8 research outputs found
Direct comparison of current-induced spin polarization in topological insulator Bi2Se3 and InAs Rashba states
Three-dimensional topological insulators (TIs) exhibit time-reversal symmetry
protected, linearly dispersing Dirac surface states. Band bending at the TI
surface may also lead to coexisting trivial two-dimensional electron gas (2DEG)
states with parabolic energy dispersion that exist as spin-split pairs due to
Rashba spin-orbit coupling (SOC). A bias current is expected to generate spin
polarization in both systems arising from their helical spin-momentum locking.
However, their induced spin polarization is expected to be different in both
magnitude and sign. Here, we compare spin potentiometric measurements of bias
current-generated spin polarization in Bi2Se3(111) films where Dirac surface
states coexist with trivial 2DEG states, with identical measurements on
InAs(001) samples where only trivial 2DEG states are present. We observe spin
polarization arising from spin-momentum locking in both cases, with opposite
signs of the spin voltage. We present a model based on spin dependent
electrochemical potentials to directly derive the signs expected for the TI
surface states, and unambiguously show that the dominant contribution to the
current-generated spin polarization measured in the TI is from the Dirac
surface states. This direct electrical access of the helical spin texture of
Dirac and Rashba 2DEG states is an enabling step towards the electrical
manipulation of spins in next generation TI and SOC based quantum devices
Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal
When a polarized light beam is incident upon the surface of a magnetic
material, the reflected light undergoes a polarization rotation. This
magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of
ferro- and ferrimagnetic materials because it provides a powerful probe for
electronic and magnetic properties as well as for various applications
including magneto-optical recording. Recently, there has been a surge of
interest in antiferromagnets (AFMs) as prospective spintronic materials for
high-density and ultrafast memory devices, owing to their vanishingly small
stray field and orders of magnitude faster spin dynamics compared to their
ferromagnetic counterparts. In fact, the MOKE has proven useful for the study
and application of the antiferromagnetic (AF) state. Although limited to
insulators, certain types of AFMs are known to exhibit a large MOKE, as they
are weak ferromagnets due to canting of the otherwise collinear spin structure.
Here we report the first observation of a large MOKE signal in an AF metal at
room temperature. In particular, we find that despite a vanishingly small
magnetization of 0.002 /Mn, the non-collinear AF metal
MnSn exhibits a large zero-field MOKE with a polar Kerr rotation angle of
20 milli-degrees, comparable to ferromagnetic metals. Our first-principles
calculations have clarified that ferroic ordering of magnetic octupoles in the
non-collinear Neel state may cause a large MOKE even in its fully compensated
AF state without spin magnetization. This large MOKE further allows imaging of
the magnetic octupole domains and their reversal induced by magnetic field. The
observation of a large MOKE in an AF metal should open new avenues for the
study of domain dynamics as well as spintronics using AFMs.Comment: 30 pages, 4 figure
Magnetic Field-Induced Spin Nematic Phase Up to Room Temperature in Epitaxial Antiferromagnetic FeTe Thin Films Grown by Molecular Beam Epitaxy
Electronic nematicity, where strong correlations drive
electrons
to align in a way that lowers the crystal symmetry, is ubiquitous
among unconventional superconductors. Understanding the interplay
of such a nematic state with other electronic phases underpins the
complex behavior of these materials and the potential for tuning their
properties through external stimuli. Here, we report magnetic field-induced
spin nematicity in a model system tetragonal FeTe, the parent compound
of iron chalcogenide superconductors, which exhibits a bicollinear
antiferromagnetic order. The studies were conducted on epitaxial FeTe
thin films grown on SrTiO3(001) substrates by molecular
beam epitaxy, where the bicollinear antiferromagnetic order was confirmed
by in situ atomic resolution scanning tunneling microscopy
imaging. A 2-fold anisotropy is observed in in-plane angle-dependent
magnetoresistance measurements, indicative of magnetic field-induced
nematicity. Such 2-fold anisotropy persists up to 300 K, well-above
the bicollinear antiferromagnetic ordering temperature of 75 K, indicating
a magnetic field-induced spin nematic phase up to room temperature
in the antiferromagnet FeTe
Room-Temperature Spin Filtering in Metallic Ferromagnet–Multilayer Graphene–Ferromagnet Junctions
We
report room-temperature negative magnetoresistance in ferromagnet–graphene–ferromagnet
(FM|Gr|FM) junctions with minority spin polarization exceeding 80%,
consistent with predictions of strong minority spin filtering. We
fabricated arrays of such junctions <i>via</i> chemical
vapor deposition of multilayer graphene on lattice-matched single-crystal
NiFe(111) films and standard photolithographic patterning and etching
techniques. The junctions exhibit metallic transport behavior, low
resistance, and the negative magnetoresistance characteristic of a
minority spin filter interface throughout the temperature range 10
to 300 K. We develop a device model to incorporate the predicted spin
filtering by explicitly treating a metallic minority spin channel
with spin current conversion and a tunnel barrier majority spin channel
and extract spin polarization of at least 80% in the graphene layer
in our structures. The junctions also show antiferromagnetic coupling,
consistent with several recent predictions. The methods and findings
are relevant to fast-readout low-power magnetic random access memory
technology, spin logic devices, and low-power magnetic field sensors