分子磁体

分子基材料是化学和材料科学中的重要分支．分子磁体有着与传统块材磁体不同的特性和用途．文章简要介绍了分子磁体的基本原理、主要研究领域和当前的进展．     

分子磁体与环境的纠缠和退相干

本文研究了双轴分子磁体在耗散环境中的相干量子隧穿,作为环境的声子库抑制了相干量子隧穿,从而引起分子磁体中薛定谔猫态的退相干. 而环境内部声子之间的相互作用会导致分子磁体与热库之间退耦合,于是对退相干有一定的抑制作用. 在绝热近似和非绝热近似下,借助于约化密度矩阵计算了超Ohmo耗散中分子磁体与环境之间的纠缠度,当纠缠达到最大时,相干隧穿被完全抑制.     

单分子磁体中交换常数计算的新方法

采用基于密度泛函理论(DFT)的第一性原理计算方法,在现有的单分子磁体交换耦合常数计算方法的基础上提出了一种新的计算方法.采用新方法计算两个原子之间的交换耦合常数时,首先分别设置这两个原子的自旋取向,然后再同时设置两个原子的自旋取向,计算出总能即可,而不用选取不同的自旋组态并求解线性方程组.方法特别适合含有数目很大的磁性原子的单分子磁体交换耦合常数的计算.采用新方法计算了Fe7和Fe20配合物分子的交换耦合常数.结果表明,Fe7和Fe20分子中的交换常数都为负,即反铁磁耦合时能量更低,这与类似分子的相关研究相一致.     

单分子磁体中交换常数计算的新方法

采用基于密度泛函理论(DFT)的第一性原理计算方法,在现有的单分子磁体交换耦合常数计算方法的基础上提出了一种新的计算方法.采用新方法计算两个原子之间的交换耦合常数时,首先分别设置这两个原子的自旋取向,然后再同时设置两个原子的自旋取向,计算出总能即可,而不用选取不同的自旋组态并求解线性方程组.方法特别适合含有数目很大的磁性原子的单分子磁体交换耦合常数的计算.采用新方法计算了Fe7和Fe20配合物分子的交换耦合常数.结果表明,Fe7和Fe20分子中的交换常数都为负,即反铁磁耦合时能量更低,这与类似分子的相关研究相一致.     

超导磁体失超过程中Quench-Back效应的数值模拟研究北大核心CSCD

Quench-Back是一种诱发超导磁体内产生新的失超区从而加快失超传播的现象.合理利用Quench-Back效应进行失超保护,需要准确理解Quench-Back作用下磁体内部的传热与电磁过程.以实际工程中某超导螺线管磁体为例,建立了失超过程三维瞬态耦合热-电磁-电路模型.整个模型采用两个顺序耦合的子模型实现,子模型一为三维传热模型,子模型二为轴对称电磁-电路直接耦合模型.得到了失超过程电流,热点温度和内电压变化曲线,分析了Quench-Back效应对失超过程关键参数的影响,研究了Quench-Back加速失超传播的具体过程.研究结果表明:Quench-Back效应可以有效控制失超过程中磁体内的热点温度,降低磁体内的过电压;Quench-Back效应与骨架材料,线圈结构和失超起始位置关系密切.     

有机强磁性研究

在对有机强磁性体的概仿、源及其所特有的物理和作简要介绍的基础上，阐述了有机材料的强磁性研究的重要意义以及有机强磁体作为新材料可能具有的应用前景，评述了国内外目前有机强磁性的理论和实验研究现状，讨论了有机强磁体的获取途径和分子设计方法，关着重讨论了纯有机强磁性体的模型设计和结构分析等方面的工作。     

基于交换耦合模型纳米双相(硬磁/软磁)自旋体系的磁性

采用MonteCarlo方法对由异类自旋组成混合Heisenberg自旋体系进行了数值计算,以模拟和预测基于交换耦合模型纳米双相(硬磁软磁)磁性体系的磁性.区别于以往所报道的那些直接针对硬磁NdFeB与软磁纳米αFe复合磁体的微磁学计算工作,着眼于符合Heisenberg模型的两类完全不同的原子自旋集团(硬磁自旋和软磁自旋)之间的直接交换耦合作用,模拟计算了由这两类自旋组成的复合自旋体系的内禀矫顽力Hc、剩余磁化强度Mr、最大磁能级(M×H)max等宏观磁性参量随软磁自旋集团的尺度和体积百分比、两类自旋集 关键词： Heisenberg模型 MonteCarlo模拟 纳米复合磁体 Nd-Fe-B     

