4 research outputs found
Spin Relaxation in Single-Layer Graphene with Tunable Mobility
Graphene is an attractive material for spintronics due
to theoretical
predictions of long spin lifetimes arising from low spin–orbit
and hyperfine couplings. In experiments, however, spin lifetimes in
single-layer graphene (SLG) measured via Hanle effects are much shorter
than expected theoretically. Thus, the origin of spin relaxation in
SLG is a major issue for graphene spintronics. Despite extensive theoretical
and experimental work addressing this question, there is still little
clarity on the microscopic origin of spin relaxation. By using organic
ligand-bound nanoparticles as charge reservoirs to tune the mobility
between 2700 and 12 000 cm<sup>2</sup>/(V s), we successfully
isolate the effect of charged impurity scattering on spin relaxation
in SLG. Our results demonstrate that, while charged impurities can
greatly affect mobility, the spin lifetimes are not affected by charged
impurity scattering
Strontium Oxide Tunnel Barriers for High Quality Spin Transport and Large Spin Accumulation in Graphene
The
quality of the tunnel barrier at the ferromagnet/graphene interface
plays a pivotal role in graphene spin valves by circumventing the
impedance mismatch problem, decreasing interfacial spin dephasing
mechanisms and decreasing spin absorption back into the ferromagnet.
It is thus crucial to integrate superior tunnel barriers to enhance
spin transport and spin accumulation in graphene. Here, we employ
a novel tunnel barrier, strontium oxide (SrO), onto graphene to realize
high quality spin transport as evidenced by room-temperature spin
relaxation times exceeding a nanosecond in graphene on silicon dioxide
substrates. Furthermore, the smooth and pinhole-free SrO tunnel barrier
grown by molecular beam epitaxy (MBE), which can withstand large charge
injection current densities, allows us to experimentally realize large
spin accumulation in graphene at room temperature. This work puts
graphene on the path to achieve efficient manipulation of nanomagnet
magnetization using spin currents in graphene for logic and memory
applications
Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit
Monolayer
van der Waals (vdW) magnets provide an exciting opportunity
for exploring two-dimensional (2D) magnetism for scientific and technological
advances, but the intrinsic ferromagnetism has only been observed
at low temperatures. Here, we report the observation of room temperature
ferromagnetism in manganese selenide (MnSe<sub><i>x</i></sub>) films grown by molecular beam epitaxy (MBE). Magnetic and structural
characterization provides strong evidence that, in the monolayer limit,
the ferromagnetism originates from a vdW manganese diselenide (MnSe<sub>2</sub>) monolayer, while for thicker films it could originate from
a combination of vdW MnSe<sub>2</sub> and/or interfacial magnetism
of α-MnSe(111). Magnetization measurements of monolayer MnSe<sub><i>x</i></sub> films on GaSe and SnSe<sub>2</sub> epilayers
show ferromagnetic ordering with a large saturation magnetization
of ∼4 Bohr magnetons per Mn, which is consistent with the density
functional theory calculations predicting ferromagnetism in monolayer
1T-MnSe<sub>2</sub>. Growing MnSe<sub><i>x</i></sub> films
on GaSe up to a high thickness (∼40 nm) produces α-MnSe(111)
and an enhanced magnetic moment (∼2×) compared to the
monolayer MnSe<sub><i>x</i></sub> samples. Detailed structural
characterization by scanning transmission electron microscopy (STEM),
scanning tunneling microscopy (STM), and reflection high energy electron
diffraction (RHEED) reveals an abrupt and clean interface between
GaSe(0001) and α-MnSe(111). In particular, the structure measured
by STEM is consistent with the presence of a MnSe<sub>2</sub> monolayer
at the interface. These results hold promise for potential applications
in energy efficient information storage and processing
NaSn<sub>2</sub>As<sub>2</sub>: An Exfoliatable Layered van der Waals Zintl Phase
The
discovery of new families of exfoliatable 2D crystals that
have diverse sets of electronic, optical, and spin–orbit coupling
properties enables the realization of unique physical phenomena in
these few-atom-thick building blocks and in proximity to other materials.
Herein, using NaSn<sub>2</sub>As<sub>2</sub> as a model system, we
demonstrate that layered Zintl phases having the stoichiometry ATt<sub>2</sub>Pn<sub>2</sub> (A = group 1 or 2 element, Tt = group 14 tetrel
element, and Pn = group 15 pnictogen element) and feature networks
separated by van der Waals gaps can be readily exfoliated with both
mechanical and liquid-phase methods. We identified the symmetries
of the Raman-active modes of the bulk crystals <i>via</i> polarized Raman spectroscopy. The bulk and mechanically exfoliated
NaSn<sub>2</sub>As<sub>2</sub> samples are resistant toward oxidation,
with only the top surface oxidizing in ambient conditions over a couple
of days, while the liquid-exfoliated samples oxidize much more quickly
in ambient conditions. Employing angle-resolved photoemission spectroscopy,
density functional theory, and transport on bulk and exfoliated samples,
we show that NaSn<sub>2</sub>As<sub>2</sub> is a highly conducting
2D semimetal, with resistivities on the order of 10<sup>–6</sup> Ω·m. Due to peculiarities in the band structure, the
dominating p-type carriers at low temperature are nearly compensated
by the opening of n-type conduction channels as temperature increases.
This work further expands the family of exfoliatable 2D materials
to layered van der Waals Zintl phases, opening up opportunities in
electronics and spintronics