Arthur E. Clark

Extraordinary magnetoelasticity and lattice softening in bcc Fe-Ga alloys

Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ100 exhibits two peaks as a function of Ga content. At room temperature, λ100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b1| of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c11−c12) which approaches zero near 27% Ga. λ111 is significantly smaller in magnitude than λ100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%.

Effect of quenching on the magnetostriction on Fe/sub 1-x/Ga/sub x/ (0.13x<0.21)

The magnetostriction (/spl lambda//sub 100/) of b.c.c. Fe is increased over 10-fold at room temperature by the substitution of /spl sim/20% gallium for Fe. Fe/sub 1-x/Ga/sub x/ alloys with x between 0.19 and 0.214 that are quenched from 800/spl deg/C exhibit magnetostrictions /spl sim/25% higher than those furnace-cooled at 10/spl deg//min. We propose that this great increase of magnetostriction above that of Fe in Fe-Ga alloys is not due to conventional magnetoelastic effects but due to the substitutive presence of asymmetrically shaped clusters of the Ga atoms. As the concentration of solute atoms approaches 25%, the lattice becomes relaxed with formation of a more ordered structure and the magnetostriction decreases in value.

Structural transformations in quenched Fe–Ga alloys

Abstract It has been speculated that the large increase in magnetostriction in Fe–Ga alloys results from local short-range ordering of the Ga atoms along specific crystallographic directions in the disordered Fe structure. The structural transitions associated with different cooling rates from the high temperature disordered state were investigated with X-ray diffraction of oriented single crystals of Fe–19 at% Ga. Results are presented for long-range ordering during slow cooling and indirect evidence of local short-range ordering of Ga atoms in the disordered state when the alloys are quenched is also presented. In the latter case, the short-range ordering of Ga atoms leads to a tetragonal distortion of the lattice. The dependence of the magnetostrictive response of Fe–Ga alloys on thermal history has been found to be directly related to these structural transformations in Fe–19 at% Ga alloys and experimental support for the proposed magnetostriction model based on Ga–Ga pairing along [100] crystallographic directions is presented.

Magnetoelasticity of Fe–Ga and Fe–Al alloys

Abstract Measurements of the saturation magnetostriction λ l 0 0 on single crystals of Fe 1− x Ga x with 0.21⩽ x ⩽0.35 are presented. The temperature dependences of λ l 0 0 and the magnetization of the x =0.35 sample are discussed along with prior results in terms of a collapse of the magnetization into an inhomogeneous arrangement of iron moments (cluster glass). The anomalous behaviors of λ l 0 0 and c 11 – c 12 vs. x seen in both Fe 1− x Ga x and Fe 1− x Al x alloys are attributed to internal stresses associated with short-range atomic ordering.

Temperature and stress dependencies of the magnetic and magnetostrictive properties of Fe0.81Ga0.19

It was recently reported that the addition of nonmagnetic Ga increased the saturation magnetostriction (λ100) of Fe over tenfold while leaving the rhombohedral magnetostriction (λ111) almost unchanged. To determine the relationship between the magnetostriction and the magnetization we measured the temperature and stress dependence of both the magnetostriction and magnetization from −21 °C to +80 °C under compressive stresses ranging from 14.4 MPa to 87.1 MPa. For this study a single crystal rod of Fe0.81Ga0.19 was quenched from 800 °C into water to insure a nearly random distribution of Ga atoms. Constant temperature tests showed that compressive stresses greater than 14.4 MPa were needed to achieve the maximum magnetostriction. For the case of a 45.3 MPa compressive stress and applied field of 800 Oe, the maximum magnetostriction at 80 °C decreases from its value at −21 °C by 12.9%. This small magnetostrictive decrease is consistent with a correspondingly small 3.6% decrease in magnetization over the sam...

Magnetic field dependence of galfenol elastic properties

Elastic shear moduli measurements on Fe100−xGax (x=12–33) single crystals (via resonant ultrasound spectroscopy) with and without a magnetic field and within 4–300 K are reported. The pronounced softening of the tetragonal shear modulus c′ is concluded to be, based on magnetoelastic coupling, the cause of the second peak in the tetragonal magnetostriction constant λ100 near x=28. Exceedingly high ΔE effects (∼25%), combined with the extreme softness in c′ (c′<10GPa), suggest structural changes take place, yet, gradual in nature, as the moduli show a smooth dependence on Ga concentration, temperature, and magnetic field. Shear anisotropy (c44∕c′) as high as 14.7 was observed for Fe71.2Ga28.8.

