R. Senderak

Annealing behaviour of structural and magnetic properties of evaporated Co thin films

Cobalt thin films of 50 nm nominal thickness were e-beam evaporated on silicon substrates covered with thermal oxide. Two series of independent and cumulative vacuum annealings up to 600 ◦ C and 650 ◦ C, respectively, were performed. The x-ray diffraction, specular and non-specular x-ray reflectivity and longitudinal magneto-optical Kerr effect measurements were applied to probe the annealing behaviour of the film structure and magnetic properties. A gradual transition from the hexagonal close-packed (hcp) to the face-centred cubic (fcc) structure was observed. Evolution of the in-plane magnetic anisotropy is dominated by residual stresses which relax during the structural transformation. The coercivity follows the stress behaviour in the hcp phase up to 300 ◦ C and increases abruptly above 400 ◦ C due to improving the magneto-crystalline anisotropy in the growing fcc crystallites and enhanced surface/interface roughness. (Some figures in this article are in colour only in the electronic version)

Giant magnetoresistance in granular Ag–Co films irradiated by excimer laser

Abstract A series of Ag 100− x Co x films ( x varying from 10 to 30 at.%) were codeposited by two electron beam sources in UHV system at the Si/SiO 2 substrate temperature ≤40°C. The Ag 75 Co 25 films were thermally processed by rapid thermal annealing (RTA) at 500, 750 and 1000°C for 30 s and by excimer laser irradiation at the fluences F =0.1, 0.15 and 0.2 J cm −2 and number of pulses n varying from 1 to 100. The samples were analyzed by X-ray diffraction (XRD) and grazing incidence XRD (GIXRD). The electrical resistance R ( H ) was measured up to 50 kOe with magnetic field perpendicular or parallel to the film plane and with current in plane of the sample, mostly at 4.2 K and 300 K. With increasing Co content in the film, a different degree of fcc Ag and fcc Co separation in the as-deposited films was found, and for x →30%, small fcc Co peaks appeared. With RTA, a pronounced phase separation and increasing grain size with increasing annealing temperature was observed. On the contrary, after laser irradiation, more random and less equilibrium structures with smaller grain size and even negative temperature coefficient of the resistance developed. The giant magnetoresistance (GMR) of Ag 75 Co 25 films is ≈74% at 4.2 K and 11.3% at 300 K in the as-deposited state. The GMR at 4.2 K can be increased by XeCl irradiation to 82% at the most severe irradiation conditions ( F =0.2 J cm −2 , n =10) and to 13.2–13.5% at 300 K at the applied fluences. The increase of GMR is ascribed to the formation of more random structure with smaller grain size.

Thermally activated interface shift in the tungsten/silicon multilayers

The in situ x‐ray reflectivity measurements during linear and isothermal annealings of a W/Si multilayer (ML) were performed, and successive disappearance and reappearance of the second and third ML Bragg maxima between 400 and 500 °C were observed. Such behavior is direct evidence of a long‐range interface shift, and was found to be connected with a substantial decrease of the electronic density of the expanding originally W layers and the ML period. Surprisingly, the changes take place without smearing‐out the interfaces. The results are explained by the Si diffusion into the W layers.

Characterization of obliquely deposited tungsten/silicon multilayers

Abstract W/Si multilayers (MLs) with periods 6.6–15 nm were evaporated both perpendicularly and obliquely at incident angles 20° and 40° to the normal. MLs were deposited at 70–100 °C onto oxidized silicon. The samples were studied by TEM, CS-TEM, SAXD and resistometry in the temperature range 1.5–300 K. Experimental data were completed by simulation of the SAXD spectra. The MLs were found to be amorphous and superconducting with Tc ⩽ 2.84 K. With increasing angle of deposition the thickness of the intermixed regions at the interfaces decreased and the roughness of the interfaces increased. The reflectivity of MLs was practically independent of the deposition angle, probably due to the mutual compensation of the two above-mentioned effects. The 2D–3D crossover in the weak localization and interaction behaviour observed in the temperature range Tc−30 K correlates with the pure amorphous Si layer thickness reduced due to the mutual penetration of W and Si at perpendicular deposition as well as at oblique deposition for the incident angle 20°.

Thermal stability of W1-xSix/Si multilayer reflective coatings under high intensity excimer laser pulses

Abstract UHV e-beam evaporated W 1 − x Si x Si multilayers (MLs) with four different compositions (x = 0, 0.33, 0.5 and 0.66) were XeCl laser irradiated at different fluences F ≤ 0.6 J cm−2 with number of pulses N ≤ 100. The samples were analyzed by soft X-ray reflectivity, X-ray diffraction and resistometry measurements. It was shown, that the hardness of the samples against laser irradiation increases with increasing x up to x = 0.5, then goes down again. This behavior is explained by the interplay of the suppression of the interdiffusion at the interfaces of MLs with increasing content of Si in W and the gradual decrease of the refractory nature of W when it is diluted by Si.

