Random External Magnetic Field Research Articles

Effect of a Gaussian random external magnetic field with spatio temporal variation on compensation in Ising spin-1/2 trilayered square ferrimagnets

Effect of a Gaussian random external magnetic field with spatio temporal variation on compensation in Ising spin-1/2 trilayered square ferrimagnets

Effect of a uniform random external magnetic field with spatiotemporal variation on compensation in Ising spin-1/2 trilayered square ferrimagnets.

Trilayered spin-1/2 Ising ferrimagnets are interesting thin systems for compensation phenomenon. In this work, a Metropolis Monte Carlo study is performed on the magnetic and thermodynamic response of such a system on square Bravais lattice, driven by uniform random external magnetic field with spatiotemporal variations. In two distinct configurations, the surface layers are made up of A and the middle layer is made up of B atoms in an ABA-type stacking while in AAB-type stacking, the top layer and the middle layer is made up of A atoms while the bottom layer is made up of B atoms. The magnetic coupling between the like atoms (A-A and B-B) is ferromagnetic while between the unlike atoms (A-B), it is antiferromagnetic. For the time-dependent external uniform random field, the mean is always set to zero and the standard deviation is varied until spin-field energy is comparable to the dominant cooperative energy of the system. The findings show that the observed compensation and critical points shift and steady-state magnetic behaviors shift among N, L, P, and Q, etc., type of ferrimagnetic behaviors, depending on the strength of external uniform random field. The compensation phenomenon even vanishes after crossing a finite threshold of standard deviation of the magnetic field for particular choices of the other controlling parameters. Thus islands of ferrimagnetic phase without compensation appear within the phase area with compensation of field-free case, in the two-dimensional Hamiltonian parameter space. For both the configurations, the areas of such islands even grow with increasing standard deviation of the external field, σ, obeying a scaling relation of the form: f(σ,A(σ))=σ^{-b}A(σ) with b_{ABA}=1.958±0.122 and b_{AAB}=1.783±0.118.

Modeling of Three-Axis Hall Effect Sensors Based on Integrated Magnetic Concentrator

In this paper, we develop computational model to analyze the magnetic concentrating effect of integrated magnetic concentrator (IMC) on surrounding external magnetic field. We present an IMC-based three-axis Hall sensor model that enables to measure both inclination angles and absolute strength of a random external magnetic field. An IMC changes surrounding parallel magnetic components into perpendicular components, and therefore allows the horizontal Hall plates to measure both the strength and inclination angles of parallel external magnetic fields. We develop a finite element method (FEM) based model in COMSOL Multiphysics for the three-axis Hall sensor. Key factors influencing IMC's magnetic concentrating effect, including material property and sensor structure, are investigated and discussed using the developed model. Comparing to traditional IMC-based three-axis angular sensors, a reference permanent magnet is no longer needed in the sensor. A measurement accuracy of 0.8 and 1.2 degrees are achieved respectively for the angles of α and θ of external magnetic field.

Deterministic quantum state transfer of atoms in a random magnetic field

We propose a method for transferring atoms to a target quantum state for a multilevel quantum system with sequentially increasing, but otherwise unknown, energy splitting. This is achieved with a feedback algorithm that processes off-resonant optical measurements of state populations during adiabatic rapid passage in real-time. Specifically, we reliably perform the transfer $|F=2,m_F=2\rangle \rightarrow |1,1\rangle \rightarrow |2,1\rangle$ for a sample of ultracold $^{87}$Rb in the presence of a random external magnetic field.

A New Approach to Calculate the Shielding Factor of Magnetic Shields Comprising Nonlinear Ferromagnetic Materials under Arbitrary Disturbances

To enable the realization of ultra-low magnetic fields for scientific and technological research, magnetic shielding is required to create a space with low residual magnetic field and high shielding factors. The shielding factors of magnetic shields are due to nonlinear material properties, the geometry and structure of the shields, and the external magnetic fields. Magnetic shielding is used in environments full of random realistic disturbances, resulting in an arbitrary and random external magnetic field, and in this case, the shielding effect is hard to define simply by the shielding factors. A new method to simulate and predict a dynamic internal space magnetic field wave is proposed based on the Finite Element method (FEM) combined with the Jiles-Atherton (JA) model. By simulating the hysteresis behavior of the magnetic shields and establishing a dynamic model, the new method can simulate dynamic magnetic field changes inside magnetic shields as long as the external disturbances are known. The shielding factors under an AC external field with a sine wave and certain frequencies are calculated to validate the feasibility of the new method. A real-time wave of internal magnetic flux density under an AC triangular wave external field is simulated directly with the new method versus a method that splits the triangular wave into several sine waves by a Fourier transform, divides the shielding factors, and then adds the quotients together. Moreover, real-time internal waves under some arbitrary fields are measured. Experimental internal magnetic flux density waves of a 4-layer magnetically shielded room (MSR) at the Harbin Institute of Technology (HIT) fit the simulated results well, taking experimental errors into account.

