Experimental verification and quantitative analysis of the temperature (phonon) breakdown phenomenon in the high-field magnetoresistivity of compensated metals

The transverse magnetoresistivity of tungsten single crystals with resistivity ratios of up to 80.000 was measured in the temperature range from 4.2 to 60 K and in magnetic fields up to 15 T. The experiments show that the temperature (phonon) breakdown effect occurs in compensated metals as a result of strong electron-phonon scattering. This leads to a transformation of closed electron orbits into quasi-open ones and, consequently, to anomalies of the galvanomagnetic properties. A large anisotropy of the magnetoresistivity (up to 83%), i.e. a dependence of resistivity on current direction, is observed in a cubic crystal. The power index n in the field dependence of the magnetoresistivity decreases and the characteristic intersheet gap temperatures T₀, which are determined from an exponential temperature dependence of the magnetoresistivity, formally depend on magnetic field. A detailed analysis of the experimental results allows us to explain all these anomalies by the temperature breakdown effect and to obtain new information on this phenomenon in metals at high magnetic fields.     

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%.     

Anomalies in the Galvanomagnetic Properties of Fe1.9V1.1Al Pseudogap Semiconductors at High Magnetic Fields

The high-field dependence of the galvanomagnetic properties of Fe_1.9 V_1.1Al compounds was measured in the temperature range from 2.5 to 100 K and in magnetic fields of up to 15 T. We observe that the Hall coefficient decreases with magnetic field, increasingly at low temperatures (2.5–15 K). The magnetoresistivity is negative, but no saturation occurs at fields of up to 15 T. The observed peculiarities of the galvanomagnetic properties can be explained theoretically and confirm that Fe_1.9V_1.1Al compounds show a pseudogap semiconductor behavior.     

Magnetic ordering in CeCu6−x Au x single crystals: Thermodynamic and transport properties

We report on measurements of the magnetizationM, specific heatC, resistivity , and magnetoresistivity of CeCu _6–x Au _x single crystals (x=0.3 and 0.5) grown by the Czochralski method. Antiferromagnetic ordering is observed inM andC for temperatures less thanT _N=0.48 K (x=0.3) and 0.95 K (x=0.5), similar to the ordering temperatures found previously for polycrystalline samples. As a function of magnetic fieldB, M(B) andC(T, B) are strongly anisotropic, with the easy axis along the crystallographicc direction (orthorhombic notation) as for pure CeCu ₆ . For large magnetic fields where the magnetic ordering is suppressed, the specific heat can be described by the resonance-level model suggestive of a single-ion Kondo effect, similar to CeCu ₆ where for largeB the short-range magnetic correlations are suppressed. The averaged Kondo temperature as determined from a number of properties decreases with increasingx, withT _K=4.0 K forx=0.3 and 3.0 K forx=0.5, compared to 5.8 K forx=0. The magnetoresistivity shows a negative contribution arising from incoherent Kondo scattering and a positive contribution associated with the magnetic order.     

High-field galvanomagnetic properties of compensated metals under electron-surface and intersheet electron-phonon scattering (tungsten)

The field and temperature dependence of the transverse magnetoresistivity and the Hall coefficient for single-crystal tungsten with a resistivity ratio _293K/_4.2K varying from 780 to 80800 are measured at temperatures between 4.2 K and 50 K in magnetic fields up to 150 kOe. The role of the static skin effect and the temperature breakdown in the galvanomagnetic properties of the compensated metals is evaluated through experiments with tungsten. The experimental data shows that the galvanomagnetic properties of the compensated metals in the high magnetic fields are not dependent only on the Fermi surface geometry. A substantial role in their formation is played by the processes of the electron-surface and the intersheet electron-phonon scatterings.     

Search for thermometers with low magnetoresistive effects: platinum-cobalt alloy

Measurements performed on thermometers made of dilute ferromagnetic alloys of Pt with 0.45, 0.50, 0.75, 1.06 and 2.15 at% Co, in the temperature range 2–28 K and in magnetic fields up to 6 T showed a large region of negative magnetoresistivity that limits the magnetic error at low temperatures and field values. With 0.5 at% Co or less, the change of the magnetic error with T and B was smooth, while a magnetic transition has been observed with higher cobalt concentrations. Curie temperature has been found to occur at 6.0 K, 14.2 K and, by extrapolation, 32 K for alloys with 0.75, 1.06, and 2.15 at%Co respectively. The transition resulted in a second order discontinuity of the R-T characteristics at B = 0 and B = 3.8 ± 0.2 T with 0.75 at%Co and 8.0 ± 0.5 T with 1.06 at%Co, while for other field values the discontinuity was first-order. Hence, this alloy can be used for thermometry only with very low cobalt concentrations: with 0.3 at%Co it would be possible to limit the magnetic error within ±0.2 K up to 2 T and done to 5 K.     

