Deborah L. Schlagel

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.

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.

Magnetostriction of ternary Fe–Ga–X alloys (X=Ni,Mo,Sn,Al)

Investigations were made into the effect of small additions of Ni, Mo, Sn, as well as larger additions of Al on the magnetostriction of single crystal Fe100−xGax alloys (x≅13). The Fe–Ga and Fe–Al systems are seemingly unique among the Fe-based alloys in having very large magnetostrictions in spite of Ga and Al being nonmagnetic. In this paper, we show how additions of Ni, Mo, Sn, and Al affect λ100 and λ111 of the binary Fe–Ga alloys. We substituted small amounts of Ni into a binary Fe–Ga alloy in an attempt to reduce the magnitude of the negative λ111, as Ni does in Fe, in order to improve the magnetostriction of polycrystals. The measured λ111’s were reduced to a very small value, ∼3 ppm, but λ100 fell dramatically to +67 ppm for Fe86Ga11Ni3. Mo was substituted for Ga to determine the effect of a partially filled 4d shell on the magnetostriction. Here |λ111| is affected the most, increasing to a value greater than all known α-Fe-based alloys (λ111=−22 ppm for Fe85Ga10.2Mo4.8). We find that the addition...

Magnetic Field Induced Phase Transitions in Gd5(Si1.95Ge2.05) Single Crystal and the Anisotropic Magnetocaloric Effect

Magnetization measurements using a Gd5(Si1.95Ge2.05) single crystal with the magnetic field applied along three crystallographic directions, [001], [010] and [100], were carried out as a function of the applied field (0–56 kOe) at various temperatures (∼5–320 K). The magnetic field (H)–temperature (T) phase diagrams were constructed for the Gd5(Si1.95Ge2.05) single crystal with field along the three directions. A small anisotropy was observed. The magnetocaloric effect was calculated from isothermal magnetization data, and the observed anisotropy correlates with the H–T phase diagrams. The results are discussed in connection with the magnetic field induced martensitic-like structural transition observed in Gd5(Si2Ge2)-type compounds.

Lattice dynamics in magnetic superelastic Ni-Mn-In alloys. Neutron scattering and ultrasonic experiments

Neutron scattering and ultrasonic methods have been used to study the lattice dynamics of two single crystals of Ni-Mn-In Heusler alloys close to Ni50Mn34In16 magnetic superelastic composition. The paper reports the experimental determination of the low-lying phonon dispersion curves and the elastic constants for this alloy system. We found that the frequencies of the TA2 branch are relatively low and it exhibits a small dip anomaly at a wave number n= 1/3, which softens with decreasing temperature. Associated with the softening of this phonon, we also observed the softening of the shear elastic constant C0 = (C11 C12)=2. Both temperature softenings are typical for bcc based solids which undergo martensitic transformations and re ect the dynamical instability of the cubic lattice against shearing of f110g planes along h1 10i directions. Additionally, we measured low-lying phonon dispersion branches and elastic constants in applied magnetic fields aimed to characterize the magnetoelastic coupling.

Influence of annealing and phase decomposition on the magnetostructural transitions in Ni50Mn39Sn11

Magnetic and structural transitions in the Ni50Mn50−xSnx (x=10–25) ferromagnetic shape memory alloys are currently of interest. As in Ni–Mn–Ga, these alloys feature high-temperature austenite and low-temperature martensite phases, where the magnetic state is strongly composition dependent. To study the role of chemical ordering in fine-tuning their magnetostructural properties, they were first annealed for 4 weeks/1223 K to achieve structural and compositional homogeneity, and were then further annealed for 1 week (∼150 K below the reported B2 to L21 transition) at 773 K to increase the degree of chemical ordering. For x=11, this anneal resulted in a dramatic change in the magnetic ordering temperature. Following the 1223 K anneal, the sample exhibited ferromagnetic ordering at 140 K. After the 773 K anneal, the ferromagnetic transition is at 350 K, a characteristic of the ferromagnetic austenite phase with 15

Lattice dynamics and phonon softening in Ni-Mn-Al Heusler alloys

Inelastic and elastic neutron scattering have been used to study a single crystal of the Ni{sub 54}Mn{sub 23}Al{sub 23} Heusler alloy over a broad temperature range. The paper reports the experimental determination of the low-lying phonon dispersion curves for this alloy system. We find that the frequencies of the TA2 modes are relatively low. This branch exhibits an anomaly (dip) at a wave number {xi}{sub 0}=1/3{approx}0.33, which softens with decreasing temperature. Associated with this anomalous dip at {xi}{sub 0}, an elastic central peak scattering is also present. We have also observed satellites due to the magnetic ordering.

Giant magnetostriction behavior at the Curie temperature of single crystal Gd5(Si0.5Ge0.5)4

We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd5(Si0.5Ge0.5)4 prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results reveal that giant magnetostriction in Gd5(Si0.5Ge0.5)4 only occurs as a result of the magnetic/structural transformation.We report results of thermal expansion (TE) and magnetostriction (MS) measurements on a single crystal sample of Gd5(Si0.5Ge0.5)4 prepared by the Bridgman method. TE and MS were measured along the c axis by the strain gauge method and the temperature was controlled using a closed cycle helium refrigerator. From the TE measurements the magnetic structural phase transition temperature was found to be 259.5 K on cooling and 261.5 K on heating. The abrupt change in strain and the temperature hysteresis indicate that it is a first order transition. MS measurements were conducted at 15, 258, and 265 K. At 15 K, the magnetostriction amplitude was 3–4 ppm, whereas at 258 K it was 100 ppm. At 265 K, which is just above the Curie temperature, a giant magnetostriction of 2000 ppm was found. This unusual behavior is due to the fact that the external magnetic field can increase the transition temperature above 265 K, resulting in a first order magnetic/structural phase transition. The results reveal that giant magneto...

Field-driven structural phase transition and sign-switching magnetocaloric effect in Ni–Mn–Sn

Depending on the starting equilibrium temperature, the application of a magnetic field on a sample of Ni–Mn–Sn produces sample heating or cooling during adiabatic experiments. The competition between endothermal and exothermal effects is observed close to the martensite-to-austenite magnetostructural phase transition. A model assuming the coexistence of two phases and a field dependence of their volume allows to compute the evolution of entropy and heat capacity during the phase transition. The correct fitting of the results suggests that the field-induced reduction of the martensite-to-austenite transition temperature is responsible for the observed sign switching of the magnetocaloric effect.