Spin glasses in a random external magnetic field

A spin glass placed in a random external magnetic field is investigated. The magnetic relaxation of the spin glass is believed to follow a power law. It is demonstrated that, in this case, the glass magnetization variation is a non-Markovian process. Statistical characteristics of the magnetization fluctuations are obtained.

Random-field-induced order in a bosonic t-J model

In this paper, we study the effect of a random quenched external field for spin order and also multiple Bose–Einstein condensation (BEC). This system is realized by the cold atomic gases in an optical lattice. In particular, we are interested in the strong-repulsion region of the two-component gases for which the bosonic t-J model is a good effective model. In the bosonic t-J model, a long-range order of the pseudo-spin and also BEC of atoms appear quite naturally as in the fermion t-J model for the high-temperature superconducting materials. Random Raman scattering between two internal states of a single atom plays a role of the random external field, and we study its effects on the pseudo-spin order and the BEC by means of quantum Monte-Carlo simulations. The random external field breaks a continuous U(1) symmetry existing in the original bosonic t-J model and it induces new orders, named random-field-induced order (RFIO). We show a phase diagram of the bosonic t-J model with the random external magnetic field and study the robustness of the RFIO states. We also study topological excitations like vortices and the domain wall in the RFIO state. Finally, we point out the possibility of a quantum bit by the RFIO.

Optimized GPU simulation of continuous-spin glass models

We develop a highly optimized code for simulating the Edwards-Anderson Heisenberg model on graphics processing units (GPUs). Using a number of computational tricks such as tiling, data compression and appropriate memory layouts, the simulation code combining over-relaxation, heat bath and parallel tempering moves achieves a peak performance of 0.29 ns per spin update on realistic system sizes, corresponding to a more than 150 fold speed-up over a serial CPU reference implementation. The optimized implementation is used to study the spin-glass transition in a random external magnetic field to probe the existence of a de Almeida-Thouless line in the model, for which we give benchmark results.

Covariant Gauge on the Lattice: A New Implementation

We derive a new implementation of linear covariant gauges on the lattice, based on a minimizing functional that can be interpreted as the Hamiltonian of a spin-glass model in a random external magnetic field. We show that our method solves most problems encountered in earlier implementations, mostly related to the no-go condition formulated by Giusti [Nucl. Phys. B498, 331 (1997)]. We carry out tests in the SU(2) case in four space-time dimensions. We also present preliminary results for the transverse gluon propagator at different values of the gauge parameter xi.

Effects of double-Gaussian disorder on the dynamics of the quantumXYchain

We investigate the dynamics of the random one-dimensional spin-1/2 quantum isotropic $XY$ model in high-temperature limit by the recurrence relations method. Both the time-dependent transverse autocorrelation function and the corresponding spectral density are calculated for the cases that the exchange couplings between spins or transverse fields satisfy the double-Gaussian distribution. It is found that when the value of ${\ensuremath{\sigma}}_{J}$ (standard deviation of random exchange coupling) or ${\ensuremath{\sigma}}_{B}$ (standard deviation of random external magnetic field) is small, the dynamics of the system undergoes a crossover between a central-peak behavior and a collective-mode one. However, when ${\ensuremath{\sigma}}_{J}$ or ${\ensuremath{\sigma}}_{B}$ is large enough, the crossover vanishes, and the system shows the most disordered behavior or a central-peak behavior. We also investigate the cases that the couplings or fields satisfy the bimodal distribution and Gaussian distribution, which are the two special cases of double-Gaussian distribution, and find that the dynamics of the system is similar to that of the one-dimensional transverse Ising system.

On the mean-field Ising model in a random external field

We use a method developed by van Hemmen to obtain the free energy of the mean-field Ising model in a random external magnetic field. Some results of previous mean-field calculations are confirmed and generalized. The tricritical point in the global phase diagram is discussed in detail. We also consider different probability distributions of the random fields and provide some proofs regarding the conditions for the existence of a tricritical point.

The density of states of a two-dimensional electron gas in a random external magnetic field

An argument is presented to explain the observed increase in the density of states at low energy of a hamiltonian describing a charged particle with spin 1 2 and with a g-factor = 2 in the presence of a random magnetic field in two dimensions. This effect is equivalent to chiral symmetry breaking in the quenched massless euclidean Schwinger model.

Random Magnetic Fields, Supersymmetry, and Negative Dimensions

We prove the equivalence, near the critical point, of a $D$-dimensional spin system in a random external magnetic field with a ($D\ensuremath{-}2$)-dimensional spin system in the absence of a magnetic field. This is due to the hidden supersymmetry of the associated stochastic differential equation. We identify a space with one anticommuting coordinate with a space having negative dimensions -2.