200 research outputs found
Anisotropic magnetoresistance in a 2DEG in a quasi-random magnetic field
We present magnetotransport results for a 2D electron gas (2DEG) subject to
the quasi-random magnetic field produced by randomly positioned sub-micron Co
dots deposited onto the surface of a GaAs/AlGaAs heterostructure. We observe
strong local and non-local anisotropic magnetoresistance for external magnetic
fields in the plane of the 2DEG. Monte-Carlo calculations confirm that this is
due to the changing topology of the quasi-random magnetic field in which
electrons are guided predominantly along contours of zero magnetic field.Comment: 4 pages, 6 figures, submitted to Phys. Rev.
Maintenance and digital health control in smart manufacturing based on condition monitoring
Smart manufacturing is the modern form of manufacturing that utilises Industry 4.0 enablers for decision making and resources planning by taking advantage of the available data. Therefore, the state of the art technologies are either replaced or improved using the newly introduced manufacturing paradigm. In practice, condition monitoring is an on-going activity that preserves the manufacturing facility capability to deliver its production aims and decrease the production discontinuity as much as possible. Against this background, this paper discusses the state of the art condition monitoring and proposes a framework of fault detection and decision making at different levels namely component and station. The introduced framework relies on Virtual Engineering (VE) and Discrete Event Simulation (DES) in smart manufacturing environments. The application of the suggested methodology and its implementation is demonstrated in a case study of a battery module assembly line
Ballistic transport in random magnetic fields with anisotropic long-ranged correlations
We present exact theoretical results about energetic and dynamic properties
of a spinless charged quantum particle on the Euclidean plane subjected to a
perpendicular random magnetic field of Gaussian type with non-zero mean. Our
results refer to the simplifying but remarkably illuminating limiting case of
an infinite correlation length along one direction and a finite but strictly
positive correlation length along the perpendicular direction in the plane.
They are therefore ``random analogs'' of results first obtained by A. Iwatsuka
in 1985 and by J. E. M\"uller in 1992, which are greatly esteemed, in
particular for providing a basic understanding of transport properties in
certain quasi-two-dimensional semiconductor heterostructures subjected to
non-random inhomogeneous magnetic fields
Determining Curie temperatures in dilute ferromagnetic semiconductors: high Curie temperature (Ga,Mn)As
In this paper, we use simultaneous magnetometry and electrical transport measurements to critically examine ways in which the Curie temperature (TC) values have been determined in studies of dilute magnetic semiconductors. We show that, in sufficiently homogeneous samples, TC can be accurately determined from remanent magnetization and magnetic susceptibility and from the positions of the peak in the temperature derivative of the resistivity. We also show that the peak of the resistivity does not occur at TC, as illustrated by a (Ga,Mn)As sample for which the peak of the resistivity is at 21361K when TC is only 17861
Non-invasive detection of the evolution of the charge states of a double dot system
Coupled quantum dots are potential candidates for qubit systems in quantum
computing. We use a non-invasive voltage probe to study the evolution of a
coupled dot system from a situation where the dots are coupled to the leads to
a situation where they are isolated from the leads. Our measurements allow us
to identify the movement of electrons between the dots and we can also identify
the presence of a charge trap in our system by detecting the movement of
electrons between the dots and the charge trap. The data also reveals evidence
of electrons moving between the dots via excited states of either the single
dots or the double dot molecule.Comment: Accepted for publication in Phys. Rev. B. 4 pages, 4 figure
On the character of states near the Fermi level in (Ga,Mn)As: impurity to valence band crossover
We discuss the character of states near the Fermi level in Mn doped GaAs, as
revealed by a survey of dc transport and optical studies over a wide range of
Mn concentrations. A thermally activated valence band contribution to dc
transport, a mid-infrared peak at energy hbar omega approx 200 meV in the ac-
conductivity, and the hot photoluminescence spectra indicate the presence of an
impurity band in low doped (<<1% Mn) insulating GaAs:Mn materials. Consistent
with the implications of this picture, both the impurity band ionization energy
inferred from the dc transport and the position of the mid-infrared peak move
to lower energies and the peak broadens with increasing Mn concentration. In
metallic materials with > 2% doping, no traces of Mn-related activated
contribution can be identified in dc-transport, suggesting that the impurity
band has merged with the valence band. No discrepancies with this perception
are found when analyzing optical measurements in the high-doped GaAs:Mn. A
higher energy (hbar omega approx 250 meV) mid-infrared feature which appears in
the metallic samples is associated with inter-valence band transitions. Its
red-shift with increased doping can be interpreted as a consequence of
increased screening which narrows the localized-state valence-band tails and
weakens higher energy transition amplitudes. Our examination of the dc and ac
