H. Grimmer

Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers

Abstract A novel fabrication process for Ni–Ti shape-memory alloy thin films is presented. This process is based on the appropriate annealing of sputter-deposited Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the two constituents results either in the amorphization of the multilayer structure after annealing at 330°C or in the recrystallization as a Ni–Ti intermetallic compound after annealing at temperatures above 400°C. A single 30 min annealing step in the temperature range from 400 to 800°C is sufficient to obtain Ni–Ti films showing martensitic phase transformations and the shape-memory effect. The influence of increasing annealing temperature on the transformation behavior is investigated by differential scanning calorimetry. The evolution of the transformation temperatures is found to be qualitatively similar to conventional sputter-deposited Ni–Ti films. The corresponding microstructure is studied by transmission electron microscopy. A very fine-grained structure is observed even after annealing at 800°C. The film composition can be varied by adjusting the thickness ratio of the individual Ti and Ni layers. Transformation curves of films with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is demonstrated that Ni–Ti films made up from multilayers may possess an intrinsic “two-way” shape-memory effect, which is a very interesting feature in view of the development of thin film micro-actuators.

W/C, W/Ti, Ni/Ti, and Ni/V multilayers for the soft-x-ray range: experimental investigation with synchrotron radiation

An experimental investigation of W/C, W/Ti, Ni/Ti, and Ni/V multilayers is presented that uses synchrotron radiation in the soft-x-ray energy region between 100 and 1500 eV with special emphasison the water window. The multilayers were designed as normal incidence reflectors and for polarimetry purposes around the Brewster angle. Both reflection and transmission multilayers were prepared for use as linear polarizers and phase retarders, respectively, to produce or analyze circularly polarized light. Their period was optimized to achieve maximum reflectance at the 1s absorption edge of C (284 eV) and the 2p edges of Ti (454 eV) and V (512 eV), respectively. At these edges the multilayers show an enhancement of reflectance and energy resolution that is in accordance with theoretical calculations.

Development of Ni/Ti multilayer supermirrors for neutron optics

Artificial multilayer neutron supermirrors are composed of discrete sets of bilayer structures based on Ni and Ti layers with graded thicknesses and with high scattering contrast between individual layers. Various samples consisting of NiXxTiYy (X = C, N, dry air, Y = H, V) periodic multilayers and supermirrors, as well as NiXx and TiYy monolayers were deposited on normal float-glass, borkron glass and on silicon substrates using d.c. magnetron sputtering. The structure and stability of these thin films is the object of a detailed study mainly by means of X-ray diffraction, TEM, AFM and neutron reflectometry. The addition of N to the Ni layers yields a Ni lattice dilatation, a reorientation of the Ni grains from {111} to {200} parallel to the layer surface, a reduction of the grain size, a smoothing of the layer surfaces (Rr.m.s.∼0.3 nm for d = 10 nm), a sharpening of the multilayer interfaces and an enhancement of the scattering length density (nb=9.53×10−4nm−2 for Ni− N). Similar results are obtained with C and dry air, which in addition suppress the diffusion of Ni into Ti. The addition of V to the Ti layers shows a smoothing of the layer surfaces and helps to suppress interdiffusion in multilayers, leading also to sharp and stable interfaces. Large neutron supermirrors made of 40 bilayers of NiNO/TiV with a reflectivity in the range of 90% to 94% at glancing angles up to more than twice the critical angle of total reflection from an ordinary Ni surface have been achieved and are fabricated in mass production.

Twinning by reticular pseudo-merohedry in trigonal, tetragonal and hexagonal crystals.

Twin laws for trigonal, tetragonal and hexagonal crystals describing twins with principal axes inclined by an angle Phi > 0 are analysed. Twins by reticular merohedry (i.e. obliquity delta = 0) are possible only for certain values s of the axial ratio c/a. For any other axial ratio r, the laws describe twinning by reticular pseudo-merohedry, i.e. with obliquity delta > 0. It is shown that (a) tandelta is a product of two factors, one of which is sinPhi, the other depends only on the relative deviation of r from s; (b) tandelta approximately epsilon, where epsilon denotes the deformation parameter introduced by Bonnet & Durand [Philos. Mag. (1975), 32, 997-1006]. The angle Phi is listed for all cases of reticular merohedry of trigonal, tetragonal and hexagonal (i.e. optically uniaxial) crystals with twin index Sigma </= 5. Mallards criterion requires that twin laws by (reticular) pseudo-merohedry have Sigma </= 5 and delta </= 6 degrees. Le Page [J. Appl. Cryst. (2002), 35, 175-181] has written a program determining laws with twin index Sigma </= Sigma(max) and obliquity delta </= delta(max) for any given lattice geometry. Here those solutions are analysed and completed for optically uniaxial crystals. Their lattices are characterized by the Bravais class (tP, tI, hP or hR) and the axial ratio c/a = r. For small delta(max), most solutions are related to (reticular) merohedry for an appropriate value s approximately r of the axial ratio. It is argued that other solutions, which are not related to (reticular) merohedry, are not needed to explain observed laws of growth twinning but may be important to interpret observed laws of deformation twinning.

