Amorphous rare earth alloys: Random-anisotropy ferromagnetism in the pair approximation

A self-consistent two-spin cluster approximation is used to calculate magnetization curves and correlation functions for a ferromagnetic amorphous alloy within the random-anisotropy model for the case of unit total angular momentum (j = 1) and correlations between the anisotropy axes.     

Magnetization curves of randomly oriented ferromagnetic single-domain nanoparticles with combined symmetry of magnetic anisotropy

The magnetization curves of randomly oriented nanoparticles with combined symmetry of magnetic anisotropy were studied. The composite mode of the Stoner–Wolfarth model has been used. In terms of this model each nanoparticle is characterized by random cubic crystalline magnetic anisotropy and by random uniaxial magnetic anisotropy. The series of simulated magnetization curves have been obtained. Each curve corresponds to different contributions of cubic and uniaxial magnetic anisotropy energy to the full energy of an individual nanoparticle k_u. Within this series we discuss the values of remnant magnetization, coercive force, both initial and maximal susceptibilities as the function of k_u. It is found that the magnetic properties are not monotonous functions of k_u. We discuss the possibility of comparing the calculated magnetization curves with the experimental curves in order to obtain new information on the magnetic constant.     

Extracting intergranular exchange coupling in perpendicular magnetic recording media

A micromagnetic numerical technique has been used to demonstrate how intergranular exchange coupling and intrinsic anisotropy field dispersion can be extracted from measuring two types of M-H curves. A realistic grain configuration formed by planar Voronoi cells is used to simulate perpendicular magnetic media. This technique effectively separates the effects of intergranular exchange coupling and anisotropy dispersion by finding their correlation to differentiated M-H curves with different initial magnetization states, even in the presence of thermal fluctuation. The validity of this method is investigated with a series of intergranular exchange couplings and anisotropy dispersions for different media thickness. A relationship between the auto-correlation function of an ac-erased sample and dispersion of the exchange interaction is demonstrated. Utilizing magnetization auto-correlation functions, the magnetic intergranular exchange coupling statistics show a correlation with the auto-correlation function shape in terms of zero-cross and undershoot values.     

Frequenzabhängige magnetische Suszeptibilitäten und renormierte Spinwellenenergien von GdCl3

GdCl₃ is a ferromagnet with two equivalent sub-lattices in which the dipolar interaction is comparable with the exchange interaction. In the Callen decoupling approximation the time and temperature dependent Green’s functions are derived and the transversal complex susceptibilitiesχ _xx(ω,h ₀) andχ _yx(ω,h ₀) are calculated. The energies of the acoustical and the optical spinwave modes following from the Green’s functions are renormalized by the magnetization and the spin correlation. The spinwave spectrum, showing an unusual angular dependence, is calculated analytically in the whole Brillouin zone and numerically in the long wavelength region.     

Spin excitations and thermodynamics of the <Emphasis Type="Italic">t</Emphasis>-<Emphasis Type="Italic">J</Emphasis> model on the honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on the honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures and hole dopings, the electronic spectrum of excitations, the spin-excitation spectrum and thermodynamic quantities (two-spin correlation functions, staggered magnetization, magnetic susceptibility, correlation length) are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature and doping dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. Our results on the doping dependencies of the magnetization and susceptibility are analyzed in comparison with previous results for the t-J model on the square lattice.     

Thermodynamic properties of the ferromagnetic spinel ZnpCd1−pCr2Se4

The magnetic properties of the B-spinel ferromagnetic Zn_pCd_1−pCr₂Se₄ compound are studied via a cluster series expansion approximation with nearest and next-nearest exchange integrals J₁ and J₂, respectively. Using the minimization of the free-energy expressions, the magnetization, the magnetic susceptibility, the two-spin correlation functions and the specific heat are obtained and computed numerically as a function of temperature and for each composition of the system. The magnetization curves are used to determine the critical temperatures T_c. Using the power laws in the vicinity of the critical regions the critical exponents β, γ and α associated, respectively, with the magnetization, the magnetic susceptibility and the specific heat are numerically calculated. The critical temperatures T_c obtained are in very good agreement with those predicted by the magnetic measurements and the values of the critical exponents may be compared with other theoretical results based on the 3D Heisenberg model.     

The magnetic properties of disordered Fe-Al alloy system

Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of disordered Fe-Al alloys on the basis of a site-diluted quantum Heisenberg spin model are examined. We calculated the critical temperature and the hysteresis loops for this system. We find a number of characteristic phenomena. In particular, the effect of concentration c of magnetic atoms and the reduced exchange anisotropic parameter η on both the critical temperature and magnetization profiles are clarified.     

Spin excitations and thermodynamics of the antiferromagnetic Heisenberg model on the layered honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic Heisenberg model on a stacked honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures, the thermodynamic quantities (two-spin correlation functions, internal energy, magnetic susceptibility, staggered magnetization, Néel temperature, correlation length) and the spin-excitation spectrum are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. The Néel temperature is calculated for arbitrary interlayer couplings. Our results are in a good agreement with numerical computations for finite clusters and with available experimental data on the β-Cu₂V₂O₂ compound.     

