On a theory of spin systems with competing anisotropies

On the basis of a quantum treatment of Me_1?cFe_cX (Me = Mn, Ni, Co, X = Cl₂, CO₃, etc.) type random systems it is shown that parallel with order parameters such as magnetizations of mixed magnetic crystal ordering subsystems there is one more order parameter determining the angle between them.     

Ordered states magnetic resonances in the random mixture with competing spin anisotropies Fe1−xCoxCl2

Antiferromagnetic and ferromagnetic resonance experiments at submillimeter wavelengths have been performed on random mixtures Fe_1?xCo_xCl₂ of two metamagnets with competing orthogonal spin anisotropies. The results show clearly the competition of the anisotropies in these alloys. The anisotropy in these mixtures decreases with concentration in both the iron rich and cobalt rich regions.     

Mössbauer study on a two-dimensional random mixture with competing spin anisotropies K2Ni1−x Fe x F4

Mössbauer measurements have been made on a two-dimensional (2D) random mixture K₂Ni_1?x Fe _x F₄ with competing spin anisotropies. The concentration versus temperature phase diagram predicted by Oguchi and Ishikawa for mixed systems with competition between orthorhombic anisotropies has been shown to exist in K₂Ni_1?x Fe _x F₄. The coexistence of two kinds of Mössbauer spectra is seen in the transition regions, and is believed to be an intrinsic property of this system.     

Uniform and localized magnetic modes in the random mixture with competing spin anisotropies,Fe(1−x)CoxBr2

Electron spin resonance experiments at millimeter and submillimeter wavelengths have been performed on the randomly mixed antiferromagnet with competing spin anisotropies, Fe_(1?x)Co_xBr₂. Two antiferromagnetic resonance (AFMR) modes and extra resonances have been observed in the Co-rich concentration region. From the analysis of the AFMR, the anisotropy in the mixtures is shown to become small with concentration. This is a clear evidence for the competition of the anisotropies in these alloys. The extra modes are qualitatively explained as arising from a localized excitation of an Fe²⁺ spin.     

Magnetization reversal dynamics in Co nanowires with competing magnetic anisotropies

We present the magnetization reversal dynamics of Co nanowires with competing magnetic anisotropies. The aspect ratio (R) of the nanowires is varied between 2.5 and 60, and we observe a cross-over of the directions of the magnetic easy and hard axes at R=6.8. Micromagnetic simulations qualitatively reproduce the observed cross-over and give detailed insight into the reversal mechanisms associated with the cross-over. The reversal mechanism for a field applied along the long axis of the nanowire exhibits a quasi-coherent rotation mode and a corkscrew-like mode, respectively, above and below the cross-over, with the formation of a Bloch domain near the cross-over region. For a field applied along the short axis, the reversal occurs by nucleation and propagation of reversed domains from the two ends of the nanowires for very high values of the aspect ratio down to the cross-over region, but it transforms into quasi-coherent rotation mode for smaller aspect ratios (below the cross-over region).     

Absence of ordering in a class of random spin systems,including simple models for spin glasses,and a new inequality for random ferromagnets

Absence of ordering is proved for a class of one- and two-dimensional quenched random systems including the Edwards-Anderson model for spin-glasses; a new inequality for random ferromagnets is proved using FKGP-inequalities and very recent correlation inequalities due to Sherman.     

Phase transitions and multidomain states in magnetic nanostructures with competing anisotropies

The evolution of the magnetic states in nanostructures with competing surface and bulk anisotropies was studied under a variation in the applied magnetic field. For spatially uniform magnetic states, the energy functional reduces to the energy of a bulk magnet in which the effective anisotropy depends on the thickness of the ferromagnetic layer. This dependence results in spin reorientation as the nanolayer thickness varies. The lines of first-order phase transitions and the equilibrium parameters of multidomain structures and domains of competing phases were determined. The calculated magnetic phase diagrams are used to analyze magnetic states and to construct magnetization curves.     

Temperature-induced magnetic phase transitions in crystals with competing single-ion and interionic magnetic anisotropies

Temperature-induced phase transitions in a uniaxial ferromagnetic system of spins S = 1 with competing one-particle and two-particle anisotropies are studied. It is shown that, in the case where easy-plane single-ion anisotropy dominates over easy-axis two-particle anisotropy, the transition from the paramagnetic state to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is a second-order displacive magnetic phase transition. In the opposite case, where two-particle anisotropy dominates over single-particle anisotropy, the transition to a ferromagnetic state with magnetization perpendicular to the anisotropy axis is also continuous but of the order-disorder type. In a system with competing second-order one-and two-particle anisotropies, the orientational first-order phase transition can occur to a state with the magnetization directed along or perpendicular to the anisotropy axis.     

Energy barrier distributions for magnetic nanoparticles with competing cubic and uniaxial anisotropies

We report in this study the effect of the competition between cubic and uniaxial anisotropies on the magnetic properties of magnetic nanoparticles. We have employed Monte Carlo simulations in our calculations and we have seen that the observed behavior is very different for the cases where easy uniaxial axes are completely random oriented or parallel to an external magnetic field. We have also calculated the effective energy barrier distribution probed during the isothermal magnetic relaxation and a two peak structure is observed only for a random orientation of uniaxial axes.     

