ESR line broadening in Cr3+/MgO at liquid helium temperatures

Line broadening has been observed at 9 GHz in the ESR absorption spectrum of Cr³⁺ in MgO in the liquid helium temperature range, for a range of Cr³⁺ concentrations from 800 7400 ppm. The broadened linewidths at 4.2 K are about two times larger than those at 77 K and depend on polar angle. The lineshapes are Gaussian, in contrast to the Lorentzian lineshape between 293 and 77 K. The broadening is interpreted by the combined effects of strain, due to the charge misfit of Cr³⁺ and the host cation, and temperature which causes both exchange striction and departure from the cubic symmetry to a lower symmetry. Analysis of the linewidth data gives D=(5.25±0.40)×10^–4 cm^–1 and confirms that the total linewidth, H _t, is given by H _t=H ₀+(2D/g) cos _H where H ₀ is the linewidth independent of concentration, temperature and polar angle, _H.     

Densification of LaGaO3 at low sintering temperatures via Fe3+ substitution at Ga3+ site

Attempts to achieve near theoretical density in polycrystalline LaGaO₃ compacts using Fe₂O₃ as a dopant have been made. The B-site substituted lanthanum gallates with iron doping ranging from 15 to 55 mole% FeO_1.5 (Fe₂O₃) were synthesized via solid state reaction using oxide and nitrate precursors and sintered at 1200°C for 24 to 72 h and at 1300°C for 12 to 36 h. Systematic variation in the lattice constants and microstructural aspects was followed as a function of mole fraction of Fe₂O₃. It was found that the parent LaGaO₃ structure could take up as high as 55 mole% of Fe₂O₃ retaining the orthorhombic crystal structure. Compositions containing 35 mole% Fe₂O₃ soaked for 48 h or more at 1200°C and for as low as 12 h at 1300°C resulted in perfectly dense compacts. Bodies containing lower dopant concentrations (x_{Fe 2 O 3} 0.25) were found to have a substantial degree of porosity with good intergranular connectivity, while a significant amount of grain growth and broad grain size distribution was an interesting feature in samples containing mole fractions higher than 0.35.     

Ion transport in LaF3 at low temperatures

Thermal stimulated polarization and depolarization currents of LaF₃, undoped and CaF₂ doped, were obtained: They confirm on one hand conductivity data obtained by Murin et all. (9). On the other hand they prove the electrolytic nature of the conductivity. They show finally that Ca²⁺-anion vacancy complexes, capable of reorientation, are formed with an activation energy of reorientation of about 0.19eV.     

The ESR spectrum of Gd3+/MgO

Electron spin resonance studies have been made at 9.1 GHz on Gd³⁺/MgO single crystals grown by electrofusion and containing low gadolinium concentrations. A single isotropic line havingg=1.992±0.00024 was observed in contrast to the seven line spectrum reported by Abraham. The experimental peak-to-peak linewidth for the −1/2↔+1/2 transition at 293 K was 0.3 mT and was independent of polar angle. This was nearly one hundred times less than the calculated dipolar linewidth and analysis verified that the lineshape was Lorentzian indicating exchange narrowing. The linewidth was independent of temperature from 4.2 to 293 K and the exchange energy derived for a gadolinium concentration of 310 ppm was 15 GHz. A discussion is given of the cubic field splittings of Gd³⁺ in oxide crystals and a comparison made of the exchange energies of transition group ions in the MgO lattice.     

Effective viscosity of3He-B at low temperatures

We show that there exists a nontopological soliton in the superfluid ³He-A phase near T _AB. The possibility that the soliton causes the ³He A-B transition is examined. Some properties of the soliton are considered.     

Calibration of the au-0.03at% Fe/Chromel thermocouple for damping measurements at low temperatures

The Au-0.03at% Fe/Chrornel P thermocouple was calibrated between 1 and 300 K to monitor the temperature of freely vibrating specimens in measurements of elastic energy dissipation. In the intermediate temperature range (50–23 K) the calibration points were obtained by means of a miniature sealed cell containing oxygen, which gave three fixed points: the O₂ triple point, 54.3610 K; the β-γ transition, 43.802 K; and the α-β transition, 23.873 K.     

