88 research outputs found
Progress and prospects in the quantum anomalous Hall effect
The quantum anomalous Hall effect refers to the quantization of Hall effect
in the absence of applied magnetic field. The quantum anomalous Hall effect is
of topological nature and well suited for field-free resistance metrology and
low-power information processing utilizing dissipationless chiral edge
transport. In this Perspective, we provide an overview of the recent
achievements as well as the materials challenges and opportunities, pertaining
to engineering intrinsic/interfacial magnetic coupling, that are expected to
propel future development of the field.Comment: Invited for APL Materials, Special Topic - Materials Challenges and
Synthesis Science of Emerging Quantum Material
Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach
The quantum anomalous Hall effect (QAHE) has been recently demonstrated in
Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures
below 100 mK. In those materials, the spins of unfilled d-electrons in the
transition metal dopants are exchange coupled to develop a long-range
ferromagnetic order, which is essential for realizing QAHE. However, the
addition of random dopants does not only introduce excess charge carriers that
require readjusting the Bi/Sb ratio, but also unavoidably introduces
paramagnetic spins that can adversely affect the chiral edge transport in QAHE.
In this work, we show a heterostructure approach to independently tune the
electronic and magnetic properties of the topological surface states in
(BixSb1-x)2Te3 without resorting to random doping of transition metal elements.
In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium
iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we
find that the TI surface in contact with YIG becomes ferromagnetic via
proximity coupling which is revealed by the anomalous Hall effect (AHE). The
Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is
uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is
varied, the AHE resistivity scales with the longitudinal resistivity. In this
approach, we decouple the electronic properties from the induced ferromagnetism
in TI. The independent optimization provides a pathway for realizing QAHE at
higher temperatures, which is important for novel spintronic device
applications.Comment: Accepted by Nano Letter
Absence of Magnetic Fluctuations in the Ferromagnetic/Topological Heterostructure EuS/BiSe
Heterostructures of topological insulators and ferromagnets offer new
opportunities in spintronics and a route to novel anomalous Hall states. In one
such structure, EuS/BiSe 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 BiSe
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 BiSe.
This could provide an additional pathway for manipulating the magnetic,
optical, or electronic response of topological heterostructures.Comment: 6 pages, 3 figure
Spatially modulated magnetic structure of EuS due to the tetragonal domain structure of SrTiO
The combination of ferromagnets with topological superconductors or
insulators allows for new phases of matter that support excitations such as
chiral edge modes and Majorana fermions. EuS, a wide-band-gap ferromagnetic
insulator with a Curie temperature around 16 K, and SrTiO (STO), an
important substrate for engineering heterostructures, may support these phases.
We present scanning superconducting quantum interference device (SQUID)
measurements of EuS grown epitaxially on STO that reveal micron-scale
variations in ferromagnetism and paramagnetism. These variations are oriented
along the STO crystal axes and only change their configuration upon thermal
cycling above the STO cubic-to-tetragonal structural transition temperature at
105 K, indicating that the observed magnetic features are due to coupling
between EuS and the STO tetragonal structure. We speculate that the STO
tetragonal distortions may strain the EuS, altering the magnetic anisotropy on
a micron-scale. This result demonstrates that local variation in the induced
magnetic order from EuS grown on STO needs to be considered when engineering
new phases of matter that require spatially homogeneous exchange
Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet
When a topological insulator (TI) is in contact with a ferromagnet, both time
reversal and inversion symmetries are broken at the interface. An energy gap is
formed at the TI surface, and its electrons gain a net magnetic moment through
short-range exchange interactions. Magnetic TIs can host various exotic quantum
phenomena, such as massive Dirac fermions, Majorana fermions, the quantum
anomalous Hall effect and chiral edge currents along the domain boundaries.
However, selective measurement of induced magnetism at the buried interface has
remained a challenge. Using magnetic second harmonic generation, we directly
probe both the in-plane and out-of-plane magnetizations induced at the
interface between the ferromagnetic insulator (FMI) EuS and the
three-dimensional TI Bi2Se3. Our findings not only allow characterizing
magnetism at the TI-FMI interface but also lay the groundwork for imaging
magnetic domains and domain boundaries at the magnetic TI surfaces.Comment: 26 pages, 7 figures Journal reference added; title modifie
Unconventional Planar Hall Effect in Exchange-Coupled Topological Insulator-Ferromagnetic Insulator Heterostructures
The Dirac electrons occupying the surface states (SSs) of topological
insulators (TIs) have been predicted to exhibit many exciting magneto-transport
phenomena. Here we report on the first experimental observation of an
unconventional planar Hall effect (PHE) and an electrically gate-tunable
hysteretic planar magnetoresistance (PMR) in EuS/TI heterostructures, in which
EuS is a ferromagnetic insulator (FMI) with an in-plane magnetization. In such
exchange-coupled FMI/TI heterostructures, we find a significant (suppressed)
PHE when the in-plane magnetic field is parallel (perpendicular) to the
electric current. This behavior differs from previous observations of the PHE
in ferromagnets and semiconductors. Furthermore, as the thickness of the 3D TI
films is reduced into the 2D limit, in which the Dirac SSs develop a
hybridization gap, we find a suppression of the PHE around the charge neutral
point indicating the vital role of Dirac SSs in this phenomenon. To explain our
findings, we outline a symmetry argument that excludes linear-Hall mechanisms
and suggest two possible non-linear Hall mechanisms that can account for all
the essential qualitative features in our observations.Comment: 17 pages, 4 figures, accepted by Phys. Rev.
Band structure of topological insulators from noise measurements in tunnel junctions
The unique properties of spin-polarized surface or edge states in topological
insulators (TIs) make these quantum coherent systems interesting from the point
of view of both fundamental physics and their implementation in low power
spintronic devices. Here we present such a study in TIs, through tunneling and
noise spectroscopy utilizing TI/AlO/Co tunnel junctions with bottom TI
electrodes of either BiTe or BiSe. We demonstrate that features
related to the band structure of the TI materials show up in the tunneling
conductance and even more clearly through low frequency noise measurements. The
bias dependence of 1/f noise reveals peaks at specific energies corresponding
to band structure features of the TI. TI tunnel junctions could thus simplify
the study of the properties of such quantum coherent systems, that can further
lead to the manipulation of their spin-polarized properties for technological
purposes
Interfacial control of vortex-limited critical current in type-II superconductor films
In a small subset of type-II superconductor films, the critical current is determined by a weakened Bean-Livingston barrier posed by the film surfaces to vortex penetration into the sample. A film property thus depends sensitively on the surface or interface to an adjacent material. We theoretically investigate the dependence of vortex barrier and critical current in such films on the Rashba spin-orbit coupling at their interfaces with adjacent materials. Considering an interface with a magnetic insulator, we find the spontaneous supercurrent resulting from the exchange field and interfacial spin-orbit coupling to substantially modify the vortex surface barrier, consistent with a previous prediction. Thus, we show that the critical currents in superconductor-magnet heterostructures can be controlled, and even enhanced, via the interfacial spin-orbit coupling. Since the latter can be controlled via a gate voltage, our analysis predicts a class of heterostructures amenable to gate-voltage modulation of superconducting critical currents. It also sheds light on the recently observed gate-voltage enhancement of critical current in NbN superconducting film
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