Magnetization In Ferromagnetic Materials Research Articles

Spin-lattice dynamics simulation of external field effect on magnetic order of ferromagnetic iron

Modeling of field-induced magnetization in ferromagnetic materials has been an active topic in the last dozen years, yet a dynamic treatment of distance-dependent exchange integral has been lacking. In view of that, we employ spin-lattice dynamics (SLD) simulations to study the external field effect on magnetic order of ferromagnetic iron. Our results show that an external field can increase the inflection point of the temperature. Also the model provides a better description of the effect of spin correlation in response to an external field than the mean-field theory. An external field has a more prominent effect on the long range magnetic order than on the short range counterpart. Furthermore, an external field allows the magnon dispersion curves and the uniform precession modes to exhibit magnetic order variation from their temperature dependence.

Nonlinear low-frequency oscillations of magnetization in ferromagnetic materials near phase transition

The paper deals with the study of nonlinear oscillations of magnetization, which evolve in ferromagnetic materials at low frequencies in the vicinity of the spin-orientation transition. It is shown that these effects occur when easy-axis anisotropy is realized, as occurs in the case of the magnetic impedance in wires covered with a magnetic coating and when anisotropy is caused by the specimen shape alone.

Modeling plastic deformation effect on magnetization in ferromagnetic materials

Based on the Sablik-Landgraf model, an integrated model has been developed which provides a description of the effect of plastic deformation on magnetization. The modeling approach is to incorporate the effect of plastic deformation on the effective field and that on the model parameters. The effective field incorporates the contributions of residual stress, stress demagnetization term, and the plastic deformation. We also consider the effect of plastic deformation on the model parameters: pinning coefficient, the scaling constant and the interdomain coupling coefficient. The computed magnetization exhibits sharp change in the preliminary stage of plastic deformation, and then decreases slowly with the increase of plastic strain, in agreement with experimental results.

Physical model of plastic deformation on magnetization in ferromagnetic materials

Traditional magnetic/stress coupling models are based on the magnetoelastic theory, which cannot explain magnetic phenomena when ferromagnetic materials are subjected to a plastic deformation. To promote the development of magnetomechanical theories, a new physical-based magnetic/stress coupling model was proposed in this paper, where the effects of the applied magnetic field, elastic and plastic deformations on the magnetization are, respectively, considered in terms of different acting mechanisms. Moreover, the influences of variants on the strength in different magnetization stages are also calculated.

Paradigm of the time-resolved magneto-optical Kerr effect for femtosecond magnetism

The magneto-optical Kerr effect (MOKE) is a powerful tool for studying changes in the magnetization of ferromagnetic materials. It works by measuring changes in the polarization of reflected light. However, because the conventional theoretical basis for interpreting a MOKE signal assumes measurement with continuous-wave light1,2, its use for understanding high-speed magnetization dynamics of a material probed with femtosecond optical pulses3,4 has been controversial5,6,7,8,9,10. Here we establish a new paradigm for interpreting time-resolved MOKE measurements, through a first-principles investigation of ferromagnetic nickel. We show that the time-resolved optical and magnetic responses energetically follow their respective optical and magneto-optical susceptibilities. As a result, the one-to-one correspondence between them sensitively depends on the incident photon energy. In nickel, for photon energies below 2 eV the magnetic response is faithfully reflected in the optical response, but above 2 eV they decouple. By constructing a phase-sensitive polarization versus magnetization plot, we find that for short pulses the magnetic signals are delayed by 10 fs with respect to the optical signals. For longer pulses, the delay shortens and the behaviour approaches the continuous-wave response. This finally resolves the long-standing dispute over the interpretation in the time-resolved MOKE measurements and lays a solid foundation for understanding femtomagnetism3,4.

