R. C. O’Handley

Large magnetic‐field‐induced strains in Ni2MnGa single crystals

Strains of nearly 0.2% have been induced along [001] in unstressed crystals of Ni2MnGa with magnetic fields of 8 kOe applied at 265 K. These stains are associated with the superelastic motion of twin boundaries in the martensitic phase that is stable below about 274 K.

Model for strain and magnetization in magnetic shape-memory alloys

The large magnetic-field-induced strains observed in martensitic phases based on Ni2MnGa and in other magnetic shape memory alloys are believed to arise from a process of twin-boundary motion rather than magnetostriction. The dependence of strain on magnetization, e(M), generally shows a large component that is linear (rather than quadratic) in M below saturation (quadratic dependence being typical of magnetostrictive strain). A simple phenomenological model for the magnetization process and field-induced strain by twin-boundary and phase-boundary motion is proposed for both the strong and weak anisotropy cases. The model is shown to account for the nearly linear dependence of strain on magnetization in the martensitic phases of these materials. It shows the field dependence of the magnetization and strain to be functions of an effective stiffness constant, C, the transformation strain, e0, and the magnetic anisotropy of the martensitic phase, Ku, through two reduced field parameters, he=MsH/Ce02 and ha=M...

Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials (invited)

Ferromagnetic shape-memory alloys have recently emerged as a new class of active materials showing very large magnetic-field-induced extensional strains. Recently, a single crystal of a tetragonally distorted Heusler alloy in the NiMnGa system has shown a 5% shear strain at room temperature in a field of 4 kOe. The magnetic and crystallographic aspects of the twin-boundary motion responsible for this effect are described. Ferromagnetic shape-memory alloys strain by virtue of the motion of the boundaries separating adjacent twin variants. The twin-boundary motion is driven by the Zeeman energy difference between the adjacent twins due to their nearly orthogonal magnetic easy axes and large magnetocrystalline anisotropy. The twin boundary constitutes a nearly 90° domain wall. Essentially, twin-boundary motion shorts out the more difficult magnetization rotation process. The field and stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression includin...

Empirical mapping of Ni–Mn–Ga properties with composition and valence electron concentration

A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K

Large field induced strain in single crystalline Ni–Mn–Ga ferromagnetic shape memory alloy

A room temperature free shear strain of 5.7% is reported in a single crystal of Ni–Mn–Ga having a composition close to the Heusler alloy Ni2MnGa. A twin boundary was created in a 2 mm×2 mm×25 mm single crystal using a permanent magnet with surface field strength of about 320 000 A/m. A sharp 6.5° bend occurs in the sample at the twin boundary. The surface magnetization changes abruptly across this boundary. By moving the sample relative to the edge of the magnet, we were able to sweep the boundary back and forth along the crystal length. Surface magnetization was measured using a Hall probe and the results confirm that the easy axis is the tetragonal c axis. Powder x-ray diffraction shows that the fcc to body-centered-tetragonal bct martensitic transition of this material involved a 6% reduction of the bct cell c/a ratio, from √ to about 1.33. The maximum achievable strain is thus estimated to be 6.2%. The twin planes in the system are the {112}bct and were observed to lie almost normal to the long axis o...

Physics of ferromagnetic amorphous alloys

After some introductory comments on materials classification and a brief historical outline, this review seeks to identify those fundamental physical phenomena that underlie the major technical properties of ferromagnetic amorphous alloys. Those phenomena are reviewed and an attempt is made to compare existing data and, where possible, to synthesize new perspectives. A fundamental approach is taken in the sense that we move from consideration of the local atomic structure to its consequences for the electronic structure and from the electronic structure finally to the physical properties it determines. We focus on the aspects of the structure‐property sequence that set amorphous metallic alloys apart from their crystalline counterparts. Such a review would be incomplete without addressing the new issues and insights raised by the discovery of the quasicrystalline state. The very existence and better understood atomic arrangements of quasicrystals help to define and illuminate their amorphous cousins.

Pulsed magnetic field-induced actuation of Ni–Mn–Ga single crystals

The field-induced actuation of Ni–Mn–Ga single crystals through twin-boundary motion has been demonstrated with magnetic fieldpulses of various intensities lasting 620 μs. It is shown that the complete field-induced strain can be obtained in 250 μs, which implies the possibility of full 6% cycling of Ni–Mn–Ga at 2 kHz, for crystals having dimensions in the range of a few millimeters. The final extension increases with the peak driving force, which is not linear with the field and saturates at 7.85 kOe. An increase of the field beyond the saturation level produces no additional strain but reduces the time for field-induced detwinning.

Giant surface magnetostriction in polycrystalline Ni and NiFe films

We have measured the effective magnetoelastic coupling coefficients, Beff, of polycrystalline NiFe/Ag/Si, NiFe/Cu/Si, and Ni/SiO2/Si films in situ as functions of magnetic layer thickness over the range from 1.5 to 50 nm using magneto‐optic Kerr effect and applied static strain. The Beff’s agree well with bulk values at large thicknesses but take on anomalously large values for thicknesses below about 5 nm. The data are well fit by a Neel model, Beff=Bbulk+Bsurf/(t−t0), where t0 may be related to intermixing at the interface with the substrate (verified by Auger depth profiling). These data suggest that the surface conditions are of enhanced importance in controlling magnetic properties, particularly anisotropy, permeability, and noise, even in films whose compositions are nominally of zero magnetostriction.

Domain wall kinetics in soft ferromagnetic metallic glasses

Domain wall kinetics are studied in glassy iron‐base wire and iron‐nickel–base ribbon. These materials exhibit high wall mobilities. The wire is examined in a single‐wall regime below the nucleation field. Here reversal takes place by propagation of a domain wall of 2.5‐cm axial length along the wire. The mobility normal to the wall is 270 cm/sec Oe. In the case of glassy iron‐nickel–base ribbon the number of walls taking part in reversal is measured to be two. Using this number, the wall mobility is 1700 cm/sec Oe. The viscosity limiting the wall mobility is partitioned into a (calculated) eddy‐current contribution and a spin‐relaxation contribution. The latter constitutes about 40% of the total viscosity in the wire and about 50% in the ribbon samples. Gilbert damping constants for these samples, 0.44 (wire) and 0.19 (ribbon), are small compared with similarly obtained values for comparable polycrystalline alloys.

FIELD-INDUCED STRAIN UNDER LOAD IN NI-MN-GA MAGNETIC SHAPE MEMORY MATERIALS

Single-crystal Ni2MnGa shows a nearly 0.2% strain under a magnetic field of 8 kOe at −8 °C. Polycrystalline samples have been prepared near stoichiometry to study the composition dependence of the magnetic and elastic properties. A narrow band of compositions has been found having a range of Curie and martensitic transformation temperatures, Tc and T0, extending to above room temperature. The compressive stress–strain characteristics in variable transverse field were studied in samples selected to have T0 just below room temperature. Stress-induced martensite was observed as expected and the magnetic field was applied under fixed load for various stresses. A transverse field of 3200 Oe caused the sample to strain under load doing work that increased up to 1.3 J/kg with increasing volume fraction of stress-induced martensite.