单分子磁体

单分子磁体指的是那些在阻塞温度以下出现磁化强度慢弛豫的分子。自从1993 年报道了 第一例单分子磁体[Mn12O12(O2CCH3)16(H2O)4] ￠ 4H2O ￠ 2CH3CO2H (Mn12) 以来,人们在探索 新颖的单分子磁体及其在信息存储、自旋电子学和量子计算等方面的潜在应用方面付出了极大的努 力,已经合成得到许多具有单分子磁体性质的单核和多核金属簇合物。本文将重点介绍基于氧桥联 的锰或铁簇合物、单核过渡金属化合物以及稀土配合物的单分子磁体。     

磁爆加载下强磁体的磁场研究

 为了研究磁爆压缩发生器加载下强磁体形成的磁场,对磁爆加载过程进行分析,建立了磁爆加载下强磁体形成磁场的理论模型。按照此模型,对6种不同结构强磁体的磁场进行对比研究,得到了强磁体的磁场变化规律。结果表明：在加载初始阶段,磁爆压缩发生器的自身参数为主要影响因素,各磁体的磁场峰值和范围差别较小;在磁通压缩阶段,电路过程的改变使得磁体结构的影响逐渐显著,各磁体的磁场峰值和范围发生了明显变化;磁体结构对磁场的空间分布具有决定性作用,磁场分布不受加载过程的影响。     

分子磁体中的量子隧穿及宏观量子效应

文章介绍了分子磁体中的量子隧穿和宏观量子效应理论和实验研究的新进展．分子磁体既有宏观磁体特性也呈现纯量子行为，例如磁化矢量的量子隧穿．文章作者解释了如何通过量子隧穿实现宏观量子相干(即薛定谔猫态的相干叠加)和量子态位相干涉．对隧穿率计算的瞬子方法，特别是有限温度隧穿理论及其在分子磁体量子隧穿中的应用也做了简要的阐述．     

Quenching of exciton diamagnetic shifts in polar,layered materials

A theory for Wannier excitons in a magnetic field or arbitrary strength, including both anisotropy and exciton-phonon coupling, is presented. Both the anisotropy and the exciton-phonon coupling are shown to considerably quench the shifts of the energy levels in the magnetic field. Results are presented for PbI₂ and compared with recent experimental data.     

Electric field control of magnetism in multiferroic heterostructures

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.     

Inelastic light scattering in magnetic dots and wires

An overview of the current status of the study of spin-wave excitations in arrays of magnetic dots and wires is given. We describe both the status of theory and recent inelastic light scattering experiments addressing the three most important issues: the modification of magnetic properties by patterning due to shape anisotropies, anisotropic coupling between magnetic islands, and the quantization of spin waves due to the in-plane confinement of spin waves in islands.     

梯度磁场中自旋-轨道耦合旋转两分量玻色-爱因斯坦凝聚体的基态研究

利用准二维Gross-Pitaevskii方程,研究了在梯度磁场中具有自旋-轨道耦合的旋转两分量玻色-爱因斯坦凝聚体的基态结构.探索了自旋-轨道耦合作用和梯度磁场对基态的影响.结果发现,在梯度磁场下,随着自旋-轨道耦合强度增大,基态结构由skyrmion格子逐渐过渡为skyrmion列.对于弱自旋-轨道耦合和小旋转频率情况,增大磁场梯度强度可导致基态由平面波相转变为half-skyrmion;对于强自旋-轨道耦合和大旋转频率情况,梯度磁场可诱导hidden涡旋的产生.梯度磁场、自旋-轨道耦合和旋转作为体系的调控参数,可用于控制不同基态相间的转化.     

