强磁场中Fermi气体的稳定性及顺磁性

运用理论解析和数值模拟的方法,研究强磁场中Fermi气体在低温下的稳定性及顺磁性, 分析温度、磁场对稳定性及磁化率的影响.研究结果显示,强磁场引起系统稳定性的改变, 这种改变与温度有关,而温度对稳定性改变及磁化率的影响又与自由气体的化学势即粒子数密度有关. 当自由系统的化学势为偶数时,磁场弱化系统的稳定性,温度升高可降低系统的磁化率; 当自由系统的化学势为奇数时,磁场强化系统的稳定性,温度升高可增大系统的磁化率. 升高温度,可以弱化磁场对系统稳定性的影响;加强磁场,可以使得系统稳定性的改变发生振荡, 使系统磁化率的振荡中心下移.     

Crystal growth, structure and ferromagnetic properties of a Ce3Pt23Si11 single crystal

A high-quality single crystal of Ce₃Pt₂₃Si₁₁ has been grown using the Czochralski method. The crystal structure is presented and the chemical composition has been checked using an electron microprobe analyzer. Measurements of the electrical resistivity and magnetic susceptibility performed at low temperature show a ferromagnetic transition at T_c=0.44 K.     

Magnetic ordering in TmTe

Neutron diffraction experiments were performed on the divalent thulium compound TmTe. Magnetic structure determination leads to a type II antiferromagnet below the Néel temperature T_N = .43 K. No crystal field excitations were observed.     

Far-infrared reflectivity of the mixed-valence compounds TmTe and TmSe

Far-infrared reflectivity measurements have been made on the mixed valence compounds TmTe and TmSe between 1.3 and 300 K. For TmTe at 1.3 K a single Reststrahlen band is observed at frequencies expected for divalent Tm in TmTe. At 4.2 K new weak structure appears which may be due to trivalent Tm. TmSe shows a surprisingly low reflectivity, but no phonon structure. The plasma edge is at ～ 10cm^-1 indicating a strongly frequency-dependent conductivity.     

Anharmonic properties of TmTe

Data on elastic constants and associated properties at high temperature for TmTe crystal are presented and discussed starting from primary physical parameters viz. nearest neighbour distance and hardness parameter assuming long- and short-range potentials. When the values of the higher order elastic constants are known for a crystal, many of the anharmonic properties of the crystal can be treated within the limit of the continuum approximation in a quantitative manner. In this study, higher order elastic constants and related properties are computed upto 1000 K for TmTe. The first-order pressure derivatives of second- and third-order elastic constants, the second-order pressure derivatives of second-order elastic constants and partial contractions are also evaluated at different temperatures. The results thus obtained are compared with other available data and found in well agreement with present values.     

Magnetocaloric properties of manganites in alternating magnetic fields

The magnetocaloric effect has been measured in manganites of various chemical compositions in weak alternating magnetic fields. The capabilities of a simple method for measuring the magnetocaloric effect by modulating the magnetic field have been demonstrated. The dependence of the magnetocaloric effect on the temperature, magnetic field, and chemical composition of samples is interpreted.     

Faraday and Cotton-Mouton effects for acoustic phonons in paramagnetic Rare Earth compounds

A theory is developed for the Faraday effect and Cotton-Mouton effect of acoustic phonons in paramagnetic Rare Earth systems. The effects occur if degenerate transverse acoustic phonons are propagating in the direction of an applied magnetic field and perpendicular to it. The theory is formulated for cubic crystals but a generalization to other crystal symmetries is straightforward. The theoretical findings are applied to TmTe and it is demonstrated that both effects are large enough to be observable.     

Phase diagram of the Bi-Sn system in the liquid and solid states

Results are given for the specific resistance, thermo-emf, and magnetic susceptibility in the range from room temperature up to 500 °C. The methods of measurement have previously been reported [1]. Each property is presented as a function of composition; the results are compared with ones for the system Bi-Cd. The temperature dependence of the three quantities is calculated for the alloy of eutectic composition.     

Magnetic properties of the cerium monochalcogenides

Single crystals of CeS, CeSe and CeTe have been investigated by means of measurements of the specific heat, the thermal expansion, the magnetic susceptibility and the magnetization at low temperatures. Crystal-field effects on some of these properties are used to derive the crystal-field splitting of the 4f electron ground state. It is found that chemical composition and crystal perfection strongly influence the magnetic properties of these compounds.     

Microstructure and magnetic properties of strontium titanate implanted with iron

The magnetic phase composition of strontium titanate surface layers implanted with iron ions is studied by means of Mössbauer spectroscopy, and by measuring alternating current magnetic susceptibility. It is shown that the interaction between α-Fe nanoclusters at high concentrations of the implanted admixture produces ferromagnetic order in samples at room temperature.     

Crystal field effect in the thermal expansion of cubic rare earth compounds

We report on evidence of crystal-electric-field effects in the low temperature thermal expansion of various rocksalt-type rare earth compounds (PrSb, SmSb, ErSb, TmSb, CeTe, TmTe). From our measurements we deduce the volume dependence of the energy splitting between the lowest crystal field levels and we calculate magnetoelastic coupling parameters involving the volume strain.     

Relaxation phenomena in ensembles of CoFe2O4 nanoparticles

Collective magnetic behavior of CoFe₂O₄ nanoparticles with diameters of 76, 16, 15 and 8 nm, respectively, prepared by different chemical methods has been investigated. Particle composition, size and structure have been characterized by inductive coupled plasma (ICP), transmission electron microscopy (TEM) and powder X-ray diffraction (PXRD). Basic magnetic properties have been determined from the temperature dependence of magnetization and magnetization isotherms measurements. The three samples exhibit characteristic of a superparamagnetic system with the presence of strong interparticle interactions. Magnetic relaxation phenomena have been examined via frequency-dependent ac susceptibility measurements and aging and memory effect experiments. For the particles coated with oleic acid, it has been demonstrated that the sample reveals all attributes of a super-spin glass (SSG) system with strong interparticle interactions.     

