拓扑绝缘体和铁磁体的异质结构为自旋电子学提供了新的机会，并为新颖的反常霍尔态提供了途径。在一种这样的结构中，最近观察到EuS/Bi $_{2}$ Se $_{3}$居里温度的显著提高。我们对一组类似的薄膜进行拉曼光谱，以研究磁性和晶格激发。界面应变通过其对Bi $_{2}$ Se $_{3}$声子模式的影响进行监测，而磁性系统通过EuS拉曼模式进行探测。尽管它的外观在裸EuS，异质结构缺乏相应的EuS拉曼信号。通过数值计算，排除了法布里-珀罗干涉对模的抑制作用。我们将EuS中没有磁信号归因于与Bi $_{2}$ Se $_{3}$的大电荷转移。这可以提供一个额外的途径来操纵拓扑异质结构的磁，光或电子响应。
英文摘要：
Heterostructures of topological insulators and ferromagnets offer new opportunities in spintronics and a route to novel anomalous Hall states. In one such structure, EuS/Bi$_{2}$Se$_{3}$ a dramatic enhancement of the Curie temperature was recently observed. We performed Raman spectroscopy on a similar set of thin films to investigate the magnetic and lattice excitations. Interfacial strain was monitored through its effects on the Bi$_{2}$Se$_{3}$ phonon modes while the magnetic system was probed through the EuS Raman mode. Despite its appearance in bare EuS, the heterostructures lack the corresponding EuS Raman signal. Through numerical calculations we rule out the possibility of Fabry-Perot interference suppressing the mode. We attribute the absence of a magnetic signal in EuS to a large charge transfer with the Bi$_{2}$Se$_{3}$. This could provide an additional pathway for manipulating the magnetic, optical, or electronic response of topological heterostructures.
In a recent work, signatures of a pair of Majorana bound states (MBS) were found in a new experimental platform formed by EuS islands deposited on top of a gold surface which was made superconducting through proximity coupling to a superconductor. In this work, we provide a theoretical understanding for how MBS can be formed in EuS/Au/superconductor heterostructures. We focus on the strip geometry where a narrow ferromagnetic strip is deposited on a planar structure. We first explicitly map out the topological phase diagram of the EuS/Au/superconductor heterostructure using the lattice Green's…
Detection of the interfacial exchange field at a ferromagnetic insulator-nonmagnetic metal interface with pure spin currents
在铁磁金属（NM）和铁磁绝缘体（FI）之间的界面处，自旋电流可以与磁化相互作用，导致自旋电流的调制。在这些FI-NM界面的界面交换场可以通过将该界面放置成与横向自旋阀（LSV）器件的自旋输运通道接触并观察额外的自旋弛豫过程来探测。研究了铁磁绝缘体EuS（EuS-LSV）和钇铁石榴石YPDMATHABMM Fe $_5$ O $_{12}$（YIG-LSV）附近的Cu自旋输运通道的横向自旋阀器件中的界面交换场.将自旋信号与在具有MgO或AlO $_x$覆盖的单晶硅/SiO $_2$衬底上制造的参考横向自旋阀器件进行比较。EuS-LSV和YIG-LSV中的非局域自旋阀信号分别低约4倍和6倍。自旋信号的抑制归因于由于界面自旋轨道场而在Cu-EuS（或Cu-YIG）界面处增强的表面自旋翻转概率。除了自旋信号的抑制外，我们还发现在$T \sim$ 30 K处的自旋信号中广泛观察到的低温峰在与EuS或YIG接触的情况下向更高的温度移动。不同注入器-检测器距离的自旋信号的温度依赖性揭示了在这些界面处的波动交换场导致额外的自旋退相干，其除了常规的自旋弛豫源之外还限制了自旋弛豫时间。结果表明，纯自旋电流随温度变化的测量方法可以用来探测铁磁绝缘体-铁磁金属界面的界面交换场。
We performed x-ray magnetic circular dichroism (XMCD) measurements on heterostructures comprising topological insulators (TIs) of the (Bi,Sb)$_2$(Se,Te)$_3$ family and the magnetic insulator EuS. XMCD measurements allow us to investigate element-selective magnetic proximity effects at the very TI/EuS interface. A systematic analysis reveals that there is neither significant induced magnetism within the TI nor an enhancement of the Eu magnetic moment at such interface. The induced magnetic moments in Bi, Sb, Te, and Se sites are lower than the estimated detection limit of the XMCD measurements …
