Artificial Single Variant Martensite in Freestanding Fe70Pd30 Films Obtained by Coherent Epitaxial Growth

2010 WILEY-VCH Verlag Gmb Microactuators and sensors based on magnetic shape-memory (MSM) alloys will benefit from the large strain close to 10% obtained in these materials. These strains exceed the values obtainable by magnetostriction or piezoelectricity by more than one order of magnitude. Thus, they can be used directly for most applications, avoiding additional complications of mechanical amplification. As the highest strains to-date are obtained in bulk single crystals, the use of epitaxial films is most promising for microsystems, owing to their single-crystal-like microstructure. With reduced actuator size, however, the influence of interfaces, and in particular of oxidation, becomes more important. Though the prototype Ni2MnGa system is relatively inert, an oxide surface layer may hinder the martensitic transformation in thin films. For the Fe70Pd30 system [8] oxidation is expected not to be critical due to the high content of a noble element. In Fe70Pd30 the martensitic transformation occurs around room temperature (RT). First reports indicate that epitaxial growth of thin Fe70Pd30 films can be obtained already at RT. For epitaxial growth of the NiMnGa system, a minimum temperature of 350 8C is required. Recently it was shown that epitaxial growth of Fe70Pd30 is possible on various metallic buffers. Due to coherent growth, huge tetragonal distortions were stabilized in 50 nm thick films, covering most of the Bain transformation path from face-centered cubic (fcc) to bodycentered cubic (bcc) structure. Here, we show how this approach can be extended to obtain freestanding films of micrometer thickness, thus fulfilling both key requirements for the integration into microsystems as well as prerequisites for MSM films, that is, martensitic, ferromagnetic at RT, freestanding, and single-crystalline-like. In addition, our experiments reveal that the transformation behavior in these films differs from in the bulk. While this topic has been an extensive playground for theory, experiments are rare, since a detailed analysis often requires epitaxial films. Recently, experiments on epitaxial films, for example, have revealed a variant selection by the rigid interface to the substrate or by reduction of the magnetic stray field energy in freestanding films. The present experiments are more fundamental since they show that circumventing the forward martensitic transformation by forming the martensitic structure directly at RT hinders the nucleation of the reverse transformation to austenite. This remarkable suppression of the transformation not only gives a better understanding of the martensitic transformation but also opens innovative routes for microsensors. Films of Fe70Pd30, 1.2mm thick, were deposited with a low deposition rate of 0.024 nms 1 at 30W sputtering power in a magnetron sputtering system on Au-buffered MgO(001) oriented, epi-polished substrates. The crystal structure of the Fe70Pd30 films was analyzed by four-circle and temperature-dependent two-circle X-ray diffraction (XRD), where x and w denote the tilt and rotational angles, respectively. The u–2u scans (see Fig. 1a) show the 200 Au reflection of the buffer layer (2u1⁄4 44.448) as well as the Fe70Pd30 002 reflection (57.388). Assuming a constant volume of the Fe70Pd30 unit cell compared to the cubic austenite, [15] the lattice parameters a and c were calculated to be 0.287 nm and 0.321 nm ( 0.001 nm), respectively, which constitutes a c/a ratio of 1.12. Temperature-dependent XRD measurements showed no change in the crystal structure in the accessible temperature range between 150 and 375K. The pole figure of the 101 reflection reveals a four-fold symmetry (Fig. 1b). The maximum intensity is obtained at an average of 47.278 at w1⁄4 458. The peak in the w direction is rather sharp with a small full width at half maximum (FWHM), indicating well-oriented growth of the body-centered tetragonal (bct) unit cell rotated by 458compared to the edges of the MgO cell. The larger FWHM in the x direction indicates relaxation of the lattice. There are no indications that twinning has occurred in the film. The sample for transmission electron microscopy (TEM) investigations was prepared by focused ion beam (FIB) lift-out