Reversible fcc M bcc transformation in freestanding epitaxially grown Fe–Pd ferromagnetic shape memory films T. Edler, a S. Hamann, b A. Ludwig b and S.G. Mayr c,⇑ a I. Physikalisches Institut, Georg-August-Universita ¨t Go ¨ ttingen, Friedrich-Hund-Platz 1, D-37077 Go ¨ ttingen, Germany b Werkstoffe der Mikrotechnik, Institut fu ¨ r Werkstoffe, Fakulta ¨t fu ¨ r Maschinenbau, Ruhr-Universita ¨ t Bochum, Universita ¨ tsstr. 150, D-44801 Bochum, Germany c Leibniz-Institut fu ¨ r Oberfla ¨ chenmodifizierung e.V., Translationszentrum fu ¨ r regenerative Medizin und Fakulta ¨t fu ¨ r Physik und Geowissenschaften, Universita ¨ t Leipzig, Permoserstrasse 15, D-04318 Leipzig, Germany Received 7 August 2010; revised 8 September 2010; accepted 9 September 2010 Available online 17 September 2010 Using temperature-dependent X-ray diffraction and magnetization measurements, a reversible face-centered cubic (fcc) to body- centered cubic (bcc) structural transformation was confirmed in freestanding epitaxially grown Fe 70 Pd 30 films after lift-off from their MgO (1 0 0) substrates—a transformation generally considered irreversible in bulk samples. The latter is accompanied by a distinct change of the sample magnetization. In contrast, substrate constraints were found to suppress the thermoelastic fcc to bcc transfor- mation in substrate-attached films. Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Martensitic phase transformation; Magnetic thin films; Molecular beam epitaxy; Shape memory alloys Fe–Pd alloys have attracted significant interest recently due to their martensitic transformation [1,2] and the magnetic field-induced strain effect (MFIS) [3– 6]. Generally the latter is based on an alignment of mar- tensitic variants in the low-temperature phase along the direction of an external magnetic field, yielding a maxi- mum strain of 3% for Fe 70 Pd 30 [7]. Coherent epitaxial growth of thin Fe–Pd films has been achieved by various methods, including sputter, pulsed laser deposition (PLD) [8,9] and vapor deposition [10]. While PLD grown films suffer from a breakdown of epitaxy for thicknesses <20 nm [11] due to misfit-related stresses, growth of epitaxial films up to the lm range is feasible by vapor and sputter deposition on heated MgO sub- strates [10,13]. For actuator or sensor applications it is of great interest to eliminate substrate constraints by fabricating freestanding, single-crystalline films that ex- hibit a reversible thermoelastic transformation. In this paper, we present studies on structural and magnetic properties of such Fe 70 Pd 30 films that reveal a novel reversible face-centered cubic (fcc) to body-centered cu- bic (bcc) transformation generally considered irrevers- ible in bulk samples [12]. Homogeneous epitaxial Fe 70 Pd 30 films with thick- nesses of 200 nm were condensed epitaxially in the meta- stable fcc structure on MgO (1 0 0) substrates at 963 K by vapor deposition, as described in Ref. [10]. After preparation, parts of the films were released from the substrate by chemically dissolving the MgO, while leav- ing the film structurally and chemically intact [14]. Room temperature pole figure measurements (using a Phillips X’Pert system) of freestanding films verify a sin- gle-crystal structure illustrated by the well-defined (1 1 1) fcc austenite peak in every quadrant in Figure 1. In the as-deposited state no martensitic transformation was observed for the substrate-attached and the freestanding sample down to 130 K using temperature dependent X-ray diffraction (XRD(T) - with a PANalytical X’Pert PRO X-ray diffraction system (PIXEL detector, Cu K a radiation) equipped with an Anton Paar TTK 450). However, the Invar effect, which is known to occur in the Fe–Pd system [15], was observed for temperatures above T ¼ 300 K: Figure 1 presents the lattice parame- ters as a function of T for both the substrate-attached and the freestanding thin films. For T > 300 K the latter virtually remains constant, while the former increases with temperature, although at slightly reduced slope. 1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.09.013 ⇑ Corresponding author. Tel.: +49 341 235 3368; fax: + 49 341 235 2595; e-mail addresses: tedler@gwdg.de; Stefan.Mayr@IOM-Leipzig.DE Available online at www.sciencedirect.com Scripta Materialia 64 (2011) 89–92 www.elsevier.com/locate/scriptamat