Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations,we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.

Theoretical Investigation of Influence of Mechanical Stress on Magnetic Properties of Ferromagnetic/Antiferromagnetic Bilayers Deposited on Flexible Substrates

Effect of mechanical stress on magnetic properties of an exchange-biased ferromagnetic/antiferromagnetic bilayer deposited on a flexible substrate is investigated. The hysteresis loops with different magnitudes and orientations of the stress can be classified into three types. The corresponding physical conditions for each type of the loop are deduced based on the principle of minimal energy. The equation of the critical stress is derived, which can judge whether the loops show hysteresis or not. Numerical calculations suggest that except for the magnitude of the mechanical stress, the relative orientation of the stress is also an important factor to tune the exchange bias effect.

Model of hybrid interfacial domain wall in ferromagnetic/antiferromagnetic bilayers

A general model of a hybrid inteffacial domain wall （HIDW） in ferromagnetic/antiferromagnetic exchange biased bilayers is proposed, where an inteffacial domain wall is allowed to extend into either the ferromagnetic or antiferromagnetic layer or across both. The proposition is based on our theoretical investigation on thickness and field dependences of ferromagnetic domain wall （FMDW） and antiferromagnetic domain wall （AFDW）, respectively. Good match of the simulation to the hysteresis loops of a series of NiFe/FeMn exchange-biased bilayers confirms the existence of the HIDW, where the AFDW part is found to preferentially occupy the entire antiferromagnetic layer while the FMDW shrinks with the increased magnetic field as expected. The observed asymmetry between the ascending and descending branches of the hysteresis loop is explained naturally as a consequence of different partition ratios between AFDW and FMDW.

Magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulators

Control and detection of antiferromagnetic topological materials are challenging since the total magnetization vanishes.Here we investigate the magneto-optical Kerr and Faraday effects in bilayer antiferromagnetic insulator Mn Bi2Te4.We find that by breaking the combined mirror symmetries with either perpendicular electric field or external magnetic moment,Kerr and Faraday effects occur.Under perpendicular electric field,antiferromagnetic topological insulators(AFMTI)show sharp peaks at the interband transition threshold,whereas trivial insulators show small adjacent positive and negative peaks.Gate voltage and Fermi energy can be tuned to reveal the differences between AFMTI and trivial insulators.We find that AFMTI with large antiferromagnetic order can be proposed as a pure magneto-optical rotator due to sizable Kerr(Faraday)angles and vanishing ellipticity.Under external magnetic moment,AFMTI and trivial insulators are significantly different in the magnitude of Kerr and Faraday angles and ellipticity.For the qualitative behaviors,AFMTI shows distinct features of Kerr and Faraday angles when the spin configurations of the system change.These phenomena provide new possibilities to optically detect and manipulate the layered topological antiferromagnets.

Dynamic magnetic behavior of the mixed spin (2, 5/2) Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.

Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Exchange bias in ferromagnet/antiferromagnet bilayers

Since the exchange bias （EB） effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet （FM）/antiferromagnet （AFM） bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.

Coexistence of antiferromagnetic and ferromagnetic in Mn-doped anatase TiO_2 nanowires

A series of Mn-doped TiO2 nanowires(NWs) were prepared by hydrothermal method at the mole fraction of Mn changing from 0 to 12.0%.X-ray powder diffraction(XRD) analysis shows that all the samples have pure anatase structure.SEM and TEM studies show that the diameter and the length of the Mn-doped TiO2 NWs are larger than those of the undoped TiO2 NWs.Energy dispersive X-ray spectroscopy(EDX) reveals that the samples are composed of Ti,Mn and O.According to magnetization measurements,all samples show ferromagnetic behavior,but only the undoped TiO2 NWs are completely ferromagnetic with a saturated magnetization about 1.0 mA·m2/kg.Mn-doped TiO2 samples exhibit antiferromagnetic and ferromagnetic(AF-FM) behaviors simultaneously.Photoluminescence(PL) spectra demonstrate the existence of MnO2 sublattice.These observations indicate that an AF-WF crossover is induced by the coexistence of TiO2 sublattice and MnO2 sublattice.

关于Planar Ferromagnets and Antiferromagnets泛函的径向极小元的注记

就Bethuel,Brezis和Helein提出的问题讨论了Planar Ferromagnets and Antiferromagnets泛函在H={u(x)=(sinf(r)|xx|,cosf(r))∈H1(B1,S2);f(0)=0,f(1)=2π,r=|x|}中的径向极小元的一些性质,其中包括此泛函的径向极小元的零点的分布及若干个上界估计,并给出了这一问题的肯定回答.

