Oscillations of conductivity and the hall coefficient in thin bismuth films

It was shown that in the Abrikosov-Cohen spectrum, the conductivity and the Hall coefficient in thin bismuth films have an oscillatory dependence on the film thickness even if it is assumed that the relaxation time does not depend on the energy.     

Effects of energy band model on magnetoresistance in bismuth

Using the quantum statistical method, a study is made of the effect of a dc magnetic field on the longitudinal and transverse magnetoresistance in the very low-temperature limit, and numerical results are presented for the MNENP, NENP, ENP, and EP models. The results show that the larger the magnetic field, the greater is the amplitude of the oscillations. The amplitude of oscillations in the transverse magnetoresistance is much larger than that in the longitudinal magnetoresistance. There are spikelike discontinuities in the transverse magnetoresistance. The quantum oscillations of both magnetoresistances for the MNENP model show up much more clearly than those for the NENP, ENP, and EP models.     

Non-linearity in NaCl single-crystal films

Epitaxial Ag/NaCl/Ag capacitors have been fabricated on hot NaCl substrates and their dielectric properties have been measured. Only one loss peak in the relaxation spectrum of the epitaxial films has been observed, which has been attributed to the blocking of cation vacancies at the electrode-dielectric interfaces. The activation energy associated with the loss peak has been calculated, and is of the order of 0.83 ± 0.1 eV. The charge carrier concentration has been estimated to be about 10²⁴ m^?3. The increase in capacitance observed at low frequencies suggests that the formation of space charge is responsible for the polarization in alkali halides.A strong non-linearity is associated with the polarization and has been observed in the form of variations of dielectric loss and capacitance in alkali halide films. Harmonic generation and cut-off in the current through the alkali halide epitaxial capacitors have been observed. All these observations imply that the space charge polarization mechanism is predominant in alkali halide films. A built-in potential of several hundred millivolts across the freshly prepared epitaxial capacitor has been observed, and this causes an asymmetry in the electrical behaviour. No evidence of vacancy pair orientation has been found and the results have been attributed to the migration of cation vacancies through the whole thickness of the epitaxial film with subsequent blocking at the electrodes to form a space charge.     

Electrodeposited single-crystal cobalt films

Electrodeposition of single-crystal thin films of cobalt from a high concentration metal ion bath is described. The crystal structures observed by RHEED are related to the current densities and pH of the plating bath and the deposit thickness. Emphasis is on the parameter values within which single-crystal h.c.p. cobalt may be obtained with the c-axis perpendicular to the film plane.     

Anomalous anisotropic magnetoresistance in topological insulator films

Topological insulators are insulating in the bulk but possess spin-momentum locked metallic surface states protected by time-reversal symmetry. The existence of these surface states has been confirmed by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). Detecting these surface states by transport measurements, which might at first appear to be the most direct avenue, was shown to be much more challenging than expected. Here, we report a detailed electronic transport study in high quality Bi₂Se₃ topological insulator thin films. Interestingly, measurements under an in-plane magnetic field, along and perpendicular to the bias current show anomalous opposite magnetoresistance.      

Epitaxial growth of metal single-crystal films

A review of the epitaxial growth of metals in high and ultra-high vacuum leading to single-crystal films is presented. The conditions for obtaining such films are described and tabulated for two groups of substrates: (i) metals, Si, Ge (strong interfacial bonding) and (ii) alkali halides and MgO cleavage faces (weak interfacial bonding). Information on the crystal defects and their relation with the growth parameters and annealing procedures is also given.     

Giant magnetoresistance in electrodeposited Co-Ag granular films

We report on the electrodeposition of Co-Ag granular films with GMR from a chloride-based electrolyte. Two different electrochemical techniques (chronoamperometry and pulse plating) were used to prepare the films. Both electrochemical and TEM analysis have revealed the heterogeneity of the as-deposited samples. GMR values up to 7% at room temperature were measured. The longitudinal (LMR) and transverse (TMR) magnetoresistance were practically indistinguishable indicative of real granular systems. An appropriate numerical analysis of the magnetoresistance curves showed high superparamagnetic contribution to the total MR.     

Deviations from Matthiessen's rule and longitudinal magnetoresistance in copper

Deviations from Matthiessen's rule (MR) due to phonons inCuAu andCuNi alloys were measured as a function of temperature between 4°K and 300°C. They can amount to 40% of the residual resistivity of the doped samples. A second group of measurements concerns deviations from MR that occur inCuNi during a neutron irradiation at 4.6°K. It turned out that there are two essential reasons for these deviations. The first one is the influence of the defects on the temperature-dependent part of the electrical resistivity. It plays an important role mainly at low temperatures. We found experimentally and theoretically aT ⁵ dependence for this additional resistivity at low temperatures. The second reason is the existence of anisotropies in the Fermi surface of Cu and in the scattering potential of the defects. These deviations can be described, as suggested by J. M. Ziman, by a two-band model for the Fermi surface. Furthermore, we could show that the enhanced resistivity increase in theCuNi alloys during neutron irradiation at 4.6°K, which was generally interpreted as an enhanced defect production, is essentially caused by deviations from MR. Measurements of the longitudinal magnetoresistance of the doped copper at 4.2°K were very useful for discriminating between the different causes of deviations from MR.     

