The origin of noncollinear anisotropies and asymmetric magnetization reversal in FM/AFM bilayer system

The effects of the magnitude of the uniaxial anisotropy of a ferromagnet and the cooling field on the noncollinearity between uniaxial anisotropy and induced unidirectional anisotropy in a ferromagnet/antiferromagnet bilayer system are investigated. A diagram of noncollinear anisotropies and relative negative (positive) exchange bias field dependence upon cooling field and uniaxial anisotropy of the ferromagnet is obtained. The numerical result shows that the emergence of noncollinear anisotropies originates from the action of the cooling field and uniaxial anisotropy of the ferromagnet. The noncollinearity strongly depends on the magnitude of cooling field and uniaxial anisotropy of the ferromagnet. Moreover, the effect of noncollinear anisotropies and applied field on asymmetric magnetization reversal is also investigated. Amazingly, when the magnetic field is applied collinearly with unidirectional anisotropy, the hysteresis loop of ferromagnet/antiferromagnet bilayers is always symmetric even if there are noncollinear anisotropies. Our results indicate that the asymmetry of the hysteresis loop only originates from the noncollinearity between the induced unidirectional anisotropy and the applied field, rather than from the noncollinearity between the uniaxial and unidirectional anisotropies.     

The paramagnetic properties of one-dimensional spin-1 single-ion anisotropic ferromagnet

One-dimensional single-ion anisotropic ferromagnet with spin-1 is investigated by means of Green's function treatment in this paper. The model Hamiltonian includes a Heisenberg ferromagnetic term, an external magnetic field, and a second-order single-ion anisotropy. The magnetic properties of the system are treated by the random phase approximation for the exchange interaction term and the Anderson-Callen approximation for the anisotropy term. Our paramagnetic results are in agreement with the other theoretical results.     

Magnetostatic surface waves produced by an inhomogeneity of the anisotropy with a turning point of the spectral function on a ferromagnet surface

Magnetostatic surface waves with fixed frequency and wave vector are predicted to exist in a ferromagnet with an inhomogeneity of the magnetic anisotropy such that the spectral function has a turning point on the surface. This result is most important for the case when an external magnetic field magnetizes the ferromagnet perpendicular to its surface. The frequency of the surface wave is determined by the frequency of the magnetostatic volume wave at the surface of the ferromagnet, and the wave vector is determined by the surface values of the local magnetic anisotropy field and its derivative. Zh. Tekh. Fiz. 68, 118–123 (June 1998)     

Thermodynamic Properties of Random Transverse Field Mixed Spin System in the Presence of Single-Ion Anisotropy

We study the thermodynamic properties of random transverse field mixed spin system in the presence ofsingle-ion anisotropy on a square lattice. By making use of the effective field theory and a cutting approximation, thedetailed phase diagrams are described and some interesting results are found under trimodal random transverse fielddistribution. A smallsingle-ion anisotropy can magnify magnetic ordering region at low temperatures and existence ofa large transverse field can assist the occurrence of reentrant phenomena. With increasing disorder, second-order phasetransitions are shown to change into first-order phase transitions. The trajectory of the tricritical point in the phasespace as a function of disorder is presented. These indicate a strong correlation with the corresponding to trimodaltransverse field distribution.     

Thermodynamic Properties of Random Transverse Field Mixed Spin System in the Presence of Single-Ion Anisotropy

We study the thermodynamic properties of random transverse field mixed spin system in the presence of single-ion anisotropy on a square lattice. By making use of the effective field theory and a cutting approximation, the detailed phase diagrams are described and some interesting results are found under trimodal random transverse field distribution. A small single-ion anisotropy can magnify magnetic ordering region at low temperatures and existence of a large transverse field can assist the occurrence of reentrant phenomena. With increasing disorder, second-order phase transitions are shown to change into first-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. These indicate a strong correlation with the corresponding to trimodal transverse field distribution.     

Spin waves in two-dimensional ferromagnet with large easy-plane anisotropy

Spin waves in easy-plane two-dimensional ferromagnet when anisotropy is much stronger than exchange are investigated. The spectra of magnons, the spin–spin and quadrupolar correlation functions have been derived. It is shown that in such a system there exist spin waves at low temperatures. Some properties of the quadrupolar ordering in ferromagnets are discussed.     

