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     

磁场及体各向异性对面心立方双层铁磁薄膜能带的影响

利用二次量子化方法研究了由相同面心立方结构(100)材料构成的铁磁性双层薄膜,重点讨论了磁场及体各向异性对面心立方双层铁磁薄膜能带的影响.结果表明,无论界面为铁磁性界面交换作用或反铁磁性界面交换作用,磁场及各向异性场对对称薄膜中的自旋波存在方式及整个能带形状没有影响,只是影响整个激发模的能量.磁场和各向异性常数越大,自旋波能量越高.     

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     

涡流对侧向反铁磁/非磁超晶格表面推迟模的影响

使用等效介质理论研究了在Voigt位形下,由反铁磁半导体和绝缘非磁物质构成的半无限侧向反铁磁/非磁超晶格中的表面推迟模.讨论了在外磁场为零和非零两种情况下涡流对推迟模的影响,给出了同时考虑传导电流和位移电流时,侧向反铁磁/非磁超晶格的频率与波矢的关系曲线以及自旋波衰减随波矢k的变化曲线. 关键词： 推迟模 侧向超晶格 自旋波衰减     

混合自旋XY系统热纠缠的研究

本文研究了两格点、三格点以及四格点混合自旋（1／2,1）XY系统的热纠缠,通过数值计算,探讨了纠缠随温度和外磁场的变化关系．发现在外磁场不存在或比较弱的情况下,纠缠随着温度的升高单调减小．对于两格点和四格点系统,不论是铁磁还是反铁磁情况,纠缠都在同一临界温度下消失,并且这一临界值不受外磁场变化的影响．对于三格点系统,临界温度也不受外磁场的影响,但是铁磁情况下系统的临界温度高于反铁磁情况．在温度极低的环境中,纠缠在一定的磁场范围内形成稳定的平台,但是当磁场强度超过某一临界值时,纠缠完全消失．本文还对混合自旋和单一自旋系统的热纠缠进行了对比分析,发现在混合自旋系统中存在多层次的能级交错现象．     

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。     

磁场诱导的单离子各向异性反铁磁链的比热

利用改进的Schwinger玻色子平均场理论,研究了横向外加磁场下自旋为1的单粒子各向异性海森堡反铁磁链的热力学性质.发现在外加磁场下预示这一体系中存在Luttinger液体相—完全极化相相变的比热奇性,并给出了相应的相图.研究还发现,单粒子各向异性对比热的影响较小,这和纵向场的结果完全不同.以上结果对整自旋的反铁磁链具有普适性,对实验研究具有指导性. 关键词： 反铁磁链 热力学性质 Schwinger 玻色子     

用外磁场控制电流感应磁化翻转效应中的临界电流方法

应用基于磁动力学方程的宏观唯象模型,研究了弱外磁场下纳米尺度赝自旋阀结构的电流感应磁化翻转效应.在统一考虑铁磁/非磁界面的自旋相关散射以及铁磁层中的自旋积累和弛豫过程后,给出了赝自旋阀结构在弱外磁场下的磁化翻转条件和临界电流.对该效应的数值计算解释了弱外磁场下赝自旋阀结构的电阻-电流回线的偏移,并给出了用外磁场控制电流感应磁化翻转效应中的临界电流方法. 关键词： 电流感应磁化翻转 外磁场 临界电流 赝自旋阀     

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。     

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。     

二维带电自旋-1/2费米气体的磁性质

通过引入描述电荷-磁场和自旋-磁场相互作用竞争关系的自旋因子,研究了磁场和简谐势阱双重约束的二维带电自旋-1/2费米气体的磁性质.结果表明,当自旋因子很小时,系统显示出抗磁性,随着自旋因子的进一步增大,系统逐渐转变为顺磁性.自旋因子的临界值将磁化强度划分为抗磁性区和顺磁性区,临界值随磁场和温度的增大仅发生微小的改变.     

Spin waves in nickel nanorings of large aspect ratio

The spin dynamics of high-aspect-ratio nickel nanorings in a longitudinal magnetic field have been investigated by Brillouin spectroscopy and the results are compared with a macroscopic theory and three-dimensional micromagnetic simulations. Good agreement is found between the measured and calculated magnetic field dependence of the spin wave frequency. Simulations show that as the field decreases from saturation, the rings switch from a "bamboo" to a novel "twisted bamboo" state at a certain critical field, and predict a corresponding dip in the dependence of the spin wave frequency on the magnetic field.     