transport characteristics of GaAs:Mn is accompanied by comparisons with its
shallow acceptor counterparts, confirming the disordered valence band picture
of high-doped metallic GaAs:Mn material.Comment: 10 pages, 12 figure
Prospects of high temperature ferromagnetism in (Ga,Mn)As semiconductors
We report on a comprehensive combined experimental and theoretical study of
Curie temperature trends in (Ga,Mn)As ferromagnetic semiconductors. Broad
agreement between theoretical expectations and measured data allows us to
conclude that T_c in high-quality metallic samples increases linearly with the
number of uncompensated local moments on Mn_Ga acceptors, with no sign of
saturation. Room temperature ferromagnetism is expected for a 10% concentration
of these local moments. Our magnetotransport and magnetization data are
consistnent with the picture in which Mn impurities incorporated during growth
at interstitial Mn_I positions act as double-donors and compensate neighboring
Mn_Ga local moments because of strong near-neighbor Mn_Ga-Mn_I
antiferromagnetic coupling. These defects can be efficiently removed by
post-growth annealing. Our analysis suggests that there is no fundamental
obstacle to substitutional Mn_Ga doping in high-quality materials beyond our
current maximum level of 6.2%, although this achievement will require further
advances in growth condition control. Modest charge compensation does not limit
the maximum Curie temperature possible in ferromagnetic semiconductors based on
(Ga,Mn)As.Comment: 13 pages, 12 figures, submitted to Phys. Rev.
Experimental observation of the optical spin transfer torque
The spin transfer torque is a phenomenon in which angular momentum of a spin
polarized electrical current entering a ferromagnet is transferred to the
magnetization. The effect has opened a new research field of electrically
driven magnetization dynamics in magnetic nanostructures and plays an important
role in the development of a new generation of memory devices and tunable
oscillators. Optical excitations of magnetic systems by laser pulses have been
a separate research field whose aim is to explore magnetization dynamics at
short time scales and enable ultrafast spintronic devices. We report the
experimental observation of the optical spin transfer torque, predicted
theoretically several years ago building the bridge between these two fields of
spintronics research. In a pump-and-probe optical experiment we measure
coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by
circularly polarized laser pulses. During the pump pulse, the spin angular
momentum of photo-carriers generated by the absorbed light is transferred to
the collective magnetization of the ferromagnet. We interpret the observed
optical spin transfer torque and the magnetization precession it triggers on a
quantitative microscopic level. Bringing the spin transfer physics into optics
introduces a fundamentally distinct mechanism from the previously reported
thermal and non-thermal laser excitations of magnets. Bringing optics into the
field of spin transfer torques decreases by several orders of magnitude the
timescales at which these phenomena are explored and utilized.Comment: 11 pages, 4 figure
Nonvolatile ferroelectric control of ferromagnetism in (Ga,Mn)As
There is currently much interest in materials and structures that provide
coupled ferroelectric and ferromagnetic responses, with a long-term goal of
developing new memories and spintronic logic elements. Within the field there
is a focus on composites coupled by magnetostrictive and piezoelectric strain
transmitted across ferromagnetic-ferroelectric interfaces, but substrate
clamping limits the response in the supported multilayer configuration favoured
for devices. This constraint is avoided in a ferroelectric-ferromagnetic
bilayer in which the magnetic response is modulated by the electric field of
the poled ferroelectric. Here, we report the realization of such a device using
a diluted magnetic semiconductor (DMS) channel and a polymer ferroelectric
gate. Polarization reversal of the gate by a single voltage pulse results in a
persistent modulation of the Curie temperature as large as 5%. The device
demonstrates direct and quantitatively understood electric-fieldmediated
coupling in a multiferroic bilayer and may provide new routes to nanostructured
DMS materials and devices via ferroelectric domain nanopatterning. The
successful implementation of a polymer-ferroelectric gate fieldeffect
transistor (FeFET) with a DMS channel adds a new functionality to semiconductor
spintronics and may be of importance for future low-voltage spintronics devices
and memory structures.Comment: 19 pages, 5 figure
Deterministic control of magnetic vortex wall chirality by electric field
Concepts for information storage and logical processing based on magnetic domain walls have great potential for implementation in future information and communications technologies. To date, the need to apply power hungry magnetic fields or heat dissipating spin polarized currents to manipulate magnetic domain walls has limited the development of such technologies. The possibility of controlling magnetic domain walls using voltages offers an energy efficient route to overcome these limitations. Here we show that a voltage-induced uniaxial strain induces reversible deterministic switching of the chirality of a magnetic vortex wall. We discuss how this functionality will be applicable to schemes for information storage and logical processing, making a significant step towards the practical implementation of magnetic domain walls in energy efficient computing
- …