Library

Notes: Recently it was discovered that composites of Terfenol-D alloys with an insulating binder produce very large magnetostrictions. Resistivities of these composites reach high values, making them attractive for high-frequency applications which require small eddy current losses. In this paper the magnetostriction, magnetization, and Young's moduli measurements made under constant magnetic field conditions and under constant flux conditions are reported. From these measurements, magnetomechanical coupling factors are calculated. The properties are compared to those of ordinary metallic Terfenol-D and nickel. Two different types of composites were investigated. In the first type the composite has an isotropic structure and in the second type, anisotropic. It is shown that the anisotropic type is more desirable since it possesses both higher magnetostriction and higher coupling factors. It is also clearly shown that the magnetization process for the anisotropic type can be explained by a 180° domain wall motion followed by a magnetization rotation.

Notes: Rare-earth–iron alloys, R0.9Fe0.1, R0.72Fe0.28, and R0.42Fe0.58 (R=Tb0.6Dy0.4), containing the R/RFe2 eutectic composition were prepared by Bridgman and free-standing zone-melting techniques. Magnetization measurements were made in fields up to 800 kA/m between 55 and 300 K. A huge increase in magnetization below 210 K occurs as the R component becomes ordered. At low applied magnetic fields there is clear identification of both the ferromagnetic ordering temperature TC and the Néel spiral ordering temperature TN of R. (For Tb0.6Dy0.4, TC=165 K, TN=210 K.) Magnetization and magnetostriction measurements reveal very large magnetocrystalline anisotropies for both the R and the RFe2 components. Unexpectedly, at 77 K, were the rare-earth component of the eutectic system is ordered and the magnetostriction is large (λhγ(approximately-greater-than)0.6%), the magnetostriction is largest in the samples containing the largest amount of the RFe2 phase. Young's modulus measurements reveal the reduction in the stiffness with the addition of the softer rare earth to the stiff RFe2 compound.

Notes: Idealized giant Barkhausen (BG) jumps in amorphous wires result in a simple rectangular hysteresis loop. There is a range of domain wall depinning fields so that if the depinning field for magnetization in the central section of the wire is high enough, magnetization reversal takes place by domains becoming depinned at the end of the wire. This leads to the rectangular hysteresis loop characteristic of GB. We have observed a different kind of Barkhausen effect in Fe-B-Si amorphous wires under a variety of conditions. The magnetization starts linearly with field, then displays a roughly quadratic field dependence, saturating at lower fields than that given by the low field linear extrapolation. In the quadratic region the plot of log(Hs−H) vs log(Ms−M), where Hs is the apparent saturation magnetization taking place at field Hs, is linear with a slope of 1/2. This is typical of simple second order instability processes. Thus, instead of a first order giant Barkhausen jump we have observed a second order Barkhausen process that is continuous. The only jump that occurs is in demagnetizing. This is caused by hysteresis. We believe this is the first time that this second order process has been described. From the details of the measurement we believe that the magnetization takes place by a longitudinal wall moving radially rather than a radial wall moving longitudinally as in the GB. The circumstances under which this takes place are in well-annealed wires magnetized with an imposed twist and longitudinal stress. In as-cast wires we imposed a curvature by bending along the longitudinal axis. The solenoid of the exciting field is bent the same way. The magnetization in wires with the imposed curvature displays a GB behavior for high and low curvatures. In an intermediate regime of curvature a behavior midway between GB and our second order effect is displayed. The low field magnetization looks like the second order effect but has a small jump to apparent saturation in the quadratic region described above. In summary we have found a range of depinning fields that lead to different magnetization processes when the depinning field values are less than that for central magnetization.

Notes: The elastic moduli of the highly magnetostrictive TbxDy1−x alloys (x=0.5, 0.6, and 0.67) were measured at 77 K under conditions of constant magnetic field and constant magnetic induction. From these values the magnetic contribution to the moduli (intrinsic ΔE effect) and magnetoelastic coupling factor k were calculated. For Young's moduli measured under constant flux density (magnetically blocked conditions), it was found that EB∼20 to 50 GPa. For measurements made while maintaining a constant magnetic field (magnetically free conditions), it was found that Young's moduli EH minima range from ∼3 to 5 GPa. Such large differences between EB and EH yield magnetoelastic coupling factors in excess of 0.9. Theoretical expressions of the magnetic contribution to the elastic compliance, (1/EH−1/EB), were derived using the single vector magnetization rotation model.