Theory of two-magnon dissipation in disordered ferromagnets I. Relation to sample statistics

The theory of two-magnon dissipation in disordered ferromagnetic materials is developed. Fluctuations of one-ion anisotropy energy due to the disorder are assumed to be the source of the linewidth broadening (relaxation of magnons by two-magnon scattering). Ordinary two-magnon theory is reconsidered on the basis of the sample statistics. The main idea of it is to describe the macroscopic properties of materials by means of quantities, whose relative fluctuations over the ensemble of configurations are negligibly small for sufficiently large sample. The energies of magnons — modes with well defined wave vectork — and their lifetimes are shown to be such variables. Further, possible models for a two-component ferromagnet are considered in the approximation of high external field. The energies of magnons in mixed crystal are obtained as linear combinations of those in parent crystals. Finally, the dependence of magnon lifetimes on the ordering is separated from the dependence on the magnitude of anisotropy variations.     

Self-consistent many-body theory for the standard basis operator Green's functions

We propose a self-consistent many-body theory for the standard basis operator Green's functions and obtain an exact Dyson-type matrix equation for the interacting many-level systems. A zeroth order approximation, which neglects all the damping effects, is investigated in detail for the anisotropic Heisenberg model, the isotropic quadrupolar system and the Hubbard model. In the case of the anisotropic Heisenberg ferromagnet with both exchange and single-ion anisotropy the low-temperature renormalization of the spin-waves for the uniaxial ordering agrees with the Bloch-Dyson theory. For the spin-1 easy-plane ferromagnet, the critical parameters for the phase transition at zero temperature are determined and compared with other theories. The elementary excitation spectrum of the spin-1 isotropic quadrupolar system is calculated and compared with the random phase approximation and Callen-like decoupling schemes. Finally, the theory is applied to the study of the single-particle excitation spectrum of the Hubbard model.     

Magnetic properties of amorphous heisenberg ferromagnet with random anisotropy

Magnetization, susceptibility, and the critical temperature for a spin-one amorphous Heisenberg ferromagnet with random local anisotropy are calculated in the molecular field approximation for the exchange interactions. The effects of random fluctuations of the magnitude of the local anisotropy about its crystalline value are studied for various probability distributions. It is found that for a given randomness, measured in terms of the root mean square fluctuation of the magnitude of the local anisotropy, the magnetization and the critical temperature depend more on the range, denned as the magnitude of the maximum value of the fluctuation from its mean value, than the shape of the probability distribution function. The effect of randomness decreases as the crystalline value of the anisotropy increases. Calculation of the susceptibility shows that above the critical temperature, randomness has no effect.     

Solving the triangular Ising antiferromagnet by simple mean field

Few years ago, application of the mean field Bethe scheme on a given system was shown to produce a systematic change of the system intrinsic symmetry. For instance, once applied on a ferromagnet, individual spins are no more equivalent. Accordingly a new loopwise mean field theory was designed to both go beyond the one site Weiss approach and yet preserve the initial Hamitonian symmetry. This loopwise scheme is applied here to solve the triangular antiferromagnetic Ising model. It is found to yield Wannier's exact result of no ordering at non-zero temperature. No adjustable parameter is used. Simultaneously a non-zero critical temperature is obtained for the triangular Ising ferromagnet. This simple mean field scheme opens a new way to tackle random systems. Received 14 November 2001 / Received in final form 22 March 2002 Published online 19 July 2002     

A new method of ab initio calculation of the magnetocrystalline anisotropy energy in relativistic ferromagnets

A differential technique of calculating the magnetocrystalline anisotropy energy based on the spin-polarised relativistic calculation of the electron states in a ferromagnet is developed. The formalism of the relativistic version of KKR method is shown to be the most convenient for such calculations.     

An investigation of string-like cooperative motion in a strong network glass-former

The phase states of a non-Heisenberg two-dimensional ferromagnet are studied, in which the long-range magnetic order is stabilized by the magnetoelastic interaction. It is shown that in this system, together with the phases of nonzero magnetic order, there exists a quadrupolar phase characterized by a tensor order parameter at zero external field. The transition temperature from the quadrupolar phase to the paramagnetic phase is determined.Received: 24 November 2003, Published online: 18 June 2004PACS: 75.10.-b General theory and models of magnetic ordering - 75.30.Kz Magnetic phase boundaries (including magnetic transitions, metamagnetism, etc.)     

Low-temperature magnetization and susceptibility of an amorphous ferromagnet

The zero temperature magnetization and susceptibility are considered for an amorphous ferromagnet in which the local anisotropy field H_A has random directions. Results are used to estimate H_A from existing data in amorphous TbFe₂.     

Effects of Field Orientation on the Driven Lattice Gas

Steady states of the driven lattice gas (DLG) on triangular, hexagonal and square lattices with the field at several fixed orientations to the principal lattice vectors were studied by Monte Carlo simulation. In most cases a strong field suppressed change to a low-temperature ordered phase. On each lattice, one field orientation that caused nonequilibrium ordering was identified. On triangular and hexagonal lattices, dependence of energy and anisotropy on field strength was studied at those orientations. Anisotropic ordering along the field developed at intermediate temperatures under weak fields. Partial ordering along the field persisted to low temperature under strong fields.     