Faraday rotation of terbium-yttrium ferrite garnet

The authors report Faraday rotation (FR) measurements performed on single crystals of terbium-yttrium iron garnets at 1.15-μm wavelength for three terbium contents (1.98, 1.02, and 0.35) in the temperature range 6-300 K, under magnetic field up to 20 kOe applied along the [111] and [100] directions. The spontaneous FR (associated with the spontaneous magnetization) is isotropic while the FR susceptibilities are anisotropic at low temperature; this anisotropy originates mainly from spin reorientation transitions. To analyze the terbium contribution to FR, the Fe contribution to the YIG data is evaluated. The terbium electric dipole transition coefficient is found to consist in a linear temperature dependence in the temperature range 100-300 K as previously observed in pure TbIG     

Magnetic properties of β-FeSi2 single crystals

We investigated temperature and magnetic field dependences of the magnetization of β-FeSi₂ single crystals in the temperature range of 5-300 K in magnetic fields up to 15 kOe. The temperature dependence of the magnetic susceptibility of the Cr- and Ni-doped sample can be explained by temperature-dependent contributions due to paramagnetic centres and due to the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature as well as activation energy of the donor and acceptor levels are estimated.     

Temperature breakdown phenomenon in tungsten single crystals at high magnetic fields

The transverse magnetoresistance and the Hall effect of tungsten single crystals with resistivity ratios of up to 95 000 were measured in the temperature range from 4.2 to 60 K and in magnetic, fields up to 15 T. It is shown that anomalies of the galvanomagnetic properties appear under strong intersheet electron-phonon scattering conditions. An exponential temperature dependence of the magnetoresistance (Peierls' exponent) is observed; the type of the field dependence of the magnetoresistance depends on the transport current direction in the crystal; the temperature behaviour of the Hall coefficient is determined by the type of the electron quasiorbits prevailing under these conditions. The data allow us to conclude that the temperature breakdown effect, i.e. a fundamental change of the electron trajectories in high magnetic fields, takes place in tungsten single crystals under conditions of intersheet electron-phonon scattering.     

Angular dependence of the magnetoresistivity of Pd2Si

We present the magnetoresistivity at high magnetic field measured on single crystals of Pd₂Si of very good quality. The magnetoresistance depends on the orientation of field and current directions relative to the crystallographic axis. It shows that this suicide behaves like a compensated metal (i.e. equal numbers of electrons and holes) with open orbits along the main crystallographic directions.     

A unified model for the cohesive enthalpy, critical temperature, surface tension and volume thermal expansion coefficient of liquid metals of bcc, fcc and hcp crystals

First the cohesive enthalpy of pure liquid metals is modeled, based on experimental critical temperatures of alkali metals. The cohesive enthalpies are scaled to the melting points of pure metals. The temperature coefficient of cohesive enthalpy is the heat capacity of the liquid metal. The surface tension and its temperature coefficient for pure liquid metals are modeled through the excess surface enthalpy, excess surface entropy and molar surface area supposing that the outer two surface layers of liquid metals are similar to the {1 1 1} plane of fcc crystals. The volumetric thermal expansion coefficient of liquid metals is scaled to the ratio of the heat capacity and cohesion enthalpy. From known values of melting point, heat capacity and molar volume the following calculated properties of liquid metals are tabulated: (i) cohesive enthalpy at melting point, (ii) cohesive energy of the solid metal at 0 K, (iii) critical temperature, (iv) surface tension at melting point, (v) volume thermal expansion coefficient, and (vi) temperature coefficient of surface tension. The present models are valid only for liquid metals of bcc, fcc or hcp crystals as only their structure and nature of bonding are similar enough to be treated together.     

