Gabriela Petculescu

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...

Chapter three – Magnetoelasticity of bcc Fe–Ga Alloys

Publisher Summary The study of Fe–Ga and related alloys is an active frontier research effort. This chapter provides some insightful understanding of the large magnetostriction observed in this family of materials. Excellent agreement between theory and experiment has been achieved for the Fe–Ga zone I composition alloys. Here, Ga atoms are far apart from each other and they enhance the magnetostriction “locally.” The large magnetostriction of Fe–Ga in this region is caused by intrinsic electronic properties, rather than by the formation of heterogeneous precipitations or nanoparticle. Density functional studies and rigid band model analysis are useful to address the most fundamental issues regarding magnetostriction in intermetallic alloys; valuable predictions are made available for experimentation.

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...

Magnetoelastic coupling in Fe100−xGex single crystals with 4<x<18

In this paper we examine the elastic (c′ and c44) and magnetostrictive (λ100 and λ111) behaviors of Fe100−xGex for 4<x<18, quantities used further to find the fundamental magnetoelastic coupling constants b1 and b2 at room temperature. The x dependence of b1 and b2 for Fe100−xGex is contrasted to those of Fe100−xGax and Fe100−xAlx. While the rhombohedral shear elastic constant c44 is almost insensitive to the type and amount of solute, the tetragonal shear constant c′ shows a pronounced and rapid softening with increasing x for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction λ111 behavior is analogous for all three alloy systems, showing a sign change from negative to positive at the onset of chemical order, the tetragonal magnetostriction λ100 behavior differs. For the Ga and Al alloys, λ100 maintains positive values over the entire x range, both curves showing large peak values, whereas λ100 of Fe100−xGex exhibits a moderate positive peak followe...

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...

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...

Rhombohedral magnetostriction in dilute iron (Co) alloys

Iron is a well-utilized material in structural and magnetic applications. This does not mean, however, that it is well understood, especially in the field of magnetostriction. In particular, the rhombohedral magnetostriction of iron, λ111, is anomalous in two respects: it is negative in sign, in disagreement with the prediction of first principles theory, and its magnitude decreases with increasing temperature much too rapidly to be explained by a power law dependence on magnetization. These behaviors could arise from the location of the Fermi level, which leaves a small region of the majority 3d t2g states unfilled, possibly favoring small internal displacements that split these states. If this view is correct, adding small amounts of Co to Fe fills some of these states, and the value of λ111 should increase toward a positive value, as predicted for perfect bcc Fe. We have measured the magnetostriction coefficients (λ111 and λ100) of pure Fe, Fe97Co3, and Fe94Co6 single crystals from 77 K to 450 K. Reson...

Magnetostrictive and elastic properties of Fe100−xMox (2<x<12) single crystals

In this paper we investigate the magnetostrictive [(3/2)λ100 and (3/2)λ111] and elastic (c′ and c44) behavior of single crystalline alloys Fe100−xMox for 2<x<12; the magnetoelastic coupling constants (−b1 and −b2) are computed from the measurements. Similar to other Fe–X (X=Al, Ga, and Ge) alloys, the tetragonal magnetostriction (3/2)λ100 increases monotonically from ∼70×10−6 at ∼2.5 at. % Mo to a maximum of either ∼100×10−6 at ∼8 at. % Mo for the slow cooled crystals or ∼125×10−6 at ∼11 at. % Mo for quenched crystals. A sharp decrease after the peak is observed for the slow cooled crystals due to the formation of a second phase. The rhombohedral magnetostriction (3/2)λ111 of the Fe–Mo alloys is found to be insensitive to the Mo content. This behavior is distinctly different from other Fe–X (X=Al, Ga, and Ge) alloys where a slight decrease in magnitude and a sign reversal upon chemical ordering was observed for (3/2)λ111. Both shear elastic constants (c′ and c44) for Fe–Mo are remarkably insensitive to th...

Iron‐gallium (Galfenol) transduction alloys: Magnetic and mechanical properties.

Galfenol, Fe100−xGax (∼5 500 MPa. In this presentation, temperature, field and stress dependent magnetic and elastic properties such as permeability, piezomagnetic d constants, Young’s modulus and coupling factors will be presented for several Fe–Ga alloys.

Experimental Investigation of Galfenol Elastic Properties and Auxetic Behavior

Iron-gallium alloys (known as Galfenol) are one of only a few metal alloys known to exhibit large auxetic or negative Poisson’s ratio behavior. The mechanical properties of Galfenol, including the auxeticity, are strongly dependent on the composition of the alloy. This research seeks to measure the elastic properties of Galfenol through a range of practical compositions in order to create a database as well as present trends in the elastic properties. This is achieved through tensile testing of single-crystal Galfenol dog-bone-shaped specimens and through Resonant Ultrasound Spectroscopy (RUS) of small parallelepiped samples. The effects of heat treatment were also studied. This project will enable future researchers to refer to the elastic properties of the alloy measured using two different techniques, as well as enable them to select the alloy with optimum elastic properties for their applications.© 2008 ASME