三维随机点阵三态矢量Potts模型Monte Carlo模拟

在三维随机格点阵上对三态矢量Potts模型进行了Monte Carlo数值模拟,构造了N×N×N(N=8,10,12)三种不同大小点阵,分别计算了该自旋系统的能量密度、磁化强度、比热和磁化率等的温度曲线。结果表明该自旋系统在β₆≈0.16处存在着能表征一阶Z(3)对称破缺相变的能量热滞图和比热陡峰,磁化强度的跃变规律亦支持该结论,从而显示出该模型在随机点阵上的相变行为与正方点阵的已有结果的相似性。     

镉取代对纳米镍铁氧体结构和磁性的影响

采用湿化学共沉淀法合成了Ni_1-xCd_xFe₂O₄ (0≤x≤0.5)的混合铁氧体,利用X射线衍射对其结构和晶相进行表征．结果表明：随着镉Cd浓度的增加晶格参数逐渐增大．扫描电子显微镜研究微观结构,TG/DTA研究了硫酸共沉淀物．揭示了在650 oC合成铁氧体同时进行分解．测量了纳米粒子的磁化强度和交流磁化率．结果表明,Ni_1-xCd_xFe₂O₄混合铁氧体的磁化率、居里温度和有效磁矩随着镉含量的增加下降。     

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.     

外磁场对纳米管上Blume-Capel模型磁热性能的研究

利用有效场理论研究了纳米管上外磁场中Blume-Capel模型格点的磁化强度、磁化率、内能、比热和自由能,得到了系统格点的磁化强度、磁化率、内能、比热和自由能与外磁场和晶场的关系.结果表明:外磁场强度、交换相互作用和晶场强度等诸多因素相互竞争,使系统表现出比没有外磁场作用的Blume-Capel模型更为丰富的磁学特性;外磁场能够增大系统格点的磁化强度,导致系统的二级相变消失;负晶场作用系统时,系统会发生一级相变;晶场强度、外磁场强度不同时,系统的磁化率、内能、比热和自由能也呈现出复杂性.     

限制在一维谐振势下的三维自由电子气的一些热力学性质

利用围路积分表达的计算系统热力学势的一个公式,得到被一维谐振子势限制、有垂直磁场作用的三维自由电子气的在任意温度下的热力学势的精确解析表达式.然后利用其研究了不同温度和尺度区域内磁化强度、磁化率和比热随磁场强度的变化情况.研究表明,低温下磁化强度、磁化率和比热随磁场强度变化出现振荡现象,其中比热还会出现两种不同的振荡模式.     

(Bi,Pb)-Sr-Ca-Cu-O体系110K超导体的相转变及超导电性

品质优良的110K超导体(Bi,Pb)₂Sr₂Ca₂Cu₃O_y的单相多晶样品已制备,其零电阻温度为107K。讨论了2201相,2212相和2223相的形成和转变以及淬火温度对其的影响。测量了样品的复磁化率,结果表明超导体晶粒间的Josephson结的耦合强度与样品的制备过程有关,对不同热处理的样品测其比热;△c(107K)=5.3J/molK,索末菲常数v=34.6mJ/molK,比热跳跃与热处理     

Blume–Emery–Griffiths纳米管的热力学与相变性质

利用有效场理论研究了圆柱形纳米管上Blume-Emery-Griffiths系统的热力学与相变性质,得到了系统的磁化强度、磁化率、比热和相图.讨论了四次交换作用与二次交换作用的比值 与晶格场对系统热力学量和相图的影响.研究发现:系统存在三临界点,且三临界点由参数 和晶格场共同决定,即若确定了参数 ,则三临界点所对应的晶格场也能确定.随着参数 的增加,系统出现三临界点时所对应的温度和晶格场也相应增大.     

