J. Buschbeck

Adaptive modulations of martensites.

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape-memory alloys. To understand the origin of these phases, we employ and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase, and tetragonal martensite in Ni-Mn-Ga magnetic shape-memory alloy epitaxial films. We show that the modulated martensite can be constructed from nanotwinned variants of the tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes metastability, the sequence of 6M-10M-14M-NM intermartensitic transitions, and the magnetocrystalline anisotropy.

Magnetically induced reorientation of martensite variants in constrained epitaxial Ni-Mn-Ga films grown on MgO(001)

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni-Mn-Ga films. Ni-Mn-Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45 relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing microstructural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.

Full tunability of strain along the fcc-bcc bain path in epitaxial films and consequences for magnetic properties.

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.

Epitaxial Ni–Mn–Ga films deposited on SrTiO3 and evidence of magnetically induced reorientation of martensitic variants at room temperature

Epitaxial Ni–Mn–Ga films were grown on SrTiO3 by sputter deposition. The films deposited at 673K are ferromagnetic and martensitic at room temperature. Pole figure measurements indicate that the twinned orthorhombic martensite microstructure of the film has a lower symmetry compared to bulk. Magnetically induced reorientation or magnetic shape memory effect is indicated by magnetization curve measurements. Though the overall extension of the film is constrained by a rigid substrate, the reorientation is possible due to the additional degree of freedom in the orthorhombic phase.

Stress induced martensite in epitaxial Ni–Mn–Ga films deposited on MgO(001)

Biaxial tensile stress in epitaxial Ni52Mn23Ga25 films on MgO(001) was measured at different temperatures using an adapted x-ray stress analysis. A stress of up to 105MPa originates from different thermal expansions of substrate and film and partially from the substrate-film misfit. The film transforms to twinned orthorhombic martensite at 319K. The stress increases the martensitic transformation temperature of about 63K. This is in good agreement with respect to the transformation temperature expected from the composition of the film. The presence of biaxial tensile stress leads to twin boundary selection, hence, there are no twin planes perpendicular to the substrate.

Thermodynamics and kinetics during pulsed laser annealing and patterning of FePt films

Laser annealing using 25ns pulses of a KrF excimer laser was applied to electrochemical and pulsed laser deposited FePt films of 700nm and 40 or 80nm thicknesses, respectively. The dependence of phase formation and magnetization behavior on the film thickness and the as-deposited state are studied. Models of the film heating are compared to the experimentally observed results. Furthermore a time temperature transformation (TTT) diagram for the ordering of FePt is proposed, summarizing the phase formation in dependence on annealing time and temperature. In agreement with the TTT diagram, laser annealing leads to a disordering of L10-ordered films. By only partially disordering the L10 phase, this approach provides control over the coercivity of hard magnetic FePt films. Local disordering is applied for magnetic patterning of the FePt films with dot patterns in the micrometer size range.

Mechanisms of stress generation and relaxation during pulsed laser deposition of epitaxial Fe?Pd magnetic shape memory alloy films on MgO

Mechanical stress generation during epitaxial growth of Fe–Pd thin films on MgO from pulsed laser deposition is a key parameter for the suitability in shape memory applications. By employing in situ substrate curvature measurements, we determine the stress states as a function of film thickness and composition. Depending on composition, different stress states are observed during initial film growth, which can be attributed to different misfits. Compressive stress generation by atomic peening is observed in the later stages of growth. Comparison with ex situ x-ray based strain measurements allows integral and local stress to be distinguished and yields heterogeneities of the stress state between coherent and incoherent regions. In combination with cross-sectional TEM measurements the relevant stress relaxation mechanism is identified to be stress-induced martensite formation with (111) twinning.

Correlation of phase transformations and magnetic properties in annealed epitaxial Fe–Pd magnetic shape memory alloy films

Single-crystal-like films are promising candidates for magnetic shape memory (MSM) applications on the microscale. For defect reduction and stress relaxation, we apply a heat treatment to pulsed laser deposited, partial epitaxial Fe–Pd films with different compositions. By recrystallization starting from the epitaxial interface, single-crystal-like films are obtained. Deformation twins being present in the as-deposited state are completely eliminated. The epitaxial (100) orientation allows clear monitoring of the transformation from face centered cubic (fcc) austenite to face centered tetragonal (fct) martensite by x-ray diffraction experiments. Transformation from fcc austenite to fct martensite is hindered by constraints from the substrate. At temperatures down to 125 K residual fcc austenite is present. Magnetic measurements performed down to 50 K indicate that during further cooling the phase transformation to body centered tetragonal martensite occurs. The results show that annealing of laser deposit...

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

Fe–Pd thin films as a model system for self-organized exchange coupled nanomagnets

In equilibrium the Fe–Pd system on the iron rich side of the phase diagram demixes into Fe and L10-ordered FePd. Here, we examine the suitability of the demixing process for self-organized formation of exchange coupled thin film magnets. In this way the benefit of the high magnetization of Fe is combined with the high magnetocrystalline anisotropy of FePd. By using combinatorial methods the influence of composition and thickness on structure, microstructure, and magnetic properties is analyzed. Experiments show the different thermodynamic and kinetic conditions required for demixing and ordering. In particular, for nanostructures the interface energy during demixing must be considered.