A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature T_c before writing, followed by rapid cooling. Requirements are a suitable T_c, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh₂CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m⁻³ is combined with a saturation magnetization of μ₀M_s = 0.52 T at 2 K (2.2 MJ m⁻³ and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μ_B on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its T_c of 450 K, together with a thermal conductivity of 20 W m⁻¹ K⁻¹, make Rh₂CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.⁻².     

Analysis of the lattice thermal conductivity of thin films by means of a modified Mayadas-Shatzkes model: The case of bismuth films

The lattice thermal conductivity λ_øx(d, T) of bismuth films in a direction of the film plane (x direction) with the thicknesses d ranging from 20 to 400 nm was determined in the temperature range 80 K ⩽ T ⩽ 400 K from the measured total thermal conductivity λ_x(d, T) and the calculated charge carrier contribution λ_ex(d, T). A modified Mayadas-Shatzkes model of phonon scattering on polycrystalline films is presented. Following this model the thickness and temperature dependence of λ_øx(d, T) can be interpreted.     

In-field Conductivity Fluctuations in Ba<Subscript>0.72</Subscript>K<Subscript>0.28</Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript> Single Crystals

Fluctuations in the conductivity of Ba_0.72K_0.28Fe₂As₂ single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T _c. 3D fluctuations in superconductivity are realized near T _c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T ₀, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε ₀), as functions of magnetic field were used to determine the coherence length and the lifetime, τ _φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba_0.72K_0.28Fe₂As₂ single crystal gives a value of 3.76 × 10^??12 s for the τ _φ in the absence of magnetic field and it decreases to 2.4 × 10^??12 s in magnetic field of 13 T.     

Heat conduction in Ba1−xKxBiO3

We have investigated heat conduction of single crystal Ba_1?xK_xBiO₃ in the temperature range of 2–300 K and in a magnetic field of up to 6 Tesla. Temperature dependence of thermal conductivityκ(T) reveals the participation of both electrons and phonons with their relative contributions that depend critically on the potassium doping concentration. Crystals underdoped with potassium (samples with higherT _c) exhibit a strong suppression ofκ and a glass-like temperature dependence. In contrast, those with a higher potassium content (lowerT _c) show an increase as temperature decreases with a peak near 23 K. Field dependence ofκ(H) is also very sensitive to the level of potassium doping. Crystals exhibiting a large phonon contribution show an initial drop inκ(H) at low fields followed by a minimum and then a slow rise to saturation as the field increases. The initial drop is due to the additional phonon scattering by magnetic vortices as the sample enters a mixed state. The high field behavior ofκ(H), arising from a continuous break-up of Cooper pairs, exhibits scaling which suggests the presence of an unconventional superconducting gap structure in this material.     

Thermodynamic and transport properties of UPt3

In the normal phase of UPt₃, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT ² dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromT _c to 146 mK (the lowest experimental temperature), a largeT ² contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.     

Dielectric and thermal properties of a machinable glass—ceramic at low temperatures

Measurements of the dielectric properties (2–300 K), specific heat (2–20 K), and thermal conductivity (2–22 K) are reported for a mica-containing glass-ceramic which has a machinability in the range from brass to low-carbon steel. The dielectric constant increases with increasing temperature and is field independent for field strengths up to at least 70 kV cm^?1 at low temperatures. Power-supply-limited attempts to measure the dielectric breakdown strength at low temperatures are consistent with the reported strength at room temperature (1.4 MV cm^?1). The thermal properties are similar to fused SiO₂ with two exceptions: the thermal conductivity does not show the ‘knee’ at ～ 10 K typical of amorphous materials, and the specific heat deviates strongly from a T³ law below 3.5 K     

Effect of Disorder Induced by Heavy-Ion Irradiation on CeCoIn5 Superconductivity

We present a study of the effect of heavy-ion irradiation on a thinned single crystal of the unconventional heavy fermion superconductor CeCoIn₅. Magnetic susceptibility and low-temperature specific heat results show that the superconducting transition temperature (T _c) changed only 4% with irradiation of 1×10¹² ions/cm², Energy=1 GeV, while the specific heat jump at T _c, ΔC, divided by C _n, where C _n is the normal state specific heat just above T _c, was reduced to 3.6 from ΔC/C _n=4.5 for unirradiated CeCoIn₅. The increase of low-temperature magnetic susceptibility and the saturation in magnetization suggest that the defects induced by heavy-ion irradiation are magnetic in nature, as was seen in the case of neutron irradiation on the heavy fermion superconductors, UBe₁₃ and UPt₃. The non-Fermi liquid behavior of the irradiated sample, based on the temperature dependence of the low temperature magnetic susceptibility, is significantly altered.     

