Rotational Magnetization Process Research Articles

Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression including Zeeman energy, magnetic anisotropy energy, internal elastic energy, and external stress. Models indicate the limits to the magnitude of the field-induced strain and point to the material parameters that make the effect possible. The field-induced strain in ferromagnetic shape-memory alloys is contrasted with the more familiar phenomenon of magnetostriction.

Theoretical calculation for a two-axis magnetometer based on magnetization rotation

In this work we present a theoretical study for the improvement of fluxgate-like magnetometers. We study an alternative procedure to measure weak magnetic fields by using magnetization rotation processes. To obtain these magnetization processes, we propose to use as cores circular discs of a very thin soft magnetic material. These cores will be magnetized by a rotating magnetic field high enough to saturate them. Thus, the magnetization processes will be due to magnetization rotation and the only losses are due to macro-eddy currents. These kind of devices can also be used to measure the magnetic field in two perpendicular directions by using a single core, and, in principle, the exciting field frequency can be greater than in conventional fluxgate magnetometers. The maximum calculated sensitivity of this device is 5.3 times higher than a conventional fluxgate magnetometer of similar dimensions.

The effect of microstructure on rotational magnetization processes in track edges

A micromagnetic model of longitudinal recording media has been used to investigate rotational magnetization effects. The model indicates that rotational magnetization processes are very sensitive to microstructure and that reversal can be achieved with rotating fields of magnitude significantly lower than the major loop coercivity.

Effect of exchange coupling of polycrystalline antiferromagnetic layers on the magnetization behavior of soft magnetic layers

The complementary nature of the exchange bias field and the coercivity enhancement in ferromagnetic (F)-antiferromagnetic (AF) exchange-coupled layer systems is explained based on an extended Fulcomer and Charap's model, in which the directional distribution of the easy axes plays a decisive role in governing the magnetization behaviors. The model was further extended to deal with the AF-layer thickness ranging from zero to infinity taking into account of Neel's spin fanning model. A comprehensive explanation of the influence of exchange coupling on the magnetization behavior of the F-layer is given. It is shown that a rotational magnetization process is inherent for the samples in which the exchange coupling effect is dominant. The same model explains the temperature dependence of the exchange bias field and coercivity and the memory effect observed after annealing. The varieties of phenomena are mostly ascribed to the existence of distribution in the magnetic properties, net spin moment at the AF-grain surface and AF-grain size.

Burst magnetostriction in Tb0.3Dy0.7Fe1.9

The magnetostriction and magnetic induction calculated by a continuous, anisotropic, anhysteric, magnetization model are compared with magnetostriction and magnetic induction measurements on burst and nonburst magnetostrictive Tb0.3Dy0.7Fe1.9 twinned single crystal rods. The model shows that the magnetostriction and permeability suppression occurring at low applied field is the result of the rotation, and subsequent capture, of initial field antiparallel magnetization into field transverse [111̄] or [1̄1̄1] local magnetoelastic energy minima. The model further shows that the interval of high magnetostriction applied field derivative, dλ/dH, characteristic of burst magnetostrictive material, is the result of the rotation of field transverse [111̄] or [1̄1̄1] oriented magnetization into the [111] near field magnetocrystalline minima. The occurance of burst magnetostriction is therefore contingent on obtaining sufficient magnetocrystalline anisotropy and sufficiently tight magnetization energy distribution in experimental Tb0.3Dy0.7Fe1.9 twinned single crystal rods so as to minimize the applied field interval over which this magnetization rotation process occurs. A final analysis shows that the present model is able to correctly approximate the applied field dependence of the burst magnetostriction response and the applied field dependence of the simultaneous magnetostriction and permeability suppression with a single set of parameters for a range of constant [112] applied compressive stresses. The model additionally exhibits approximately correct saturation magnetostrictions for a range of experimentally applied compressive stresses. However, the model fails to match the experimental behavior above a simultaneous dλ/dH, permeability and field hysteresis transition, located approximately 1000 microstrain from the saturation magnetostriction. The experimental transition clearly indicates a change in magnetization mechanism not accommodated by the present model.

Method based on the saturation approach law for monitoring the quality of texture in 3% Si–Fe

The magnetic saturation of a polycrystalline material is achieved as a result of rotation of the magnetization vector toward the direction of the applied magnetic field, against the magnetocrystalline energy. The magnetization rotation process in a grain-oriented material depends on the values of anisotropy constants, on the distribution of grain orientations, and on the direction of magnetization. Based on the magnetization rotation process, a simple instrument was devised and tested to monitor the differences of texture strength in grain-oriented 3% Si–Fe laminates. The simplicity and robustness of this instrument are remarkable and it can easily work on line.