Galvanomagnetic and magnetic properties of evaporated thin nickel films I: Field dependence of the hall voltage and the magnetoresistivity

Thin nickel films with thicknesses between 10 and 2000 Å were evaporated under ultrahigh vacuum conditions and were condensed at room temperature on glass substrates. They were investigated both before and after annealing at 523 K. The variation of the Hall voltage and the magnetoresistivity with magnetic field (<13 kG) was measured. From these curves the ordinary and the extraordinary Hall coefficient, the saturation magnetization, the total magnetoresistivity and the “ordinary” magnetoresistivity were determined.     

Interfering electron quantum states in ultrapure magnesium

A detailed analysis is presented of the large-amplitude temperature-insensitive oscillations in the transverse magnetoresistivity of ultrapure Mg single crystals resulting from the direct interference of electron quantum states. A calculation of the relative harmonic content of these interference signals based on the transmission characteristics of a magnetic breakdown-generated interferometer is used to quantitatively study certain aspects of the electron states in Mg as well as details of magnetic breakdown. In particular, values of magnetic breakdown parametersH ₀are determined without invoking the complexities of transport theory. An absolute lower limit for the electron quantum state lifetime of τ ? 0.5 nsec is obtained (forT=1.5° K), although a best fit to the data gives a value an order of magnitude larger, τ～5 nsec, which corresponds to quantum phase coherence extending over a distance of 3 mm in these crystals. In addition, this work provides direct experimental verification of the π/2 phase difference between transmitted and reflected electron states at a magnetic breakdown junction. Comparison of the results of this experiment with previous work via an existing semiempirical band structure calculation demonstrates the complete consistency of these measurements with previous Fermi surface data.     

Single crystal growth of actinide compounds

During recent years, the importance of solid state actinide research has been increasingly recognized. Further progress in actinide solid state physics depends on the availability of pure and perfect single crystals. Actinide compounds have large magnetic anisotropy with anisotropy fields of 8 × 10⁷ A·m^?1 or higher. Investigation of the mechanism responsible for such unique behaviour requires large single crystals of high purity for magnetization, neutron diffraction, angular and energy dependent photoemission measurements. Materials of interest for actinide solid state research are the metals and compounds with simple crystal structures like dioxides (CaF₂ structure), monopnictides and monochalcogenides (NaCl structure), and intermetallic compounds (Laves phases).This article gives an overview of actinide single crystal growth facilities in Karlsruhe and Geel. The actinide compounds are prepared by direct synthesis from the purest elements available using non-contaminating techniques. The reaction occurs in vacuum sealed quartz tubes where the actinide metal reacts with the vapour of the other element, or by levitation melting in a Hukin crucible. Different techniques have been developed to grow single crystals of actinide metals and compounds. High temperature solution growth from molten salts is used to prepare actinide dioxide single crystals. Oxides, pnictides and chalcogenides are grown by chemical vapour transport. Large single crystals of the monopnictides and monochalcogenides are obtained with the recrystallization (or mineralization) technique in sealed tungsten crucibles. Single crystals of congruently melting intermetallic compounds are pulled from levitated or semilevitated melts by the Czochralski method. Selected single crystals are characterized, orientated and encapsulated for safe handling during measurements.     

Temperature stability of magnetic field-induced strain and field-controlled shape memory effect on Ni/sub 52/Mn/sub 16.4/Fe/sub 8/Ga/sub 23.6/ single crystals

The temperature dependence of the magnetic field-induced strain (MFIS) and the field-controlled shape memory effect in Ni/sub 52/Mn/sub 16.4/Fe/sub 8/Ga/sub 23.6/ single crystals were investigated by measuring the MFIS and measuring the magnetic field-enhanced transformation strain with a field bias applied in the [001] and [010] directions of the parent phase, respectively. The results show that such material combined with the martensitic transformation can product large field-enhanced transformation strain and large MFIS. The strain accompanying the martensitic transformation is -1.61% in zero field and can be enhanced to -3.30% by a field of 960 kA/m. A MFIS of 1.04% has been induced along [001] in unstressed crystals with saturated magnetic field of 600 kA/m applied along the same direction at near martensitic transformation temperature. It was found that the MFIS is almost temperature independent; the maximum decrease of the saturated MFIS is less than 10%, from 265 K to 100 K. This well-behaved temperature response makes this alloy particularly valuable for industrial and military smart actuators and transducers. Furthermore, it was found that the direction in which the MFIS has the largest value is always the [001], namely, the growth direction of the crystals.     