Monte Carlo study of half-magnetization plateau and magnetic phase diagram in pyrochlore antiferromagnetic Heisenberg model

The antiferromagnetic Heisenberg model on a pyrochlore lattice under external magnetic field is studied by classical Monte Carlo simulation. The model includes bilinear and biquadratic interactions; the latter effectively describes the coupling to lattice distortions. The magnetization process shows a half-magnetization plateau at low temperatures, accompanied with strong suppression of the magnetic susceptibility. Temperature dependence of the plateau behavior is clarified. Finite-temperature phase diagram under the magnetic field is determined. The results are compared with recent experimental results in chromium spinel oxides.     

Ferroelectricity generated by spin-orbit and spin-lattice couplings in multiferroic DyMnO3

While the ferroelectricity in type-II multiferroic rare-earth manganites is believed to be generated by the inverse Dzyaloshinskii-Moriya (DM) interaction (spin-orbit coupling) associated with the Mn spiral spin order, recent results revealed the strong spin-lattice coupling arising from the Dy-Mn spin interaction in DyMnO₃, which may also be an ingredient contributing to the ferroelectricity. In this work, we summarize our recent experiments on this issue by performing a series of rare-earth site nonmagnetic Y and magnetic Ho substitutions at Dy site for DyMnO₃. It is demonstrated that the Dy-Mn spin interaction contributes to the ferroelectric polarization through the symmetric exchange striction mechanism (spin-lattice coupling). A coexistence of the spin-orbit coupling and spin-lattice coupling in one compound is confirmed. At the same time, the independent Dy antiferromagnetic spin order at low temperature can be effectively suppressed by the substitutions, beneficial to the polarization enhancement.     

Dynamics of magnetic skyrmions

We review the recent progress on the magnetic skyrmions in chiral magnetic materials.The magnetic skyrmion is a topological spin configuration with localized spatial extent,which could be thought of as an emergent rigid particle,owing to its particular topological and chiral properties.Static skyrmionic configurations have been found in various materials with different transport and thermodynamic properties.The magnetic skyrmions respond to externally applied fields in a very unique way,and their coupling to other quasiparticles in solid-state systems gives rise to the emergent electrodynamics.Being not only theoretically important,the magnetic skyrmion is also very promising to be the information carrier in next generation spintronic devices.     

Artificial multiferroic heterostructures for an electric control of magnetic properties

The control of magnetism by electric fields is an important goal for future low-power spintronics devices. This partly explains the intensified recent interest for magnetoelectric multiferroic materials and heterostructures. The lack of ferro- or ferrimagnetic–ferroelectric materials with large magnetoelectric coupling between the two orders has spurred intensive research on artificial multiferroics combining ferroelectric or piezoelectric materials and ferromagnets. In this paper we review synthetically the potential of thin-film-based heterostructures in which a magnetic film is in contact with a ferroelectric or piezoelectric one to obtain an electric control of magnetic properties. This electric control either results from a strain-induced magnetoelectric coupling, a charge-driven one, or from the modulation of an interfacial exchange-bias interaction.     

The Two Dimensional Kondo Model with Rashba Spin-Orbit Coupling

We investigate the effect that Rashba spin-orbit coupling has on the low energy behaviour of a two dimensional magnetic impurity system. It is shown that the Kondo effect, the screening of the magnetic impurity at temperatures T<T _K , is robust against such spin-orbit coupling, despite the fact that the spin of the conduction electrons is no longer a conserved quantity. A proposal is made for how the spin-orbit coupling may change the value of the Kondo temperature T _K in such systems and the prospects of measuring this change are discussed. We conclude that many of the assumptions made in our analysis invalidate our results as applied to recent experiments in semi-conductor quantum dots but may apply to measurements made with magnetic atoms placed on metallic surfaces.     

Martensitic transformation and related magnetic effects in Ni-Mn-based ferromagnetic shape memory alloys

Ferromagnetic shape memory alloys, which undergo the martensitic transformation, are famous multifunctional materials. They exhibit many interesting magnetic properties around the martensitic transformation temperature due to the strong coupling between magnetism and structure. Tuning magnetic phase transition and optimizing the magnetic effects in these alloys are of great importance. In this paper, the regulation of martensitic transformation and the investigation of some related magnetic effects in Ni-Mn-based alloys are reviewed based on our recent research results.