Monte Carlo study of magnetization reversal in the model of a hard/soft magnetic bilayer

Magnetization reversal in the model of a hard/soft magnetic bilayer under the action of an external magnetic field has been investigated by the Monte Carlo method. Calculations have been performed for three systems: (i) the model without a soft-magnetic layer (hard-magnetic layer), (ii) the model with a soft-magnetic layer of thickness 25 atomic layers (predominantly exchange-coupled system), and (iii) with 50 (weak exchange coupling) atomic layers. The effect of a soft-magnetic phase on the magnetization reversal of the magnetic bilayer and on the formation of a 1D spin spring in the magnetic bilayer has been demonstrated. An inf lection that has been detected on the arch of the hysteresis loop only for the system with weak exchange coupling is completely determined by the behavior of the soft layer in the external magnetic field. The critical fields of magnetization reversal decrease with increasing thickness of the soft phase.     

Effect of the phonon and magnetic anharmonicity on the thermal and elastic properties of nearly magnetic δ-plutonium

The temperature dependences of the total heat capacity and the lattice components of the bulk modulus, the volume thermal expansion coefficient, and the mean-square deviation of atoms from the equilibrium positions of nearly magnetic δ-plutonium (using the Pu_0.96Ga_0.04 alloy as an example) have been calculated within the framework of the self-consistent thermodynamic model. The electronic heat capacity has been calculated using the results obtained in terms of the self-consistent spin-fluctuation theory based on the inclusion of the strong magnetic anharmonicity, which leads to a splitting of the electronic spectra by fluctuating exchange fields. On this basis, the effect of phonon anharmonicity not only on the lattice heat capacity but also on other thermal and elastic properties has been considered.     

强磁场中弱相互作用费米气体的热力学性质

基于赝势法和局域密度近似研究了强磁场中弱相互作用费米气体的热力学性质,得出化学势、总能和热容量的解析式,同时分析了磁场及相互作用对系统热力学性质的影响.研究表明,无论是高温情况还是低温情况下,磁场都能调节相互作用的影响.低温下,与无磁场的系统相比,磁场降低系统的化学势、总能和热容量;与无相互作用系统相比,排斥作用增加化学势而降低总能及热容量.高温下,磁场和排斥作用均可降低系统的总能而增加热容量,强磁场可以改变相互作用对总能及热容量的影响. 关键词： 强磁场 弱相互作用 费米气体 热力学性质     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

Magnetization reversal mechanism of magnetic tunnel junctions

Using the ion-beam-sputtering technique,we have fabricated Fe/Al2O3/Fe magnetic tunnelling junctions (MTJs).We have observed double-peaked shapes of curves,which have a level summit and a symmetrical feature,showing the magnetoresistance of the junction as a function of applied field.We have measured the tunnel conductance of MTJs which have insulating layers of different thicknesses.We have studied the dependence of the magnetoresistance of MTJs on tunnel conductance.The microstructures of hard-and soft-magnetic layers and interfaces of ferromagnets and insulators were probed.Analysing the influence of MJT microstructures,including those having clusters or/and granules in magnetic and non-magnetic films,a magnetization reversal mechanism(MRM) is proposed,which suggests that the MRM of tunnelling junctions may be explained by using a group-by-group reversal model of magnetic moments of the mesoscopical particles.We discuss the influence of MTJ microstructures,including those with clusters or/and granules in the ferromagnetic and non-magnetic films,on the MRM.     

Magnetic properties of bilayer nano-stanene-like structure with Ruderman–Kittel–Kasuya–Yoshida coupling

A bilayer nano-stanene-like structure with Ruderman–Kittel–Kasuya–Yoshida (RKKY) coupling described by the Ising model is proposed. The magnetic and thermodynamic properties are studied using the effective-field theory with correlations. The exchange coupling, longitudinal magnetic field, number of non-magnetic layers, and anisotropies had major influences on the magnetization, specific heat, and internal energy. Different saturation magnetizations are observed on the magnetization curve. The variation in the system blocking temperature is studied. The results provide theoretical guidance for the magnetic investigation of nanomaterials with RKKY coupling.     

On the calculation of thermodynamic quantities in the Holstein model for homogeneous polynucleotides

The dynamics of a system for different types of polarons, i.e., in polythymine nucleotides (large-radius polaron), in polyadenine fragments (small-radius polaron), and in polyguanine DNA (intermediate case) at different thermostat temperatures are calculated using the semi-classical Holstein model. The temperature dependences of the thermodynamic equilibrium values of the total energy, the energy of an excess charge, and the electronic heat capacity have been obtained. For all polaron types, the peak of the electronic heat capacity dependence on temperature separates two modes (polaron and delocalized state). The electronic part of the energy is estimated in the high-temperature limit. In all cases, the electron heat capacity at high temperatures decreases in inverse proportion to the square of the temperature.     