Tetragonal magnetostriction and magnetoelastic coupling in Fe-Al, Fe-Ga, Fe-Ge, Fe-Si, Fe-Ga-Al, and Fe-Ga-Ge alloys

This paper presents a comparative study on the tetragonal magnetostriction constant, λγ,2, [ = (3/2)λ100] and magnetoelastic coupling, b1, of binary Fe100-xZx (0 < x < 35, Z = Al, Ga, Ge, and Si) and ternary Fe-Ga-Al and Fe-Ga-Ge alloys. The quantities are corrected for magnetostrains due to sample geometry (the magnetostrictive form effect). Recently published elastic constant data along with magnetization measurements at both room temperature and 77 K make these corrections possible. The form effect correction lowers the magnetostriction by ∼10 ppm for high-modulus alloys and by as much as 30 ppm for low-modulus alloys. The elastic constants are also used to determine the values of the magnetoelastic coupling constant, b1. With the new magnetostriction data on the Fe-Al-Ga alloy, it is possible to show how the double peak magnetostriction feature of the binary Fe-Ga alloy flows into the single peak binary Fe-Al alloy. The corrected magnetostriction and magnetoelastic coupling data for the various alloys...

Magnetostriction of ternary Fe–Ga–X (X=C,V,Cr,Mn,Co,Rh) alloys

Binary iron-gallium (Galfenol) alloys have large magnetostrictions over a wide temperature range. Single crystal measurements show that additions of 2at.% or greater of 3d and 4d transition elements with fewer (V, Cr, Mo, Mn) and more (Co, Ni, Rh) valence electrons than Fe, all reduce the saturation magnetostriction. Kawamiya and Adachi [J. Magn. Magn. Mater. 31–34, 145 (1983)] reported that the D03 structure is stabilized by 3d transition elements with electron∕atom ratios both less than iron and greater than iron. If D03 ordering decreases the magnetostriction, the maximum magnetostriction should be largest for the (more disordered) binary Fe–Ga alloys as observed. Notably, addition of small amounts of C (0.07, 0.08, and 0.14at.%) increases the magnetostriction of the slow cooled binary alloy to values comparable to the rapidly quenched alloy. We assume that small atom (C, B, N) additions enter interstitially and inhibit ordering, thus maximizing the magnetostriction without quenching.

Quasi-Static Transduction Characterization of Galfenol

The objective of this work is to characterize the magnetoelastic transduction properties of single-crystal and textured polycrystalline Fe-Ga alloys (Galfenol) under controlled mechanical, magnetic, and thermal conditions. Polycrystalline samples of interest include a directionally solidified specimen, which possesses a favorable saturation magnetostriction output, and an extruded specimen, whose magnetostriction properties are significantly reduced by annealing. A brief discussion of the thermally controlled transducer used for the magnetic testing is presented first. Thereafter, the single-crystal response to major-loop cyclic magnetic fields under different temperature and stress conditions, as well as its response to minor-loop cyclic magnetic fields and major-loop cyclic stress are examined. Next, the magnetic and magnetostrictive responses to major-loop cyclic magnetic field conditions are compared for the directionally solidified, extruded, and single-crystal specimens. The paper concludes with a magnetic characterization summary of the different Fe-Ga alloys examined.

Magnetostriction of ternary Fe–Ga–X alloys (X=Ni,Mo,Sn,Al)

Investigations were made into the effect of small additions of Ni, Mo, Sn, as well as larger additions of Al on the magnetostriction of single crystal Fe100−xGax alloys (x≅13). The Fe–Ga and Fe–Al systems are seemingly unique among the Fe-based alloys in having very large magnetostrictions in spite of Ga and Al being nonmagnetic. In this paper, we show how additions of Ni, Mo, Sn, and Al affect λ100 and λ111 of the binary Fe–Ga alloys. We substituted small amounts of Ni into a binary Fe–Ga alloy in an attempt to reduce the magnitude of the negative λ111, as Ni does in Fe, in order to improve the magnetostriction of polycrystals. The measured λ111’s were reduced to a very small value, ∼3 ppm, but λ100 fell dramatically to +67 ppm for Fe86Ga11Ni3. Mo was substituted for Ga to determine the effect of a partially filled 4d shell on the magnetostriction. Here |λ111| is affected the most, increasing to a value greater than all known α-Fe-based alloys (λ111=−22 ppm for Fe85Ga10.2Mo4.8). We find that the addition...