α-LiIO3单晶体在静电场作用下X射线双晶衍射的研究

本文用X射线双晶摄谱仪观察了α-LiIO₃单晶体的不同晶面在各种方式外加电压作用下,双晶衍射曲线半高宽W,最大反射系数k_B(0)以及积分反射能力R的变化。对于k_B(0)随所加电压和加电压时间的改变有一最大值的现象,我们提出了α-LiIO₃单晶体在静电场作用下IO₃^-离子沿z轴移动和绕z轴转动的可能解释。     

Blume-Emery-Griffiths纳米管的热力学与相变性质

利用有效场理论研究了柱形纳米管上Blume-Emery-Griffiths系统的热力学与相变性质,得到了系统的磁化强度、磁化率、比热和相图.讨论了四次交换作用与二次交换作用的比值r与晶格场对系统热力学量和相图的影响.研究发现:系统存在三临界点,且三临界点由参数r和晶格场共同决定,即若确定了参数r,则三临界点所对应的晶格场也能确定.随着参数r的增加,系统出现三临界点时所对应的温度和晶格场也相应增大.     

超导膜的能隙和磁场的依赖关系

本文计算出超导膜在磁场中的能隙。所得到的表达式适用于所有的温度,但磁场要满足((ehp₀/(mc) A/(πk_BT_c))²《1条件,其中A是矢势,hp₀是Fermi动量。理论结果和实验进行了比较,符合得还好。     

Landau levels of holes in a two-dimensional channel

A method for calculating the Landau levels in a two-dimensional channel is proposed. This method is aimed at construction of an effective two-dimensional Hamiltonian of holes H(k_x, k_y, B). The Hamiltonian is expanded in powers of the magnetic field B and noncommuting momenta k_x and k_y. The results indicate an interconnection between effective masses and g-factors of the two-dimensional holes. It is possible to account for the effects of band warping in a nonperturbative manner. The shift of phase in the Shubnikov-de Haas oscillations is explained.     

Experimental determination of magnetic landau parameters in ErRh4B4

From d.c. magnetization data obtained between 0.45 and 13 K we derive the bare Curie temperature, the temperature dependence of the paramagnetic susceptibility, and the coefficients of the higher order terms in the Landau expansion of the magnetic free energy for ErRh₄B₄. We find the unexpected result that the fourth order term is small or zero. Consequently the sixth order term plays the dominant role in stabilizing the magnetic phase.     

Magnetization anomaly in metamagnetic superconductors

The magnetization in the tetragonal c-plane was measured below T_N using a single crystal of bct ErRh₄B₄. The magnetization decreases with increasing magnetic field in a field range just below the metamagnetic transition. This magnetization anomaly is explained by the fact that the entrance of the vortices is suppressed by an increase of the vortex core energy due to the precursor metamagnetic transition in the core around metamagnetic transition field.     

纳米复合永磁Pr9Fe74Co12B5Snx(x=0, 0.5)的磁化行为与磁黏滞性

用熔体快淬法制备了纳米复合永磁样品Pr₉Fe₇₄Co₁₂B₅ 与Pr₉Fe₇₄Co₁₂B₅Sn_0.5，分析了样品的起始磁化、反磁化过程，测得样品的总磁化率、可逆磁化率以及样品的磁黏滞性.结果表明，两样品在室温下均表现为单一硬磁相磁化行为，在低温下表现为双相行为，且由于添加Sn后使晶粒均匀化从而导致样品低温下的双相行为更加明显.添加Sn后引起样品中软磁相含量和软磁相晶粒尺寸的增加，使磁化反转中可逆磁化部分增多，且使反磁化形核场降低.磁黏滞性研究表明，热激活体积与软磁相晶粒的大小有关. 关键词： 纳米复合永磁 磁化反转 磁粘滞     

Magnetization measurements of magnetic two-dimensional electron gases

We directly measure the magnetization of both the conduction electrons and Mn2+ ions in (Zn,Cd,Mn)Se two-dimensional electron gases (2DEGs) by integrating them into ultrasensitive micromechanical magnetometers. The interplay between spin and orbital energy in these magnetic 2DEGs causes Landau level degeneracies at the Fermi energy. These Landau level crossings result in novel features in the de Haas-van Alphen oscillations, which are quantitatively reproduced by a simple model.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Quasiparticle lifetime of electron–electron interactions for two-dimensional electron gases with magnetic field

