Planes of Continuous-Wave Oscillations From an Electron Nanocontact Spin-Transfer Device

We have found that the frequencies of continuous-wave (CW) microwave oscillations in nanocontact spin-transfer (ST) devices occur in well-defined planes of frequencies, as a function of the dc current bias, the direction of the magnetic field, and the magnitude of the magnetic field. The frequency, f, of these technologically significant peaks for each magnetic field direction is described empirically by the equation of a plane, f=a|Boarr|+bI+c, where |Boarr| is the magnitude of the magnetic field, I is the dc bias current, and a, b, and c are constants of the plane. The primary frequency plane described by this equation is accompanied by a smaller secondary plane. The empirical equation describing the primary plane serves as a guide for efficiently locating CW oscillations within the independent variable space     

Dynamic quantized fracture mechanics

A new quantum action-based theory, dynamic quantized fracture mechanics (DQFM), is presented that modifies continuum-based dynamic fracture mechanics (DFM). The crack propagation is assumed as quantized in both space and time. The static limit case corresponds to quantized fracture mechanics (QFM), that we have recently developed to predict the strength of nanostructures. DQFM predicts the well-known forbidden strength and crack speed bands – observed in atomistic simulations – which are unexplained by continuum-based approaches. In contrast to DFM and linear elastic fracture mechanics (LEFM), that are shown to be limiting cases of DQFM and which can treat only large (with respect to the “fracture quantum”) and sharp cracks under moderate loading speed, DQFM has no restrictions on treating defect size and shape, or loading rate. Simple examples are discussed (i) strengths predicted by DQFM for static loads are compared with experimental and numerical results on carbon nanotubes containing nanoscale defects; (ii) the dynamic fracture initiation toughness predicted by DQFM is compared with experimental results on microsecond range impact failures of 2024-T3 aircraft aluminum alloy. Since LEFM has been successfully applied also at the geophysics size-scale, it is conceivable that DQFM theory can treat objects that span at least 15 orders of magnitude in size. International Conference on Fracture XI–Symposium 34, on Physics and Scaling in Fracture     

Nanocontact heteroepitaxy of thin GaSb and AlGaSb films on Si substrates using ultrahigh-density nanodot seeds

A film of GaSb grown epitaxially on a Si substrate is a direct transition semiconductor useful for application as a light source in Si photonics and channel material in next-generation field effect transistors because its energy bandgap is close to the optical fibre communication wavelength and it possesses high carrier mobility. Here, we report a novel method for heteroepitaxial growth of high-quality GaSb/Si films, despite having a lattice mismatch as large as ～ 12%, using elastically strain-relaxed GaSb nanodots with ultrahigh density as seed crystals for film growth. The nanodot seed crystals were grown epitaxially by restricted contact with the Si substrate through nanowindows in an ultrathin SiO(2) film on the Si substrate. A light-emitting diode containing GaSb/Si films with a thickness of ～ 90 nm fabricated by this method operated at room temperature. The growth method was also used to fabricate AlGaSb films of high quality. Our method provides a new avenue for heteroepitaxial growth of high-quality film in systems with large lattice mismatch.     

High-field magnetoresistance of AuGa

Measurements of the high-field magnetoresistance in AuGa indicate the existence of open orbits in [001]. The nearly free-electron model predicts open orbits in [001] and [100], but evidence for the existence of the latter only begins to appear at the highest fields used in this work.Work supported by the National Science Foundation.     

Linear magnetoresistance of aluminum

The transverse magnetoresistivity of pure single crystals of aluminum (RRR 22,000) has been studied at 4.2 K and in high magnetic fields using the hard helicon technique. The measured magnetoresistivity is separated into a saturating and a linearly increasing part, and the effect of deformation and quenching on the linear component is studied. The linear component appears to be particularly sensitive to strains, and it is suggested that the dislocation structure is important. The Hall coefficient is also measured and is found to be field independent and equal to the free-electron value 1.023×10^–10 Vm/AT within the experimental uncertainty of 1%.     

Classical and quantum routes to linear magnetoresistance

The hallmark of materials science is the ability to tailor the microstructure of a given material to provide a desired response. Carbon mixed with iron provides the steel of buildings and bridges; impurities sprinkled in silicon single crystals form the raw materials of the electronics revolution; pinning centres in superconductors let them become powerful magnets. Here, we show that either adding a few parts per million of the proper chemical impurities to indium antimonide, a well-known semiconductor, or redesigning the material's structure on the micrometre scale, can transform its response to an applied magnetic field. The former approach is purely quantum mechanical; the latter a classical outgrowth of disorder, turned to advantage. In both cases, the magnetoresistive response--at the heart of magnetic sensor technology--can be converted to a simple, large and linear function of field that does not saturate. Harnessing the effects of disorder has the further advantage of extending the useful applications range of such a magnetic sensor to very high temperatures by circumventing the usual limitations imposed by phonon scattering.     