New PLAD apparatus and fabrication of epitaxial films and junctions of functional materials: SiC, GaN, ZnO, diamond and GMR layers

Abstract We have developed a new pulsed laser ablation–deposition (PLAD) apparatus and techniques for fabricating films of high-temperature or functional materials, including two short-wavelength lasers: (a) a YAG 5th harmonic (213 nm) and (b) Raman-shifted lasers containing vacuum ultraviolet light; also involved are (c) a high-temperature heater with a maximum temperature of 1350 °C, (d) dual-target simultaneous ablation mechanics, and (e) hybrid PLAD using a pico-second YAG laser combined with (c) and/or (d). Using the high-T heater, hetero-epitaxial films of 3C–, 2H– and 4H–SiC have been prepared on sapphire-c. In situ p-doping for GaN epitaxial films is achieved by simultaneous ablation of GaN and Mg targets by (d) during film growth. Junctions such as pGaN (Mg-doped)-film/n-SiC(0 0 0 1) substrate and pGaN/n-Si(1 1 1) show good diode characteristics. Epitaxial films with a diamond lattice can be grown on the sapphire-c plane by hybrid PLAD (e) with a high-T heater using a 6H-SiC target. High quality epitaxial films of ZnO are grown by PLAD by introducing a low-temperature self-buffer layer; magnetization of ferromagnetic materials is enforced by overlaying on a ferromagnetic lattice plane of an anti-ferromagnetic material, showing the value of the layer-overlaying method in improving quality. The short-wavelength lasers are useful in reducing surface particles on functional films, including superconductors.

Lie symmetry of the Landau-Lifshitz-Gilbert equation and exact linearization in the Minkowski space

In this paper we present a linear representation of the Landau-Lifshitz-Gilbert equation for describing the magnetization of ferromagnetic materials. According to Lie’s theory, we prove that this equation admits a superposition principle and its formula is derived. The underlying vector space of the Landau-Lifshitz-Gilbert equation is found to be a projective Minkowski space denoted by $$P {\Bbb M}^{3+1},$$ of which the projective proper orthochronous Lorentz group PSO o(3,1) left acts. By the Lie symmetry a group preserving scheme is developed, which improves the computational accuracy and efficiency.

Numerically efficient steady-state finite-element analysis of magnetically saturated electromechanical devices

We present a numerically efficient steady-state finite-element solver for electromechanical devices incorporating magnetic saturation in which the magnetization of ferromagnetic materials is modeled as a field-dependent equivalent current density. We use a Lagrangian representation of continuum variables, thereby removing numerical instabilities due to the Peclet effect but precluding the use of standard harmonic balance methods. The shooting-Newton method is therefore used to calculate the steady-state behavior. A matrix-free Krylov-subspace linear solver, the generalized minimum residuals method (GMRES), dramatically reduces the computational burden by eliminating the need to calculate the shooting-Newton Jacobian. Simulation results from a synchronous reluctance motor model with 10 288 nodes and 4935 elements confirm that the proposed method requires much less computation time than running transient analysis until convergence.

Magnetic hysteresis modeling of electronic sensor devices and components

The non-linear dependence on history of magnetic field and magnetization in ferromagnetic materials is described by statistical domain behaviour using phenomenological parameters. These parameters are related to macroscopic hysteresis features (as coercivity, initial susceptibility, saturation, and remanence), which allows fast and easy identification. Consequently, these parameters are related to spontaneous magnetization, magnetocrystalline anisotropy, magnetostriction, and microstructure geometry, using dimensionless microscopic constants. This step allows correct interpretations of the parameter dependence and the macroscopic hysteresis on stress, temperature, and direction of magnetization. Examples of model applications are the prediction of the magnetic behaviour of thin films in magnetoresistive sensors, stress dependence of magnetization in amorphous ribbons, and Barkhausen noise.

Prediction and measurements of a current derivative sensor response for voltage pulses applied to induction machines

Current derivative sensors (CDI’s) can advantageously be used in modern drives to determine the position of the rotor or flux in induction machines if no sensor at the shaft is available or to realize an effective condition monitoring of the drive. This is achieved by exploiting the transient current slope of the machine due to voltage pulses impressed by the switching of the inverter. To enable a CDI optimization with reference to the demands given by the drive application, a simulation model is presented which describes the nonlinear dependence on the history of the magnetic field and magnetization in ferromagnetic materials by statistical domain behavior using phenomenological parameters. In addition, measurements and simulations of the developed sensor output signals are presented which are used in a 15 kW induction machine drive. The optimized CDI sensitivity is S=0.35 μΩs and the bandwidth exceeds the characteristic of standard current sensors by five times.

