M. Wun-Fogle

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...

Anisotropy compensation and magnetostriction in TbxDy1−x(Fe1−yTy)1.9 (T=Co,Mn)

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 ...

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.

Induced Magnetic Anisotropy in Stress-Annealed Galfenol Alloys

Values of the uniaxial anisotropy, cubic anisotropy, saturation magnetic induction, and saturation magnetostriction were obtained from measurements of the magnetization and magnetostriction of stress-annealed Galfenol (Fe_100-xGa_x,x=12.5,18.4, and 22.0)and Fe₈₁Al₁₉ as a function of compressive stresses <100 MPa. The values were derived from fitting magnetization and magnetostriction curves to the energy expression E_i=-mu ₀M_sHalpha_z+K_cubic(alpha _x ⁴+alpha_y ⁴+alpha_z ⁴)+K_un_iaxialalpha_z ²-lambda_satTalpha_z ², where M_s,lambda_sat and the Ks are fit parameters. The alpha_is are the direction cosines of the magnetization direction with respect to the field and stress direction (z).H is the magnetic field and T is the measurement stress (compressive T<0). Data are fit with high precision by only the above four constants plus a smoothing constant. Importantly, K_uniaxial enables a prediction of the maximum usable tensile stress