Magnetostrictive Tb Research Articles

Crystal growth of giant magnetostrictive Tb_Dy_Fe alloy

Crystal growth of giant magnetostrictive Tb_Dy_Fe alloy

Effect of the pulverization processes on the magnetostrictive characteristics on giant magnetostrictive Tb 0.3Dy 0.7Fe 2 thin film

Two different processes by ball milling and cyclic hydriding-dehydriding were examined for the preparation of the pseudo-binary Tb 0.3Dy 0.7Fe 2 powder sample with the C-15 Laves phase. The cyclic treatment resulted in both the reduction of the particle size and the disproportionation of the alloy into rare earth dihydrides RH 2 (R=Tb, Dy) and α-Fe. However, subsequent vacuum annealing treatment of the disproportionated sample at 700 K in a vacuum of 5×10 −4 Pa yielded a clean and less strained stoichiometric powder sample compared with ball milled samples. The magnetostrictive characteristics of Tb 0.3Dy 0.7Fe 2 films prepared by the flash evaporation process were found to be influenced by the prior processes for the powder preparation.

Microsegregation phenomenon in Terfenol-D rods grown by electron beam zoning method

A new electron beam zoning method was developed to prepare rods of the high magnetostrictive Tb 0.27Dy 0.73Fe 2− x . Rods with good 〈112〉 orientation of the individual grains can be obtained by this method. The samples contain a dendrite morphology with the {111} planes parallel to the dendrite sheet plane. High magnetostrictive properties were achieved in these samples with directional growth. Compositional microsegregation was observed by analytical electron microscopy. This result is helpful for perparing highly magnetostrictive materials.

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.

Directional solidification of TbDyFe magnetostrictive alloy

Grain-aligned rods of the huge magnetostrictive TbDyFe alloy have been prepared by directional solidification techniques. Axial grain orientation, phase spacing and elemental distribution of the alloy rods have been investigated. It was found that unseeded rods usually show a strong 〈112〉 orientation at higher growth rate regime. At lower growth rate regime, 〈110〉 axial grain orientation was often observed. In all the rods, a range of phase spacing existed. The average spacing was more sensitive to the temperature gradient than the growth rate. Large spacing was achieved with lower temperature gradient and lower growth rates. It was also found that different partition of Tb and Dy in the Laves phase and RE-rich phase existed, as well as the microsegregation of Tb and Dy. These led to a higher Tb/Dy atomic ratio in the RE-rich phase and a variation of the Tb/Dy ratio in the cellular Laves phase sheets. To explain the different axial grain orientations, a model was proposed. An orientation selection principle was adopted such that an orientation with a higher local interface temperature among competing orientations was preferred. The effects of the kinetic attachment anisotropy and the surface tension anisotropy were considered.

Magnetostrictive properties of polymer bonded Terfenol-D

A resin bonding route has been used to produce rods of the highly magnetostrictive alloy Tb0.27Dy0.73Fe1.93. The influence of the resin load on the magnetic properties has been studied. Two phenomena could be observed during the measurement of the magnetomechanical factor. The first was a threshold value of applied magnetic field. The second was abnormal behaviour of the characteristic frequencies with applied bias field.

Enhancement of piezomagnetic response of highly magnetostrictive rare earth-iron alloys at kHz frequencies

The effects of Al and Si additions on the frequency response of highly magnetostrictive Tb–Dy–Fe alloys have been studied. These elements reduced the electrical conductivity of the material, thereby increasing the depth of penetration of ac magnetic fields and extending the operational frequency range. Complex permeability measurements were made on doped and undoped polycrystalline samples with the objective of studying the improvement in energy conversion efficiency at kilohertz frequencies as a result of the alloying additions. The resulting complex permeability was compared with the values for single crystal specimens of Tb0.3Dy0.7Fe1.92 at a range of frequencies from 10 Hz to 50 kHz.

Giant magnetostrictive materials: thin film formation and application to magnetic surface acoustic wave devices

This paper points out current problems in the research and development of giant magnetostrictive materials (GMMs), and the use of thin film GMMs instead of bulk is emphasized because of the ready and inexpensive production and high applicability. One of the most prominent applications using GMM films is the magnetic surface acoustic wave device with tunability under an external magnetic field. Thin films of the giant magnetostrictive Tb 0.3Dy 0.7Fe 2 alloy were prepared by vacuum flash evaporation. The film structure was able to be changed from amorphous to crystalline by controlling the substrate temperature and deposition rate. The crystalline films exhibited much higher magnetization and larger magnetostrictions than did the amorphous films. The contamination of the films in vacuum facilitated amorphization and markedly reduced the magnetostrictions of the films. X-ray diffraction and Auger electron spectroscopy analyses revealed that the ratio of vacuum to deposition rate is an appropriate indicator to assess the contamination effect of films.

Theoretical modelling of the effects of anisotropy and stress on the magnetization and magnetostriction of Tb0.3Dy0.7Fe2

The alloy Tb0.3Dy0.7Fe2 is known to have a useful combination of high magnetostriction and low anisotropy energy. This means that relatively high strain amplitudes, exceeding 1 × 10−3, can be obtained at low to moderate field strengths of less than 240 kA/m. This paper is concerned with the application of the anisotropic rotation model of the magnetization mechanism in this material. The model comprises the underlying assumptions which correspond to the Stoner-Wohlfarth model and its derivatives. In this case, the magnetic moments of the domains are constrained by the anisotropy to lie along the 〈111〉 directions within the crystal. It is shown from symmetry considerations that the fractional occupancies of six of the eight 〈111〉 directions exhibit pair degeneracy, leaving only five independent parameters to describe the distribution of domains among those directions. The changes in magnetization and magnetostriction which occur as the material is magnetized by a field along the [112] axis were determined both under zero stress and under a range of uniaxial stress levels applied along the [112] axis by following the changes in direction of the local minima in the sum of anisotropy, field and magnetostrictive energies.