Anisotropy of magnetization and flux pinning at 77 K in melt textured YBCO

We have studied the zero-field cooled (MZFC), field cooled (MFC) and remanent magnetization (MREM) variations as a function of orientation and field at 77 K in melt textured Y1Ba2Cu3O7-x superconductors. We find a lesser preference of the remanent flux for the c-axis, as compared with low temperatures, which is interpreted as being due to more three-dimensional behaviour of vortices. The three-dimensional behaviour is more pronounced at higher fields. Field cooled magnetization does not in general satisfy the relation MFC=MZFC+MREM, indicative of lack of rigidity and continued depinning of vortices during the (field cooled) magnetization rotation process. The differences between MFC and MZFC+MREM are discussed as a function of field and rotation angle. Data are explained in terms of weaker pinning and decreased effects of anisotropy as compared with low temperatures. We also find evidence for intergrain flux pinning at H=10 Oe in zero-field cooled magnetization.

Analysis of magnetization processes in nanocrystalline FeCoZr ferromagnets

By annealing an amorphous Fe 60Co 30Zr 10 alloy above the crystallization temperature, depending on the annealing temperature, different grain sizes can be produced. For small grain sizes, d ≤ 40 nm, the magnetic properties strongly depend on d. It is shown that H c, χ 0 and the Rayleigh constant α R follow a d 0, d −6 and a d −17 power law, respectively, and can be described within the framework of the random anisotropy model (ram). The experimental relation between H c, χ 0 and α R shows good agreement with the statistical potential theory for domain wall movements. For larger grains d ≅ 60 nm we find reversible rotational magnetization processes.

Loss optimization for iron-rich metallic glasses

We optimized the losses in iron-rich metallic glasses by surface crystallization and by annealing in a field perpendicular to the ribbon surface. Both methods lead to a weak perpendicular anisotropy and rotational magnetization processes. The origin of surface crystallization could be traced to the effects of oxygen during quenching and annealing. While in surface crystallized samples the ratio between measured losses and classical eddy current losses was at best 6 at 2000 Hz (0.2 T), a much better behaviour could be achieved by field annealing. With careful sample preparation and annealing, a Ni-content of 2.5% proved to be sufficient to obtain low losses between 2-3 times the classical eddy current losses in a large range of magnetization amplitudes and frequencies at permeabilities in excess of 10000. Domain observations show uniform magnetization processes.

Rotational magnetic processes in a dirty superconductor.

Magnetization-vector ($\mathbf{M}$) measurements were made on a thin disk of impure Nb rotated relative to a fixed field $\mathbf{H}$ at 4.2 K. For all $H$, $\mathbf{M}$ is found to be separable into a nonrotating diamagnetic component ${\mathbf{M}}_{d}$ plus a penetrating-flux component ${\mathbf{M}}_{p}$ that turns rigidly with the sample for small rotation angles. For $H$ below ${H}_{c1}$ on a hysteresis loop, ${\mathbf{M}}_{p}$ diminishes in size but continues to rotate rigidly up to 360\ifmmode^\circ\else\textdegree\fi{}. For $H$ between ${H}_{c1}$ and ${H}_{c2}$, ${\mathbf{M}}_{p}$ rotates up to and then remains at some critical angle, indicating a constant frictional torque between ${\mathbf{M}}_{p}$ and the sample, which presumably arises from vortex-flux jumps between pinning centers. Moreover, ${\mathbf{M}}_{d}$ closely equals $\frac{\ensuremath{-}\mathbf{H}}{4\ensuremath{\pi}}$ for $H$ above as well as below ${H}_{c1}$.

A fundamental parameters XRF approach for the determination of the composition and thickness of multilayer metal films (abstract)

Analysis of the composition and thickness of magnetic thin films is important in controlling the properties of these materials. For instance, of all factors which determine the overall behavior of Permalloy films used in magnetic recording, composition is perhaps the most critical. For a film near the magnetomechanical zero point, a change in the composition of as little as 0.3% can alter the nature of the magnetization rotation process to the point where errors in readback occur. For this reason it is a necessity to perform accurate and precise analyses of the film composition. One means of performing rapid and precise analysis of these films is x-ray fluorescence (XRF). General algorithms have been developed to provide fundamental parameter corrections for XRF data from single and multilayer metal films. The corrections yield precise values for both thickness and composition. For example, the precision for a replicate analysis on a Permalloy sample was measured to be 80.34 ± 0.07% (weight) Ni (1σ). The fundamental parameter algorithms are quite general and are capable of dealing with variable compositions in each of the films in a multilayer stack. Previously, the restrictions were such that: (1) each element analyzed must appear only once in the films or substrate, and (2) the signal from each element is not obscured by that of another element. We have developed a procedure for the case where the substrate contains an element which is repeated once in the films, for example, nickel over alloy 42. In either case, the accuracy and precision of the measurement is improved if a substrate signal is observed.