Detection and classification of material attributes-a practical application of wavelet analysis

We describe a method for classifying material properties from measurements of the Barkhausen effect, which originates from a fast magnetization of ferromagnetic materials using alternating currents. We use wavelet analysis to develop a tool box for evaluating Barkhausen measurements. The described wavelet techniques allow detection of extremely weak signals in the Barkhausen noise voltage. By using a statistical classification rule, we show that the detected structures are directly related to material properties.

Implications of Io's magnetic signature: Ferromagnetism?

The Galileo spacecraft recently measured an ≈ 40% decrease of Jupiter's magnetic field near Io from a distance of 0.5 RIo [Kivelson et al., 1996], a signature that cannot be explained in terms of the previously known Alfven wing current system alone. The Galileo observations can be explained by an additional magnetic moment of Io aligned with the ambient Jovian field. There is no Earth‐like magnetosphere at Io because its magnetic moment is not sufficient to stand off the Jovian field far above Io's atmosphere. The Alfven wing current system must close within the Io interior, implying an electrical conductivity in the outer layers much higher than that expected for solid rock alone. Since widespread sulfur‐rich aquifers are present to support Io's extensive volcanism, the thermal and electrical conductivities in Io's outer layers will be higher than would be the case for solid rock. We suggest that on average the temperature remains below the Curie point to a depth on the order of at least 300 km. Io's magnetic moment could then be explained as thermoremanent magnetization of ferromagnetic material. In this model, Io's outer layers cooled through the Curie temperature in a magnetic field about 15 times the current ambient field at Io. At that time, Io was closer to Jupiter, and/or Jupiter's dynamo magnetic moment was stronger.

Detection of irradiation effects of reactor vessel steels by magneto-acoustic emission : 47722 Kwon, O.-Y.; Ono, K. Journal of Acoustic Emission, Vol. 9, No. 4, pp. 227–236 (Oct. – Dec. 1991)

Abstract : The irradiation effects of reactor vessel steels were studied by magneto-acoustic emission (MAE). MAE is a new method of non-destructive evaluation. It is generated primarily by domain wall motions during magnetization of ferromagnetic materials in the alternating magnetic field. Since MAE activity is sensitive to the microstructural changes, it is applied to determine the status of steel samples exposed to neutron irradiation. Two commercial steels, A302B plate and A533B weld, and two model alloys were irradiated at four different conditions; high and low flux, two fluence levels and three temperatures. For each material/condition and unirradiated control samples, MAE intensity in rms voltage with increasing magnetic field was measured. Direct and envelope-detected MAE waveforms were also recorded. Measurements were also made for the samples unirradiated but aged at 288 C up to 278 hours in order to separate the effects of thermal aging from irradiation effects. Barkhausen effects were also investigated using a surface probe. The results presented here show that MAE waveform analysis can be a promising non-destructive evaluation technique for monitoring the microscopic changes in the steel components subjected to neutron irradiation.

A new method for fluxgate magnetometers using the coupling property of odd and even harmonics

A new method suitable for the single core fluxgate magnetometer has been proposed. The method makes use of the coupling property of odd and even harmonics generated by the self-product of the induced voltage during the magnetization of ferromagnetic material by an external field superposed on the AC driving magnetic field. The power difference between the two half-periods of magnetization was expressed by the coupling of odd and even harmonics, which is proportional to the external field under the condition that Hext<<HAC and HAC is constant, where Hext and HAC are the external magnetic field and AC driving field amplitude respectively.

Recent developments in modeling of the stress derivative of magnetization in ferromagnetic materials

The effect of changing stress on the magnetization of ferromagnetic materials leads to behavior in which the magnetization may increase, or decrease, when exposed to the same stress under the same external conditions. A simple empirical law seems to govern the behavior when the magnetization begins from a major hysteresis loop. The application of the law of approach, in which the derivative of the magnetization with respect to the elastic energy supplied dM/dW is proportional to the magnetization displacement Man−M, is discussed.