Measurement of magnetic susceptibility and calculation of shape factor of NMR samples

A method for accurate measurement of magnetic susceptibility and determination of the shape factor in an NMR tube is shown. The combination of accurate shape factor determination with susceptibility measurement leads to improved accuracy when measuring chemical shift. This is important for comparing samples in different solvents or under different conditions, such as temperature, solvent, and pH.     

Lattice vibrational properties of TmTe

We have investigated the lattice dynamical properties of a TmTe compound by using a breathing shell model suitable for this compound. The calculated phonon dispersion curves (PDC) reveal that this compound does not show any anomaly in their phonon properties. Our results on PDC, phonon density of states and lattice specific heat reveal that the phonon properties of this compound are like the other rare earth chalcogenides, particularly Eu-chalcogenides. We emphasize the need of measurements of the complete PDC of TmTe to support the present results on the calculated phonon properties.     

Magnetic susceptibility of (IPA)2FeCl4, (IBuA)2FeCl4 and (IPA)2CuCi4 (IPA = isopropylammonium ion and IBuA = isobutylammonium ion)

The magnetic susceptibility of (IPA)₂FeCl₄, (IBuA)₂FeCl₄ and (IPA)₂CuCl₄, where IPA = (CH₃)₂CHNH⁺₃ and IBuA = (CH₃)₂CHCH₂NH⁺₃, were measured in the temperature range form 80 K up to a temperature near the melting points of the samples. The results obtained were interpreted in termw of a canted two-dimensional antiferromagnet. The variation of the magnetic susceptibility with the change in the composition was also discussed.     

Bethe-Weiss方法在固溶体铁磁性统计理论中的应用

Bethe-Weiss方法是Bethe对合金有序现象所创建的近似理论被移植于铁磁性海森堡模型的统计理论的结果。在这方法中,我们选出由一个中心原子与它所有的最近邻原子所组成的集团作为处理的对象,它们之间的交换作用得到精确的对待,而集团以外的原子的作用就近似地用一有效场代替,后者由自洽条件来推。我们应用这一方法来计算由磁性原子(自旋为1/2)与非磁性原子组成的代换固溶体在较高温度段内的铁磁性,得出简单立方和体心立方晶格居里点T_c随非磁性原子浓度f的增加而下降的曲线,居里点以上的磁化率与磁矩的短程有序能与温度的关系。由后者看出,当f增加时,在有较高的短程有序的情况下才出现铁磁性。从居里点的计算中发现:这一方法只能应用在非磁性原子的浓度较低的情况,我们讨论了这一事实出现的原因。     

Monte Carlo investigation of the correlation between magnetic and chemical ordering in NiFe alloys

The Ni_xFe_1−x alloys close to the stoichiometric Ni₃Fe composition are modeled by means of Monte Carlo simulations. To describe the atomic and magnetic configurations, the Ising and Heisenberg models with nearest-neighbor interactions have been used, respectively. The pairwise interactions have been fitted to the experimentally measured Curie and Kurnakov temperatures, the Fe-Fe magnetic exchange interaction has been considered antiferromagnetic. The mutual influence of the magnetic and chemical ordering is evidenced and a good agreement with the phase diagram is obtained. Our numerical results show that the magnetic order is able to increase the Kurnakov temperature and, reciprocally, the chemical order is responsible for a rise in the Curie temperature. Also, the influence of the applied magnetic field on the chemical order is investigated and an increase of the Kurnakov temperature with the external field is observed.     

Ferromagnetic ordering in alkali-metal iron antimonides: NaFe4Sb12 and KFe4Sb12

New alkali-metal compounds with the filled-skutterudite structure were synthesized and their chemical and physical properties investigated. X-ray diffraction, microprobe, and chemical analysis established the structure and the composition without defects on the cation site. Magnetization, ac susceptibility, specific heat, resistivity, and NMR or NQR demonstrated NaFe4Sb12 to be ferromagnetic below approximately 85 K and to exhibit an additional magnetic anomaly around 40 K. Band structure calculations find a large density of states at the Fermi energy and a ferromagnetic ground state. Similar behavior was observed for KFe4Sb12.     

Nuclear magnetic resonance and static magnetic susceptibility of AsF5 - intercalated graphite

NMR and static magnetic susceptibility (χ) measurements on stage 1 AsF₅ intercalated graphite (C₈AsF₅) are presented. The relaxation times, T_i and T₂, of the F¹⁹ nuclear magnetic resonance were measured over the temperature range 136K – 295K; χ was measured over the range 80K – 295K. The NMR results indicate a motionally narrowed line with gradual ordering of the intercalant as the temperature is decreased. The magnetic susceptibility is independent of temperature with no observable Curie law controbution. The absence of localized moment behavior for the intercalant is interpreted either in terms of chemical disproportionation of the AsF₅ to closed shell ions or in terms of the Anderson model of localized moments in metals.     

Magnetic properties of Fe1−xCoxO

Measurements are reported for the magnetic susceptibility of solid solutions Fe_1-xCo_xO for compositions with x =0.10 to 0.95 and for the temperature range 4.2 to 600 K. It is found that the Curie-Weiss law is obeyed at high temperatures while the dependence of the susceptibility and transition temperature on composition appear to be in good agreement with the “virtual crystal” approximation.