We study the electronic properties of InAs/EuS/Al heterostructures as explored in a recent experiment [S. Vaitiekenas \emph{et al.}, Nat. Phys. (2020)], combining both spectroscopic results and microscopic device simulations. In particular, we use angle-resolved photoemission spectroscopy to investigate the band bending at the InAs/EuS interface. The resulting band offset value serves as an essential input to subsequent microscopic device simulations, allowing us to map the electronic wave function distribution. We conclude that the magnetic proximity effects at the Al/EuS as well as the InAs/…
Hybrid semiconducting nanowire devices combining epitaxial superconductor and ferromagnetic insulator layers have been recently explored experimentally as an alternative platform for topological superconductivity at zero applied magnetic field. In this proof-of-principle work we show that the topological regime can be reached in actual devices depending on some geometrical constraints. To this end, we perform numerical simulations of InAs wires in which we explicitly include the superconducting Al and magnetic EuS shells, as well as the interaction with the electrostatic environment at a self-…
We study a simple microscopic model for thin, ferromagnetic, metallic layers on semi-infinite bulk superconductor. We find that for certain values of the exchange spliting, on the ferromagnetic side, the ground states of such structures feature spontaneously induced spin polarized currents. Using a mean-field theory, which is selfconsistent with respect to the pairing amplitude $χ$, spin polarization $\vec{m}$ and the spontaneous current $\vec{j}_s$, we show that not only there are Andreev bound states in the ferromagnet but when their energies $E_n$ are near zero they support spontaneous curr…
Magnetic-field effect in the heterostructure Ba$_{0.8}$Sr$_{0.2}$TiO$_3$/LaMnO$_3$
研究了在单晶锰氧化物LaMnO $_3$上沉积的铁电Ba $_{0.8}$ Sr $_{0.2}$ TiO $_3$薄膜的输运和磁输运性质。在界面处产生的二维电子气将反铁磁锰氧化物的界面区域转变为具有重定向磁矩的铁磁状态。我们得到，这些铁磁区域的异质结构的电阻的贡献似乎是依赖于外部磁场施加垂直于界面（和平行于{ \it c}轴的铁磁状态的LaMnO $_3$）。我们认为，所研究的异质结构的电阻的下降的影响下的外部磁场是由于在界面区域的铁磁秩序的外观。这导致相对较小的阻力。反过来，所施加的磁场的循环导致铁磁有序区域的增加，并且因此导致铁磁无序处的载流子的散射减少。
Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures
我们证明了拓扑绝缘体化合物Bi 2 Te 3在铁磁绝缘体Cr 2 Ge 2 Te 6上的范德华外延。这些层的取向为Bi 2 Te 3的（001）平行于Cr2 Ge 2 Te 6的（001），Bi 2 Te 3的（110）平行于Cr2 Ge 2 Te 6的（100）。横截面透射电子显微镜表明形成了尖锐的界面。在低温下，由Bi_2 Te_3/Cr_2Ge_2 Te_6组成的双层膜表现出很大的反常霍尔效应（AHE）。倾斜场的AHE的研究表明，易轴位于沿c轴的异质结构，与体Cr2 Ge 2 Te 6的磁化测量一致。Cr2 Ge 2 Te 6的61 K居里温度和近化学计量比材料的使用可能导致基于AHE的自旋电子器件的发展。
The search of novel topological phases, such as the quantum anomalous Hall insulator (QAHI) or the axion insulator, has motivated different schemes to introduce magnetism into topological insulators. One scheme is to introduce ferromagnetic dopants in topological insulators. However, it is generally challenging and requires carefully engineered growth/heterostructures or relatively low temperatures to observe the QAHI due to issues such as the added disorder with ferromagnetic dopants. Another promising scheme is using the magnetic proximity effect with a magnetic insulator to magnetize the to…