Magnetic viscosity, fluctuation fields, and activation energies (invited)

Time dependence of magnetization in ferromagnetic materials was first described towards the end of the 19th century. Subsequently, two types of mechanisms responsible for time dependent behavior were identified and became known as ‘‘diffusion’’ and ‘‘fluctuation’’ after-effect or viscosity. The former depends on thermally induced motion of impurity atoms. The latter is a consequence of thermal activation of irreversible domain processes such as domain-wall motion and the nucleation of domains of reverse magnetization. Fluctuation viscosity affects, to a greater or smaller extent, all magnetic materials subject to hysteresis. In the late 1940s descriptions of magnetic viscosity in terms of fluctuation fields (Néel) and activation energy distributions (Street and Woolley) were developed. The two approaches will be described. An analysis of the time dependent phenomena exhibited by magneto-optical films will be presented as a simple example of the application of activation energy modeling.

Modeling the effects of stress on magnetization in ferromagnetic materials (abstract)

In previous papers, it has been shown how stress affects the magnetic properties of materials. Although a model theory has been developed for the case of changing H field under constant stress, so far no attempts have been made to quantitatively model the effects of changing stress under constant field. The main principle in this case is the approach of the magnetization to the anhysteretic. The anhysteretic is itself stress dependent, and therefore presents a moving target for the bulk magnetization. The displacement of the bulk magnetization in isotropic media is determined by the elastic energy supplied ΔW=(1/E)(Δσ)2, where E is the elastic modulus, and Δσ the change in stress. The displacement of the magnetization D=Man(H,σ)−M(H,σ) decays with W according to dD/dW=−ξD, where ξ is the decay coefficient. Solving this equation gives,Man(H,σ)−M(H,σ)=[(Man(H, σ)−M(H,σ0)]exp[(−ξ/E)(Δσ)2], which can be rewritten in terms of the change in magnetization with stress ΔM(H,σ,σ0) the magnetomechanical effect,ΔM(H,σ,σ0)=[Man(H,σ)−M(H, σ)]{1−exp[(−ξ/E)(Δσ)2]}. The stress dependent anhysteretic Man(H,σ) can then be determined from the unstressed anhysteretic through the addition of the extra effective field term Hσ: Hσ=(3/2)(σ/μ0)(dλ/dM) so that Man(H,σ)=Man(H + Hσ). Through the use of the last three equations it is possible to describe the changes in magnetization of the material arising from changing stress.

A differential equation approach to minor loops in the Jiles-Atherton hysteresis model

The reason for the failure of the differential equations to yield physical minor loops involved in the magnetization in ferromagnetic materials is explained. A modified solution for minor loops is presented which retains the features of D.C. Jiles and D.L. Atherton's (1983) original motor loops, but only requires knowledge of the initial point on each portion of the loop to obtain the solution. This yields a general differential equation formulation for the Jiles-Atherton theory that can be used with circuit simulations having arbitrary excitations and initial conditions for ferromagnetic components. >

Stress effect on the magnitude of acoustic emission during magnetization of ferromagnetic materials

The effect of stress on the magnitude of acoustic emission during magnetization of mild steel and nickel has been investigated. It has become clear that the magnitude of acoustic emission during magnetization shows a strong dependence on stress. This effect should have nondestructive testing applications suited for field inspection.

Acoustic emission characteristics during magnetization of ferromagnetic materials

The characteristics of acoustic emission generated during magnetization of typical ferromagnetic materials (steel, nickel, and cobalt) in alternating magnetic field of 50 Hz were investigated. Acoustic emission parameters measured were root‐mean‐square voltage, frequency spectrum, and waveform. The data were taken as a function of stress, magnetic field strength, and annealing time. It became clear that the magnitudes of acoustic emission during magnetization of steel and nickel were strongly dependent on stress and the features of these stress dependencies changed with magnetic field strength. It was found that the magnitude of acoustic emission during magnetization of cobalt was much smaller than those of steel and nickel. This experimental result and the theoretical calculation of elastic energy emitted by domain wall motion show that the motion of 90° domain wall is the most probable source for the acoustic emission during magnetization of ferromagnetic materials.