Molar-Volume Dependence of the Pressure in hcp Solid 3He in Magnetic Fields

Pressure measurements have been made in a hcp solid ³ He at molar volumes from 18.90 to 19.45 cm ³ /mol, at temperatures down to 1 mK and in magnetic fields up to 0.16 Tesla. The results are analyzed by a high-temperature series expansion with multiple-exchange processes. The molar volume dependence of the multiple-exchange interactions has been determined by pressure measurements in magnetic fields. The results indicate that the exchange frequencies determined by the pressure measurements in a hcp solid ³ He are consistent with those obtained by magnetization measurements.     

Magnetic Ordering in 3He Nano-Clusters

Simultaneous measurements of magnetic susceptibility from 0.5 to 10 mK and pressure from 2.88 to 3.54 MPa have been made in ³ He nano-clusters embedded in a ⁴ He matrix, following phase separation. The susceptibility of the 3.54 MPa, all-solid sample behaves similarly to that of bulk ³ He for v=21.3 cm ³ /mole, with a Weiss constant =–250K. For the 2.88 MPa, liquid-droplet sample, =140K, indicating a ferromagnetic tendency, similar to 2-D films at some coverages. At intermediate pressures, has a peak near 1.05 mK, but without a discontinuity. For all samples, had a solid-like contribution to the lowest temperatures. Magnetic ordering in nano-clusters appears to be different than the U2D2 phase of bulk ³ He.     

Spin-Fluctuation Mediated Thermal Conductivity Around the Magnetic Instability of CeNi2Ge2

Thermal conductivity has been measured from 0.03 K to 0.8 K in single crystal CeNi ₂ Ge ₂ which is quite near the quantum critical point (QCP). The Wiedemann–Franz law is observed at low temperatures (T<0.1 K) for both directions (jc and j c), indicating that quasiparticles are scattered elastically as T0 K. With increasing temperature, a deviation from the Wiedemann–Franz law due to inelastic spin-fluctuation contributions is observed. The effect of proximity to the QCP in CeNi ₂ Ge ₂ appears in the spin-fluctuation contributions to the thermal and electrical conductivities.     

Dependence of Magnetization on Temperature, Magnetoresistance Effect and Magnetic Phase Diagram for the Layered Gd5Ge4 Compound

The dependence of dc magnetization and electrical transport on temperature were systemically studied for the layered Gd₅Ge₄ compound. A complex coexistence behavior of ferromagnetism (FM) with antiferromagnetism (AFM) phase and significant magnetoresistance effect were found under the inducement by the applied magnetic field. The results show an unusual magnetoresistance (MR) effect with opposite behavior at the high and low temperatures, which is positive at high temperature region and negative at low temperature region for the layered Gd₅Ge₄ alloy. And its MR maximum is close to 50%, which can be a very large numerical value e.g. for giant magnetoresistance (GMR) materials. From the magnetization loop under different temperatures, also a magnetization step behavior was found below ∼6.2 K. The results proved an existence of reentrance AFM phase at low temperature. From experimental data, also the magnetic phase diagram is obtained. This kind of complex magnetization is analyzed and discussed in the frame of phase separation. The experimental results opened also the possibility of application of the layered Gd₅Ge₄ compound in the fields of information record and sensor technique.      

Neutron Diffraction Studies of Nuclear Magnetic Ordering in Silver

Nuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of ¹⁰⁹ Ag by neutron absorption and diffraction techniques. Below the Neel temperature T_N , a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vector k = (2/a)(001) (up-down structure). The entropy at the transition, ln 2 in zero magnetic field, corresponding to a critical polarization P_c = 0.75 and T_N = 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions of B: [001] and The critical field extrapolated to S = 0 is 100 ± 10 T, and for both field directions S_c is highest around B = 30 T. The transition to the paramagnetic state is presumably of second order. The nuclear magnetization was measured by transmission of unpolarized neutrons, and the dimensionless static volume susceptibility in SI units was found to be 0.36 ± 0.01 in the ordered state independently of B and S. The ac susceptibility at 7.9 Hz showed a kink at the transition only when the sample was not exposed to neutrons.     

Magnetic Pair Breaking in Superconducting SrPd2Ge2 Investigated by Scanning Tunnelling Spectroscopy

SrPd₂Ge₂ has been structurally identical with the 122-type iron pnictide compounds with iron completely replaced by Pd and pnictogen replaced by Ge yielding the superconducting transition below 3 K. Our previous works showed that in contrast to iron pnictides this system is electronically fully three-dimensional and revealed conventional superconductivity, even of type I. Here, by means of sub-Kelvin STM spectroscopy, we show that the dirty limit occurs in the sample driving it to type-II superconductivity. STM also proves that in magnetic field the sample undergoes the phase transition to the Abrikosov vortex-lattice state. The de Gennes–Maki theory of gapless superconductivity for dirty superconductors is successfully applied to model the tunnelling spectra of the superconducting density of states in magnetic field. We show that this theory is applicable in a wide range of magnetic fields starting from 60 % of the upper critical field H _c2.     

