一维亚铁磁混合自旋链的磁特性的Monte Carlo模拟

基于一维自旋链模型,采用Monte Carlo方法对具有反铁磁相互作用的自旋一I和自旋-3／2交替混合的亚铁磁系统进行模拟计算,研究单离子各向异性对系统磁特性的影响．模拟结果发现：基态时系统的磁化强度在外加磁场作用下呈现出阶梯状,并获得了自旋-1和自旋-3／2交替混合的亚铁磁系统相图．最后,通过对系统能量和自旋组态的研究,解释了该系统中出现的阶梯效应和相图．     

金属离子掺杂对CuO基纳米复合材料的交换偏置调控

为了研究反铁磁基体中掺杂的金属离子对交换偏置效应的影响, 本文采用非均相沉淀法制备了纳米复合材料. X射线衍射图(XRD)和场发射扫描电子显微镜(SEM) 照片清晰表明CuO纳米复合样品具有统一的颗粒尺寸, 约为80 nm. 通过体系中掺杂磁性金属离子Ni和Fe, 实现了亚铁磁MFe₂O₄ (M=Cu, Ni)晶粒镶嵌在反铁磁(AFM) CuO 基体中. 在CuO基体中加入少量的Ni能改变两相交界面的磁无序从而生成类自旋玻璃相, 相应提高对铁磁相磁矩的钉扎作用. 同时, 场冷过程中反铁磁相内形成磁畴, 冻结在原始状态或磁场方向上, 畴壁也起到钉扎铁磁自旋的作用, 进而提高交换偏置效应. 随后加入的Ni 会生成各向异性能较大的NiO, 也能够提高交换偏置场. 在带场冷却下, 所有样品均发生垂直交换偏置, 也证明了样品在场冷过程中形成了自旋玻璃相, 正是由于亚铁磁与自旋玻璃相界面上的磁交换耦合, 才导致回线在整个测量范围内发生了向上偏移. 零场冷却和场冷却(ZFC/FC)情况下磁化强度与温度变化曲线(M-T)说明在这些复合材料中的交换偏置效应是由于存在亚铁磁颗粒和类自旋玻璃相界面处的交换耦合作用. 研究发现随着持续掺杂Ni离子, 交换偏置场先缓慢增加后又急剧增加, 生成各向异性能高的反铁磁相NiO 和反铁磁相内的畴态组织是这一结果的原因.     

含三体相互作用的海森堡自旋链纠缠研究

研究了含三体相互作用的海森堡自旋链的能级和相应本征态的纠缠情况,详细分析了含有三个粒子的海森堡自旋链的纠缠,发现所有的纠缠态都为W态。给出三粒子海森堡自旋链在温度为零时的基态纠缠,发现反铁磁自旋链的基态不随各向异性、耦合强度等参数发生变化,铁磁自旋链的基态纠缠随各向异性因子和三自旋耦合项而改变。讨论了三比特自旋链在低温时的热纠缠,给出纠缠度表达式及其随温度、各向异性因子和耦合强度的变化关系。     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

C掺杂FePt铁磁薄膜光诱导超快退磁动力学研究

FePt合金薄膜由于具有较强的磁各向异性而在磁信息和磁光信息存储中具有重要的应用.C掺杂可精确调控薄膜的磁各向异性,从而可有效地改变薄膜的矫顽场.通过超短激光脉冲与铁磁薄膜相互作用,可以获得非平衡状态下电子、自旋和晶格等自由度之间的动态耦合参数,这是研究超快磁记录材料的物理基础.本文基于瞬态磁光Kerr效应,研究了两种C掺杂浓度下FePt薄膜的超快磁光响应.实验结果表明:瞬态Kerr信号与外加磁场正相关,磁场反向,Kerr信号反号,而瞬态反射率与外加磁场无关;不同C掺杂的FePt薄膜的矫顽场不同,软磁的退磁时间显著小于硬磁薄膜的退磁时间.我们还观测到超快激光在铁磁薄膜中诱导频率约为49 GHz的相干声学声子,该声子的频率与外加磁场无关.实验结果为设计和研制新型磁光薄膜提供了实验依据.     

交换偏置系统中的反铁磁磁化与自旋波

采用自由能变分法研究了铁磁/反铁磁双层膜系统在反铁磁层存在净磁化下的自旋波谱.本模型中铁磁薄层具有单轴磁晶各向异性和立方磁晶各向异性,反铁磁层仅具有单轴磁晶各向异性,但厚度有限,推导出了系统铁磁共振频率的表达式.结果表明：系统的自旋波谱分光学模和声学模两种,其中光学模仅在反铁磁层存在净磁化时得到激发.自旋波谱可按外磁场强度的变化情况分为强弱两支;区分强磁场和弱磁场的临界场依赖于铁磁/反铁磁间的交换作用,反铁磁层的磁化强度以及反铁磁层的厚度等.交换偏置场对光学模的影响明显于声学模,而反铁磁的净磁化和其厚度对系统的影响紧密联系,难以区分.但当反铁磁层净磁化很小可忽略时,系统只存在声学模激发. 关键词： 铁磁/反铁磁双层膜 反铁磁层净磁化 光学模 声学模     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

磁场诱导制备Bi-Mn合金的结构和低温磁性研究

系统研究了在外加诱导磁场下制备的Bi-Mn 6%合金的结构和低温磁特性.结果表明,在外加诱导磁场下制备的Bi-Mn 6%合金呈现典型的双相结构和各向异性特征,MnBi相c轴沿外加诱导磁场方向定向排列.发现MnBi相的矫顽力随温度的升高而增大,而饱和磁化强度则随温度的升高而减小.MnBi相的自旋重定向温度TSR随测量磁场的增大逐渐向低温区移动,在高的外加测量磁场下这种自旋重定向特征消失,发现了TSR随外加诱导制备磁场的增大而逐渐向高温区移动.对磁场诱导制备织构化MnBi相和该类材料磁各向异性能的物理机制进行了分析和讨论.     

