skip to main content

Attention:

The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.


Title: Magnetization switching using topological surface states
Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi 2 Se 3 and an insulating ferromagnet BaFe 12 O 19 . A charge current in Bi 2 Se 3 can switch the magnetization in BaFe 12 O 19 up and down. When the magnetization is switched by a field, a current in Bi 2 Se 3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe 12 O 19 . These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.  more » « less
Award ID(s):
1710512 1727044
NSF-PAR ID:
10162493
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more » ; ; « less
Date Published:
Journal Name:
Science Advances
Volume:
5
Issue:
8
ISSN:
2375-2548
Page Range / eLocation ID:
eaaw3415
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. We present measurements of thermally generated transverse spin currents in the topological insulator Bi2Se3, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi2Se3/CoFeB. We find that Bi2Se3does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient isJsxT= (4.9 ± 0.9) × 106(2e)A m2K μm1, and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi2Se3is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi2Se3can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy.

     
    more » « less
  2. Abstract This work focuses on the low frequency Drude response of bulk-insulating topological insulator (TI) Bi 2 Se 3 films. The frequency and field dependence of the mobility and carrier density are measured simultaneously via time-domain terahertz spectroscopy. These films are grown on buffer layers, capped by Se, and have been exposed in air for months. Under a magnetic field up to 7 Tesla, we observe prominent cyclotron resonances (CRs). We attribute the sharp CR to two different topological surface states from both surfaces of the films. The CR sharpens at high fields due to an electron-impurity scattering. By using magneto-terahertz spectroscopy, we confirm that these films are bulk-insulating, which paves the way to use intrinsic TIs without bulk carriers for applications including topological spintronics and quantum computing. 
    more » « less
  3. Topological surface states (TSSs) coexist with a rapidly formed two-dimensional electron gas (2DEG) at the surface of Bi 2 Se 3 . While this complex band structure has been widely studied for its interactions between the two states in terms of electrical conductivity and carrier density, the resulting thermopower has not been investigated as thoroughly. Here, we report measurements of the temperature dependent Seebeck coefficient ( S) and electrical conductivity ( σ) on an undoped 10 nm thin Bi 2 Se 3 film over the temperature range of 100–300 K to find an overall metal-like behavior. The measured S is consistent with the theory when assuming that both the TSS and the 2DEG contribute to thermoelectric transport. Our analysis further shows that the coefficient corresponds to a Fermi level situated well above the conduction band minima of the 2DEG, resulting in comparable contributions from the TSS and the 2DEG. The thermoelectric power factor ( S 2 σ) at 300 K increases by 10%–30% over the bulk. This work provides insights into understanding and enhancing thermoelectric phenomena in topological insulators. 
    more » « less
  4. Abstract

    The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr‐doped (Bi,Sb)2Te3(CBST) grown on an uncompensated antiferromagnetic insulator Al‐doped Cr2O3. Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al‐Cr2O3surface spins fixing interfacial magnetic moments perpendicular to the film plane. The interfacial coupling results in an exchange‐biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al‐Cr2O3layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH‐based spintronics.

     
    more » « less
  5. A quantum anomalous Hall (QAH) insulator is a topological phase in which the interior is insulating but electrical current flows along the edges of the sample in either a clockwise or counterclockwise direction, as dictated by the spontaneous magnetization orientation. Such a chiral edge current eliminates any backscattering, giving rise to quantized Hall resistance and zero longitudinal resistance. Here we fabricate mesoscopic QAH sandwich Hall bar devices and succeed in switching the edge current chirality through thermally assisted spin–orbit torque (SOT). The well-quantized QAH states before and after SOT switching with opposite edge current chiralities are demonstrated through four- and three-terminal measurements. We show that the SOT responsible for magnetization switching can be generated by both surface and bulk carriers. Our results further our understanding of the interplay between magnetism and topological states and usher in an easy and instantaneous method to manipulate the QAH state. 
    more » « less