Highlights

Ni₂MnGa and Mn₂NiGa: Heusler shape memory compounds as thin films

Investigators: Catherine Jenkins, Claudia Felser, Gerhard Fecher, Benjamin Balke, Gerhard Jakob, Hans-Joachim Elmers, Tobias Eichhorn, R. Ramesh

The ternary intermetallic nickel—manganese—gallium has been known for over a decade as a ferromagnetic shape memory compound. In other words, this material both undergoes a martensitic transition and the magnetocrystalline anisotropy in the low-temperature phase couples the magnetic easy axis to the axis of tetragonal distortion strongly enough that the martensitic variants can be changed by the application of a magnetic field. The theoretically achievable strain between the cubic (high temperature) and orthorhombic or modulated tetragonal (low temperature) phases is around 10%. Depending on the relative orientation of the crystallographic plane between the martensitic variants and the surface of the crystal, the mode of actuation can be lengthening, kink propagation, or bending. In thin film form, released cantilevers of sputtered shape memory material have been shown to exhibit a response similar to that of bulk single crystals.

Epitaxial NMG films (XRD peak width less than 0.1°) are investigated with x-ray magnetic circular dichroism (XMCD) and photoemission electron microscopy (PEEM) at the BESSY Berlin, ANKA Karlsruhe, and SPring-8 Japan synchrotrons, in addition to the full complement of standard magnetic and structural techniques. Details of the thickness dependence of the twinning structure and the phase transition are being sought.

With a precise control of the deposition conditions, the martensitic transition in off-stoichiometric films can be tailored to be above room temperature. This makes the topographic changes associated with the lowering in symmetry from Fm-3m to I4/mmm available to both optical and atomic force microscopes (AFM), and extensive AFM imaging is therefore undertaken during this effort.

Manganese—nickel—gallium also orders in the Heusler structure, similar to NMG, and was shown to undergo an equivalent martensitic transition only in 2004. However, it has two distinct advantages: stoichiometric MNG is in the martensitic or active phase at room temperature without the need for complicated compositional alterations, and the theoretical strain is more than 21%, with a possible amplification of the effect assisted by the application of a magnetic field. The world’s first thin films are being grown by dc sputtering in collaboration with the Johannes Gutenberg-University in Mainz, Germany, and the Chinese Academy of Sciences in Beijing, and theoretical considerations are examined using Wien2K software.

Freestanding cantilevers of Ni₂MnGa have been fabricated by focused ion beam (FIB) etching. Bending in or out of plane are possible modes of response to an applied magnetic field.

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