Mianliang Huang

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.

Effect of interstitial additions on magnetostriction in Fe–Ga alloys

The additions of trace amounts of small interstitial atoms (carbon, boron, and nitrogen) to Fe–Ga (Galfenol) alloys have a small but beneficial effect on the magnetostriction of Fe–Ga alloys especially at high Ga compositions. The saturated magnetostrictions [(3∕2)λ100’s] of both slow cooled and quenched single crystal Fe–Ga–C alloys with Ga contents >18at.% are about 10%–30% higher than those of the comparable binary Fe–Ga alloys. For boron and nitrogen additions, the magnetostrictions of slow cooled alloys with Ga content >18at.% were approximately 20% higher than those of the binary Fe–Ga alloys. We assume that these small atoms enter interstitially into the octahedral site as in pure α-Fe and inhibit chemical ordering, resulting in increased λ100. Thermal analysis of the Fe–Ga binary alloys and Fe–Ga–C ternary alloys indicates that the addition of C into the Fe–Ga system decreases the formation kinetics of D03 and extends the disordered region beyond the maximum for slow cooled binary samples.

Short range ordering in Fe–Ge and Fe–Ga single crystals

We report on the nature of short range ordering (SRO) in slow cooled (100) and (111) Fe81.6Ga18.4 and Fe81Ge9 single crystals determined by x-ray θ-2θ scans. The SRO of the Ga atoms has at least some D03 character. In contrast Ge atoms only exhibit B2 chemical ordering symmetries in the short range order. It has been proposed that the D03 character in the SRO is important for the enhancement in magnetoelasticity in Fe-based alloys; however the presence of B2 character in Fe–Ge alloys suggest that the SRO is not of primary importance in increasing the magnetostriction.

Effect of carbon addition on the single crystalline magnetostriction of Fe-X (X = Al and Ga) alloys

The effect of carbon addition on the magnetostriction of Fe–Ga and Fe–Al alloys was investigated and is summarized in this study. It was found that the addition of carbon generally increased the magnetostriction over binary alloys of Fe–Ga and Fe–Al systems. The formation of carbide in the Fe–Ga–C alloys with a composition near D03 phase region decreased the magnetostriction drastically. Fe–Al–C and Fe–Ga–C alloys responded differently to thermal treatments; the magnetostriction in the quenched Fe–Al–C alloys is equal to or slightly lower than that of the slow cooled as is observed in binary Fe–Al alloy; in contrast, the magnetostriction is generally higher in quenched Fe–Ga–C alloys than slow cooled condition, consistent with the behavior of binary alloys of Fe–Ga. A significant increase in magnetostriction between 25% and 165% depending on the phase region in Fe–Ga–C alloys by quenching was observed in the A2+D03 two-phase region and D03 single phase region.

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...

Kinetic study on the phase transformations in sputter deposited NiMn thin films

The phase transformations of NiMn films were investigated using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and x-ray diffraction (XRD). The sputter deposited NiMn thin films onto room temperature substrates were observed to possess a metastable, chemically disordered, mixture of amorphous and face centered cubic (fcc) phases. DSC measurements revealed four exothermic peaks. TEM studies determined that the DSC peaks were associated with the oxidation of the Mn, two phase transformation reactions (amorphous to fcc and fcc to L10) and grain growth of L10 phase. Using DSC, the enthalpy (activation energy) for amorphous to fcc and fcc to L10 transformations has been measured yielding values of −1.4, −4.7kJ∕mol (121±25 and 140±25kJ∕mol), respectively. The enthalpy of grain growth has been determined as −1.1kJ∕mol. The kinetics of the reaction was simulated using the Johnson–Mehl–Avrami analysis, where the necessary parameters were determined by the Kissinger method on DSC d...

Reversible tuning of the surface state in a pseudobinary Bi2(Te-Se)3 topological insulator

We use angle-resolved photoemission spectroscopy to study a nontrivial surface state in a pseudobinary Bi2Te2.8Se0.2 topological insulator. We show that, unlike previously studied binaries, this is an intrinsic topological insulator with the conduction bulk band residing well above the chemical potential. Our data indicate that under a good vacuum condition there are no significant aging effects for more than two weeks after cleaving. We also demonstrate that the shift of the Kramers point at low temperature is caused by UV-assisted absorption of molecular hydrogen. Our findings pave the way for applications of these materials in devices and present an easy scheme to tune their properties.

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...

Low temperature heat capacity of Fe1−xGax alloys with large magnetostriction

The low temperature heat capacity C{sub p} of Fe{sub 1-x}Ga{sub x} alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron ({gamma}T) and phonon ({beta}T{sup 3}) contributions. The Debye temperature {Theta}{sub D} decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of {Theta}{sub D} from lattice dynamical models and from measured elastic constants C{sub 11}, C{sub 12}, and C{sub 44} are in agreement with the measured data. The linear coefficient of electronic specific heat {gamma} remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level.