Interface study of W-Si/Si and obliquely deposited W/Si multilayers by grazing-incidence high-resolution x-ray diffraction

X-ray reflectivity and diffuse scattering measurements at grazing incidence including 2 theta and omega scans were done using Cu K alpha 1 radiation on W/Si multilayers deposited obliquely at different angles and on W1-xSix/Si multilayers of different compositions. The reflectivity and omega scans were simulated by the Fresnel optical computational code and within the distorted-wave Born approximation, taking the vertical roughness correlation into account, respectively. The root-mean-square interface roughness increases by about 15% while lateral correlation length decreases by nearly one order of magnitude on increasing deposition angle of the W/Si multilayers up to 47 degrees . Simultaneously the vertical roughness correlation decreases, which is shown also by 2 theta scans. Thus rougher, less correlated interfaces and an increased tendency for agglomeration appear. No root-mean-square interface roughness change was detected for W1-xSix/Si multilayers with x up to x=0.67 while the lateral correlation length decreased by one order of magnitude, interfaces being more correlated vertically. The interfaces do not exhibit the fractal behaviour in either type of multilayer.

Scanning magneto-optical Kerr microscope with auto-balanced detection scheme

We have developed a scanning magneto-optical Kerr microscope dedicated to localization and measurement of the in-plane magnetization of ultra-thin layered magnetic nanostructures with high sensitivity and signal-to-noise ratio. The novel light detection scheme is based on a differential photodetector with automatic common mode noise rejection system with a high noise suppression up to 50 dB. The sensitivity of the developed detection scheme was tested by measurement of a single Co layer and a giant magnetoresistance (GMR) multilayer stack. The spatial resolution of the Kerr microscope was demonstrated by mapping an isolated 5×5 μm spin-valve pillar.

Electronic transport properties of amorphous W/Si multilayers

Abstract The temperature dependence of the sheet resistance of e-beam evaporated W/Si multilayers was investigated. A positive temperature coefficient of resistivity (TCR) 190–600 ppm/K was obtained for multilayers with crystalline tungsten layers (dw > 4 nm). Multilayers with amorphous tungsten layers 1–4 nm thick revealed nom-metallic resistivity behaviour with a negative TCR, -(100–300) ppm/K in the temperature range 1.8–300 K, and were superconducting with the superconducting transition temperature Tc up to 4.21 K. The resistivity behaviour of the amorphous W/Si multilayers with Si layer thicknesses 1–10 nm can be described within the framework of 3D weak localization and interaction models. At temperatures close to Tc superconducting fluctuations decrease the resistance of these samples. At temperatures above Tc both effects can be comparable, resulting in a maximum in the temperature dependence of the resistance. A relatively strong coupling between metallic layers was observed. This coupling could originate from mixing at the W/Si interfaces and interdiffusion during the deposition and subsequent annealing. The different resistivity behaviour of amorphous tungsten monolayers strongly supports this assumption.

Effect of magnetic flux distribution on GMR in Ag/Co multilayers

Abstract Since the discovery of giant magnetoresistance (GMR) in multilayers (MLs) various applications of this effect are studied. One practical feature of ML GMR sensing elements is the magnetic flux distribution in the direction perpendicular to the surface of ML in the current-in-plane geometry. It results from the fact that top and bottom ferromagnetic layers concentrate more magnetic flux than central layers of the stack. This effect is studied in this work in evaporated Ag/Co MLs deposited onto Si substrates. The thickness of Co was 1.2–1.4 nm, i.e. Co layers were continuous. Ag layers were 5.4–5.8 nm thick. Number of Ag/Co periods was only ≤5 to expose the relative influence of top and bottom parts of ML. To make the ML structure symmetrical, an additional Co layer was deposited as first on the substrate. Ag/Co MLs were analyzed by X-ray diffraction (XRD) and X-ray reflectivity (XRR). The magnetic flux distribution was simulated. GMR measurements were performed in dynamic conditions in the external ac field (50 Hz) with increasing swing. By simulations of XRR spectra the effective roughness of ML interfaces 0.4–0.8 nm was obtained. The GMR signal at RT increased with number of periods and the external field ≤16 kA/m up to approximately 2%. The influence of distribution of the magnetic flux density on GMR was inferred from the behavior of the positions of maxima of GMR on the magnetic field axis vs. number of periods. It can be explained by the statistical distribution of coercivity of different domains in Co layers.

Characterization of laterally nanopatterned W/Si multilayers

We present a structural study of two amorphous periodic W/Si multilayer gratings obtained by etching the planar multilayer up to the multilayer substrate based on scanning-electron and atomic force microscopy observations and X-ray reflectivity measurements of coherent grating truncation rods. Two different exposure modes to prepare resist mask were examined to optimize the resulting grating structure. The real structural parameters of a more perfect grating were extracted from fitting the measured grating truncation rods. The simulations are based on the matrix modal eigenvalue approach of the dynamical theory of reflectivity by gratings which generalizes the Fresnel transmission and reflection coefficients for lateral diffraction. The interface imperfections are taken into account by the coherent amplitude approach which averages the propagation matrices of the wave field over random interface displacements.