J.E. Evetts

Activation energy spectra and relaxation in amorphous materials

A theoretical model for relaxation in glassy materials, in particular metallic glasses, based on a spectrum of available processes distributed in activation energy is presented. The model is used to discuss “In t” kinetics, “reversibility” and “crossover” effects which have been observed experimentally. Where direct comparison is possible between the theory and experiment the agreement is good over all these various observed phenomena.

Magnetoresistance of artificial La0.7Sr0.3MnO3 grain boundaries as a function of misorientation angle

The resistance of polycrystalline doped LaMnO3 materials has been shown to be highly sensitive to low magnetic fields. To enable direct study of the properties of isolated grain boundaries we have grown epitaxial La0.7Sr0.3MnO3 films on a series of bicrystal substrates with different misorientation angles and patterned the films into a Wheatstone bridge geometry. We show that the grain boundary resistance and its magnetic field dependence vary strongly with the misorientation angle. The temperature dependence of the grain boundary resistance is also presented. We have obtained resistance changes of over 3% in fields of 2 mT at 300 K.

Differential scanning calorimetry study of solid‐state amorphization in multilayer thin‐film Ni/Zr

Differential scanning calorimetry (DSC) has been used to study solid‐state amorphization and subsequent crystallization in sputtered multilayer Ni/Zr thin films. Initial results provide quantitative information about the thermodynamics and kinetics of these processes. An analysis of DSC data enables the activation energy and pre‐exponential factor for interdiffusion of Ni and Zr in a‐NiZr to be found.

Properties of carbon nitride films deposited by magnetron sputtering

Abstract Carbon nitride films have been deposited by r.f. and d.c. planar balanced magnetron sputtering under various conditions. The films were analysed by Rutherford back-scattering (RBS), electron energy loss spectroscopy (EELS), and Fourier transform infrared (FTIR) spectroscopy. The deposition rates of the films synthesised by r.f. magnetron sputtering increased with nitrogen content in the sputtering gas. The nitrogen content of the films also increased as the nitrogen content in the sputtering gas was increased to 50 vol.%, and then remained constant. Absorption bands at 1250, 1550 and 2200 cm−1 were detected in FTIR spectra, corresponding to Raman-reactive disordered sp2 carbon, graphite-like sp2 carbon, and nitrile or isocyanate groups, respectively. Additionally, broad absorption around 1000–1200 cm−1, which may be associated with sp3 hybridised carbon, was observed in films deposited with low nitrogen content in the sputtering gas. Sputtering gas total pressure and sputtering power were also varied. The results were interpreted in terms of the collision frequency of the sputtered species with reactive gases in the system. The films deposited with >70% nitrogen in the sputtering gas showed higher friction coefficients and lower wear resistance in sliding friction tests than films grown with lower gas nitrogen content, suggesting that these properties of the films were associated with their structure.

Magnetic force microscopy of Co-Pd multilayers with perpendicular anisotropy

The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films is 55 A and the domain‐wall energy is 14 mJ/m2.The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films i...

Sputter deposition of giant magnetoresistive Co-Cu multilayers

Abstract We have used UHV dc magnetron sputtering to deposit giant magnetoressive Co-Cu multilayers on Si substrates. The largest magnetoresistances are obtained in films which are deposited using the lowest sputtering pressures, and which therefore experience the most energetic bombardment during deposition. The large magnetoresistances are caused not by comparatively small resistivities in an applied magnetic field, but by relatively large zero-field resistivities. Structural features associated with low sputtering pressures allow substantial antiferromagnetic alignment of Co layers in zero field. The smaller the sputtering pressure, the greater the antiferromagnetic alignment, the greater the zero-field resistivity and the further the sample resistivity is able to fall when a magnetic field changes the alignment from antiferromagnetic to ferromagnetic. Multilayers of Co-Cu made by evaporation may not show substantial magnetoresistance because, unlike sputtering, evaporation transfers little energy to the instantaneous surface of a growing film.

Resistance of a domain wall in La0.7Ca0.3MnO3

Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area...

BaZrO3 and BaHfO3: preparation, properties and compatibility with YBa2Cu3O7-x

Single-phase BaZrO3 and BaHfO3 polycrystalline powders were prepared by solid-state reaction and by spray-drying methods. BaHfO3 ceramic was fabricated at 1300°C for 24 h after pre-sintering HfO2 and BaCO3 powders at 1040°C for 4 h. Homogeneous powders of BaZrO3 and BaHfO3 were also prepared by a spray-drying route. The interaction between YBa2Cu3O7-x (YBCO) and BaHfO3 and BaZrO3 was investigated in the temperature range 900–1060°C using heat-treatment cycles appropriate to composite reaction processing and melt-texturing. The results indicate that neither compound reacts significantly with YBCO at 950°C, and BaHfO3 is still unreactive up to 1000°C. Both of them are also very stable during the melting-texture process. BaHfO3 and BaZrO3 are thus very promising substrate materials and buffer layers for the deposition of thin and thick films and as container materials for bulk YBCO superconductors; BaHfO3 seems to be a more stable material than BaZrO3.

The effect of anodization on the electromigration drift velocity in aluminum films

The electromigration drift velocity has been measured, using the Blech–Kinsbron [Thin Solid Films 25, 327 (1975)] edge displacement method, for aluminum thin films under different thicknesses of anodization. The drift velocity is found to decrease with increasing anodization thickness. The results are interpreted in terms of a change in the self‐diffusivity of aluminum as a result of the compressive stresses imposed by the anodized layer. The effect of the variation of diffusivity with stress on the stress distribution within a drifting thin‐film sample is discussed.The electromigration drift velocity has been measured, using the Blech–Kinsbron [Thin Solid Films 25, 327 (1975)] edge displacement method, for aluminum thin films under different thicknesses of anodization. The drift velocity is found to decrease with increasing anodization thickness. The results are interpreted in terms of a change in the self‐diffusivity of aluminum as a result of the compressive stresses imposed by the anodized layer. The effect of the variation of diffusivity with stress on the stress distribution within a drifting thin‐film sample is discussed.

A transient nucleation model for solid state amorphisation

Abstract Solid state amorphisation in diffusion couples is analysed with particular reference to the Ni-Zr system. We show that there is no nucleation barrier for formation of the first phase, and discuss why that phase is amorphous. We postulate that formation of the second product phase (a crystalline intermetallic compound) is suppressed by consumption of sub-critical clusters of that phase by the growing amorphous layer. We derive approximate expressions for the critical thickening rate of the amorphous phase (above which suppression is possible), and for the maximum thickness that the phase can attain before the crystalline compound nucleates.