Theoretical study of the thermodynamic properties of lithium, sodium, and potassium nitrates

The thermal properties of lithium, sodium, and potassium nitrates have been studied in a gradient approximation of the density functional theory using the method of linear combination of atomic orbitals of the CRYSTAL09 program package. The long-wave frequencies and corresponding mode Grüneisen parameters are calculated. The quasi-harmonic Debye-Einstein model is used to calculate the parameters of the equation of states and also the dependences of the thermodynamic potentials, the entropy, the heat capacity, the thermal expansion coefficient, and the Grüneisen parameter on pressure and temperature. The role of external and intramolecular vibrations in the interpretation of thermodynamic properties is determined. The obtained results agree well with the available experimental data.     

Imaging the magnetic spin structure of exchange-coupled thin films

We have investigated the magnetic spin structure of a soft-magnetic film that is exchange-coupled to a hard-magnetic layer to form an exchange-spring layer system. The depth dependence of the magnetization direction was determined by nuclear resonant scattering of synchrotron radiation from ultrathin 57Fe probe layers. In an external field a magnetic spiral structure forms that can be described within a one-dimensional micromagnetical model. The experimental method allows one to image vertical spin structures in stratified media with unprecedented accuracy.     

核壳结构磁性复合纳米材料的可控合成与性能

具有核/壳结构的磁性复合纳米材料是十分重要的功能材料,其综合物性受材料微结构的影响,而这很大程度上又取决于复合体系的可控合成.本文综述了近二十年来有关核/壳磁性复合纳米材料的制备、表征及性能研究方面的进展,讨论的体系主要有：铁氧体基永磁/软磁（反铁磁）复合纳米材料、非磁性体包覆磁性核而成的复合纳米材料、用磁性颗粒催化合成的碳基复合纳米材料、基于交换偏置效应而设计的复合纳米材料、核-壳同轴结构的一维复合纳米材料和核/壳/壳三元结构的磁性复合纳米材料等.构建复合体系的组分包括M型永磁铁氧体、3d过渡金属（及其合金、氧化物、碳化物）、多铁化合物、非磁性体（比如绝缘体、半导体、有机分子）和碳材料等,着重分析了复合纳米材料的热稳定性、光致发光性能、光电催化能力、电化学特性、微波吸收性能、磁电阻效应、永磁体性能、高频软磁特性、交换偏置效应及其相关现象.最后,对核/壳结构磁性复合纳米材料的未来发展趋势进行了展望,并在基础研究和改性应用方面提出了一些建议.     

Effect of magnetoelastic interaction on the thermodynamics of ferromagnets: Simulation by the method of spin-lattice dynamics

Thermodynamic properties of systems with coupled magnetic and lattice degrees of freedom are analyzed by the numerical spin-lattice dynamics (SLD) method. A scheme of numerical integration is developed for SLD equations in a thermostat, that follows the earlier formulated approaches and is modified to describe systems with realistic interatomic interactions. The method proposed allows one to calculate the spectral density of oscillations, heat capacity, magnetization, and thermal expansion coefficient within a single scheme. It is established that, due to short-range magnetic order, the interplay between magnetic and lattice degrees of freedom contributes to the thermodynamic properties of the system even in the paramagnetic state. It is shown that there exist two mechanisms how the spin-lattice interaction influences the thermodynamic properties: static and dynamic mechanisms; the first is determined by its contribution to the thermal expansion of the lattice, and the second, by the dynamic interaction between magnetic moments and crystal lattice vibrations.     

反常磁矩对弱磁场弱相互作用费米气体热力学性质的影响

考虑费米子的反常磁矩, 运用赝势法和热力学理论, 导出弱磁场中弱相互作用费米气体自由能的解析式, 以此为基础给出高温和低温情况下系统热力学性质, 分析反常磁矩对热力学性质的影响机理. 研究表明: 反常磁矩对热力学性质的影响与温度相关, 而且这种影响随温度的上升在低温区是增大的, 在高温区是减小的; 对于系统的化学势、内能, 反常磁矩加强了磁场的影响, 弱化了相互作用的影响; 对于系统的热容量, 反常磁矩在低温区使其减小, 在高温区使其增加.     

磁场中非广延相对论费米系统的统计性质

基于广义外势中的非广延统计理论,运用理论解析与数值模拟方法,研究磁场中非广延极端相对论费米气体的热力学性质,给出总能、热容量、化学势的解析式,分析非广延参数、极端相对论效应、磁场及温度对系统热力学性质的影响机理.研究显示,非广延参数不仅对热力学性质有直接的影响,而且也影响着磁场的物理效应. 随温度的升高,非广延参数及磁场对热力学性质的影响均被放大.极端相对论效应对化学势及热容量有特别显著的影响.     