A Model for Magnetic Ordering in Quasi-Low-Dimensional Magnet CsDy(MoO4)2

For quasi-low-dimensional magnet CsDy(MoO4)2 a model for the equilibrium spin arrangement in the magnetically ordered state is proposed on the ground of the symmetry analysis of neutron diffraction data. Low crystal symmetry of monoclinic syngony results in several spatial orientations of strongly anisotropic magnetic centers in the crystal and leads to the form of an order parameter more complicated than simply ferromagnetic or antiferromagnetic one. Measurements of magnetization along crystal axes have been performed both above and below the Neel point.     

Magnetic Field Dependence of the Energy Gap in Nanotubes

The dependence of the energy gap in the band structure of a carbon nanotube on the flux of an external uniform magnetic field is studied in the framework of the zero-range potential method. Taking into account a renormalization procedure for the coupling constant of the zero-range potential, we obtain an estimate for the maximal gap between the conductivity band and the valence one; this estimate is closer to experimental data when compared to the value given by the tight-binding approximation. It is shown numerically that decreasing the diameter of nanotubes breaks down the flux periodicity of the gap structure.     

Temperature and Pressure Dependence of the Speed of Sound in Seawater

The speed of sound in sea water has been measured for various salinities over the temperature range 0.1–30°C at excess pressures of 0–60 MPa. The results are presented in tables and an equation derived by least-squares fitting that gives the difference in the speed of sound between distilled water and seawater. The data are compared with calculations on the Chen-Millero algorithm, which was used in compiling the UNESCO international tables for the speed of sound. Suggestions are made for setting up GSSSD tables Seawater: Speed of sound.     

High Magnetic Field Dependence of Magnetodielectric Properties in Sr2CoSi2O7 Crystal

We have investigated magnetic and dielectric properties of åkermanite Sr₂CoSi₂O₇ single crystal in which the orbital hybridization is controllable by external magnetic fields. Sr₂CoSi₂O₇ shows the electric polarization that is originated from asymmetric orbital hybridization induced by magnetic field. This induced electric polarization was observed even at temperatures considerably higher than the magnetic transition temperature. The polarization data obtained for the paramagnetic phase are scaled with the square of the induced magnetization. By the discussion of the crystal symmetry, the space group of a polar state of åkermanite materials is probably Cmm2.     

Pressure Dependence of UO2 Melting Measured by Double-Pulse Laser Heating

The melting point of uranium dioxide was measured as a function of isostatic pressure in an autoclave filled with helium up to 0.25 GPa. Containerless laser surface-heating was applied to measure the melting point by thermal arrest measurements during sample cooling. Precise control of the cooling rate, aimed at a precise determination of the freezing plateau, was obtained with a two-pulse method, where two Nd:YAG high power laser pulses are mixed within the same optical fiber and then focused on the sample surface. Calibration of the power of the two pulses permits the measurement of the melting point with very low uncertainty (0.5%). The measured melting slope of UO₂ is 170 and 200 KGPa^–1, a value which is markedly higher than that predicted by the Clausius–Clapeyron equation, which expresses this slope in terms of the enthalpy of fusion and the melting expansion. Possible reasons for this discrepancy are discussed.     

Superfluid, Superconducting, and Magnetic Ordering in Mesoscopic Quantum Dots

The nature of superfluid, superconducting, and magnetic ordering is elucidated for mesoscopic systems in which the single-particle level spacing is much larger than both the temperature and the critical temperature of ordering. Ordering is defined as a spontaneous breaking of symmetry, the gauge invariance and time reversal being by definition symmetries broken in superfluidity (superconductivity) and magnetism contexts, respectively. Superfluidity and superconductivity are realized in thermodynamic equilibrium states with a nonintegral average number of particles and are accompanied by the spontaneous breaking of time homogeneity. In Fermi systems, two types of superfluidity and superconductivity are possible which are characterized by the presence of pair or single-particle condensates. The latter is remarkable in that spontaneous breaking of fundamental symmetries such as spatial 2 rotation and double time reversal takes place. Possible experiments on metallic nanoparticles and ultracold atomic gases in magnetic traps are discussed.     