Thermal conductivity of Ta-Nb alloys in superconducting,mixed, and normal states

The thermal conductivity of the type II superconductors Ta₉₅Nb₅, Ta₈₀Nb₂₀, Ta₆₀Nb₄₀, and Ta₂₀Nb₈₀ has been measured as a function of magnetic field up to 14 kOe and of temperature between 0.5 and 4.5 K. The temperature dependence of the thermal conductivity in the superconducting state is well accounted for on the basis of the Bardeen, Rickayzen, and Tewordt theory above 0.4T _c . The lattice thermal conductivity limited by dislocation scattering is observed below 0.3T _c . The thermal conductivity near the upper critical fieldH _c2 shows a linear dependence on magnetic field as predicted theoretically by Caroli and Cyrot. After the correction of the phonon contribution, the experimental results for the dirtiest sample, Ta₆₀Nb₄₀, are found in good quantitative agreement with the theory. Deviations from the theory for less dirty alloys depend on the electron mean free path.     

Anisotropy of the upper critical field near Tc and the properties of URu2Si2 and UBe13 in the normal state

Simultaneous measurements of the electrical resistivity Seebeck coefficientS, and thermal conductivity k of URu₂Si₂ and UBe₁₃ are carried out. For URu₂Si₂ a strong anomaly in the k(T) dependence near the temperature of the antiferromagnetic transition was observed. The thermal conductivity of UBe₁₃ exhibits only a small (about 30%) change of the derivativedk/dT near 8–9 K, which may be related to an increase of the characteristic length of antiferromagnetic correlations below this temperature. Studying STM spectra in the normal and magnetic phase of a URu₂Si₂ single crystal we observed that the magnetic gap, partially opened at the Fermi surface below the Néel temperature,T _N =17.5 K, is strongly anisotropic: gapped states mainly correspond to the tetragonalab-plane. Anisotropy of the upper critical field nearT _c (H<4kOe), studied for URu₂Si₂ and UBe₁₃ single crystals, was found to be in a good correspondence with the character of the antiferromagnetic transition.     

Inelastic Scattering on the Thermal Conductivity of Ba<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>As<Subscript>2</Subscript>

The thermal conductivity of Ba_1−x K _x Fe₂As₂ shows a large peak at a temperature somewhat below half the value of its superconducting state critical temperature T _c. A corresponding peak was observed in the microwave conductivity. In this case, the peak was understood as the composite effect of a very anisotropic superconducting gap on at least one of the superconducting bands with nodes, possibly on the electron pocket at the M point of the Brillouin zone, combined with a rapid decrease in the inelastic scattering rate with decreasing temperature T. In this paper, we show that the thermal conductivity peak also follows from the same model. These results point to a commonality with the cuprates and, as in that instance, is evidence for an electronic mechanism. In such a mechanism, the fluctuation spectrum responsible for the scattering and pairing glue becomes gapped with the onset of superconductivity, and consequently, the low temperature inelastic scattering rapidly tends to zero.     

Experimental study of organic zero-gap conductor α-(BEDT-TTF)2I3

Abstract

A zero-gap state with a Dirac cone type energy dispersion was discovered in the organic conductor α-(BEDT-TTF)₂I₃ under high hydrostatic pressures. This is the first two-dimensional (2D) zero-gap state discovered in bulk crystals with a layered structure. In contrast to the case of graphene, the Dirac cone in this system is highly anisotropic. The present system, therefore, provides a new type of massless Dirac fermion system with anisotropic Fermi velocity. This system exhibits remarkable transport phenomena characteristic to electrons on the Dirac cone type energy structure. The carrier density, written as n∝T², is a characteristic feature of the 2D zero-gap structure. On the other hand, the resistivity per layer (sheet resistance R_S) is given as R_S=h/e² and is independent of temperature. The effect of a magnetic field on samples in the zero-gap system was examined. The difference between zero-gap conductors and conventional conductors is the appearance of a Landau level called the zero mode at the contact points when a magnetic field is applied normal to the conductive layer. Zero-mode Landau carriers give rise to strong negative out-of-plane magnetoresistance.     