NMR in superfluid3He at very low temperatures

Just a few years ago it was generally believed that the NMR properties of superfluid³He far below T_c would not be qualitatively different from those found at higher temperatures. Surprisingly, the strange enhancement of relaxation processes at around 0.4 T_c named catastrophic relaxation, was then found. Recently an extremely long lived induction signal with new dynamical properties was discovered in the region of 100 K. Some of the newly discovered properties can be partly explained, although there are still many unsolved problems. The main aim of this article is to raise new questions for future investigations at the lowest temperatures presently possible.     

High frequency dielectric properties of MgO,Fe/MgO and Cr/MgO

The study of the dielectric constant and loss in pure single-crystal MgO and in Fe- and Cr-doped crystals, the low frequency results of which have been reported [1], has now been extended to cover frequencies from 500 Hz to 9 GHz. Measurements were made on the same specimens, at room temperature, using bridge, cavity, slotted-line and cavity-resonator techniques. Over the whole frequency range, a good fit is obtained to the Universal Laws of dielectric response: _ac ⁿ and (–) ^n–1 with n=0.98±0.02 for both pure and doped MgO. At any point in this frequency range the addition of iron or chromium increases and decreases , the changes being more pronounced with iron. These effects are discussed in terms of hopping mechanisms.     

The dielectric behaviour of single-crystal MgO,Fe/MgO and Cr/MgO

The dielectric constants and loss factors,, for pure single-crystal MgO and for Fe-and Cr-doped crystals have been measured at frequencies, , from 500 Hz to 500 kHz at room temperature. For pure MgO at 1 kHz the values of and the loss tangent, tan , (9.62 and 2.16×10^–3, respectively) agree well with the data of Von Hippel; the conductivity, , varies as ⁿ withn=0.98±0.02. In Fe-doped crystals increases with Fe-concentration (at any given frequency); for a crystal doped with 12800 ppm Fe, was about four times the value for pure MgO. At all concentrations the variation of log with log was linear andn=0.98±0.02. A decrease in with increasing Fe-concentration was also observed. A similar, although less pronounced, behaviour was found in Cr-doped crystals. The effects are discussed in terms of hopping mechanisms.     

Interfacial interaction in Fe/MgO/Fe magnetic tunneling junctions

We present a study on interfacial structures and tunneling magnetoresistance (TMR) in Fe/MgO/Fe junctions using a MgO(111) film with {100} facets. It is shown using X-ray photoelectron spectroscopy that a FeO layer occurs at MgO/Fe rather than Fe/MgO interface, which could be used to tune the TMR effect. At the Fe/MgO interface, such a change in electronic structure is attributed to the band bending associated with a change in thickness of Fe films. The present study provides a new understanding on the Fe/MgO/Fe interfacial behavior and metal/oxide barriers involving electron transport.     

Thermal conductivity of superconducting UPt3 at low temperatures

We study the thermal conductivity within the E_1g and E_2u models for superconductivity in UPt₃ and compare the theoretical results for electronic heat transport with recently measured results reported by Lussier, Ellman and Taillefer. The existing data down to T/T_c 0.1 provides convincing evidence for the presence of both line and point nodes in the gap, but the data can be accounted for either by an E_1g or E_2u order parameter. We discuss the features of the pairing symmetry, Fermi surface, and excitation spectrum that are reflected in the thermal conductivity at very low temperatures. Significant differences between the E_1g and E_2u models are predicted to develop at excitation energies below the bandwidth of the impurity-induced Andreev bound states. The zero-temperature limit of the axis thermal conductivity, lim_T0 k_c/T, isuniversal for the E_2u model, but non-universal for the E_1g model. Thus, impurity concentration studies at very low temperatures should differentiate between the nodal structures of the E_2u and E_1g models.     