Direct Measurements of Magneto-caloric Effect of Gd5Si2Ge2 Alloys in Low Magnetic Field

Direct measurements of the magneto-caloric effect of Gd₅Si₂Ge₂ alloys under heat treatment conditions are investigated by measuring adiabatic temperature change (ΔT _ad) in a magnetic field of 1.5 T using a homemade magnetocaloric effect measuring apparatus. The crystal structure, microstructure as well as the chemical composition of the alloys are measured using X-ray diffraction (XRD), scanning electron microscope (SEM) with energy dispersive X-ray Detector (EDX). It is found that the microstructure of the alloys could be fully homogenized and the impurities in the alloys could be remarkably removed via appropriate heat treatment. As a result, the maximum adiabatic temperature change (ΔT _ad) of the alloy annealed at 1573 K increases by 105% from 1.9 to 3.9 K for a magnetic field change from 0 to 15 kOe when compared to the as arc-melted material, while the magnetic ordering temperature slightly reduced.     

Pressure Dependence of the Thermophysical Properties of n-Pentadecane and n-Heptadecane

The speed of sound in liquid n-pentadecane and n-heptadecane was measured using a pulse technique operating at 3 MHz. The measurements were carried out at pressures up to 150 MPa in the temperature range from 293 to 383 K. The experimental results have been used to evaluate various thermophysical properties such as density and isentropic and isothermal compressibilities up to 150 MPa with the help of additional density and heat capacity data at atmospheric pressure.     

Magnetic Ordering of Dy3+ Ions in RbDy(WO4)2 Single Crystal

The specific heat C(T) of the monoclinic RbDy(WO ₄ ) ₂ crystal has been studied at very low temperatures 0.2T1.9 K and in magnetic fields 0H0.38 T. The Neel temperature was shown to be equal to T_N = 0.818 ± 0.005 K. The experimental value of the effective exchange parameter was obtained to be equal to J/k = – 0.798 K. The C T) dependence below Neel temperature 0.5T_N0.99T_N ) is well described by 2D Ising model, whereas in the temperature region close above T_N (1.01 T_N2T_N) it can be described by neither 2D, nor 3D Ising model. The experimental and theoretical H-T_N diagrams for field H a are in a reasonable agreement for simple quadratic lattice.     

Temperature Dependence of Magnetic Parameters in FePt Nanoparticles

Here, particular focus is placed on the atomic alignment and the order–disorder phase transition of the FePt alloy with the aid of a model that can describe realistic phenomena. Here, we present a method to study the order–disorder phenomena of FePt alloys. We will discuss the increase in the coercivity of FePt nanoparticles by an increasing annealing temperature for annealed FePt nanoparticles. According to the experimental evidence, we will present a model for explaining spin-glass-like behavior of these particles during the annealing procedure. In the phase transition from disordered FCC to ordered FCT in FePt nano-particles can be treated by first-order phase transitions. So, the mean field approach can be used in order to model this kind of phase transition. In nanoparticles, which are synthetized by sol–gel methods, the short range parameter is predominant because in these kinds of preparations, the long range order takes a lot of time to happen. By using a short range parameter, we are able to use the mean field approach, which considers the diffusion of atoms to the near neighborhood sites. The effects of random exchange and random magnetocrystalline anisotropy are known as the main results of coercivity reduction in magnetic nanoparticles. These effects are strongly dependent to the annealing temperature of nanoparticles. During disorder–order transition, these two effects are max. As transition continues, these effects vanish and coercivity of nanoparticles increases. Here, we will add these two effects to the Hamiltonian of the FePt system. With increasing the annealing temperature, the fraction of the FePt nanoparticles with a FCC disordered phase vanishes, which leads to decrease in the amount of 〈j〉. During the annealing process, the $\frac{c}{a}$ ratio varies as a function of the annealing temperature. The relation between the $\frac{c}{a}$ ratio and the annealing temperature is derived. The derived $\frac{c}{a}$ ratio and coercivity formulas are compared with experimental results. The results are in a good agreement with experimental data.     

Electronic and Magnetic Properties in the Novel Skutterudite ThPt4Ge12

Theoretical calculations on ThPt₄Ge₁₂ filled skutterudite were performed. The calculated energy bands yielded indication of metallic behavior, while the projected density of states provided indication of a hybridization formed by Th f-, Pt d- and Ge p-states. The absence of a detected minigap provides support that this material is not a good thermoelectric material. Furthermore, Crystal Orbital Overlap Population yielded indication of the absence of a ferromagnetic instability.      

Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Mn注入n型Ge单晶的磁性研究

利用离子注入方法,以离子能量为100keV,剂量为3×1016cm-2,室温下往n型Ge(111)单晶衬底注入Mn+离子,注入后的样品分别在400和600℃氮气氛下进行热处理.利用交变梯度样品磁强计室温下对样品进行了磁性测量,利用X射线衍射法分析了样品的结构特性,俄歇电子能谱法分析了热处理样品的组分特性.磁性测试结果表明热处理后的样品表现出室温下的铁磁性.结合样品的组分、结构特征进行磁性分析后认为样品在热处理后形成铁磁性半导体MnxGe1-x结构可能是样品表现室温铁磁性的主要原因.