The resistance and magnetoresistance of lithium tetra-ammine

Resistance and magnetoresistance measurements are presented for specimens of Li(NH₃)₄ with mole ratios of 4:1, NH₃:Li. Measurements were made using a probeless, mutual inductance technique from 1.5 to 4.2 K and from 10 to 100 K. The low-temperature resistivity has a largely quadratic temperature dependence, =(0)+BT². The coefficient B has only a small temperature variation from 1.5 to 25 K and is independent of the residual resistivity. Near 69 K there is a local minimum in the electrical resistivity. Thermal hysteresis is observed in this region as well. We suggest that these features are characteristic of the compound Li (NH₃)₄ and may be due to a magnetic transition near 70 K. The magnetoresistivity was determined from 0 to 100 kG at both 1.66 and 4.2 K. It is large, with a marked decrease in field dependence above 50 kG. Our results indicate that lithium tetra-ammine is probably an uncompensated metal with a high proportion of open-orbit carriers.This work was supported by the Advanced Research Projects Agency and the National Science foundation through the Materials Science Center of Cornell University, Report # 1897.Alfred P. Sloan Research Fellow.     

Quantized magnetoresistance in atomic-size contacts

When the dimensions of a metallic conductor are reduced so that they become comparable to the de Broglie wavelengths of the conduction electrons, the absence of scattering results in ballistic electron transport and the conductance becomes quantized. In ferromagnetic metals, the spin angular momentum of the electrons results in spin-dependent conductance quantization and various unusual magnetoresistive phenomena. Theorists have predicted a related phenomenon known as ballistic anisotropic magnetoresistance (BAMR). Here we report the first experimental evidence for BAMR by observing a stepwise variation in the ballistic conductance of cobalt nanocontacts as the direction of an applied magnetic field is varied. Our results show that BAMR can be positive and negative, and exhibits symmetric and asymmetric angular dependences, consistent with theoretical predictions.     

Transverse thermal magnetoresistance of potassium

We report the results of extensive thermal magnetoresistance measurements on single-crystal and polycrystalline specimens of potassium having residual resistance ratios (RRR) ranging from 1100 to 5300. Measurements were made between 2 and 9 K for magnetic fields up to 1.8 T. The observed thermal magnetoresistance cannot be understood on the basis of either semiclassical theories or from the electrical magnetoresistance and the Wiedemann-Franz law. We do, however, observe a number of interesting relationships between the thermal and electrical magnetoresistances, many of which are not immediately obvious when comparing direct experimental observations. The thermal magnetoresistance W(T, H) is given reasonably well by W(T, H)T = W(T, 0)T + AH + BH ², where both A and B are temperature-dependent coefficients. Our results show that A = A ₀ + A ₁ T ³, while B(T) cannot be expressed as any simple power law. A₀ is quite dependent upon the RRR, while a ₁ is independent of the RRR. We find two very interesting relationships between corresponding coefficients in the electrical and thermal magneto-resistance: (i) the Wiedmann-Franz law relates A ₀ to the Kohler slope of the electrical magnetoresistance and (ii) the temperature-dependent portions of the electrical and thermal Kohler slopes are both proportional to the electron-phonon scattering contribution to the corresponding zero-field resistance. The latter provides evidence that inelastic scattering is very important in determining the temperature-dependent linear magnetoresistances. Part, but by no means all, of the quadratic thermal resistance is accounted for by lattice thermal conduction. We have not been successful in generating another mechanism that gives a quadratic field dependence. After subtracting the lattice contribution, the Lorenz ratio is still strongly field dependent, decreasing with increasing field. Based on these observations and additional arguments, our general conclusion is that at least a portion of the anomalous electrical and thermal magneto-resistances is due to intrinsic causes and not inhomogeneities or other macroscopic defects.Supported by the U.S. Energy Research and Development Administration under contract no. AT(11-1)3150, technical report no. COO-3150-36. This work also benefited from use of the facilities provided by the Materials Science Center, Cornell University, supported by the National Science Foundation, grant no. GH-33637.Supported by the University of Cincinnati Research Council during the preparation of this report.     

Optimal biasing of magnetoresistance amplifiers

The magnetoresistive amplifier is usually implemented by allowing an input signal current to modulate a magnetic field which, in turn, produces a change in the resistance of an active element in the output portion of the device. Amplifiers of this type may be classified either as a conventional type, which utilizes the normal magnetoresistive effect of the active material, or as a type which employs the characteristics of a superconducting element operating in the resistive transition region. In both cases, there has been reported experimental data which indicates that the power gain for these amplifiers is a function of the biasing magnetic field (or biasing current), and that an optimal value of the bias field exists which results in a maximum value of the power gain. Analytical methods of determining these optimal values are derived and theoretical predictions based on these methods are shown to be consistent with the experimental results.     