Improved method for the investigation of the rotational magnetization process in electrical steel sheets

The magnetization processes in electrical steel sheets in rotating and alternating magnetic fields up to 1.8 T can be investigated with an instrument which is described in this paper. The components of the vectors of the magnetic induction <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\vec{B}</tex> and the magnetic field strength <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\vec{H}</tex> are measured wlth four sensors during one period and processed by a transient recorder and a computer. Some results for rotating fields are presented for a sample of non-oriented silicon steel.

Permeability and field-induced anisotropy in zero magnetostrictive amorphous alloy Fe5Co75Si4B16

Zero magnetostrictive high saturation magnetic induction material Fe5Co75Si4B16 was studied to relate the induced anisotropy Ku with the permeability μ′, for both strip and ring specimens with induced anisotropy KSu and KRu , respectively. A simple model gives the ratio of KSu/KRu=π/2 . The results show that the permeability is well expressed by μ′KRu=2πM2s derived from the reversible rotational magnetization process at higher driving field. A distinct deviation from the above relation was found at a lower field.

Tensorial description of the rotational magnetization process in anisotropic silicon steel

If an electrical steel sheet is placed in a magnetic field rotating in the plane of the sample there is an angle between the vector of the magnetic field strength and the magnetic induction due to the losses and to the anisotropy of the material. To describe this phenomenon a tensorial permeability is given in this paper. The tensor is a product of a rotating matrix related to the losses and a diagonal matrix related to the anisotropy of the sheet. In experiments performed with a measurement apparatus described elsewhere, the rotational magnetization process in a sample of nonoriented silicon steel has been examined. From these experiments the components of the tensorial permeability can be calculated.

Abstract: Transverse susceptibility and magnetization of transverse field annealed Metglas 2605CO

Amorphous Fe-B base ribbons have been shown to have large magnetoelastic coupling1 when appropriately annealed in a transverse magnetic field. The domain structure of Metglas 2605CO (Fe67Co18B14Si1), when annealed as above, was observed by a Bitter technique. The magnetization of each domain is entirely transverse with the main Bloch walls normal to the axis of the ribbon. The domain width is about 0.3 mm. Longitudinal magnetization measurements indicate a rotational magnetization process. We have measured χyy and χyz (y normal to, and z along the axis of the ribbon) by passing an ac current through the ribbon in the presence of a bias field. χyy is defined as the susceptibility determined from the voltage drop along the ribbon. χyz is defined as the susceptibility determined from the voltage of a pickup coil wound around the sample. Anomalous structure is seen in values of χyy between 10 and 100 kHz. We speculate that transverse magnetoelastic resonances are the cause of the structure. The overall reduction in χyy as a function of frequency is larger than can be explained by normal eddy current analysis. Transverse magnetization loops at several longitudinal biases have been measured and show a strong nonlinearity as a function of the current. Metglas is a registered trademark of the Allied Chemical Corporation. 1 C. Modzelewski, H. T. Savage, L. T. Kabacoff, and A. E. Clark, IEEE Trans. Magn. MAG-17, 2837 (1981).

A. C. Losses and temperature-dependent magnetic properties of an amorphous magnetic alloy

Dynamic losses at room temperature and temperature-dependent (2.5 –450 K) d.c. magnetic and electrical resistivity properties were characterized for an amorphous magnetic alloy of composition Fe 0.40Ni 0.42 P 0.17B 0.01. Dynamic losses per cycle at 4 kG exhibited a non-linear dependence on the magnetizing frequency in the range of 10 – 400 Hz, but were linear at higher frequencies. The saturation moment at 0 K was calculated to be 1.27 μ B. Square loop geometries were observed for fields below 1 Oe, while at higher fields a rotational magnetization process dominated. Electrical resistivity increased by 7% in the temperature range (2.5 – 450 K).

Investigation of rotational magnetization processes in Ni single crystals at low magnetic fields

Measurements of magnetostriction and magnetization have shown that in plastically deformed 〈100〉-Ni single crystals rotational processes and domain wall displacements take place simultaneously at small magnetic fields. The contribution of rotational processes to the total change in magnetization varies between 15% and 30%.