Contact-free determination of the current-voltage law parameters for melt-textured YBCO superconductors in orthogonal magnetic field

A contact-free method to obtain the current-voltage characteristics (CVC) of hard superconductors by measuring the relaxation of the magnetization in a perpendicular magnetic field is developed. The relaxation curves obtained for melt-textured YBCO samples are well fitted by curves calculated within the electrodynamic model using a power-law CVC. This procedure uses only two fitting parameters, namely, the critical sheet current J_c and the exponent m of the power-law CVC.     

Time correlation functions and ergodic properties in the alternating XY-chain

Explicit expressions for the time-dependent spin correlation functions are derived for the one-dimensional alternating XY-model in zero field. The ergodic properties of the wave-number dependent magnetization are discussed. It is found that the magnetization is ergodic, apart from exceptional cases, such as e.g. the homogeneous XY-model.     

Thermodynamic and magnetic properties of HoRh2Si2: A comparison between experiments and calculations in a crystal field model

The origin of two peaks observed in the magnetization and the heat capacity curves of HoRh₂Si₂ is investigated. The thermodynamic and the magnetic properties of this compound are calculated using a Hamiltonian including crystal field and isotropic exchange interaction terms which are comparable in magnitude. Two components (parallel and perpendicular to the c-axis) of the magnetization are evaluated by the molecular field approximation. The two peaks in the magnetization and the heat capacity can be reproduced by choosing appropriate values for crystal field parameters and molecular field constant. The lower transition temperature corresponds to the temperature at which the perpendicular component of the magnetization disappears, and the higher one, to that at which the parallel component of the magnetization disappears.     

Autocorrelation function of the x-component of the magnetization in the one-dimensional XY-model

Exact expressions are derived for the time-dependent correlation functions of the x and y component of the magnetization for the one-dimensional anisotropic XY-model ($\text{S=}\text{1}\text{2}$) in zero field in the high-temperature limit. As a corollary, the correlation functions for the Ising chain in a transverse field are obtained.     

Effect of the morphology on the mechanisms of the magnetization reversal of multilayered thin Co/Pd films

A comparative study of the magnetization curves of continuous and porous multilayered Pd_10nm/[Co_0.3nm]/Pd_0.55nm]15/Pd_2nm films deposited on an anodized TiO₂ template was performed by magnetometry. Based on the comparison of the dependences of coercive field H_C on angle θ between the easy-magnetization axis and the direction of external magnetic field H with theoretical dependences H_C(θ) for the magnetization reversal by domain walls motion (according to the Kondorski model) and the rotation of magnetic moments (by the Stoner–Wohlfarth model), the differences in the mechanisms of magnetization reversal for two mentioned types of the films were revealed. The correlation between the difference in the morphologies of the continuous and porous films and revealed change in the mechanisms of the magnetization reversal, as well as the changes in values of H_C and calculated constants of the magnetic anisotropy, is discussed.     

Magnetization and transport characteristics of layered high-temperature superconductors with different anisotropy parameters

The magnetization of a layered high-temperature superconductor with different anisotropy parameters has been calculated using the Monte Carlo method in the framework of a modified three-dimensional Lawrence–Doniach model with actual boundary conditions. The penetration of a magnetic flux into a bulk sample from the boundary has been simulated, and the curves of magnetization reversal of a high-temperature superconductor by an external magnetic field have been calculated for different anisotropy parameters γ and types of defects in the sample. It has been found that there are significant differences in the magnetization curves and transport properties of superconductors with different anisotropy parameters γ. The influence of tilted columnar defects on the critical current has been analyzed. A decreasing dependence of the critical current on the tilt angle of defects with respect to the c axis has been obtained. It has been shown that, as the anisotropy parameter increases, this dependence weakens and, for a specific value of γ, disappears. An explanation of the mechanism responsible for the disappearance of the dependence has been proposed.     

Characteristics of magnetism in nanoporous carbon with palladium clusters

The magnetic properties of samples of nanoporous carbon with palladium clusters prepared from polycrystalline SiC have been studied over a wide range of temperatures. The specific magnetic moment rapidly decreases with increasing temperature. The obtained experimental magnetization curves have a nonlinear character but do not reach saturation even in a field of 10 kOe. Being constructed as functions of H/T, they lie on a universal curve, which suggests superparamagnetism of the system. The experimental curves have a hysteresis. The coercive force, first, decreases with increasing temperature and, then (in the range of 5–20 K), increases, thus forming a notch singularity. On the contrary, the residual magnetization rapidly decreases with increasing temperature. A conclusion on the prevailing contribution of superparamagnetic clusters to the magnetic subsystem is made.     

Static properties of type-II superconductors at arbitrary temperature and induction

A concise expression for the free energy of clean type-II superconductors is given from which magnetization curves, gap and field are easily calculated for 0 < T < T_c and 0 < B < H_c2.     

More accurate calculations of the magnetic entropy changes

A series of Ni_44−xCo_xMn₄₅Sn₁₁ (x=0, 1, 2) alloys were prepared by means of arc melting, magnetization curves under different temperatures were measured using vibrating sample magnetometer. The bicubic interpolations method was employed to calculate the magnetization M(T, H), and the magnetic entropy changes were figured out using the thermodynamic relation of magnetic materials. Results show that the values calculated based on the bicubic interpolation method are more accurate compared with those from conventional method, and the curves obtained are relatively smooth.