X-ray reflectivity of multilayer mirrors for the water window

Multilayer structures for the soft X-ray range were designed as normal incidence mirrors or as linear polarizers at the Brewster angle. Their reflectance was measured with special emphasis on the absorption edges of the multilayer materials. The measurements not only revealed pairs of reflectance peaks at the L edges of Ti, V and Ni, which can be explained by the spin-orbit splitting of the p levels, but also a pair of peaks at the K edge of carbon.

Soft X-ray magneto-optical constants at the Fe 2p edge determined by Bragg scattering and Faraday effect

Bragg scattering on a Fe/C multilayer in L-MOKE geometry, using circularly polarised undulator radiation, was exploited to determine the optical constants of ferromagnetic Fe across the Fe 2p absorption edges. An alternative data set was obtained from Faraday experiments on identical samples measured in transmission. Both methods lead to a direct determination of both the refractive and absorptive part of the optical constant.

Polarizing Ti1-uXu/FexCoyVz supermirrors

Abstract Polarizing multilayers and supermirrors composed of Ti1−uXu and FexCoyVz have been produced using DC-magnetron sputtering. The structure of the samples has been characterized by neutron and X-ray diffraction. The results show that the smoothness of the layers can be improved by adding controlled amounts of alloying materials, an effect that has already been observed for Ni/Ti supermirrors. Flipping ratios of more than 40 and reflectivities of more than 85% at twice the critical angle of Ni are obtained, depending on the concentrations of the materials, u, x, y, z. The sputtering process can be optimized in such a way that the magnetization within the plane is anisotropic, thus allowing a saturation of the samples in low fields.

Optical components for polarization analysis of soft x-ray radiation

An instrument for measuring polarization typically consists of a phase shifter and a linear polarizer. Up to 600 eV, periodic multilayer structures can be used for this purpose. They consist of alternating layers of two materials, one of which has an absorption edge in the photon energy region of interest. The phase shift of the transmitted beam and the intensity of the reflected beam show maxima at energies just below the edge. Phase shifters on 100 nm thick Si3N4 membranes and linear polarizers on Si wafers have been made by DC magnetron sputter deposition. The polarizers were designed to work at the Brewster angle, where only s-polarized radiation is reflected. The corresponding multilayer period decreases from 8.5 nm at 100 eV to 1.4 nm at 600 eV. A reflectance of 6.8% was obtained at 512 eV for 150(V/Ni). In order to extend polarization measurements into the 1 keV region, the use of magnetic effects like magnetic circular dichroism (MCD) is being explored. This effect has been measured in Co/C and FeCoV/Ti transmission multilayers close to the L2,3 edges of Co and Fe.

Resonant magnetic scattering of linearly polarised soft X-rays from Fe-layers and Fe/C-multilayers

Abstract We report on scattering experiments in T-MOKE geometry performed at the Fe-2p absorption edge of an ex situ grown Fe layer and an Fe/C multilayer. We used linearly polarised synchrotron radiation and measured the reflected intensities upon reversal of the magnetisation perpendicular to the scattering plane. The asymmetry ratio A has been determined as a function of magnetic field strength, photon energy and angle of incidence. These data are discussed in terms of interference effects and the depth profile of the refraction index and are compared with calculations.

OPTICAL COMPONENTS FOR POLARIZATION ANALYSIS AT THE VANADIUM L3 EDGE AND THE CARBON K EDGE

A complete polarization analysis of soft X-ray radiation with primary standards can be performed using a phase shifter in transmission and an analyzer in reflection. V/Ni multilayers produced at PSI by sputter deposition showed a phase shift |△| = 7.6° at 512 eV and a reflectance ratio Rs/Rp > 1000 for E in the range 507–512 eV. For Cr/C multilayers a phase shift |△| = 21° was found at 277 eV and Rs = 20%, Rs/Rp = 1400 at 278 eV. The values show that these components are suitable for polarization analysis at the mentioned energies.