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

The tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs) is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.     

Electrical resistivity of thin bismuth films

The thickness dependence of electrical resistivity of thin bismuth films deposited on to glass substrates has been studied in the temperature range 77 to 350 K. The structural studies show that films are polycrystalline with grain size increasing with thickness. The electrical resistivity decreases with increasing temperature. This type of temperature dependence of electrical resistivity arises due to the competition between the temperature dependences of carrier density and carrier mobility, the temperature coefficients of which are opposite in sign. The electrical properties of bismuth films are greatly affected by the presence of surface states. The size effect theory of Pichard and colleagues (PTT) has been modified to investigate the variation of charge carrier concentration with thickness. The experimental results on electrical resistivity are found to be consistent with the predictions of this modified three-dimensional model (PTT). The value of specularity parameter,p, is about 0.43 and it exhibits no temperature dependence. The experimental results on grain-boundary resistivity agree fairly well with the theoretical variations. The values of transmission coefficient,t, are determined from the experimental data.     

Polar and longitudinal magneto-optical spectroscopy of bismuth substituted yttrium iron garnet films grown by pulsed laser deposition

Ferrimagnetic bismuth substituted yttrium iron garnet Bi_xY_3 − xFe₅O₁₂ (BiYIG) films with x = 1 and 2 pulsed laser deposited onto (111) Gd₃Ga₅O₁₂ (GGG) substrates were studied using magneto-optical (MO) Kerr spectroscopy in the photon energy range of 1.8-5 eV at both polar and longitudinal magnetizations. The interference at lower photon energies provided the refined film thicknesses ranging between 70 and 200 nm. The films were grown under compressive strain and displayed saturation magnetizations (μ₀M_s) lower than that of their bulk counterparts due to the presence of nanograins forming BiYIG layers and/or magnetically dead interface layers. The trends in the MO spectra agree with those deduced from the published permittivity tensor data for BiYIG using a transfer matrix model applied to a film (BiYIG)-substrate (GGG) system. Due to the reduced μ₀M_s the predicted amplitudes are typically higher. The agreement was improved using effective medium approach or by incorporating into the model MO passive interface layers. The information on MO activity at longitudinal magnetization in the garnet layers below 100 nm presents interest for MO imaging and magnetophotonic devices. The results suggest that the MO Kerr spectroscopy combined with MO Kerr magnetometry may represent a valuable, cheap and nondestructive tool for the characterization of magnetic garnet films less than 200 nm thick.     

Certifying standard specimens for light yield in bismuth germanate single-crystal scintillation detectors

Translated from Izmeritel'naya Tekhnika, No. 5, pp. 45–47, May, 1991.     

Growth of crystalline bismuth trisulphide films

Polycrystalline bismuth trisulphide films were grown by the flash evaporation method at substrate temperatures between 30 and 150°C. The structure and chemical composition were studied by transmission electron microscopy and energy dispersive analysis of X-rays. Electrical resistance measurements revealed a transition from extrinsic to intrinsic conduction occurring at ≈ 380°C.     

Electrical resistivity of thin bismuth films

For quantitative and reproducible observation of the quantum size effect (QSE) 78 samples of thin bismuth films were deposited at one time onto three muscovite substrates in ultrahigh vacuum. The electrical resistivity of these films, either in step-up series or a fixed thickness, was measured between 4.2 and 320 K. The experiments covered a thickness range from 300 to 3350 Å. The measured resistivities had a large scatter among simultaneously deposited samples of a fixed thickness. This scatter became more pronounced with decreasing temperature and it was due to the variation of the TCR above liquid nitrogen temperature. The thickness dependence of the ratio of resistivities at 4.2 K and 77 K (?_4.2/?₇₇), which had a relatively small scatter, did not show the oscillatory behavior predicted by Sandomirskii.     

巨磁电阻多层膜结构的研究进展

归纳了巨磁电阻效应发现以来在各种合金体系中出现的巨磁电阻多层膜材料,对其性能和晶体结构进行了对比分析,着重从界面粗糙度、元素混合和晶体学织构等方面,评述了巨磁电阻多层膜结构的国内外研究现状,讨论了各种结构参数对巨磁电阻效应的影响。