Collective states of interacting ferromagnetic nanoparticles

Discontinuous magnetic multilayers [CoFe/Al₂O₃] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co₈₀Fe₂₀ nanoparticles are embedded in a diamagnetic insulating a-Al₂O₃ matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.     

Effect of local field fluctuations on orientational ordering in random-site dipole systems

Some peculiarities of dipole ordering in systems with uniaxial or cubic anisotropy with an arbitrary degree of dilution are analyzed in terms of random local field theory. The approach takes into account the effect of thermal and spatial fluctuations of the local fields acting on each particle from its neighbors with an accuracy corresponding to that of the Bethe-Paierls pair clusters approach. We show that ferromagnetic (ferroelectric) structure for uniaxial Ising dipoles distributed on a simple cubic lattice is intrinsically unstable against the fluctuations of the local fields for any concentration of the dipoles. This result is quite different from the prediction of the mean-field theory which implies the possibility of ferromagnetic ordering as a metastable state in field-cooled experiments. The local field fluctuations do not exclude, however, antiferromagnetic ordering above a certain critical concentration. Ferromagnetic ordering is possible for other types of lattice geometries and for an amorphous-like dipole distribution above a certain critical concentration. A simple physical explanation of such behavior is given based on the specific angular dependence of the dipole-dipole interaction that results in a relatively high value of the local field second moment for simple cubic lattice.     

Tuning kinetic magnetism of strongly correlated electrons via a staggered flux

An interplay between kinetic process and magnetic ordering is manifested when strong correlation and electronic frustration are present: tuning a staggered flux phi from 0 to pi makes the ground state (GS) of an infinite-U Hubbard model change abruptly from a Nagaoka-type ferromagnet to a Haerter-Shastry-type antiferromagnet at a phi_(c), with both states being metallic and of kinetic origin. Intraplaquette spin correlation, as well as nonanalyticity in the GS energy, signals such a novel quantum criticality. This tunable kinetic magnetism is generic and may be experimentally realized.     

The three-dimensional three-state Potts ferromagnet exposed to random fields: evidence for a second order transition

Using large scale Monte Carlo simulations, the ordering of the three-dimensional three state Potts ferromagnet exposed to random fields is investigated. Studies of the order parameter probability distribution and of various of its moments suggest that the order of the transition depends on the strength of the random field: i.e., the first order transition of the pure ferromagnetic model persists for weak random fields, but turns into a second order transition for a range of random fields of medium strength. For large random fields the transition seems to be first order again. In this range large domains of strongly aligned Potts spins occur already in the disordered phase and the associated slow relaxation hampers significantly the Monte Carlo study of thermodynamic equilibrium phenomena. These results are discussed in the light of current theoretical concepts. Possible applications to experiments on diluted anisotropic molecular crystals and orientational glasses are briefly mentioned.     

The three-dimensional three-state Potts ferromagnet exposed to random fields: evidence for a second order transition

Using large scale Monte Carlo simulations, the ordering of the three-dimensional three state Potts ferromagnet exposed to random fields is investigated. Studies of the order parameter probability distribution and of various of its moments suggest that the order of the transition depends on the strength of the random field: i.e., the first order transition of the pure ferromagnetic model persists for weak random fields, but turns into a second order transition for a range of random fields of medium strength. For large random fields the transition seems to be first order again. In this range large domains of strongly aligned Potts spins occur already in the disordered phase and the associated slow relaxation hampers significantly the Monte Carlo study of thermodynamic equilibrium phenomena. These results are discussed in the light of current theoretical concepts. Possible applications to experiments on diluted anisotropic molecular crystals and orientational glasses are briefly mentioned.     

Magnetization of the ferromagnetic-superconductor structures

The magnetization of a structure consisting of a thin superconducting film lying on a thin ferromagnetic substrate with uniaxial anisotropy is considered. It is shown that if a ferromagnet is in a multidomain state, then due to the presence of a superconducting film, the period of its domain structures decreases, which is caused by the increase of the magnetostatic energy of the system owing to Meissner currents. In a certain range of the constant of uniaxial anisotropy, under the action of a superconducting film, a domain structure may transform from a strip structure to the structure with closure domains. It is found that due to the nonuniform magnetic field of a multidomain ferromagnet, Abrikosov vortices may exist in a thin film only at certain parameters of the magnetic field.