Magnon-mediated spin transport across nonmagnetic metal (NM) and ferromagnetic insulator (FI) interface depends critically on electron-magnon coupling. We propose a novel route to enhance electron-magnon coupling dynamically from transport viewpoint. Using non-equilibrium Green's function a theoretical formalism for magnon-mediated spin current is developed. In the language of transport, the effective electron-magnon coupling at NM/FI interface is determined by self-energy of FI lead, which is proportional to density of states (DOS) at NM/FI interface due to nonlinear process of electron-magno…
Exfoliation and van der Waals heterostructure assembly of intercalated ferromagnet Cr1/3TaS2
铁磁范德华（vdW）材料是用于具有全二维材料异质结构的自旋电子器件的需求。在这里，我们证明了机械剥离的磁性原子嵌入过渡金属二硫属化物Cr 1/3 TaS 2从其散装晶体，以前这样的插层材料被认为是难以剥离。剥离的纳米厚的薄片的磁输运揭示了低于其居里温度TC ~ 110 K的铁磁有序性以及强的面内磁各向异性;这些与其本体性质相同。此外，范德瓦尔斯异质结构组装的Cr 1/3 TaS 2与另一个插入铁磁体Fe 1/4 TaS 2证明使用干转移方法。制造的异质结组成的Cr 1/3 TaS 2和Fe 1/4 TaS 2与天然Ta 2 O 5氧化物隧道势垒之间的隧道磁阻（TMR），揭示可能的自旋注入和检测这些剥离铁磁材料通过VDW结。
Spin-Hall nano-oscillators (SHNOs) are promising spintronic devices to realize current controlled GHz frequency signals in nanoscale devices for neuromorphic computing and creating Ising systems. However, traditional SHNOs -- devices based on transition metals -- have high auto-oscillation threshold currents as well as low quality factors and output powers. Here we demonstrate a new type of hybrid SHNO based on a permalloy (Py) ferromagnetic-metal nanowire and low-damping ferrimagnetic insulator, in the form of epitaxial lithium aluminum ferrite (LAFO) thin films. The superior characteristics …
We report Coulomb blockade transport studies of semiconducting InAs nanowires grown with epitaxial superconducting Al and ferromagnetic insulator EuS on overlapping facets. Comparing experiment to a theoretical model, we associate cotunneling features in even-odd bias spectra with spin-polarized Andreev levels. Results are consistent with zero-field spin splitting exceeding the induced superconducting gap. Energies of subgap states are tunable on either side of zero via electrostatic gates.
In a topological insulator (TI)/magnetic insulator (MI) hetero-structure, large spin-orbit coupling of the TI and inversion symmetry breaking at the interface could foster non-planar spin textures such as skyrmions at the interface. This is observed as topological Hall effect in a conventional Hall set-up. While this effect has been observed at the interface of TI/MI, where MI beholds perpendicular magnetic anisotropy, non-trivial spin-textures that develop in interfacial MI with in-plane magnetic anisotropy is under-reported. In this work, we study Bi$_2$Te$_3$/EuS hetero-structure using plan…
Proximity Driven Enhanced Magnetic Order at Ferromagnetic Insulator / Magnetic Topological Insulator Interface
磁绝缘体/拓扑绝缘体（MI/TI）界面的磁交换驱动邻近效应为新现象提供了一个丰富的舞台，同时也为实现低能量耗散量子器件提供了一种途径。在这里，我们报告了一个戏剧性的增强邻近交换耦合的MI /磁TI EuS / Sb $_{2-x}$ V $_x$ Te $_3$混合异质结构，其中V掺杂是用来驱动TI（Sb $_{2}$ Te $_3$）磁性。我们观察到一个人工的反铁磁结构附近的MI/TI接口，这可能是增强的邻近耦合。邻近效应和掺杂之间的相互作用提供了深入了解可控工程的磁秩序使用混合异质结构。