Thermal conductivity of high-T c crystals: Dependence on magnitude and orientation of magnetic field,and effect of Zn doping

We present precise measurements of in-plane thermal conductivity for superconducting single crystals of YBa₂Cu₃O_7?x (YBCO) withT _c =92 and 60 K, Bi₂Sr₂CaCu₂O₈ (BSCCO), and of Zn-doped YBCO. Magnetization and thermal conductivity data obtained with the same 90-K YBCO crystal demonstrate a close relationship between the magnetic thermal resistivity and the internal magnetic fieldB in a superconductor in the mixed state. For all superconductors studied here, the magnetic thermal resistivity is a sublinear function of magnetic field. The origins of the nonlinearity are discussed. Angular dependences of the magnetic thermal resistivity have been shown to depart from the anisotropic 3D superconductor model and are in quantitative agreement with a quasi-2D model. Implications for spatial modulation of the order parameter are made.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7−x

The effects of addition to YBa₂Cu₃O_7?x of lithium halides (YBa₂Cu₃O_7?x (LiF) _y , and YBa₂Cu₃O_7?x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7?x (LiF) _y and 91.30 K in YBa₂Cu₃O_7?x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _c∝B ^?α, with a lithium-concentration-dependent α. The voltage–current characteristics follow a law typical for granular superconductors, V∝(I?I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Magnetic, Electrical and Thermodynamic Properties of UCuT x Al11−x Alloys Where T = Mn, Fe and x=4 and 5

The structure, magnetic, electrical, and thermodynamic properties of UCuT _x Al_11?x alloys, where T = Mn or Fe and x=4 or 5 are presented. The behavior of the Fe alloys is ferromagnetic-like with the Curie points amounting to 180 and 230 K, and the saturation magnetic moments under magnetic field of 5 T equal to 4.75 and 6.02 μ_B/f.u., respectively, whereas under a magnetic field of about 34 T the magnetic moments amount to 6.9 and 9.0 μ_B/f.u. for the alloys with x=4 and 5, respectively. The Curie points are reflected in the temperature dependence of the specific heat in which the anomalies are found at 180–200 and 230 K for alloys with x=4 and 5, respectively, however, it shows no reflection in the temperature dependence of the electrical resistivity. The field dependence of the magnetization at T=1.9 K for both compounds exhibits considerable hysteresis. There is a pronounced difference between ZFC and FC magnetization in its temperature dependence below the Curie point for materials with x=4 and 5. The Mn alloys exhibit ferrimagnetic-like character for which, supposedly, the interplay of the uranium and manganize sublattices is responsible. Magnetic transitions are determined at T _N =300 (x=4) and 380 K (x=5). However, those anomalies do not find confirmation in the temperature dependence of the specific heat and the electrical resistivity. Magnetic moments determined at T=1.9 K and in a magnetic field of 5 T are very low and in both cases amount to about 0.35 μ_B/f.u. and these values are slightly higher in a magnetic field of 34 T reaching a value of about 1.5 μ_B/f.u. Also for the Mn alloys the clear difference between ZFC and FC magnetization in its temperature dependence below the Curie point is observed.     

Spin Coherence Time T 2 in Metallic P-doped Si at Very Low Temperature

We report NMR measurements of transverse relaxation rate 1/T ₂ of the ³¹P nuclear spins in metallic Si:P (concentration of dopant P, n=18×10¹⁸ and 56×10¹⁸ cm^?3) at temperatures between 45 mK and 5 K in a magnetic field of 7 T. Above 1.4 K, 1/T ₂ is constant independent of temperature as well as concentration and is determined by magnetic dipolar interaction between the ³¹P and ²⁹Si nuclear spins. As temperature decreases below 1.4 K, 1/T ₂ increases over the dipolar-determined value by an order of magnitude and levels off around 0.6 K. The concentration dependence of 1/T ₂ at low temperatures suggests that 1/T ₂ below 1.4 K is determined by the coupling between the ³¹P nuclear spins. We understand 1/T ₂ at low temperatures originates from the RKKY interaction. We explain the temperature dependence of 1/T ₂ between 0.6 K and 1.4 K by the motional-narrowing expression of 1/T ₂ with a temperature-dependent correlation time of the fluctuating local field due to the RKKY interaction.     