Phase-field simulation of superconductor vortex clustering in the vicinity of ferromagnetic domain bifurcations

Superconductors and ferromagnets are highly non-compatible materials due to the natures of their respective electronic states.But when artificially brought together,they develop interesting characteristics,one of which,vortex clustering,is discussed here in this paper.Phase-field and micromagnetic simulations are performed to investigate the superconductor and ferromagnet bilayer,respectively.The ferromagnet with uniaxial anisotropy is observed to develop the maze domain,whereas the superconductor subjected to the influence of the ferromagnetic stray field displays a vortex pattern.Clustered vortices in superconductors at certain locations are observed to be precisely located over magnetic domain bifurcations.The enhanced out-of-plane stray field at bifurcations around the curved domain walls and the convergent Lorentz force due to screening currents in superconductor are attributed to the formation of clusters at bifurcation sites.Segregation of the inter-vortex spacing between straight and bifurcated domain is clearly observed.More importantly,inter-vortex spacing is predicted to serve as a precise tool to map local ferromagnet domain shapes.

热效应在电流驱动反铁磁/铁磁交换偏置场翻转中的显著作用

电流驱动的面内交换偏置场翻转具有无需外磁场辅助、抗磁场干扰以及强磁各向异性等优势,受到广泛关注.然而,在纳米级厚度薄膜系统中,反铁磁/铁磁异质结的阻塞温度较低,同时电流脉冲会产生大量的焦耳热,理论上电流热效应对于交换偏置场翻转有着显著作用,但是其作用机制缺乏相关研究和验证.我们制备了一系列反铁磁IrMn厚度不同的Pt/IrMn/Py异质结,系统性地研究了热效应在电流翻转交换偏置场中的作用机制.结果表明,在毫秒级电流脉冲下,焦耳热能够使得器件升温至阻塞温度以上,解除反铁磁/铁磁界面的交换耦合,同时电流产生的奥斯特场和自旋轨道矩能够翻转铁磁磁矩,在降温过程中完成交换偏置场的翻转.并且,在翻转过程中,反铁磁/铁磁异质结的各向异性磁阻曲线呈现与温度相关的两步磁化翻转现象,分析表明该现象起源于交换偏置耦合与铁磁直接交换作用之间的竞争关系.本文的研究结果厘清了热效应在电流驱动交换偏置场翻转过程中的重要作用,有助于推动基于电控交换偏置场的自旋电子器件发展.

双层蜂窝状海森伯铁磁体中层间交换耦合相互作用对拓扑相的影响

层状磁性拓扑材料是最小二维单元下同时具有磁序和拓扑性的材料体系,研究这一体系可能会观察到新物性和新现象的出现,因此引起了研究者们的广泛关注.本文运用线性自旋波理论,主要研究了层间铁磁耦合的双层蜂窝状海森伯铁磁体中层间交换耦合相互作用对系统拓扑相的影响.通过计算不同层间交换耦合相互作用强度下的磁子色散关系能得出,当系统达到两个强度临界值时,能量较高的两条能带和能量较低的两条能带的带隙在狄拉克点处会依次出现闭合-重新打开现象.计算能带对应的贝里曲率和陈数后,发现贝里曲率符号在相应临界值前后会发生反转,同时陈数也会发生改变,这证明系统发生了拓扑相变.此外,本文研究发现当双层蜂窝状铁磁体发生拓扑相变时,磁子热霍尔系数变化曲线会相应发生突变.本研究成果可以为利用双层蜂窝状铁磁材料制作具有更高信息传输能力的自旋电子器件提供理论支撑,也可以为其他双层铁磁系统的相关研究提供一定的理论参考.

Theoretical Study on the Ferromagnetism of Cr-doped In_2O_3

Density functional theory （DFT） calculations are performed to investigate the electronic structure and ferromagnetism of （In, Cr）2O3. The densities of states suggest that the Cr dopants provide nearly 100% polarization of the conduction carriers and the ferromagnetic ground state in Cr-doped In2O3 can be explained from p-d hybridization mechanism. The calculation results also show that the ferromagnetism is strengthened in the presence of oxygen vacancy.

Performance of switch between exchange bias and coercivity:Influences of antiferromagnetic anisotropy and exchange coupling

The study on temperature dependence of exchange bias field and coercivity is crucial to solving the writing/reading dilemma in magnetic recording.Motivated by recent experimental findings,a complete switch between exchange bias field and coercivity with temperature is proposed,and the performance,characterized by average switching temperature(T_(S))and switching temperature width(T_(W)),controlled by antiferromagnetic anisotropy(KAF)and exchange coupling(J_(AF))constants is studied based on a MonteCarlo simulation.The results show that a linear relationship between T_(S)and KAFis established when KAFis above a critical value,while T_(S)is weakly influenced by J_(AF).On the contrary,T_(W)is insensitive to KAF,while strongly depends on J_(AF).Besides overcoming thermal energy,the increase of KAFfor a small J_(AF)guarantees the completely frozen states in the antiferromagnetic layers during magnetizing at higher temperature,below which the exchange bias field exists with a negligible coercivity.Otherwise,for a large J_(AF),the uncompensated antiferromagnetic magnetization behavior during the ferromagnetic magnetization reversal becomes complicated,and the switching process in the low temperature range depends on the irreversibility of uncompensated antiferromagnetic magnetization reversal during magnetizing,while in the high temperature range mainly influenced by the field-cooling process,resulting in a large T_(W).This work provides an opportunity to control/optimize the performance of the temperatureinduced switch between unidirectional and uniaxial symmetries through precisely tuning KAFand/or J_(AF)to meet different application demands in the next generation information technology.

