Analysis Of Magnetization Processes Research Articles

First-principles calculations of magnetic properties for analysis of magnetization processes in rare-earth permanent magnets

ABSTRACT It has been empirically known that the coercivity of rare-earth permanent magnets depend on the size and shape of fine particles of the main phase in the system. Also, recent experimental observations have suggested that the atomic scale structures around the grain-boundaries of the fine particles play a crucial role to determine their switching fields. In this article, we review a theoretical attempt to describe the finite temperature magnetic properties and to evaluate the reduction of the switching fields of fine particles of several rare-earth permanent magnet materials based on an atomistic spin model that is constructed using first-principles calculations. It is shown that, over a wide temperature range, the spin model gives a good description of the the magnetization curves of rare-earth intermetallic compounds such as FeB (=Dy, Ho, Pr, Nd, Sm) and SmFe. The atomistic spin model approach is also used to describe the local magnetic anisotropy around the surfaces of the fine particles, and predicts that the rare-earth ions may exhibit planar magnetic anisotropy when they are on the crystalline-structure surfaces of the particles. The dynamical simulation of the atomistic spin model and the corresponding micromagnetic simulation show that the planar surface magnetic anisotropy causes a reduction in the switching field of fine particles by approximately 20-30%, which may be relevant to the atomic scale surface effects found in the experimental studies.

Numerical analysis of magnetization process of high-Tc superconductor undulator of X-ray FEL using power law conductivity

Numerical analysis of magnetization process of high-Tc superconductor undulator of X-ray FEL using power law conductivity

Energy contributions in magnetite nanoparticles: computation of magnetic phase diagram, theory, and simulation

In this study, we present a theoretical analysis of magnetization processes by considering energy contributions in magnetite fine particles. The focus is on the K S-driven magnetic phase transition taking place between the low surface-anisotropy ferrimagnetic state and the hedgehog configuration obtained in the high surface-anisotropy limit. Analytical expressions of energy terms (exchange, magnetocrystalline anisotropy, surface-anisotropy) are presented and their magnitudes are computed for different particle sizes. Monte Carlo simulations were also carried out for comparison purposes. A core–shell model is implemented for simulating magnetite nanoparticles between 2 and 10 nm in diameter. Our simulation framework is based on a three-dimensional classical Heisenberg-like Hamiltonian with nearest magnetic neighbors interactions. It includes exchange coupling, cubic magnetocrystalline anisotropy for core ions, and single-ion site surface-anisotropy for those atoms belonging to the shell. The magnetic phase diagram and comparisons between analytical and numerical results are presented and discussed.

Thermal Evolution of Magnetic Interactions in Ni Nanowires Embedded in Polycarbonate Membranes by Ferromagnetic Resonance

Ni polycrystalline nanowires with diameters of 50, 80, and 100 nm were electrodeposited in cylindrical pores of track-etched polycarbonate membranes. Their magnetic properties were determined as a function of temperature using ferromagnetic resonance and magnetization measurements. At room temperature, the uniaxial anisotropy is equal to the shape anisotropy whereas an additional contribution is evidenced at low temperature. This additional contribution is attributed to magnetoelastic effects induced in the nanowires due to the different thermal expansion constants of Ni and polycarbonate. The analysis of magnetization processes in Ni nanowire arrays evidenced strong dipolar interactions inside the wires due to the domain structure. The coercive field of the nanowires was shown to be nearly a linear function of the temperature and could be accounted for temperature dependence of the uniaxial anisotropy.

Analysis of magnetization processes in composite media grains

Composite grains, consisting of a subgrain with high anisotropy field coupled with a subgrain with zero anisotropy, are analyzed using a two-spin model. An analytical expression is given for the exchange coupling between the subgrains that minimizes the field required to reverse the magnetically hard layer. The results of the two-spin model are compared with those of a spin chain that represents an “exchange spring magnet.” The limitations of the two-spin model are worked out.

Effective magnetic anisotropy vector field

An effective anisotropy vector field that can be treated as an effective external field has been formulated for uniaxial anisotropies with n-fold symmetry within the plane containing the main axis. A simple method for obtaining the effective field for the two-fold symmetry uniaxial anisotropy and the calculated result according to the method are presented. Then the extension to n-fold symmetry anisotropies is given and the calculated results for four- and six-fold symmetry cases are demonstrated. The tips of those vector fields are found to trace hypocycloids moving oppositely with the rotation of magnetization. The newly introduced effective anisotropy vector field may make a new tool in various analysis of magnetization processes and help understand the effect of anisotropy on magnetization visually.

Analysis of Magnetization Processes in Nanocomposite Hard Magnetic Materials from Macroscopic Magnetic Measurements and X-RAY Magnetic Circular Dichroism

ABSTRACTA series of RExTMbalBy (3≤x≤5.5, 15≤y≤25 at%) nanocomposite magnets were prepared using the classical method of alloy rapid quenching and subsequent devitrification. Appropriate elemental additions and substitutions allowed the technical magnetic properties to be optimized. The analysis of the both hysteresis cycles and static susceptibility measurements in the range 20K-300K reveals that magnetization reversal in these materials can be interpreted schematically as a two-step process : the initial magnetization reversal is dominated by reversible processes while irreversible processes, which characterize the coercivity mechanism, occur at larger field values. To complement this study, X-Ray Magnetic Circular Dichroism (XMCD) spectroscopy was applied as an element-selective probe of magnetization reversal in nanocomposite samples. This allowed the hysteresis cycles of both hard and soft phases to be separated from the total cycle. Magnetization processes in these bulk heterogeneous systems are discussed in the light of information brought by the complementary applied experimental techniques.

Numerical analysis of magnetization processes in type II superconductors based on Ginzburg-Landau theory

This paper describes the numerical analysis of the magnetization process of type II superconductors based on the Ginzburg-Landau theory. A computer code based on the finite difference method was developed and it was applied to simulate the magnetization process of type I and type II superconductors. As regards the time integration method, the CPU times required for the computation in some cases of GL-parameter were compared for two difference schemes, the backward and the forward schemes.

An analysis of magnetization processes in metal evaporated tape

It is shown that conventional hysteresis data completely mask the true magnetic properties of metal evaporated (ME) tape. For a true demagnetization correction a continuous adjustment of the external field and its angle during measurement has to be accomplished. Using vector magnetometry, an iterative method for the determination of the static hysteresis compensated for film demagnetization of obliquely oriented thin-film recording media was developed. It is shown that the ME tape resembles a particulate medium which is highly oriented and has an extremely narrow switching field distribution. The magnetization reversal process is incoherent with a strong increase of switching fields at high angles between the field and the intrinsic easy axis which is located at an angle of 38 degrees to the film plane. It is shown that thermal activation has a strong effect on the measured data. >