弱磁场中弱相互作用费米气体的热力学性质

根据赝势法和系综理论导出弱磁场中弱相互作用费米气体的内能、化学势和热容量的小参数r的解析式.在此基础上给出高温和低温两种情况下弱磁场中弱相互作用费米气体的热力学性质,探讨磁场及粒子间相互作用对热力学性质的影响,分析磁场与三维谐振势两种约束对系统性质影响的不同及其原因. 关键词： 赝势法 费米气体 相互作用 热力学性质     

Electronic structure and thermodynamic properties of LiBC under high pressure

This paper investigates the electronic structure and thermodynamic properties of LiBC in the hexagonal structure by using the generalized gradient approximation （GGA） and local density approximation correction scheme in the frame of density functional theory. The geometric structure of LiBC under zero pressure, and the dependences of the normalized lattice parameters a/ao and c/co, the ratio e/a, the normalized primitive volume V/Vo on pressure are given. The thermodynamic quantity （including the heat capacity Cv, Debye temperature 6~D, thermal expansion a and Grfineisen parameter -y） dependences on temperature and pressure are obtained through the GGA method and the quasi-harmonic Debye model. The band structures and density of state of LiBC under different pressures have also been analysed.     

Thermodynamics of spin ice in staggered and direct (along the [111] axis) fields in the cluster approximation

We have analyzed the low-temperature thermodynamic properties of spin ice in the staggered and direct (acting along the [111] axis) fields for rare-earth oxides with the chalcolamprite structure and general formula Re₂ ³⁺Me₂ ⁴⁺O₇ ^2-. Calculations have been performed in the cluster approximation. The results have been compared with experimental temperature dependences of heat capacity and entropy for Dy₂Ti₂O₇ compound for different values of the external field in the [111] direction. The experimental data and calculated results have also been compared for the Pr₂Ru₂O₇ compound with the antiferromagnetic ordering of magnetic moments of ruthenium ions, which gives rise to the staggered field acting on the system of rare-earth ions. The calculated temperature dependences of heat capacity and entropy are in good agreement with experimental data.     

Heat capacity and entropy of two electrons quantum dot in a magnetic field with parabolic interaction

The thermodynamic properties of two electrons in two dimensional parabolic GaAs quantum dot are studied where both the magnetic field and the e–e interaction are fully considered. The e–e interaction has been treated by a model potential which makes the Hamiltonian exactly solvable. The energy spectrum is used to calculate the canonical partition function, and then we obtain the thermodynamic properties; mean energy, heat capacity and entropy as a function of temperature (T) and magnetic field (B).A steep transition from zero to 4k_B is observed in the heat capacity as a function of temperature for small values of magnetic field and saturates within a small temperature range, also the heat capacity has a peak-like structure at low temperature, while for high magnetic field heat capacity develops a shoulder at 2k_B then it approaches the saturation value with further increase in temperature. The entropy increases with increasing temperature, but at higher temperature, it remains almost independent of the magnetic field. It is shown that, at low magnetic field values, the effect of magnetic field on heat capacity is tangible and it attains a constant value with further increase in magnetic field. Entropy is almost linearly proportional with increasing magnetic field strength.     

Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.     

重力场和强磁场中费米气体的热力学性质

基于半经典近似,研究重力场和强磁场共存下费米气体的热力学性质,通过理论解析和数值模拟分析强磁场背景下重力场对系统热力学性质的影响.研究表明:与单纯强磁场相比,重力场的引入使能量及化学势都降低.随温度的上升,重力场对化学势的影响逐渐放大;对热容的影响有极大值.重力场使系统的热容随磁场的振荡几乎不变、使化学势的振荡中心下移.     

Magnetisation and remagnetisation peculiarities of Fe/Cr multilayer films caused by instability of Cr layers magnetic structure

A model to describe the magnetic properties of Fe/Cr/Fe films with nanometer-thick layers of Fe and Cr is proposed. The model rests on the assumption that there exists a magnetic order of the linearly polarized spin-density wave type in a chromium interlayer with thicknesses even less than its wavelength. This only assumption proved to be enough to describe the following characteristics, experimentally observed, of the magnetic properties of the multilayer Fe/Cr films: the existence of short and long periods, dependences of their magnetic properties on the chromium layer thickness, noncollinear orientation of the magnetizations of neighboring iron layers, the existence of the magnetization curves beyond the hysteresis loops of magnetization.