Co离子掺杂La_(2)Ti_(2-x)Co_(x)O_(7)的室温多铁性

通过向传统铁电材料La_(2)Ti_(2)O7中引入磁性Co离子,成功制备出了新型室温多铁材料La_(2)Ti_(2-x)Co_(x)O_(7)(x=0、0.03、0.05、0.07)。测试结果表明,所有样品均为单相晶体结构,在x射线探测范围内无第二相杂质出现。适当浓度的磁性Co离子掺杂不仅可有效提高材料的饱和铁磁磁化强度,还可有效降低体系漏电流,提高La_(2)Ti_(2)O7绝缘性,进而改善La_(2)Ti_(2-x)Co_(x)O_(7)铁电性和多铁性。对La_(2)Ti_(2-x)Co_(x)O_(7)等样品的顺磁、反铁磁和铁磁等复杂磁相互作用的微观机理进行了探讨。铁磁性的产生被认为与氧空位和过渡金属离子3d电子自旋之间的交换耦合作用有关。而对未掺杂的La_(2)Ti_(2)O7样品,其铁磁性的出现可能与材料内部形成的氧空位有关。由于氧空位的存在,为保持整体价态平衡,Ti^(4+)离子转变为Ti^(3+)离子,使得Ti^(3+)离子的3d轨道存在一个电子,La_(2)Ti_(2)O7样品存在弱铁磁性。

Frustrated ferromagnetic transition in AB-stacked honeycomb bilayer

In two-dimensional(2D) ferromagnets, anisotropy is essential for the magnetic ordering as dictated by the Mermin-Wagner theorem. But when competing anisotropies are present, the phase transition becomes nontrivial. Here, utilizing highly sensitive susceptometry of scanning superconducting quantum interference device microscopy, we probe the spin correlations of ABC-stacked Cr Br3under zero magnetic field. We identify a plateau feature in susceptibility above the critical temperature(TC) in thick samples.It signifies a crossover regime induced by the competition between easy-plane intralayer exchange anisotropy versus uniaxial interlayer anisotropy. The evolution of the critical behavior from the bulk to 2D shows that the competition between the anisotropies is magnified in the reduced dimension. It leads to a strongly frustrated ferromagnetic transition in the bilayer with fluctuation on the order of TC, which is distinct from both the monolayer and the bulk. Our observation demonstrates unconventional 2D critical behavior on a honeycomb lattice.

铁磁/反铁磁双层膜中交换偏置

铁磁 /反铁磁交换偏置在巨磁电阻器件中具有重要的应用 ,引起了物理学及材料学等领域内广大科学家的浓厚兴趣。本文首先阐述了交换偏置的基本性质 ;然后简述了交换偏置的实验研究方法 ;最后 ,着重介绍了几种主要的理论模型。

二维无规混合磁性系统磁特性的微磁学及Monte Carlo研究

采用能量极小原理的微磁学及MonteCarlo方法对异类自旋组成混合Heisenberg自旋体系进行模拟计算,研究了二维铁磁 反铁磁无规混合系统的磁特性.发现了二维无规混合磁性系统存在M-H磁化曲线的阶梯效应.通过一维Ising模型及系统能量、自旋组态的研究,发现小自旋数目的反铁磁耦合系统是产生M-H阶梯效应的根本原因.

外场取向对铁磁/反铁磁双层膜交换偏置的影响

利用斯通纳-沃尔法斯(Stoner-Wohlfarth,SW)模型详细讨论了铁磁/反铁磁双层膜交换偏置对外场取向角的依赖关系。解析推导出转换场和矫顽力的表达式。用两种办法计算了交换偏置场(he)和磁滞回线的半宽度(HWHL),结果分别用(he)s、(HWHL)s和(he)c、(HWHL)c表示。计算表明,(he)s、(HWHL)s和(he)c、(HWHL)c有很大区别,而只有(he)s和(HWHL)s显示出明显的阶跃现象。我们的结果还说明,转换角方程的奇点是阶跃现象的根源。另外,能量曲线的局域极小不是阶跃现象产生的必要条件。

ZnO基稀磁半导体材料研究进展

随着铁磁性半导体(如Mn掺杂InAs和GaAs)的发现,稀磁半导体(DMS)近来吸引了众多研究者的目光。传统半导体不具有磁性,而稀磁半导体可以在不改变传统半导体其它性质的情况下引入磁性,具有良好的物理化学性能。从实验和理论计算两个方面总结了ZnO基DMS的国内外研究现状,讨论了各种生长方法、基底选择、生长温度对材料磁性的影响,总结了如何通过改变实验条件来增大饱和磁化强度及提高Curie温度。