Value Of Bulk Material Research Articles

Possible Pairing without Superconductivity at Low Carrier Concentrations in Bulk and Thin-Film Superconducting Semiconductors

Simultaneous equations for the BCS parameter $\ensuremath{\Delta}$ and for the Fermi energy in the BCS state are studied at low carrier concentrations in bulk and thin-film superconducting semiconductors at $T=0$ for large cutoff energies, and solutions are plotted in the form of universal curves. For electron-electron attraction greater than some critical value in bulk material or for any attractive interaction in very thin films, the pair-binding energy tends to a constant limit for low $n$, and pairing without superconductivity is expected in some temperature range. Solutions of simultaneous equations for ${T}_{c}$ as obtained by formal application of the BCS theory, and for the Fermi energy at ${T}_{c}$, are also obtained in bulk material; but it is shown that at low carrier concentrations the temperature ${T}_{p}$ at which pairing takes place is given by ${T}_{p}=2{T}_{c}$, while it is thought that superconductivity will not set in until a lower temperature, of the order of the Bose-Einstein condensation temperature of the pairs, is reached. By combining the above theory with a previously published model for superconductivity in Zr-doped SrTi${\mathrm{O}}_{3}$, predictions for this material in the region of low carrier concentrations are made.

Structural Properties and Ferromagnetic Resonance Linewidth in Single-Crystal Nickel Ferrite Films

We have prepared single-crystal nickel ferrite films, several microns thick, on cleaved MgO surfaces both by chemical transport and by reactive sputtering. The structures were characterized by x-ray diffraction and by measurements of the static magnetic moment. The resonance field strengths parallel and perpendicular to the (100) plane of the films, and the linewidths were measured. Though the static film properties were in accordance with the corresponding values of bulk material, the resonance quantities varied over a wide range. In comparison with the theoretical values for ideal nickel ferrite films according to the Kittel resonance equation, the measured resonance fields for a smooth film may be explained by an additional quasi-isotropic internal field; a rough or granulated film, also obtained by chemical transport, is best described by a strong internal field (≥1300 Oe) which is anisotropic with regard to parallel and perpendicular film directions. The films prepared by reactive sputtering show some peculiarities. The linewidths of all films are relatively large (700–1700 Oe). We suggest two-magnon scattering for the line broadening, and for the line shape, superimposed magnetostatic and spin-wave modes excited by demagnetization fields.

Galvanomagnetic thin film devices

Thin films of In As and InSb directly evaporated on to substrates, such as glass, ceramics or ferrite, have interesting properties which make them suitable for galvanomagnetic devices as Hall probes or magneto resistors. The reasons for this are as follows: the Hall-sensitivity increases with decreasing thickness of the semiconducting layer. The higher inner resistance of thin films facilitates matching of the device to a particular circuit. Directly evaporated layers have a better heat dissipation factor than thin slices cemented to a substrate.Suitable methods of producing stoichiometric thin films of InAs and InSb with mobilities comparable to bulk material values are the flash-evaporation techniques and the three-temperature method. As the mobility shows a strong dependance on the crystallite size of the polycrystalline films, recrystallization techniques were subsequently applied to InSb films. This treatment results in larger crystallite sizes and mobilities of up to 30,000–40,000 cm2/Vs.The properties of thin film devices are discussed with special regard to Hall probes. The average values of mobility and Hall coefficient versus temperature and the resulting characteristics such as Hall-sensitivity, inner resistance, control current and temperature coefficient, are discussed with reference to several typical devices.

Ferromagnetic Resonance in Single-Crystal Nickel Films

Ferromagnetic resonance measurements have been carried out on single-crystal nickel films obtained by epitaxial evaporation on NaCl under various evaporation conditions. Resonance measurements on thin films by previous investigators have yielded anomalous values of 4πM which have tentatively been attributed to stress in the films. In this paper it is shown that there is a second factor which must be taken in consideration to explain the shift in resonance peak. Films that were grown at high temperatures (400°C) and in good vacuum (10−10 mm Hg) exhibited an apparent saturation magnetization only 60–70% of the bulk value. This is in accord with the results of ferromagnetic resonance experiments of Kuriyama et al., who attempted to explain their results by the presence of large tensile stresses in the films. However, electron microscope studies of films grown under the above conditions indicated that the films formed discrete, well-separated islands of nickel. The observed particle sizes are such that the demagnetizing factor perpendicular to the film is 60–70% of the value for a continuous planar film. This implies that the shift in the resonance peak was caused not by stresses but by a lowering of the demagnetizing field resulting from the discrete particle growth. To further substantiate this, continuous films grown at lower temperatures (300°C) and in technical vacuum (10−5 mm Hg) exhibited a magnetization approximately 60% higher than the bulk material values. X-ray diffraction indicated the presence of large compressive stresses in these films. That the increased magnetization was induced by these compressive stresses was convincingly shown by the decrease of the saturation magnetization to bulk value when the stress was relieved by the introduction of water vapor into the cavity. No such change in the external field at resonance was observed for the discontinuous films under a similar exposure to water vapor.