自旋轨道耦合和磁场对准一维铁磁/半导体/铁磁系统中电子输运性质的影响

利用传递矩阵方法，我们计算了自旋轨道耦合和磁场对准一维铁磁/半导体/铁磁系统中电子输运性质的影响。计算结果发现，透射系数的振幅随磁场增加而增大。在反铁磁排列时，即使在磁场作用下，上、下自旋电子具有相同的透射系数。与不加磁场时的情况相反，在一定的磁场和耦合强度时，铁磁排列中，上自旋电子的透射系数大于下自旋电子的，而且出现了自旋反转。     

Thermodynamic Properties of Random Transverse Field Mixed Spin System in the Presence of Single-Ion Anisotropy

We study the thermodynamic properties of random transverse field mixed spin system in the presence of single-ion anisotropy on a square lattice. By making use of the effective field theory and a cutting approximation, the detailed phase diagrams are described and some interesting results are found under trimodal random transverse field distribution. A small single-ion anisotropy can magnify magnetic ordering region at low temperatures and existence of a large transverse field can assist the occurrence of reentrant phenomena. With increasing disorder, second-order phase transitions are shown to change into first-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. These indicate a strong correlation with the corresponding to trimodal transverse field distribution.     

Thermodynamic Properties of Random Transverse Field Mixed Spin System in the Presence of Single-Ion Anisotropy

We study the thermodynamic properties of random transverse field mixed spin system in the presence ofsingle-ion anisotropy on a square lattice. By making use of the effective field theory and a cutting approximation, thedetailed phase diagrams are described and some interesting results are found under trimodal random transverse fielddistribution. A smallsingle-ion anisotropy can magnify magnetic ordering region at low temperatures and existence ofa large transverse field can assist the occurrence of reentrant phenomena. With increasing disorder, second-order phasetransitions are shown to change into first-order phase transitions. The trajectory of the tricritical point in the phasespace as a function of disorder is presented. These indicate a strong correlation with the corresponding to trimodaltransverse field distribution.     

Scaling approach to anisotropic magnetic systems statics

Scaling laws are stated for anisotropic magnetic systems, where the anisotropy parameters are either scaled or held fixed. Combining the two ways of scaling, the critical behavior of thermodynamic quantities in anisotropic systems is determined. Particular attention is drawn to the temperature range where the anisotropy becomes important, and to the dependence there of the different quantities on the anisotropy parameters. In a transverse magnetic field the phase transition of an anisotropic magnet takes place along aλ-line. Assuming the singular part of the free enthalpy to depend on the distance from theλ-line, anomalous corrections to the transverse susceptibility and magnetization are calculated. For an experimental verification of many of the results, experiments including a variation of the anisotropy parameters or a finite transverse field are necessary.     

Thermodynamic Properties of Superfluid 3 He in a Nematic Aerogel in a Strong Magnetic Field

The thermodynamic properties of the phases appearing in superfluid 3He in a nematic aerogel in a strong magnetic field are considered. The jumps in specific heat and magnetic susceptibility during the phase transitions have been found. The influence of the dipole energy on the previously obtained phase diagram has been estimated. The transverse and longitudinal NMR frequency shifts have been found for the phases under consideration and the superfluid density tensors have been derived in the Ginzburg–Landau limit.     

Field dependent specific heat and longitudinal magnetization of the quantum magnet layer Cs2CuCl4: Anisotropy effects

We have addressed the specific heat and magnetization of an anisotropic spin-1/2 triangular Heisenberg antiferromagnet Cs₂CuCl₄ in the presence of magnetic field at finite temperature. We have investigated the behavior of thermodynamic properties by means of excitation spectrum in terms of a hard core bosonic representation. The effect of in-plane anisotropy on thermodynamic properties has also been studied via the bosonic model by Green’s function approach. This anisotropy is considered for exchange constants that couple spin components perpendicular to magnetic field direction. We have found the temperature dependence of the specific heat and longitudinal magnetization in the gapped field induced spin-polarized phase for various magnetic fields and anisotropy parameters. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. Our results show temperature dependence of specific heat includes a peak so that its temperature position goes to higher temperature with increase of magnetic field. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum.     

Critical Properties of Mixed Ising Spin System with Different Trimodal Transverse Fields in the Presence of Single-Ion Anisotropy

Within the framework of an effective field approximation, the effects of single-ion anisotropy and different trimodal transverse fields of two sublattices on the critical properties of the mixed spin-1/2 and spin-1 Ising system are investigated on the simple cubic lattice. A smaller single-ion anisotropy can magnify magnetic ordering phases and a larger one can depress magnetic ordering phase for T-Ω_1/2 space at low temperatures, while a smaller single-ion anisotropy can hardly change the value of critical transverse field for T-Ω₁ space. On the other hand, influences of two different trimodal transverse fields concentrations on tricritical points and magnetic ordering phases take on some interesting results in T-D space. The main reason comes from the common action of single-ion anisotropy, different transverse fields and two trimodal distributions.     

In-plane anisotropy of upper critical field in Sr2RuO4

We investigated the behavior of the spin-triplet superconductor Sr2RuO4 ( T(c) approximately 1.5 K) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field H(c2) exhibits a clear fourfold anisotropy of about 3% at 0.35 K. Furthermore, we detected an additional transition feature below H(c2) in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane H(c2) anisotropy disappear above 0.8 K or under intentional field misalignment of less than 1 degrees. Most of these characteristics are consistent with the predicted emergence of the second superconducting phase with a line-node gap.