不同方向外磁场下海森堡自旋链的热纠缠

研究了两量子比特的海森堡XXX自旋链分别处于x方向和y方向均匀外磁场时系统的纠缠特性,并用负度N来度量。得到纠缠度N的解析表达式,并在此基础上进行数值计算。仔细讨论了磁场B、温度T和自旋耦合系数J对纠缠度N的影响。结果表明：纠缠度N会随着磁场|B|和温度T的增大而减小,但会随着自旋耦合系数J的增大而增大。另外,增大的J还会使临界磁场|Bc|和临界温度Tth变大。所以,我们可以通过调节B、T和J来控制热纠缠,这对固态系统中通过构建和选择参数调整系统的纠缠度具有一定的作用和意义。研究还发现,加在x方向均匀外磁场和加在y方向均匀外磁场对两量子比特的海森堡XXX自旋链的作用效果是一样的。     

Many-body effects in spin-polarized two-dimensional electron gas

Many-body effects on the spin polarization are studied in an n channel inversion layer on Si (1 0 0) surface in a magnetic field parallel to the surface in random phase approximation. The spin polarization exhibits a discrete jump to a full polarization at the critical magnetic field in the low-density regime and the critical field is reduced considerably from that estimated by an extrapolation based on the zero-field susceptibility.     

Thermodynamics of a Bose-Einstein condensate with free magnetization

We study thermodynamic properties of a gas of spin 3(52)Cr atoms across Bose-Einstein condensation. Magnetization is free, due to dipole-dipole interactions. We show that the critical temperature for condensation is lowered at extremely low magnetic fields, when the spin degree of freedom is thermally activated. The depolarized gas condenses in only one spin component, unless the magnetic field is set below a critical value, below which a nonferromagnetic phase is favored. Finally, we present a spin thermometry efficient even below the degeneracy temperature.     

Entanglement creation and storage in two qubits coupling to an anisotropic Heisenberg spin chain

The time evolution of the entanglement of a pair of two spin qubits is investigated when the two qubits simultaneously couple to an environment of an anisotropic Heisenberg XXZ spin chain. The entanglement of the two spin qubits can be created and is a periodic function of the time if the magnetic field is greater than a critical value. If the two spin qubits are in the Bell state, the entanglement can be stored with relatively large value even when the magnetic field is large.     

Scaling at the critical temperature of a spin glass

It is argued that the critical exponent δ for an Edwards-Anderson spin glass at its critical temperature can be extracted from the magnetic susceptibility in a weak uniform magnetic field.     

Magnetic properties of 2D nano-islands II: Ising spin model with out-of-plane magnetic field

An Ising effective field theory model is presented to calculate the magnetic properties of 2D nano-islands on a nonmagnetic substrate, subject to an externally out-of-plane applied magnetic field. The system Hamiltonian contains nearest neighbor exchange interactions, single-atom magnetic anisotropies, and the Zeeman term. The calculations yield, in particular, the single site spin correlations, the magnetizations, and the isothermal susceptibilities, for the core and periphery domains of the nano-island. The choice of a spin S=1 for the atoms of the system permits the analysis of local spin fluctuations via the single site spin correlations. We investigate in this respect the effects due to the different magnetocrystalline anisotropies and reduced dimensionalities, for the core and periphery domains, and in particular the critical influence of the applied magnetic field. Detailed theoretical results are presented for the square and hexagonal lattice symmetries, with numerical applications for the 2D monolayer Co nano-islands on a Pt substrate. It is shown that the remarkable differences between the magnetic properties of the core and periphery domains in zero field are washed out when an out-of-plane field is applied. The applied field also provokes critical discontinuities for the spin correlations and magnetization reversals, for the core and periphery domains, which are especially evident for the hexagonal lattice nano-island in the range of fields of interest. The discontinuities and magnetization reversals occur over elementary temperature widths, and shift to lower temperatures with increasing field. The field-dependant isothermal susceptibilities show new features very different from those for the susceptibilities in zero field. The present Ising model does not show any blocking temperature transition to superparamagnetism.     

Dynamics of spinor condensates and stochastic approach

The dynamics of the collective spin for Bose-Einstein condensates with nonlinear interactions, is studied within the framework of the two-component spinor. We discuss the spin resonance when the system is submitted to a periodically-modulated magnetic field at the zero temperature. In this case, the nonlinearity parameter controls the critical change between a localized and a homogeneous spin state. When the temperature is finite – or a random magnetic field is considered – the movement of the collective spin is governed by the Landau-Lifshitz-Gilbert equation, from which the complete Fokker-Planck equation is derived. This equation is the essential tool to describe the time-evolution of the probability distribution function for the collective spin. The functional integral approach is used to solve analytically examples of BEC spin behavior in a static magnetic field at finite temperature. We show how such a method can lead effectively to the complete solution of the Fokker-Planck equation for this kind of problems.     

Nonlinear 2D spin susceptibility in a finite magnetic field: spin-polarization dependence

By theoretically calculating the interacting spin susceptibility of a two-dimensional electron system in the presence of finite spin polarization, we show that the extensively employed technique of measuring the 2D spin susceptibility by linear extrapolation to a zero field from the finite-field experimental data is theoretically unjustified due to the strong nonlinear magnetic field dependence of the interacting susceptibility. Our work compellingly establishes that much of the prevailing interpretation of the 2D susceptibility measurements is incorrect, and, in general, the 2D interacting susceptibility cannot be extracted from the critical magnetic field for full spin polarization, as is routinely done experimentally.