Fish-Tail Effect and Its Evolution Under Neutron Irradiation in Li-Doped YBa2Cu3O7−x

The evolution of the critical current density of Li-doped YBa₂Cu₃O_7?x polycrystalline samples submitted to neutron irradiation is investigated as function of magnetic field (0 ≤ B ≤ 6 T) temperature (5 ≤ T ≤ 85 K) and neutron fluence (0 ≤ Φ ≤ 9.98 × 10¹⁷ cm^?2). At fluences lower than 10¹⁷ cm^?2, a second peak in j _s vs. B dependence is present (fish-tail effect). Its magnitude decreases with increasing the fluence. Above 10¹⁷ cm^?2, the second peak of current density completely disappears; instead, the logarithmic susceptibility shows a second peak at a certain field B _infl. A dependence of B _infl on fluence is proposed.     

Density and Thermal Conductivity Measurements for Silicon Melt by Electromagnetic Levitation under a Static Magnetic Field

The density and thermal conductivity of a high-purity silicon melt were measured over a wide temperature range including the undercooled regime by non-contact techniques accompanied with electromagnetic levitation (EML) under a homogeneous and static magnetic field. The maximum undercooling of 320 K for silicon was controlled by the residual impurity in the specimen, not by the melt motion or by contamination of the material. The temperature dependence of the measured density showed a linear relation for temperature as: ρ(T) = 2.51 × 10³−0.271(T−T _m) kg · m⁻³ for 1367 K < T < 1767 K, where T _m is the melting point of silicon. A periodic heating method with a CO₂ laser was adopted for the thermal conductivity measurement of the silicon melt. The measured thermal conductivity of the melt agreed roughly with values estimated by a Wiedemann–Franz law.     

Scaling Laws for the Magnetocaloric Effect in Calcium Deficiency Manganites La0.8Ca0.2−x □ x MnO3 with a Second-Order Magnetic Phase Transition

The magnetic entropy change (ΔS _M ) of polycrystalline samples La_0.8Ca_0.2?x □ _x MnO₃ (x=0.00; x=0.10 and x=0.20) with a second-order phase transition has been investigated. The field dependence of the magnetic entropy change expressed as ΔS _M ∝H ⁿ follows the phenomenological curves and at the temperature of the peak corresponds to a large field independent exponent of n=0.581; 0.642 and 0.671 for x=0.00; x=0.10 and x=0.20 samples, respectively. Then, we have constructed the phenomenological universal curve by normalizing the magnetic entropy change curves with respect to their maximum values, $\Delta S_{M_{\mathrm{max}}}$ , and rescaling the temperature axis. These universal curves collapse onto a single curve for any applied magnetic field for all samples. Moreover, we note that the universal curve for x=0.10 and x=0.20 samples collapses with a small deviation compared with x=0.00 sample. This is consistent with our previous work where we demonstrated that the magnetic phase transition relative to the samples x=0.00 belongs to the 3D-Ising universal class and to the 3D-Heisinberg class for x=0.10 and x=0.20 samples. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory by extrapolations in field or temperature.     

NMR Study of 3He Adsorbed in Zeolite Pores

The NMR properties of ³He adsorbed in the pores of MCM-41 zeolite have been studied in the temperature range 1.4 to 15 K, at pulsed NMR frequencies of 1.66 and 3.26 MHz. At a coverage x=0.84 monolayer, the linewidth 1/T ^* ₂ scales approximately linearly with the magnitude of the static magnetic field, and T ^* ₂ increases linearly with increasing temperature with an extrapolated low temperature limit of order 80 µs. However T ₂ is significantly longer, increasing from 0.7 µs at the lowest temperatures investigated in a way suggesting thermally activated motion. We attribute T ^* ₂ to static field variations between pores arising from paramagnetism of the zeolite and the random orientation of the pores. On the other hand T ₁ increases monotonically with decreasing temperature to around 80 ms at 1.6 K, indicating that the correlation time of the local magnetic field fluctuations τ _c>ω ^?1 ₀. A detailed temperature dependence of T ₁ and T ₂ has been carried out at a coverage of 0.32 monolayer. Here a minimum in T ₁ is observed at 11 K for a Larmor frequency of 3.26 MHz corresponding to a correlation time τ _c of 5×10^?8 s. T ₁ increases by around three orders of magnitude on cooling to 1.8 K. At this temperature T ₁ decreases significantly with increasing coverage while T ₂ shows a very much weaker coverage dependence.