Magnetic impurity pairs in dilute Cu(Fe) alloys at very low temperatures

The temperature-dependent static magnetization of several dilute Cu(Fe) alloys with Fe concentrations between 102 and 478 ppm has been measured in fields between 1 and 200 Oe and at temperatures between 9 mK and 0.4 K. At higher temperatures and in small fields, the magnetization displays the following concentration, field, and temperature dependences:M c ² HT ^–2/3. At lower temperatures, magnetic ordering is observed. These results are interpreted in terms of the behavior of pairs of impurities acting as individual entities. They are quite likely correlated with the anomalous low field, low-temperature observations made in previous measurements of the resistivity and specific heat of the Cu(Fe) system.This research was supported in part by the U.S. Atomic Energy Commission under Contract No. AT(04-3)-34, P.A. 143, and by the National Science Foundation.     

Interface characterization of epitaxial Fe/MgO/Fe magnetic tunnel junctions

Following predictions by first-principles theory of a huge tunnel magnetoresistance (TMR) effect in epitaxial Fe/MgO/Fe magnetic tunnel junctions (MTJs), measured magnetoresistance (MR) ratios of about 200% at room temperature (RT) have been reported in MgO-based epitaxial MTJs. Recently, a MR ratio of about 600% has been reported at RT in MgO-based MTJs prepared by magnetron sputtering, using amorphous CoFeB as the ferromagnetic electrode. These MTJs show great potential for application in spintronic devices. Fully epitaxial MTJs are excellent model systems that enhance our understanding of the spin-dependent tunneling process as the interface is well defined and can be fully characterized. Both theoretical calculations and experimental results clearly indicate that the interfacial structure plays a crucial role in the coherent tunneling across a single crystal MgO barrier, especially in epitaxial MgO-based MTJs grown by molecular beam epitaxy (MBE). Surface X-ray diffraction, Auger electron spectroscopy, X-ray absorption spectra, and X-ray magnetic circular dichroism techniques have been reported previously for interface characterization. However, no consistent viewpoint has been reached on the interfacial structures (such as FeO layer formation at the bottom Fe/MgO interface), and it is still an open issue. In this article, our recent studies on the interface characterization of MgO-based epitaxial MTJs by X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and spin-dependent tunneling spectroscopy, will be presented.     

Spin filtering effects in monocristalline Fe/MgO/Fe magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (1 0 0) Fe electrodes and MgO(1 0 0) insulating barrier are elaborated by molecular beam epitaxy. Two extreme regimes have been investigated. First, for extremely thin MgO thickness we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunneling of the minority spin interfacial resonance state. Second, for large MgO barrier thickness, the tunnel transport validates specific spin filtering effect in terms of symmetry of the electronic Bloch function and symmetry-dependent wave function attenuation in the single-crystal barrier. Within this framework, we present giant tunnel magnetoresistive effects at room temperature (125–150%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the filtering. We show that the insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance. Moreover, we provide experimental evidence for the electronic interfacial resonance states contribution to the spin polarized tunnel transport.     

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.     

The melting pressure of helium-3 at very low temperatures

The melting pressure of³ He at very low temperatures was shown earlier to be determined overwhelmingly by the solid phase. Using an extended solid³He model based on isotropic effective first-neighbor-pair antiferromagnetic and second-neighbor-pair ferromagnetic exchange interactions, the melting process is reinvestigated here. One of its motivating aspects may be said to be tied to its possible use, suggested by us earlier, for the establishment of a thermodynamic temperature scale at very low temperatures. As a consequence of the assumed multineighbor interactions, spin ordering is accelerated and, at the same temperature, the extended-model solid entropy falls below that of the simpler nearest-neighbor-pair interaction model. Equivalently, the spinordering critical transition temperature is raised over that associated with the simpler interaction scheme. The overall result is a decrease in the melting pressure variations at very low temperatures below those arising from the simple solid model. In the absence of a reliable experimental temperature scale at very low temperatures, only a qualified and cursory comparison is justified with recent experimentally estimated melting pressures. Discrepancies arise between theory and preliminary data on the temperature derivatives of the melting pressure or the entropy of the solid at melting. They might be due in part to the tentative experimental temperatures, which appear shifted toward too low temperatures when compared with the theoretical temperature scale implicit in the treatment of the generalized solid model. However, from the viewpoint which accepts the rather preliminary, scarce, very low temperature melting pressure data at face value, failure of the extended exchange model of solid³He at those temperatures must be kept in mind.Work performed under the auspices of the U.S. Atomic Energy Commission.