Ab-initio transport theory for digital ferromagnetic heterostructures
由$δ$掺杂GaAs和Mn制成的MnAs/GaAs超晶格被称为数字铁磁异质结构。在这里，我们提出了一个理论密度泛函研究的电子，磁性和输运性质的异质结构。在没有本征施主的情况下，这些系统显示出半金属态密度，交换相互作用比具有相同Mn浓度的随机合金强得多。{ \it Ab initio}弹道输运计算表明，能量接近费米能级的载流子被强烈地限制在MnAs平面周围的几个单层内。这种强约束是大交换耦合的原因。因此，该系统可以被描述为一个二维半金属与大电导的MnAs平面和小电导的垂直方向。
We derive an effective field theory model for magnetic topological insulators and predict that a magnetic electronic gap persists on the surface for temperatures above the ordering temperature of the bulk. Our analysis also applies to interfaces of heterostructures consisting of a ferromagnetic and a topological insulator. In order to make quantitative predictions for MnBi$_2$Te$_4$, and for EuS-Bi$_2$Se$_3$ heterostructures, we combine the effective field theory method with density functional theory and Monte Carlo simulations. For MnBi$_2$Te$_4$ we predict an upwards Néel temperature shift a…
Magnetic proximity effect at Bi$_2$Se$_3$/EuS interface with broken inversion symmetry
利用密度泛函理论（DFT）和低能有效模型研究了Bi $_2$ Se $_3$ /EuS界面对拓扑绝缘体（TI）Dirac表面态的磁邻近效应.我们考虑一个薄的铁磁绝缘体EuS膜堆叠在Bi $_2$ Se $_3$（111）三层或五层五重层（QL）的板的顶部，EuS的磁化垂直于界面（$z$轴），这打破了时间反转和反转对称性。结果表明，电荷转移和表面弛豫使狄拉克锥电子掺杂。对于3和5个量子点，顶面狄拉克锥的能隙为9 meV，而底面狄拉克锥保持无隙。这是由于EuS薄膜的短程感生磁矩引起的。对于5个量子阱，在底表面狄拉克点的正下方形成了一个能隙为2 meV的额外狄拉克锥，而对于3个量子阱，没有额外的狄拉克锥。我们还研究了自旋轨道纹理的狄拉克表面态与破坏反转对称性，使用DFT和有效的模型。我们发现，$p_z$轨道耦合到自旋矩的$z$分量，其符号与$p_x$和$p_y$轨道相反。$p_z$和径向$p$轨道以与切向$p$轨道相反的旋向性耦合到面内自旋织构。从有效模型得到的结果与我们的DFT计算一致。计算的自旋轨道织构可以从自旋极化角分辨光电子能谱测试。
Spin Seebeck effect and phonon energy transfer in heterostructures containing layers of a normal metal and a ferromagnetic insulator
在动力学方法的基础上的玻尔兹曼方程的声子分布函数的框架内，我们分析了声子在异质结构中的热传递包含一个层的正常金属（$ N $）和一个层的铁磁绝缘体（$ F $）。两个现实的方法，在这样的异质结中产生的温度梯度被认为是：通过加热的$N$层的电流和通过放置的$N/F$ -双层之间的大量的不同温度的双金属层。计算了$ N $ -层中的电子温度$ T_e $和$ F $ -层中的磁振子温度$ T_m $.这些温度的差异决定了塞贝克自旋效应机制中$ N $层上的电压$ V_{ISHE} $。将$ V_{ISHE} $对浴温和对$ N $和$ F $层厚度的依赖性与可用的实验数据进行比较。
Two-dimensional (2D) magnets have broad application prospects in the spintronics, but how to effectively control them with a small electric field is still an issue. Here we propose that 2D magnets can be efficiently controlled in a multiferroic heterostructure composed of 2D magnetic material and perovskite oxide ferroelectric (POF) whose dielectric polarization is easily flipped under a small electric field. We illustrate the feasibility of such strategy in the bilayer CrI3/BiFeO3(001) heterostructure by using the first-principles calculations. Different from the traditional POF multiferroic …
We demonstrate spin-valve magnetoresistance with a current-in-plane (CIP) configuration in (Ga,Fe)Sb / InAs (thickness $t_\mathrm{InAs}$ nm) / (Ga,Fe)Sb trilayer heterostructures, where (Ga,Fe)Sb is a ferromagnetic semiconductor (FMS) with high Curie temperature ($T_\mathrm{C}$). An MR curve with an open minor loop is clearly observed at 3.7 K in a sample with $t_\mathrm{InAs}$ = 3 nm, which originates from the parallel - antiparallel magnetization switching of the (Ga,Fe)Sb layers and spin-dependent scattering at the (Ga,Fe)Sb / InAs interfaces. The MR ratio increases (from 0.03 to 1.6%) with…
