Percolation thresholds, p_{c}, for site diluted spin systems on the fcc lattice are determined for exchange interactions extending up to the shell of the fourth nearest neighbors. If the interactions include the nearest, second, third, and fourth neighbors, p_{c} is equal to 0.198, 0.136, 0.061, and 0.05 respectively. These results agree with the Roberts approximate formula for p_{c}. Estimation of p_{c} to even longer-ranged couplings is presented. For instance for p_{c} = 0.01 the range of the couplings should extend at least to the eight shell of neighbors.
A quantum simulation approach facilitated by the self-consistent algorithm was applied in the present work to ferromagnetic and antiferromagnetic three-dimensional Heisenberg lattices consisting of S = 1 spins. Consequently, the calculated spontaneous magnetizations for the two sorts of lattices are precisely consistent with mean-field theory in the whole temperature range. Especially, the numerical results, such as magnetizations, total energies and total free energies per mole of spins, show no size effects. Thus, the physical properties of a huge bulk magnet can be estimated by performing simulation for a very tiny sample, so that the computational time can be greatly saved.
Quantum transfer matrix technique and numerically exact diagonalization method are applied to the Heisenberg spin systems to model ring-shaped molecules. Two cases are investigated: (i) a dozen of S = 1 spins with additional biquadratic exchange and (ii) a dimetallic molecule Cr_7Cd, where it is assumed that exchange anisotropy is determined in a local coordination system. In the latter case the calculated susceptibility is compared with experimental results.
Temperature dependences of magnetization of core-shell-type nanoparticles with non-magnetic core and ferromagnetic shell are obtained using Monte Carlo simulation. The influence of surface spin disorder of the ferromagnetic shell on overall shape of magnetization curve is analyzed. The magnetic state diagram (in shell thickness - surface anisotropy coordinates), separating collinear and non-collinear states, is determined.
We present the experimental evidence for the presence of spinodal decomposition of the magnetic ions in the Ge_{1-x-y}Cr_{x}Eu_{y}Te samples with chemical composition varying in the range of 0.015 ≤ x ≤ 0.057 and 0.003 ≤ y ≤ 0.042. The ferromagnetic transition at temperatures 50 ≤ T ≤ 57 K was observed, independent of the chemical composition. The long-range carrier mediated itinerant magnetic interactions seem to be responsible for the observed ferromagnetic order. The magnetic irreversibility with coercive field H_C = 5 - 63 mT and the saturation magnetization M_S ≈ 2 - 6 emu/g are found to strongly depend on the chemical composition of the alloy.
The phase diagram of the Askhin-Teller model in two dimensions is determined. Numerical calculations are performed for the simple square L × L lattice using transfer matrix technique. Exploiting finite-size scaling all unknown critical lines were obtained with good accuracy. An extended version of the Ashkin-Teller model is also considered within the molecular field renormalization group method and the critical surface for three-parameter odd-parity Hamiltonian is calculated.
We consider the frustrated antiferromagnetic s=1 Heisenberg quantum spin chain with regard to the Marshall-Peierls sign rule. Using exact diagonalization data we investigate the breakdown of the Marshall- Peierls sign rule in dependence on frustration and compare our findings with data for s=1/2. We calculate a critical value of frustration J_{2}^{crit}, where the Marshall-Peierls sign rule is violated. The extrapolation of this value to the infinite chain limit holds J_{2}^{crit} ≈ 0.016, lower than in the case of s=1/2 (J_{2}^{crit} ≈ 0.027). T his points to a stronger influence of frustration in the case of s=1. Nevertheless, the calculation of the weight of the Ising-states violating the Marshall- Peierls sign rule shows that the latter holds approximately even for a quite large frustration and may be used for numerical techniques.
The critical coupling and spontaneous magnetization curve for the 2D-Ising model are calculated using the effective-field method with correlations. An application of the method to the quantum S=1/2 1D-Heisenberg model is presented and reliable low-temperature estimates of the specific heat are evaluated.
The results of experimental and theoretical study of magnetic domain structure drift in low frequency oscillating magnetic field oriented perpendicular to the sample plate are presented. Experimental study was performed on uniaxial iron garnet (TbErGd)₃(FeAl)₅O₁₂ (111) plate with rhombic anisotropy for the case when orientation of domain walls of stripe domains is preserved. Dynamic domain structure was revealed by means of magnetooptic Faraday effect and registered by high speed digital camera at the speed equal to 1200 fps. Theoretical model based on the motion equations for coupled harmonic oscillators that takes into account attenuation and field inhomogeneity along the plate is proposed.
The destructive laboratory device, generating half-period sinus-shaped magnetic field pulses of 0.15-2 ms duration is investigated numerically. The coil was placed into a steel reinforcement cylinder to resist magnetic forces, while influence of thickness of the reinforcement cylinder is considered in detail. The time-dependent non-linear magneto-mechanical model and the finite element software ANSYS are employed. On the basis of the mechanical analysis, reasonable explanation of the destruction nature is provided. The numerically obtained operation threshold value was in good agreement with experimental measurements.
By using density matrix renormalization group technique we study the 1D extended anisotropic Heisenberg model. We find that starting from the ferromagnetic phase, the system undergoes two quantum phase transitions induced by frustration. By increasing the next-nearest-neighbor interaction, the ground state of the system changes smoothly from a completely polarized state to a next-nearest-neighbor correlated one. On the contrary, letting the in-plane interaction to be greater than the out-of-plane one, the ground state changes abruptly.