Percolation mechanism for colossal magnetoresistance

We propose a new mechanism to explain colossal magnetoresistance. The explanation assumes that the materials displaying colossal magnetoresistance are halfmetallic and proposes that the effect is a critical phenomenon, which is intimately connected with the ferromagnetic-to-paramagnetic phase transition present in these materials. The proposed mechanism is a percolation mechanism; the behavior of the resistance is described using a resistor network. An analysis of the percolation phase diagram and Monte Carlo calculations on the resistor network show a full qualitative correspondence with the experimentally observed features of colossal magnetoresistance.     

Hall effect and magnetoresistance in extrinsic piezoelectric semiconductors

The Hall effect and transverse magnetoresistance in extrinsic piezoelectric semiconductors such as InSb are investigated according to the scattering processes of carriers in semiconductors. These scattering processes contain the acoustic phonon scattering, the piezoelectric scattering, and the ionized-impurity scattering. The energy band structure of carriers in semiconductors is assumed to be nonparabolic. Results show that Hall angle, Hall coefficient, and transverse magnetoresistance depend strongly on the dc magnetic field and carrier density due to the energy-dependent relaxation time. Comparison with experimental data is made. It is also found that the magnetoresistance in degenerate InSb oscillates with the dc magnetic field due to the scattering of carriers with impurities in semiconductors.     

缺氧YBCO多晶陶瓷的磁电耦合与磁电阻效应

将高温超导陶瓷YBa2Cu3O6+x经过真空热处理得到氧含量x=0.13的缺氧陶瓷样品,利用HP4294A精密阻抗分析仪测量了样品介电常数温谱图,在温度为410K附近发现了介电异常现象,认为是由反铁磁相变感应铁电相变引起的.铁电测量表明缺氧YBCO多晶陶瓷在室温下有一定的铁电性.在零磁场和外加磁场条件下,采用标准四引线法分别测量了样品电阻率随温度的变化关系,发现温度低于400K时样品的磁电阻MR约为60%且基本不随温度变化,在反铁磁相变温度410K附近出现异常,认为是由于样品大量本征载流子产生并且外加磁场对顺磁区域影响较小所致.缺氧YBCO(x=0.13)陶瓷样品的磁电耦合特性,显示出它可能成为一种新的室温多铁材料从而在传感、控制和信息储存等方面发挥重要作用.     

Comparison of the quantized Hall resistance and ohm NRC

A report is given of the progress towards the establishment of a quantized Hall resistance (QHR) measurement system suitable for maintaining the NRC (National Research Center of Canada) representation of the ohm. A system using a cryogenic current comparator bridge is described and compared to the previously reported 15 T, 20-mK potentiometric system. General problems concerning the use of the quantized Hall resistance to realize a representation of the ohm are discussed     

Logic devices and circuits based on giant magnetoresistance

In this paper, a new paradigm of performing logic operations is proposed. This new approach is based on the submicrometer giant magnetoresistance (GMR) effects. The key enabling features are nonvolatility, high GMR ratio, and control over the switching fields. The possible advantages of the new approach include ultrahigh density (because of the small device size), ultrahigh speed, nonvolatility, and radiation hardness. Key limitations are also identified. Several possible ways to surmount the limitations are also discussed. The proposed approach can, in principle, be applied to other material systems with similar characteristics     

Negative magnetoresistance in SiC heteropolytype junctions

In this study, we carried out for the first time a galvanomagnetic investigation of 3C–SiC/6H–SiC heterostructures at liquid-helium temperatures and observed in n-3C–SiC low resistance of the samples and the appearance of a negative magnetoresistance in weak fields (~1 T). Analysis of the results we obtained shows that the low resistance is in all probability due to a metal—insulator transition in 3C–SiC epitaxial films. It was also found that the negative magnetoresistance magnitude decreases as the density of intertwine boundaries in a 3C–SiC epitaxial film becomes lower.     

Temperature-dependent magnetoresistance of ZnO thin film

A ZnO film was deposited, and the magnetic and the magnetoresistive (MR) properties were studied. The MR measurements reveal negative MR at 80, 50, 20, 10 and 6 K, which is supposed to be induced by the weak-localization effect, based on a logarithmic dependence of the electrical conductivity on temperature. When temperature was reduced to be 2 K, a positive MR was observed. We suggest that it is related to the spin splitting induced by exchange interaction between itinerant electrons and vacancy defects in ZnO. Through the magnetic measurement, it is found that ZnO shows ferromagnetism. It is suggested that the observed ferromagnetism is correlated with the exchange interaction.     

Precision resistance and conductance meter

Conclusions The above meter provides a considerably higher precision than any similar Soviet-made or foreign meter. Its application in practice will solve the problem of checking existing resistance and conductance meters at frequencies ranging from audio-frequencies to 1 MHz. It will also solve other problems related to the requirement of evaluating resistances and conductances in a frequency range from 1 to 1.5 MHz.