David Jiles

Theory of ferromagnetic hysteresis

Abstract A mathematical model of the hysteresis mechanisms in ferromagnets is presented. This is based on existing ideas of domain wall motion including both bending and translation. The anhysteretic magnetization curve is derived using a mean field approach in which the magnetization of any domain is coupled to the magnetic field H and the bulk magnetization M . The anhysteretic emerges as the magnetization which would be achieved in the absence of domain wall pinning. Hysteresis is then included by considering the effects of pinning of magnetic domain walls on defect sites. This gives rise to a frictional force opposing the movement of domain walls. The impedance to motion is expressed via a single parameter k , leading to a simple model equation of state. This exhibits all of the main features of hysteresis such as the initial magnetization curve, saturation of magnetization, coercivity, remanence, and hysteresis loss.

Numerical determination of hysteresis parameters for the modeling of magnetic properties using the theory of ferromagnetic hysteresis

The authors describe how the various model parameters needed to describe hysteresis on the basis of the Jiles-Atherton theory can be calculated from experimental measurements of the coercivity, remanence, saturation magnetization, initial anhysteretic susceptibility, initial normal susceptibility, and maximum differential susceptibility. The determination of hysteresis parameters based on this limited set of magnetic properties is of the most practical use since these are the properties of magnetic materials that are most likely to be available. >

Theory of ferromagnetic hysteresis (invited)

A mathematical theory of hysteresis in ferromagnetic materials is presented based on existing ideas of domain wall motion and domain rotation. Hysteresis is shown to occur as a result of impedances to changes of magnetization such as when domain walls are pinned, while the mutual interactions of the magnetic moments are shown to be of secondary importance in this respect. An equation for the anhysteretic or ideal magnetization curve is derived based on a mean field approximation and this is shown to be dependent on the mutual interactions of the moments but independent of impedances such as pinning. The introduction of a term which measures the impedance to changes in magnetization leads to a simple differential equation of state for a ferromagnet which exhibits all the features of hysteresis. Some modifications of the simple model are necessary in order to bring the solution closer to the real situation. Results are presented which show all the features of hysteresis such as initial magnetization curve, ...

Coupled magnetoelastic theory of magnetic and magnetostrictive hysteresis

It is demonstrated that hysteresis in the magnetostriction k is coupled to hysteresis in the magnetization M because of the dependence of the magnetostriction on the magnetization. At the same time, when stress is present, the magnetization is in turn coupled to the behavior of the part of the magnetostriction associated with domain moment rotation. An expression for the magnetostriction is formulated, and numerical modeling results for magnetostriction hysteresis are compared to experimental results. Although some features of the magnetostriction in iron and steel still need additional explanation, the main features of the magnetostriction are accounted for. These include liftoff (failure of the magnetostriction to return to its value in the demagnetized state as the hysteresis loop is cycled) and a magnetostriction increase after flux density B reaches its maximum and starts to decrease. A macromagnetic, multidomain formulation that yields zero magnetostriction in the demagnetized specimen is used. >

Theory of the magnetisation process in ferromagnets and its application to the magnetomechanical effect

A theory of the magnetisation process in ferromagnets, based on existing ideas of domain rotation and domain wall motion is presented. This has been developed via a consideration of the various energy terms into a mathematical description of the process leading to an equation of state for a ferromagnet. The differential equation has been solved and a solution containing terms up to the second order presented, showing the essential features of ferromagnetic hysteresis. The theory has then been used to explain the effects of stress on magnetisation. It has been found that the magnetisation approaches the anhysteretic curve when a ferromagnet is subjected to stress and this is the underlying principle behind such changes in magnetisation. The change of magnetisation with stress can not be predicted solely on the basis of the magnetostriction coefficient except in special cases when the initial (zero stress) conditions of magnetisation lie on the anhysteretic. This condition is also approximately satisfied at higher fields.

Review of magnetic methods for nondestructive evaluation

Abstract This paper provides a guideline to the literature of magnetic techniques for nondestructive evaluation. Compared with other NDE methods such as ultrasonics or eddy currents, the literature for magnetic methods is relatively small, but one of the difficulties is that it is widely scattered. This review therefore presents a fairly comprehensive summary of works on Barkhausen effect, magnetoacoustic emission, magtenic hysteresis, residual field and magnetically induced velocity change methods that have appeared to date.

Manganese-substituted cobalt ferrite magnetostrictive materials for magnetic stress sensor applications

Metal bonded cobalt ferrite composites have been shown to be promising candidate materials for use in magnetoelastic stress sensors, due to their large magnetostriction and high sensitivity of magnetization to stress. However previous results have shown that below 60°C the cobalt ferrite material exhibits substantial magnetomechanical hysteresis. In the current study, measurements indicate that substituting Mn for some of the Fe in the cobalt ferrite can lower the Curie temperature of the material while maintaining a suitable magnetostriction for stress sensing applications. These results demonstrate the possibility of optimizing the magnetomechanical hysteresis of cobalt ferrite-based composites for stress sensor applications, through control of the Curie temperature.

Theory of ferromagnetic hysteresis: determination of model parameters from experimental hysteresis loops

The authors describe how the parameters defining hysteresis in the theory of hysteresis can be obtained from a set of experimental magnetization measurements. The hysteresis parameters can be calculated from the anhysteretic susceptibility at the origin, the coercivity, the remanence, and the coordinates of the hysteresis loop tip. The ability to calculate the hysteresis parameters from a set of experimental data is a key step in the process of utilization of the hysteresis model. The algorithm that has been developed successfully determines the values of the parameters to within a few percent in most cases. >

The development of highly magnetostrictive rare earth-iron alloys

This paper reviews recent developments in highly magnetostrictive terbium-dysprosium-iron alloys. This includes discussion of domain structure, the magnetization process, microstructure, sample preparation, magnetostriction, effects of different chemical compositions on magnetic properties of the alloys, and possible applications of these materials. It has been found over recent years that there has been a steady improvement in material properties, both in terms of increased strain amplitude and reduction in variability from sample to sample. The material is now finding applications in magnetostrictive transducers, actuators and adaptive vibration control systems.

Modelling the effects of eddy current losses on frequency dependent hysteresis in electrically conducting media

In recent studies it has been shown how the effects of classical eddy current losses can be used to extend the quasi-static hysteresis model to account for the frequency dependence of hysteresis in electrically conducting media. This paper deals with the extension of the hysteresis model to include anomalous or excess losses. The instantaneous power loss due to eddy currents then consists of two terms, one of which depends on (dB/dt)/sup 2/ and the other one on (dB/dt)/sup 1.5/. The first term is the classical power loss and the second term, due to Bertotti, is the excess power loss. It is shown how these terms are incorporated into a time dependent hysteresis model. The main achievement is that a self-consistent model of hysteresis has been developed in which the quasi-static hysteresis curves are shown to be a limiting case of the more general frequency dependent model. >