1/T ₂(T) are calculated for three- and two-dimensional (3D and 2D) metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic moments in the spin-wave temperature region. The contribution of the one-magnon decay processes is strongly enhanced in comparison with the standard T-linear Korringa term, especially for the FM case. For the 3D AFM case this contribution diverges logarithmically, the divergence being cut at the magnon gap ω due to magnetic anisotropy, and for the 2D AFM case as ω^-1. The electron-magnon scattering processes yield T ²ln(T/ω) and T ²/ω^1/2-terms in 1/T ₁ for the 3D AFM and 2D FM cases, respectively. The two-magnon (“Raman”) contributions are investigated and demonstrated to be large in the 2D FM case. Peculiarities of the isotropic 2D limit (where the correlation length is very large) are analyzed. Received 29 November 1999 and Received in final form 6 June 2000     

Quantum spherical model in a random field: coherent state path integral approach

By introducing boson operators, a quantum spherical XY model in the presence of a random field has been studied by the coherent state path integral approach. The phase diagram is obtained, and the effects of the random-field fluctuations on the possibilities of the existence of a ferromagnetic phase are discussed. At the critical point, , the order parameter M describing the ordered ferromagnetic phase disappears as .Since the model is equivalent to a Bose system, we also show that the phase transition at zero temperature between the superfluid and the disordered Mott insulator phases occurs at the chemical potential , where J₀ is the strength of the exchange interaction. As the temperature T goes to zero, the asymptotic behavior of the entropy and the specific heat are and , respectively. Received: 20 May 1997 / Accepted: 20 October 1997     

The 1D spin- AF-Heisenberg model in a staggered field

We investigate the scaling properties of the excitation energies and transition amplitudes of the one-dimensional spin- antiferromagnetic Heisenberg model exposed to an external perturbation. Two types of perturbations are discussed in detail: a staggered field and a dimerized field. Received: 27 February 1998 / Accepted: 17 April 1998     

On the incommensurate phase in modulated Heisenberg chains

Using the density matrix renormalization group method (DMRG) we calculate the magnetization of frustrated S=1/2 Heisenberg chains for various modulation patterns of the nearest neighbour coupling: commensurate, incommensurate with sinusoidal modulation and incommensurate with solitonic modulation. We focus on the order of the phase transition from the commensurate dimerized phase (D) to the incommensurate phase (I). It is shown that the order of the phase transition depends sensitively on the model. For the solitonic model in particular, a k-dependent elastic energy modifies the order of the transition. Furthermore, we calculate gaps in the incommensurate phase in adiabatic approximation. Received: 9 March 1998 / Accepted: 17 April 1998     

Stabilization of the long-range magnetic ordering by dipolar and magnetoelastic interactions in two-dimensional ferromagnets

The possibility of the long-range magnetic order stabilization in two-dimensional ferromagnets with the account of dipolar and magnetoelastic interactions is investigated. The mechanisms of the magnetic order stabilization by both types of interactions are studied. The Curie temperature is estimated. The comparisons with experimental data are made. Received 22 June 2001 and Received in final form 17 January 2002     

Quantum spherical XY model with orthorhombic anisotropy in two dimensions

The quantum spherical XY model with orthorhombic anisotropy is investigated. It is shown that in contradiction with the results of reference [#!9!#], the long-range magnetic order is stabilized in two dimensions. Both analytical and numerical results are presented. The incorrect results of the work [9] are explained to be the result of improper choice of quantization axis. Received 26 August 2002 / Received in final form 24 October 2002 Published online 14 February 2003 RID="a" ID="a"e-mail: frid@tnu.crimea.ua     

Finite size corrections within the continuum limit for quantum spins: two-magnon bound states in 1D Heisenberg ferromagnet

A continuum medium approach is proposed to describe the finite size dependent effects for the 1D isotropic Heisenberg ferromagnet. The results are compared to the exact Bethe ansatz solution for the finite chain. The approach is shown to adequately account for the behaviour of the eigenfunctions and eigenenergies. The continuum is obtained by integration in Fourier space via introduction of cut-offs at the integration limits and analytical continuation from the discrete lattice to the continuous medium. It offers a new perspective on the instability of bound states, and reveals the linear behaviour of the amplitude in the critical region and other features of the model in an analytical way. We further apply this approach to investigate the long wavelength expansion of the master equation and to show the route of constructing reliable approximations valid for more complicated models. It is concluded that the approach can be useful to study mesoscopic spin systems. Received 28 May 2000 and Received in final form 6 April 2001     

Continuous canonical transformation for the double exchange model

The method of continuous canonical transformation is applied to the double exchange model with a purpose to eliminate the interaction term responsible for non conservation of magnon number. Set of differential equations for the effective Hamiltonian parameters is derived. Within the lowest order (approximate) solution we reproduce results of the standard (single step) canonical transformation. Results of the selfconsistent numerical treatment are compared with the other known studies for this model.     

