Thomas A. Lograsso

6% magnetic-field-induced strain by twin-boundary motion in ferromagnetic Ni–Mn–Ga

Field-induced strains of 6% are reported in ferromagnetic Ni–Mn–Ga martensites at room temperature. The strains are the result of twin boundary motion driven largely by the Zeeman energy difference across the twin boundary. The strain measured parallel to the applied magnetic field is negative in the sample/field geometry used here. The strain saturates in fields of order 400 kA/m and is blocked by a compressive stress of order 2 MPa applied orthogonal to the magnetic field. The strain versus field curves exhibit appreciable hysteresis associated with the motion of the twin boundaries. A simple model accounts quantitatively for the dependence of strain on magnetic field and external stress using as input parameters only measured quantities.

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...

Chemical segregation during bulk single crystal preparation of Ni–Mn–Ga ferromagnetic shape memory alloys

Abstract Preparation of bulk single crystals of the ferromagnetic shape memory alloy Ni2MnGa has been accomplished using the Bridgman method. Magnetic and magnetostrictive properties from samples sectioned from different portions of the crystal showed high variability, indicating significant chemical segregation occurred during single crystal growth. This chemical segregation during crystal growth implies that the compound Ni2MnGa does not melt congruently. Partial evaluation of the liquidus diagram of Ni–Mn–Ga by thermal analysis and microstructural evaluation has determined that the primary solidification surface for the Heusler alloy phase exists over a wide range of compositions. The primary solidification surface falls modestly in temperature with either increasing Mn or Ga concentration. The composition Ni50Mn25Ga25 melts incongruently over a temperature range of approximately 18°C. Characterization of the compositional variation along the growth direction in the single crystals was done using energy dispersive spectrometry and the results normalized against the fraction solidified. Compositional changes during solidification followed constant electron/atom (e/a) ratios over most of the length of the ingot. Under the single crystal growth conditions, the segregation pattern corresponds to a fully mixed condition and the data was fitted to a complete mixing model. Partitioning coefficients for the elemental constituents and the overall compositions were determined. The partitioning coefficients were found to be uniformly consistent between different growth runs, even though the overall compositions differed between crystals. The martensite transition temperatures along the length of the crystal were measured by differential scanning calorimetry and compared to predicted transition temperatures based on the e/a concentration.

Weak Anti-localization and Quantum Oscillations of Surface States in Topological Insulator Bi2Se2Te

Topological insulators, a new quantum state of matter, create exciting opportunities for studying topological quantum physics and for exploring spintronic applications due to their gapless helical metallic surface states. Here, we report the observation of weak anti-localization and quantum oscillations originated from surface states in Bi2Se2Te crystals. Angle-resolved photoemission spectroscopy measurements on cleaved Bi2Se2Te crystals show a well-defined linear dispersion without intersection of the conduction band. The measured weak anti-localization effect agrees well with the Hikami-Larkin-Nagaoka model and the extracted phase coherent length shows a power-law dependence with temperature (∼T−0.44), indicating the presence of the surface states. More importantly, the analysis of a Landau-level fan diagram of Shubnikov-de Hass oscillations yields a finite Berry phase of ∼0.42π, suggesting the Dirac nature of the surface states. Our results demonstrate that Bi2Se2Te can serve as a suitable topological insulator candidate for achieving intrinsic quantum transport of surface Dirac fermions.

Fe–Ga/Pb(Mg1/3Nb2/3)O3–PbTiO3 magnetoelectric laminate composites

We have found large magnetoelectric (ME) effects in long-type laminate composites of Fe–20%Ga magnetostrictive alloys and piezoelectric Pb(Mg1∕3Nb2∕3)O3–PbTiO3 single crystals. At lower frequencies, the ME voltage coefficient of a laminate with longitudinally magnetized and longitudinally polarized (i.e., L-L mode) layers was 1.41V∕Oe (or 1.01V∕cmOe). Near the natural resonant frequency (∼91kHz) of the laminate, the ME voltage coefficients were found to be dramatically increased to 50.7V∕Oe (36.2V∕cmOe) for the L-L mode. In addition, the laminate can detect a minute magnetic field as low as ∼2×10−12T at resonance frequency, and ∼1×10−10T at lower frequencies.

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.