We theoretically study the electron–electron scattering rate τ_ee⁻¹for electrons in a two-dimensional electron gas with a perpendicular magnetic field, within theGWand plasmon-pole approximations, as functions of temperatureT, impurity scattering rate Γ and magnetic fieldB. The τ_ee⁻¹increases with increasingTand increasing Γ, and shows the structure of the Landau levels asBis changed.     

Precipitation kinetics of W2B5 in (Ti0.4W0.5Cr0.1)B2 solid solutions

The isothermal precipitation kinetics of W₂B₅ secondary phase from supersaturated polycrystalline (Ti_0.4W_0.5Cr_0.1)B₂ solid solutions were investigated with X-ray diffractometry and scanning electron microscopy in the temperature range between 1500 and 1700°C. The precipitate formation is described by a modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, where W₂B₅ particles nucleate preferentially at grain boundaries and subsequently grow into the volume by a two-dimensional process controlled by volume diffusion of the transition metals. Numerical calculations are used to describe quantitatively the time dependence of the precipitated fraction and to determine a differential JMAK exponent n _diff which gives information on the nucleation and growth modes. n _diff decreases during the precipitation process from 2 to about 0.8 for all temperatures investigated. The first limit corresponds to the classical JMAK model (two-dimensional diffusional growth and constant nucleation rate) and the decrease in n _diff is the consequence of an impingement of the nucleating and growing particles in the late stages of the process. Nucleation and growth rates are determined as functions of reciprocal temperature, where the first quantity shows a non-monotonic behaviour with a maximum at about 1650°C and the second quantity exhibits an Arrhenius behaviour with an activation enthalpy of 3.6?eV. From this it can be concluded that the overall precipitate formation is dominated by the kinetics of atomic motion at low temperatures and by the thermodynamics of nucleation at high temperatures.     

Magnetization curves of randomly oriented ferromagnetic single-domain nanoparticles with combined symmetry of magnetic anisotropy

The magnetization curves of randomly oriented nanoparticles with combined symmetry of magnetic anisotropy were studied. The composite mode of the Stoner–Wolfarth model has been used. In terms of this model each nanoparticle is characterized by random cubic crystalline magnetic anisotropy and by random uniaxial magnetic anisotropy. The series of simulated magnetization curves have been obtained. Each curve corresponds to different contributions of cubic and uniaxial magnetic anisotropy energy to the full energy of an individual nanoparticle k_u. Within this series we discuss the values of remnant magnetization, coercive force, both initial and maximal susceptibilities as the function of k_u. It is found that the magnetic properties are not monotonous functions of k_u. We discuss the possibility of comparing the calculated magnetization curves with the experimental curves in order to obtain new information on the magnetic constant.     

Zener Tunneling between the Landau Levels in a Two-Dimensional Electron System with One-Dimensional Periodic Modulation

Nonlinear magnetotransport in a two-dimensional electron gas in one-dimensional lateral lattices fabricated from a selectively doped GaAs/AlAs heterostructure is investigated. One-dimensional potential modulation is imposed on the two-dimensional electron gas by means of a set of metal strips formed on the planar surface of Hall bars. The dependences of the differential resistance r_xx on the magnetic field B < 0.5 T are studied at a temperature T = 1.6 K in lattices with a period of a ≈ 200nm. It is shown that periodic oscillations in r_xx(1/B) occur in such lattices under the action of a current-induced Hall field due to Zener tunneling between Landau levels. Interference is found between Zener oscillations and commensurability oscillations of r_xx in two-dimensional electron systems with one-dimensional periodic modulation. The experimental results are qualitatively explained by the role of Landau bands in nonlinear transport at large filling factors.