3,205 research outputs found
Graphene spin capacitor for magnetic field sensing
An analysis of a novel magnetic field sensor based on a graphene spin
capacitor is presented. The proposed device consists of graphene nanoribbons on
top of an insulator material connected to a ferromagnetic source/drain. The
time evolution of spin polarized electrons injected into the capacitor can be
used for an accurate determination at room temperature of external magnetic
fields. Assuming a spin relaxation time of 100 ns, magnetic fields on the order
of mOe may be detected at room temperature. The observational
accuracy of this device depends on the density of magnetic defects and spin
relaxation time that can be achieved.Comment: 6 pages, 3 figure
Bistability in a magnetic and nonmagnetic double-quantum-well structure mediated by the magnetic phase transition
The hole distribution in a double quantum well (QW) structure consisting of a
magnetic and a nonmagnetic semiconductor QW is investigated as a function of
temperature, the energy shift between the QWs, and other relevant parameters.
When the itinerant holes mediate the ferromagnetic ordering, it is shown that a
bistable state can be formed through hole redistribution, resulting in a
significant change in the properties of the constituting magnetic QW (i.e., the
paramagnetic-ferromagnetic transition). The model calculation also indicates a
large window in the system parameter space where the bistability is possible.
Hence, this structure could form the basis of a stable memory element that may
be scaled down to a few hole regime.Comment: 9 pages, 3 figure
Unusual magnetoresistance in a topological insulator with a single ferromagnetic barrier
Tunneling surface current through a thin ferromagnetic barrier in a
three-dimensional topological insulator is shown to possess an extraordinary
response to the orientation of barrier magnetization. In contrast to
conventional magnetoresistance devices that are sensitive to the relative
alignment of two magnetic layers, a drastic change in the transmission current
is achieved by a single layer when its magnetization rotates by 90 degrees.
Numerical estimations predict a giant magnetoresistance as large as 800 % at
room temperature and the proximate exchange interaction of 40 meV in the
barrier. When coupled with electrical control of magnetization direction, this
phenomenon may be used to enhance the gating function with potentially sharp
turn-on/off for low power applications
Weak ferromagnetism of antiferromagnetic domains in graphene with defects
Magnetic properties of graphene with randomly distributed magnetic
defects/vacancies are studied in terms of the Kondo Hamiltonian in the mean
field approximation. It has been shown that graphene with defects undergoes a
magnetic phase transition from a paramagnetic to a antiferromagnetic (AFM)
phase once the temperature reaches the critical point . The defect
straggling is taken into account as an assignable cause of multiple nucleation
into AFM domains. Since each domain is characterized by partial compensating
magnetization of the defects associated with different sublattices, together
they reveal a super-paramagnetic behavior in a magnetic field. Theory
qualitatively describe the experimental data provided the temperature
dependence of the AFM domain structure.Comment: 8 pages, 2 figure
Phonon-mediated electron spin phase diffusion in a quantum dot
An effective spin relaxation mechanism that leads to electron spin
decoherence in a quantum dot is proposed. In contrast to the common
calculations of spin-flip transitions between the Kramers doublets, we take
into account a process of phonon-mediated fluctuation in the electron spin
precession and subsequent spin phase diffusion. Specifically, we consider
modulations in the longitudinal g-factor and hyperfine interaction induced by
the phonon-assisted transitions between the lowest electronic states. Prominent
differences in the temperature and magnetic field dependence between the
proposed mechanisms and the spin-flip transitions are expected to facilitate
its experimental verification. Numerical estimation demonstrates highly
efficient spin relaxation in typical semiconductor quantum dots.Comment: 5 pages, 1 figur
Non-volatile bistability effect based on electrically controlled phase transition in scaled magnetic semiconductor nanostructures
We explore the bistability effect in a dimensionally scaled semiconductor
nanostruncture consisting of a diluted magnetic semiconductor quantum dot (QD)
and a reservoir of itinerant holes separated by a barrier. The bistability
stems from the magnetic phase transition in the QD mediated by the changes in
the hole population. Our calculation shows that when properly designed, the
thermodynamic equilibrium of the scaled structure can be achieved at two
different configurations; i.e., the one with the QD in a ferromagnetic state
with a sufficient number of holes and the other with the depopulated QD in a
paramagnetic state. Subsequently, the parameter window suitable for this
bistability formation is discussed along with the the conditions for the
maximum robustness/non-volatility. To examine the issue of scaling, an
estimation of the bistabiity lifetime is made by considering the thermal
fluctuation in the QD hole population via the spontaneous transitions. A
numerical evaluation is carried out for a typical carrier-mediated magnetic
semiconductor (e.g., GaMnAs) as well as for a hypothetical case of high Curie
temperature for potential room temperature operation.Comment: 9 pages, 7 figure
Electron spin relaxation via flexural phonon modes in semiconducting carbon nanotubes
This work considers the g-tensor anisotropy induced by the flexural thermal
vibrations in one-dimensional structures and its role in electron spin
relaxation. In particular, the mechanism of spin-lattice relaxation via
flexural modes is studied theoretically for localized and delocalized
electronic states in semiconducting carbon nanotubes in the presence of
magnetic field. The calculation of one-phonon spin-flip process predicts
distinctive dependencies of the relaxation rate on temperature, magnetic field
and nanotube diameter. Comparison with the spin relaxation caused by the
hyperfine interaction clearly suggests the relative efficiency of the proposed
mechanism at sufficiently high temperatures. Specifically, the longitudinal
spin relaxation time in the semiconducting carbon nanotubes is estimated to be
as short as 30 microseconds at room temperature.Comment: 18 pages, 7 figure
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