On the Mobile Behavior of Solid 4He at High Temperatures

We report studies of solid helium contained inside a torsional oscillator, at temperatures between 1.07 K and 1.87 K. We grew single crystals inside the oscillator using commercially pure ⁴He and ³He-⁴He mixtures containing 100 ppm ³He. Crystals were grown at constant temperature and pressure on the melting curve. At the end of the growth, the crystals were disordered, following which they partially decoupled from the oscillator. The fraction of the decoupled He mass was temperature and velocity dependent. Around 1 K, the decoupled mass fraction for crystals grown from the mixture reached a limiting value of around 35%. In the case of crystals grown using commercially pure ⁴He at temperatures below 1.3 K, this fraction was much smaller. This difference could possibly be associated with the roughening transition at the solid-liquid interface.     

Device for orientation of crystals at low temperatures in a superconducting magnet for the production of single-domain horizontally-polarized specimens

A design is given for a device that can carry up to four single crystals mounted on individual three-way goniometers for mutual alignment of their crystallographic planes. The assembly, held at 4 K in the clear split of a superconducting magnet, may be rotated to a precise angle by simple external means. The rotation of the device permits the formation of horizontally-polarized, single-domain crystals of ferromagnetic materials when only a vertical magnetic field is available.     

Effect of Synthesis Process and Substrate on Electrical and Thermal Transport Properties of Bi-2212

Resistivity, thermoelectric power, and thermal conductivity have been measured for a Bi-2212 system synthesized from a glassy precursor, either with a commercially Al₂O₃ substrate or with a homemade BaZrO₃ substrate. Those measurements show that the BaZrO₃ substrate gives better superconducting properties to the Bi-2212 than the Al₂O₃ substrate. The effect of (1.0 T) weak magnetic field has been searched for. The thermal magnetoconductivity and the contributions of the magnetic field to the thermoelectrical power are studied and compared through fine measurements. An electronic contribution seems to appear already well above the critical temperature and to exist up to 200°K. The onset temperature is thus deduced.     

A size-dependent nanoscale metal-insulator transition in random materials

Insulators and conductors with periodic structures can be readily distinguished, because they have different band structures, but the differences between insulators and conductors with random structures are more subtle. In 1958, Anderson provided a straightforward criterion for distinguishing between random insulators and conductors, based on the 'diffusion' distance ζ for electrons at 0?K (ref.?3). Insulators have a finite ζ, but conductors have an infinite ζ. Aided by a scaling argument, this concept can explain many phenomena in disordered electronic systems, such as the fact that the electrical resistivity of 'dirty' metals always increases as the temperature approaches 0?K (refs?4-6). Further verification for this model has come from experiments that measure how the properties of macroscopic samples vary with changes in temperature, pressure, impurity concentration and applied magnetic field, but, surprisingly, there have been no attempts to engineer a metal-insulator transition by making the sample size less than or more than ζ. Here, we report such an engineered transition using six different thin-film systems: two are glasses that contain dispersed platinum atoms, and four are single crystals of perovskite that contain minor conducting components. With a sample size comparable to ζ, transitions can be triggered by using an electric field or ultraviolet radiation to tune ζ through the injection and extraction of electrons. It would seem possible to take advantage of this nanometallicity in applications.     

High-precision electrical resistivity of Tb,Dy, and Ho single crystals between 1 and 4 K

The electrical resistivity of Tb, Dy, and Ho single crystals has been investigated with a precision of one in 5×10⁴ in the temperature range 1–4 K and in magnetic fields up to 60 kOe. In Tb and Dy, in zero field, the change in resistance is about 0.3%. The resistance vs. temperature curve shows a point of inflection at around 3.5 K, with a more rapid decrease in resistance below this temperature. In a magnetic field, the latter part shows a gradual reduction as field increases. On the basis of the present result and previously published specific heat results, the source of this resistivity has been suggested to be the scattering of the conduction electrons by the rare earth ions in the oxide environment. The Ho crystals show a slightly stronger temperature dependence (0.4%) but no anomalous component. These results have been discussed on the basis of the spin-wave and phonon scattering. The phonon resistivity of Dy and Ho can be described by aT ³ law.