K. B. Hathaway

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...

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

Critical behavior of cubic magnetostrictive materials under stress

Critical points and lines in magnetic field space define the boundaries of different magnetization states between which the magnetization may “jump,” leading to hysteresis. We calculate the changes in these boundaries due to magnetoelastic coupling to applied stresses, and obtain the critical stresses required to eliminate jumping in specific cases. These results may provide a way of reducing or eliminating hysteresis in large magnetostriction materials.

Magnetostriction, elasticity, and D03 phase stability in Fe–Ga and Fe–Ga–Ge alloys

The contrast between the saturation tetragonal magnetostriction, λγ,2 = (3/2)λ100, of Fe1−xGax and Fe1−yGey, at compositions where both alloys exhibit D03 cubic symmetry (second peak region), was investigated. This region corresponds to x = 28 at. % Ga and y = 18 at. % Ge or, in terms of e/a = 2 x + 3 y + 1, to an e/a value of ∼1.55 for each of the alloys. Single crystal, slow-cooled, ternary Fe1−x−y GaxGey alloys with e/a ∼1.55 and gradually increasing y/x were investigated experimentally (magnetostriction, elasticity, powder XRD) and theoretically (density functional calculations). It was found that a small amount of Ge (y = 1.3) replacing Ga in the Fe–Ga alloy has a profound effect on the measured λγ,2. As y increases, the drop in λγ,2 is considerable, reaching negative values at y/x = 0.47. The two shear elastic constants c′ = (c11− c12)/2 and c44 measured for four compositions with 0.06 ≤ y/x ≤ 0.45 at 7 K range from 16 to 21 GPa and from 133 to 138 GPa, respectively. Large temperature dependence was...

The effect of partial substitution of Ge for Ga on the elastic and magnetoelastic properties of Fe–Ga alloys

Both components of the tetragonal magnetoelastic constant b1: the saturation magnetostriction, λγ,2=(3/2)λ100, and the magnetic-field saturated shear elasticity, c′=(c11−c12)/2, were investigated over a wide temperature range for the magnetostrictive Fe1−x−yGaxGey alloys, (x+y≅0.125, 0.185, and 0.245; x/y≅1 and 3). The magnetostriction was measured from 77 to 425 K using standard strain gage techniques. Both shear elastic constants (c′ and c44) were measured from 5 to 300 K using resonant ultrasound spectroscopy. Six alloy compositions were prepared to cover three important regions: (I) the disordered solute α-Fe region, (II) a richer solute region containing both disordered and ordered phases, and (III) a rich solute region containing ordered multiphases. Our observations reveal that, when the data is presented versus the total electron/atom (e/a) ratio, the above regions for both the ternary and binary alloys are in almost perfect alignment. Following this analysis, we find that the magnetoelastic coupl...

Predictions of magnetism in transition metal aluminides

The Stoner Model has been used to investigate the stability of ferromagnetism in FeAl and the analogous series 3d transition metal aluminide compounds. Stoner factors, N(EF)I, were calculated from augmented plane wave (APW) electronic structure calculations based on local density approximation (LDA), using the Janak formalism. The Stoner criteria [N(EF)I>1] predicts ferromagnetism in FeAl for lattice parameters larger than 5.44 a.u., which is less than the experimental lattice constant of 5.50 a.u., but greater than the APW–LDA minimum energy lattice constant. The Stoner factors for ordered transition metal aluminides in the B2 (CsCl) structure agree reasonably well with those calculated using a rigid band approach, starting from FeAl and shifting EF to simulate the remainder of the series. These Stoner factors predict ferromagnetism for CrAl and MnAl and, assuming the rigid band approximation, for FeAl alloyed with Cr and Mn to the level of Fe0.8Cr0.2Al and Fe0.6Mn0.4Al.

Stabilization of itinerant (band) magnetism in FeAl by Ga substitution for Al

Band structure calculations of FeGa1−xAlx have been performed, to further investigate the stability of ferromagnetism in FeAl. The Stoner parameter increases by about 20% at the FeGa end. This is also confirmed by our spin-polarized calculations. We conclude that Ga substitution for Al is likely to stabilize the elusive (or illusive) ferromagnetic state in FeAl.

Effects of Zn additions to highly magnetoelastic FeGa alloys

Fe1−xMx (M = Ga, Ge, Si, Al, Mo and x ∼ 0.18) alloys offer an extraordinary combination of magnetoelasticity and mechanical properties. They are rare-earth-free, can be processed using conventional deformation techniques, have high magnetic permeability, low hysteresis, and low magnetic saturation fields, making them attractive for device applications such as actuators and energy harvesters. Starting with Fe-Ga as a reference and using a rigid-band-filling argument, Zhang et al. predicted that lowering the Fermi level by reducing the total number of electrons could enhance magnetoelasticity. To provide a direct experimental validation for Zhangs hypothesis, elemental additions with lower-than-Ga valence are needed. Of the possible candidates, only Be and Zn have sufficient solubility. Single crystals of bcc Fe-Ga-Zn have been grown with up to 4.6 at. % Zn in a Bridgman furnace under elevated pressure (15 bars) in order to overcome the high vapor pressure of Zn and obtain homogeneous crystals. Single-crys...

Atomic structure and magnetism of ordered and disordered Al0.5Fe0.5−xMnx alloys

The equiatomic FeAl alloy has been modified by partial substitution of Mn for Fe, and its magnetic and structural properties investigated by neutron diffraction (ND), x-ray absorption fine structure (XAFS) spectroscopy, Mossbauer spectroscopy (MS), and SQUID magnetometry both for the ordered (B2) and disordered states. The unit cell volume is measured to increase linearly with Mn concentration. XAFS measurements indicate local structural displacements occur at the Mn sites in both ordered and disordered states that may act to frustrate long-range magnetic order (LRMO). Although MS and ND show no evidence of LRMO, SQUID magnetometry indicates an induced movement in the ordered state that increases with disorder but does not saturate at fields up to 5 T.

INVESTIGATION OF GRANULAR FILMS COMPOSED OF INTERDISPERSED LA1/3CA2/3MNO3 PARTICLES AND METALLIC AU PARTICLES

We have carried out a study of composite, granular films made by depositing LCMO (a colossal magnetoresistive perovskite) and pure, metallic Au onto MgO and LaAlO3 substrates. Elevated temperature caused the Au to segregate into small, micron-sized granules. The films were studied by atomic force microscopy, magnetoresistivity, superconducting quantum interference device magnetization measurements, and magnetically modulated microwave absorption.