Many-body Green's function theory for thin ferromagnetic anisotropic Heisenberg films: treatment of the exchange anisotropy

The many-body Green's function theory developed in our previous work for treating the reorientation of the magnetization of thin ferromagnetic films is extended to include the exchange anisotropy. This leads to additional momentum dependencies which require some non-trivial changes in the formalism. The theory is developed for arbitrary spin values S and for multilayers. The effects of the exchange anisotropy and the single-ion anisotropy, which was treated in our earlier work, on the magnetic properties of thin ferromagnetic films are compared. Received 31 October 2002 Published online 7 May 2003     

Moment-volume instabilities in alloys with compositions around the C14 laves phase

We measured the thermal expansion and the specific heat of Ti_xFe_100-x alloys with x = 30.5, 32.5 and 35, all with hexagonal C14 laves phase structure (MgZn₂) like TiFe₂, and determine the temperature dependence of the magnetic contributions to the thermal expansion and the specific heat c_mag. For fixed composition and c mag ( T ) show the same type of behavior, demonstrating that both anomalies are of the same microscopic nature. They originate from moment-volume fluctuations (antiferromagnetic Invar-effect) as a comparison with total energy calculations as a function of atomic volume and moment for TiFe₂ reveals. Received: 26 January 1998 / Accepted: 17 April 1998     

Quantum topological excitations: from the sawtooth lattice to the Heisenberg chain

From the recently determined structure of the delafossite YCuO_2.5, we argue that the Cu-O network has nearly independent Δ chains but with different interactions between the s = 1/2 spins. Motivated by this observation, we study the Δ chain for different ratios of the base-base and base-vertex interactions, J _bb/J _bv. By exact diagonalization and extrapolation, we show that the elementary excitation spectrum is the same for total spins S _tot = 0 and 1, but not for S _tot = 2, and has a gap only in the interval 0.4874(1) ⩽ J _bb/J _bv ⩽ 1.53(1). The gap, known to be dispersionless for J _bb = J _bv, is found to acquire increasing k-dependence as J _bb/J _bv moves away from unity. Received 29 October 2002 / Received in final form 14 January 2003 Published online 6 March 2003 RID="a" ID="a"e-mail: sblundell@cea.fr RID="b" ID="b"e-mail: nunezreg@lps.u-psud.fr     

Microscopic conditions favoring itinerant ferromagnetism: Hund's rule coupling and orbital degeneracy

The importance of Hund's rule coupling for the stabilization of itinerant ferromagnetism is investigated within a two-band Hubbard model. The magnetic phase diagram is calculated by finite-temperature quantum Monte-Carlo simulations within the dynamical mean-field theory. Ferromagnetism is found in a broad range of electron fillings whereas antiferromagnetism exists only near half filling. The possibility of orbital ordering at quarter filling is also analyzed. Received: 26 February 1998 / Accepted: 17 April 1998     

Magnetic structure of L{i_{1 - x}}N{i_{1 + x}}{o_2}

We study the magnetic structure of layered Li_1-xNi_1+xO₂ and propose a new scheme: the AF interaction between the excess Ni²⁺ in the Li layers and the Ni³⁺ ions in the Ni planes, gives rise to the formation of ferrimagnetic clusters, which control the physics of these systems. The values of the different interactions are estimated from a mean field calculation in the high temperature limit. For the small x samples studied here the method does not yield an accurate value of , but it is very sensitive to the intralayer interactions, allowing to conclude that they are ferromagnetic. The recent proposal of a quantum spin-orbital liquid in this system is discussed and the comparison with Jahn-Teller distorted NaNiO₂ is made. Received 8 December 1999     

Spin wave propagation in the domain state of a random field magnet

Inelastic neutron scattering with high wave-vector resolution has characterized the propagation of transverse spin wave modes near the antiferromagnetic zone center in the metastable domain state of a random field Ising magnet. A well-defined, long wavelength excitation is observed despite the absence of long-range magnetic order. Direct comparisons with the spin wave dispersion in the long-range ordered antiferromagnetic state reveal no measurable effects from the domain structure. This result recalls analogous behavior in thermally disordered anisotropic spin chains but contrasts sharply with that of the phonon modes in relaxor ferroelectrics. Received 2 November 2002 / Received in final form 4 February 2003 Published online 11 April 2003 RID="a" ID="a"leheny@pha.jhu.edu     

Ferromagnetic ground state in the Kondo lattice model

We present a series of rigorous examples of the Kondo lattice model that exhibit full ferromagnetism in the ground state. The models are defined in one-, two- and three-dimensional lattices, and are characterized by a range of hopping terms, specific electron filling, and large ferromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized spins could overcome the competition from mobility of a finite density of electrons and drive the system from a paramagnetic phase to a ferromagnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous results on ferromagnetism in the corresponding Hubbard model are presented. Received: 10 September 1997 / Revised: 15 October 1997 / Accepted: 17 October 1997     

Squeezed thermal spin states of magnons in the ferromagnet

In this paper, we discuss squeezed thermal spin states of magnons that are described by the Heisenberg Hamiltonian in the ferromagnet, in which the magnon system possesses a new kind of quasiparticle, which we call ferromagnon, i.e. a “dressed” quasi-particle obtained from the magnons by a Bogoliubov-Valatin transformation . Generally, the mass and noise properties of ferromagnons possess potentially important and novel effects in condensed matter physics, which have extensive application in the fields of science and technology. Moreover, it is convenient to introduce the Holstein-Primakoff method, in order to take into account the nonlinear interaction among spin waves. At last we describe the quantum fluctuations of spin-components in the squeezed thermal spin states of magnons and their temperature-dependence. Below some temperature, the squeezed thermal spin states of ferromagnons show squeeze effect.