A 3D anisotropic rotation model for magnetization and magnetostriction in Tb0.3Dy0.7Fe1.9 (abstract)

The highly magnetostrictive alloy Tb0.3Dy0.7Fe1.9 orders with its magnetic moments aligned along the 〈111〉 axes. When a magnetic field is applied along one of the 〈112〉 axes it has been shown in recent work that the principal magnetization mechanism is domain rotation. In order to produce higher magnetostriction at lower field strengths the initial domain orientations should be configured with high population of the [1̄1̄1] and [111̄] axes at right angles to the field direction. This paper is concerned with the determination of these population densities from magnetization and magnetostriction data using the domain rotation model in the presence of anisotropy. We have developed a three-dimensional anisotropy model for the domain rotation mechanism.1 This enables the direction of magnetization of domains originating along each of the 〈111〉 axes to be found as a function of magnetic field. The change in magnetization ΔM is calculated from: ΔM=∑piMs(cos θi − cos Ωi), where Ms is the saturation magnetization, Ωi is the initial orientation of a domain with respect to the field, and θi is the final orientation. The change in magnetostriction is calculated from: Δλ =3/2λ100Σi[(cos2 θf,j −cos2 θi,j)cos2 φj]Δλ= 3/232λ100Σi [(cos2 θf,j−coscos2 θi,j)cos2 φj] +3λ111Σ3j=1[(cos θf,j cos θf,j+1 −cos θi,j cos θi,j+1)cos φj cos φj+1]pi. The population densities pi of the domain in the various 〈111〉 axial directions determine the amplitude of the magnetostriction. From the model it is possible to determine the bulk magnetization and magnetostriction from the occupancies of these directions.<ks>

The effect of composition and magnetic heat treatment on the magnetostriction of TbxDy1−xFey twinned single crystals

Twinned single crystals of the giant magnetostrictive intermetallic compound TbxDy1−xFey with a 〈112〉 axial alignment have been prepared by float-zone solidification with Y varying from 1.90 to 1.98 and X being 0.32 or 0.28. The chemical compositions of the crystals were carefully analyzed. A very sharp jump in the magnetostrictive strain λ sometimes occurs in these alloys as the applied field H is increased to values of around 280 Oe under a compressive stress of 6.9 MPa. This large jump in λ is shown to depend strongly on the values of X and Y, and possible reasons for this result are discussed. A magnetic heat treatment has been developed which involves cooling the samples through the Curie temperature under a field applied transverse to the crystal axis. It is shown that this treatment is very effective at increasing the magnetostrictive properties at low applied compressive stresses for certain values of X and Y.

Ultrasound induced irreversible changes in the magnetization of highly magnetostrictive materials Tb0.27Dy0.73Fe2 and MnBi

A sample, after being saturated in a high magnetic field, is in an unstable remanent state in a low or reversed magnetic field. Application of a burst of ultrasound at this point will result in an irreversible decrease in the remanent magnetization. By subsequently changing the magnetic field, and applying a second burst of ultrasound, a reduced irreversible decrease is effected. This observed behavior is a measure of the statistical correlation between the two magnetic states of the sample. We have successfully described this behavior with a modified Preisach model. We have observed irreversible changes in the magnetization of Tb0.27Dy0.73Fe2 (λ111 = 1.6×10−3) as large as 50% of the remanent magnetization, or 0.4% of the saturation magnetization, induced by ultrasound bursts of 107 dyne/cm2 at 20 °C. In MnBi (λs = −8×10−4) crystals in Bi, irreversible effects as large as 1.8% of the remanent magnetization, or 0.6% of the saturation magnetization, have been observed at 120 K.

Magnetic anisotropy and magnetostriction of TbxGd1-xAl2

The magnetic anisotropy and the magnetostrictive properties of Tb in the metallic host GdAl2 have been studied using single crystals of TbxGd1-xAl2 where x=0.02 and 0.05. The anisotropy and magnetostriction constants are found to be proportional to x and extrapolation to x=1 yields an anisotropy constant consistent with measurements on TbAl2. The magnetostriction results are in clear conflict both with existing measurements using X-rays and with crystal field calculations; no satisfactory explanation for the discrepancies can be found.

Magnetomechanical coupling and ΔE effect in highly magnetostrictive rare earth - Fe<inf>2</inf>compounds

We have measured the magnetomechanical coupling in highly magnetostrictive Tb x Dy 1-x Fe 2-δ as a function of bias field, x, and δ. We find a peak coupling factor of 0.6 at x = .26 and bias field of 180 Oe; the largest reported for a magnetostrictive material. The corresponding material Q is 200. Because of the high magnetostriction, relative permeabilities are necessarily low; \mu_{\max} \simeq 5 . Young's modulus at constant field changes from 4.5 to ll (x1010n/m2) as the bias changes from zero to 4.5 kOe, corresponding to a change in sound velocity >50%. The magnetomechanical coupling factor was also evaluated for 0 \leq \delta \leq .3 . When δ is non-zero a eutectic second phase is present which acts as a toughening agent.

Giant Magnetically Induced Changes in the Elastic Moduli in Tb.3Dy.7Fe2

Abstract : In the report the authors discuss increases in sound velocity of greater than 60% upon magnetization of highly magnetostrictive Tb(x)Dy(1-x)Fe2, where x approximately equals 0.3. This change corresponds to a Young's modulus increase (delta E Effect) of about 160%, by far the largest known. (Modified author abstract)