Tunable in-plane spin orientation in Fe/Si(557) film by step-induced competing magnetic anisotropies

Although the spin-reorientation transition from out-of-plane to in-plane in Fe/Si film is widely reported, the tuning of in-plane spin orientation is not yet well developed. Here, we report the thickness-, temperature- and Cu-adsorptioninduced in-plane spin-reorientation transition processes in Fe/Si(557) film, which can be attributed to the coexistence of two competing step-induced uniaxial magnetic anisotropies, i.e., surface magnetic anisotropy with magnetization easy axis perpendicular to the step and volume magnetic anisotropy with magnetization easy axis parallel to the step. For Fe film thickness smaller than 32 monolayer(ML), the magnitudes of two effects under various temperatures are extracted from the thickness dependence of uniaxial magnetic anisotropy. For Fe film thickness larger than 32 ML, the deviation of experimental results from fitting results is understood by the strain-relief-induced reduction of volume magnetic anisotropy.Additionally, the surface and volume magnetic anisotropies are both greatly reduced after covering Cu capping layer on Fe/Si(557) film while no significant influence of Na Cl capping layer on step-induced magnetic anisotropies is observed.The experimental results reported here provide various practical methods for manipulating in-plane spin orientation of Fe/Si films and improve the understanding of step-induced magnetic anisotropies.     

Phase diagram of Fe(1−x)CoxCl2; a random mixture of two antiferromagnets with competing spin anisotropies

Concentration vs transition temperature phase diagram of a random mixture of two anisotropic antiferromagnets, FeCl₂ and CoCl₂, is obtained from the measurement of the susceptibility on the single crystals. Two distinct critical points, one along the c-axis and the other in the c-plane, are observed for the respective concentrations between x = 0.264 and x = 0.481. Three kinds of ordered phases, namely, Fe-rich and Co-rich antiferromagnetic phases and a new phase are found. The phase diagram shows a tetracritical point as well.A hump is observed in the temperature dependence of the susceptibility along the c-axis (c-plane for x ? 0.3) near the ordering temperature occuring in the c-plane (c-axis for x ? 0.3).     

Experimental evidence for magnetic ordering in a literally two-dimensional magnet

The electron spin resonances of literally two-dimensional magnetic structures, formed of manganese stearate by the Langmuir-Blodgett technique, have been measured down to 1.3K. In one such structure the resonance field decreased abruptly at 2.0 K by more than 1000 Oe, and was highly anisotropic below this temperature. These, and other observations can be explained by the hypothesis of weak-ferromagnetic ordering.     

Planar spin chain with competing interactions in an external magnetic field

The 1D classical planar model with nearest neighbor (NN) and next nearest neighbor (NNN) competing interactions is studied at finite temperature in presence of an external magnetic field by the transfer matrix method. In particular we are interested into helix configurations supported by an exchange competition strong enough. The short range order (SRO) is strictly related to the zero temperature long range order (LRO). Our exact calculation makes possible the comparison with the current expectation that a discontinuous helix-fan transition occurs before the continuous fan-collinear transition takes place. This feeling is based on the extrapolation of low field-high field zero temperature expansions. We find strong evidence that this expectation is fulfilled when the turn angle of the helix is lesser than /2, whereas for turn angles greater than /2 the discontinuous transition does not take place and the helix changes continuously to a collinear configuration as the magnetic field is increased. Our conclusion is based on the analysis of the SRO at finite temperature exactly calculated by the transfer matrix method and on the zero temperature exact calculation of the minimum energy of commensurate configurations.     

Microscopic theory of dipole-exchange spin waves in magnetic multilayers

A microscopic model is employed to calculate the spectrum of dipole-exchange spin waves in multilayers in which thin ferromagnetic films are separated by non-magnetic spacers. Two configurations are considered: in one the films have magnetizations parallel to each other, in the other the magnetizations are antiparallel. The calculations extend a previous microscopic formalism that allows the calculation of the dipole-exchange spin wave spectrum in thin films. The results show the splitting of the frequency bands and the mode mixing caused by the dipolar interaction between the films as a function of spacer thickness.Received: 26 May 2004, Published online: 12 August 2004PACS: 75.30.Ds Spin waves - 75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.) - 75.70.-i Magnetic properties of thin films, surfaces, and interfaces     

Nuclear spin magnetic ordering with four spin exchange in solid bcc 3He

Some consequences of four spin exchange are analysed: 1) the negative sign of the T^-3 term in the specific heat; 2) the existence of thirteen possible magnetic arrangements, in particular a pseudo-ferromagnetic pahse; 3) a susceptibility, in the ordered phase, larger than that deduced from the Heisbnberg Hamiltonian.     

Displacive magnetic phase transitions upon spin ordering in magnets with strong single-ion anisotropy

Physics of the Solid State - This paper reports on the results of investigations into the magnetic ordering of magnets with integer spins of ions and sufficiently strong easy-plane single-ion...     

Composition vs transition temperature phase diagram of a random two-component magnetic system with competing spin-anisotropies

The experimental composition vs transition temperature phase diagram for the system K₂Mn_1?xFe_xF₄ is compared with a recently proposed theory of Aharony and Fishman. The existence of two critical lines, crossing in a tetracritical point, is discussed.     

Sequence of the phase transitions in a two-dimensional ferromagnet with competing one-ion and exchange anisotropies

The phase states of a two-dimensional ferromagnet system with anisotropic exchange interaction have been investigated for various relations between the material constants. It is shown that the competition between the exchange and the one-ion anisotropies, and the account of the magnetodipolar and the magnetoelastic interactions lead to the sequence of the phase transitions. The explicit solutions of the dispersion equation have been found in the homogeneous phase states.