Notes: The temperature dependence of Young's modulus has been measured for a series of Tb1−xDyxZn pseudobinary compounds with x ranging from 0 to 1. From the sharp dips in the modulus vs temperature data, the reorientation transition temperatures have been determined, and the magnetic phase diagram deduced. Magnetization measurements taken on the same samples show less pronounced features at the corresponding temperatures. © 1996 American Institute of Physics.

Notes: The domain structures of amorphous FeSiB wires have been studied as a function of applied field. The main features of as-cast wires are: the proliferation of surface closure domains; a distinction between core and sheath domain behavior; and a reproducible switching behavior. Transverse magnetically annealed wires have larger domains, show little sign of a sheath and a greater variety of domain displacement mechanisms.

Notes: From magnetization (M) and magnetostriction (λ) measurements as a function of magnetic field and stress, the temperatures of anisotropy compensation, Tm, for technologically important TbxDy1−x(Fe1−yTy)1.9 [T=Co,Mn (0.3≤x≤0.5) (0≤y≤0.3)] were determined. Measurements of M and λ encompassing Tm were made under compressive stresses from 8.8 to 36 MPa and for temperatures from −196 to +130 °C. In agreement with earlier measurements, Tm decreases with increasing Tb. Substitution of Mn for Fe for fixed x also decreases Tm. In contrast with these observations is the increase of the anisotropy compensation temperature with the replacement of Fe by small amounts of Co. In the cases of both (1) increasing Tb content and (2) increasing Co content, the Curie temperature TC increases, yielding, in general, a higher magnetic moment and saturation magnetostriction of these alloys. Thus, compensation at a given temperature may be obtained in an improved class of Laves phase compounds, R(1)xR(2)1−x(Fe1−yCoy)2, where rare earths R(1) and R(2) are, for example, Tb and Dy.

Notes: Terfenol (Tb0.3Dy0.7Fe1.9) and related materials have proven useful as electromechanical transducers. As with other transducers, these materials exhibit hysteresis, which must be included in any model of the transducer. Preisach theory is one of the available models that includes the effects of hysteresis. The theory is able to include the hysteresis by maintaining a history of the field reversals in the material. Both the strain and magnetization hysteresis loops in a 3.8-cm-diam Terfenol rod have been measured under 20.7-MPa longitudinal stress. The data has been used to derive a Preisach model of the rod, including hysteresis effects. It is possible to predict the strain and magnetization of the rod, including minor loops, with about 10%–15% accuracy.

Notes: Magnetization, magnetostriction, and elastic modulus measurements were made on single crystal specimens of Tb1−xDyxZn. Easy axis magnetization rotation as much as 26° were observed in the (001) plane of Tb0.75Dy0.25Zn below 33 K. From these measurements, values of the K4/K8 anisotropy ratios were calculated. No easy axis magnetization rotation was observed in the x=0.6 and x=0.8 single crystals. Magnetostriction and modulus measurements at 77 K in Tb0.4Dy0.6Zn showed a saturation magnetostriction of ∼5×10−3 and a maximum magnetomechanical coupling factor of 0.96. © 1999 American Institute of Physics.

Notes: The magnetization and magnetostriction of a variety of 3/16-in.-diam Laves phase rods of TbxDyyHo1−x−yFe1.95 grown in the form of [112] oriented dendritic compounds were measured as a function of applied magnetic field −3000〈H〈3000 Oe and compressive stresses of 22 and 47 MPa at room temperature. Compared to Terfenol-D (Tb0.3Dy0.7Fe1.95), the widely used room temperature magnetostrictive material, Ho containing alloys can have substantially lower hysteresis with only slightly lower magnetostriction. The Ho concentration was kept relatively small (≤0.3) to avoid a substantial decrease in the magnetostriction, while the ratio of x and y was chosen to examine alloys spanning the line of minimum magnetic anisotropy. Most of the compositions have twice the Ho content of the previous study. As expected, alloys with higher Ho concentrations showed narrower hysteresis curves. The data shows that at 22 MPa, the Tb0.28Dy0.57Ho0.15Fe1.95 composition has a minimal (3%) loss of magnetostriction, while the hysteresis width decreased by 15%. Between 15% and 20% Ho content, the magnetostriction drops abruptly. For alloys with a fixed Ho concentration, the strain showed a peak near the expected anisotropy minimum, but the hysteresis width always increased with increasing Tb content. © 1999 American Institute of Physics.