Breaking time‐reversal symmetry by introducing magnetic order, thereby opening a gap in the topological surface state bands, is essential for realizing useful topological properties such as the quantum anomalous Hall and axion insulator states. In this work, a novel topological antiferromagnetic (AFM) phase is created at the interface of a sputtered, c‐axis‐oriented, topological insulator/ferromagnet heterostructure—Bi2Te3/Ni80Fe20 because of diffusion of Ni in Bi2Te3 (Ni‐Bi2Te3). The AFM property of the Ni‐Bi2Te3 interfacial layer is established by observation of spontaneous exchange bias in the magnetic hysteresis loop and compensated moments in the depth profile of the magnetization using polarized neutron reflectometry. Analysis of the structural and chemical properties of the Ni‐Bi2Te3 layer is carried out using selected‐area electron diffraction, electron energy loss spectroscopy, and X‐ray photoelectron spectroscopy. These studies, in parallel with first‐principles calculations, indicate a solid‐state chemical reaction that leads to the formation of Ni−Te bonds and the presence of topological antiferromagnetic (AFM) compound NiBi2Te4 in the Ni‐Bi2Te3 interface layer. The Neél temperature of the Ni‐Bi2Te3 layer is ≈63 K, which is higher than that of typical magnetic topological insulators (MTIs). The presented results provide a pathway toward industrial complementary metal−oxide−semiconductor (CMOS)‐process‐compatible sputtered‐MTI heterostructures, leading to novel materials for topological quantum devices.
Thin films of topological insulators (TIs) coupled with ferromagnets (FMs) are excellent candidates for energy-efficient spintronics devices. Here, the effect of the crystalline structural disorder of TI on the interfacial and magnetic properties of sputter-deposited TI/FM, Bi2Te3/Ni80Fe20, heterostructures is reported. Ni and a smaller amount of Fe from Py were found to diffuse across the interface and react with Bi2Te3. For improved crystalline c-axis-oriented Bi2Te3 films, a significant enhancement in Gilbert damping is observed, accompanied by an effective out-of-plane magnetic anisotropy and enhanced damping-like spin–orbit torque (DL-SOT), possibly due to the topological surface states (TSS) of Bi2Te3. Furthermore, a spontaneous exchange bias is observed in hysteresis loop measurements at low temperatures. This is caused by a topological antiferromagnetic interfacial layer formed due to a solid-state reaction between the diffused Ni with Bi2Te3 that couples with the FM, Ni80Fe20. For the increasing disorder of Bi2Te3, a significant weakening of the exchange interaction in the AFM interfacial layer is observed. These experimental results open the pathway for further exploration of crystalline-disordered TIs and their interfaces.
The dependence of the Nernst-Ettingshausen longitudinal effect on the direction of the magnetic field for the case of semimetals is studied theoretically and experimentally. A quite unique correspondence is found between the anisotropy of the effect and the band structure near the Fermi level. It is shown that the commutation effect which determines the dependence of the magneto-thermo-e.m.f. magnitude on the direction of the induction vector, permits to judge about the number and localization of band extrema in the k-space. This is an effective way for studying band structures of bismuth and its alloys a t high temperatures. The results of CE studies in bismuth and its alloys art 100 O K are given below. Theory of Thermomagnetic EffectsThe expression for the electric current density J is given as follows :where oi,(B) is the generalized electroconductivity tensor, E, the electric field, 0, T the temperature gradient.The tensor Pir(B) is determined by the correlation
Realization of the quantum anomalous Hall (QAH) effect has been demonstrated in several magnetic topological insulators (MTIs). MTIs can be synthesized by doping magnetic elements into topological insulators (TIs) [1][2][3][4][5][6][7][8][9] or growth of intrinsic MTI compounds. [10][11][12][13][14][15][16][17][18][19] However, these MTI materials require temperatures much lower than their magnetic T c to achieve quantization in the Hall effect. This deficiency has been attributed to inhomogeneity in magnetic doping which causes localized variation in surface state exchange gaps. Magnetism can also be induced in the surface of TI thin films via proximity exchange effect. This proximity induced magnetization (PIM) is envisioned by coupling TIs with a magnetic insulator (MI). PIM in TIs have been experimentally confirmed in heterostructures of TIs with magnetic insulators (MIs) such as rare earth garnets and EuS, [20][21][22][23][24][25][26][27][28] MTIs, [29,30] and telluride van der Waals (vdW) magnets. [31] However, recent studies have pointed out the complexities in measuring proximity exchange in these systems, [31] which can be affected by a number of problems such as diffusion at the interface and band bending. Thus, providing a barrier for diffusion while permitting magnetic exchange in TI/MI heterostructures will be necessary for devices, and it is shown here that a TiO x interface barrier achieves this goal. Only a single study by Watanabe et al. [31] has shown PIM and QAH in a TI/MI/TI thin film sandwich structure of (Zn,Cr)Te/(Bi,Sb) 2 Te 3 /(Zn,Cr)Te. However, even in this material system, the QAH effect was seen at extremely low temperature of 0.03 K. This low temperature is possibly due to: (1) High-quality MIs which were grown on TI thin films have very low Curie temperature, T c , such as (Zn,Cr) Te with T c < 40 K [31] and EuS with T c of ≈17 K. [25] (2) Even with 20-25% of the magnetic species, Cr in (Zn,Cr)Te, finite localized nonmagnetic nanoregions may exist in the samples.MIs such as rare-earth garnets and ferrites are excellent materials that are magnetically ordered well above room-temperature, making them ideal candidates for QAH materials coupled with TIs. However, growth of MIs on top of TIs using Combining topological insulators (TIs) and magnetic materials in heterostructures is crucial for advancing spin-based electronics. Magnetic insulators (MIs) can be deposited on TIs using the spin-spray process, which is a unique nonvacuum, low-temperature growth process. TIs have highly reactive surfaces that oxidize upon exposure to atmosphere, making it challenging to grow spinspray ferrites on TIs. In this work, it is demonstrated that a thin titanium capping layer on TI, followed by oxidation in atmosphere to produce a thin TiO x interfacial layer, protects the TI surface, without significantly compromising spin transport from the magnetic material across the TiO x to the TI surface states. First, it is demonstrated that in Bi 2 Te 3 /TiO x /Ni 80 Fe 20 heterostructures, TiO x provides an e...
As is known (1). the band structure of bismuth is satisfactorily approximated by the following energetic model: a) Three minima of the conduction band at L-points of the Brillouin zone. b) One maximum of the first valence band at H-point (heavy-hole band). c) Three maxima of the valence band (light-hole band).The role of corresponding charge carriers in transport phenomena is greatly depending on temperature and degree of doping. However, the data on this problem are contradictory (2).The paper reports experimental remlte on the influence of various groups of carriers on the anisotropy of tramport phenomena in plre and tin-doped bismuth at 77 OK.Since the general solution of the problem is very complicated we shall restrict ourselves to a particular case of cross effects when the current is parallel to the third order axes and the orientation of the msgnetic induction vector changes re-
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