Although the analogies between the electron energy loss spectra and X-ray absorption spectra were recognized long ago, the possibility to observe the magnetic circular dichroism in the electron microscope was demonstrated only recently. This technique combines the atom specificity with the high spatial resolution of the transmission electron microscope. Being a very young technique, there are still many open questions concerning the optimization of the signal to noise ratio. In this paper we study the dichroic signal dependence on the sample thickness, acceleration voltage and incoming beam tilt angle of bcc-Fe and fcc-Ni.
Starting from a uniform d-wave superconducting phase we study the energy cost due to imposed unidirectional defects with a vanishing pairing amplitude. Both renormalized mean-field theory and variational Monte Carlo calculations within the t-J model yield that the energies of inhomogeneous and uniform phases are very close to each other. This suggests that small perturbations in the microscopic Hamiltonian might lead to inhomogeneous superconducting phases in real materials as observed in recent scanning tunneling microscopy on Ca_{2-x}Na_xCuO_2Cl_2.
We present results of Monte Carlo and stochastic spin dynamics simulations of a magnetic nanoparticle model system based on experimentally produced samples. Thermodynamic investigations as well as spin dynamics studies show characteristic features, both resembling magnetic dipole glass behaviour. While spin dynamics studies at T=0 yield a multitude of low energy configurations, thermodynamic simulations show a clear transition between a paramagnetic and a frozen magnetic state. Moreover, we demonstrate the application of experimentally inspired demagnetization protocols to compute low energy configurations of the systems under consideration efficiently.
The five-dimensional ferromagnetic Ising model is simulated on the Creutz cellular automaton algorithm using finite-size lattices with linear dimension 4 ≤ L ≤ 8. The critical temperature value of infinite lattice is found to be T^{χ} (∞=8.7811 (1) using 4 ≤ L ≤ 8 which is also in very good agreement with the precise result. The value of the field critical exponent (δ =3.0067(2)) is good agreement with δ =3 which is obtained from scaling law of Widom. The exponents in the finite-size scaling relations for the magnetic susceptibility and the order parameter at the infinite-lattice critical temperature are computed to be 2.5080 (1), 2.5005 (3) and 1.2501 (1) using 4 ≤ L ≤ 8, respectively, which are in very good agreement with the theoretical predictions of 5/2 and 5/4. The finite-size scaling plots of magnetic susceptibility and the order parameter verify the finite-size scaling relations about the infinite-lattice temperature.
There are various annealing techniques to influence the magnetic anisotropy of amorphous soft magnetic alloys, applicable for magnetic sensing elements. Laser irradiation of amorphous magnetic ribbons is a kind of annealing method to affect their magnetic moment orientations and consequently change their magnetic anisotropy and magnetoimpedance effect. In this paper, we have studied the role of magnetic anisotropy in the magnetoimpedance effect of laser annealed amorphous ribbons. In our theoretical study, we took into account the variations of magnetic anisotropy constants of local magnetic moments after annealing process. We showed that the flatten magnetoimpedance response which was observed in the experiments can be justified by averaging different local anisotropies induced by laser processing.
We address the interlayer coupling in a ferromagnet/antiferromagnet bilayer where the interface of the antiferromagnet is fully compensated. We discuss the role of different types of exchange interaction for the interlayer coupling and exchange bias. We propose two types of corrections to the ideal Heisenberg Hamiltonian which may explain exchange bias. The first is a correction for the angular dependence of the exchange interactions and the second a correction due to magnetostriction and interface imperfections. The first correction contributes to an anisotropy at the interface and favors either parallel or perpendicular coupling across the interface. The second correction contributes to the exchange bias. Our analysis is based on atomic spin dynamics simulations, and our results show that small corrections to the ideal Heisenberg Hamiltonian may have macroscopic consequences in systems with frustrated interatomic interactions.
The first order phase transition line in the vicinity of the tricritical Ising point region is studied in the 3D standard Ashkin-Teller model on a cubic lattice. The large-scale Monte Carlo computer experiments using the Binder- and Challa-like cumulants, the latter modified by Musial, are proposed and performed. Specific behavior of the Challa-Musial cumulants for weak first order phase transitions is discovered and its interpretation is proposed. The paper proves the arbitrarily weak first order character of phase transitions when approaching to the Ising point.
Finite-temperature properties of the Falicov-Kimball model in two approximations were studied in the perturbative regime, i.e. for t/U ≪ 1, where t=1 is the hopping constant and U=10 denotes the Coulomb interaction strength. In our study, we determined the phase diagram of the model in the second order of the perturbation theory, where it reduces to the antiferromagnetic Ising model in the emergent magnetic field. In the fourth order, where our model constitutes the Ising model with more complicated frustrated antiferromagnetic interactions, the phase diagram was established. The Monte Carlo method was employed to investigate the phase transition lines. The existence of stripe ordering at finite temperatures is proved.
The finite-temperature static properties of the spin S=1 antiferromagnetic Heisenberg chains are extensively simulated using the quantum transfer matrix method. The zero-field susceptibility and specific heat as well as the field-dependent magnetization data are evaluated to select the microscopic parameters of a number of real quasi-one-dimensional compounds and to verify some theoretical approaches.
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