arXiv subjects

C. Gould

Publications and source records attributed to C. Gould.

54 records · Page 3Linked to original sources

arXiv2006

Lithographic engineering of anisotropies in (Ga,Mn)As

The focus of studies on ferromagnetic semiconductors is moving from material issues to device functionalities based on novel phenomena often associated with the anisotropy properties of these materials. This is driving a need for a method to locally control the anisotropy in order to allow the elaboration of devices. Here we present a method which provides patterning induced anisotropy which not only can be applied locally, but also dominates over the intrinsic material anisotropy at all temperatures.

cond-mat.mes-hall↗

arXiv2006

Magnetization-Switched Metal-Insulator Transition in a (Ga,Mn)As Tunnel Device

We observe the occurrence of an Efros-Shklovskii gap in (Ga,Mn)As based tunnel junctions. The occurrence of the gap is controlled by the extent of the hole wave-function on the Mn acceptor atoms. Using k.p-type calculations we show that this extent depends crucially on the direction of the magnetization in the (Ga,Mn)As (which has two almost equivalent easy axes). This implies one can reversibly tune the system into the insulating or metallic state by changing the magnetization.

cond-mat.str-el↗

arXiv2006

Current Assisted Magnetization Switching in (Ga,Mn)As Nanodevices

Current induced magnetization switching and resistance associated with domain walls pinned in nanoconstrictions have both been previously reported in (Ga,Mn)As based devices, but using very dissimilar experimental schemes and device geometries . Here we report on the simultaneous observation of both effects in a single nanodevice, which constitutes a significant step forward towards the eventual realization of spintronic devices which make use of domain walls to store, transport, and manipulate information.

cond-mat.mes-hall↗

arXiv2005

Tunneling magnetoresistance in devices based on epitaxial NiMnSb with uniaxial anisotropy

We demonstrate tunnel magnetoresistance (TMR) junctions based on a tri layer system consisting of an epitaxial NiMnSb, aluminum oxide and CoFe tri layer. The junctions show a tunnelling magnetoresistance of Delta R/R of 8.7% at room temperature which increases to 14.7% at 4.2K. The layers show clear separate switching and a small ferromagnetic coupling. A uniaxial in plane anisotropy in the NiMnSb layer leads to different switching characteristics depending on the direction in which the magnetic field is applied, an effect which can be used for sensor applications.

cond-mat.mtrl-sci↗

arXiv2005

Self Assembled II-VI Magnetic Quantum Dot as a Voltage-Controlled Spin-Filter

A key element in the emergence of a full spintronics technology is the development of voltage controlled spin filters to selectively inject carriers of desired spin into semiconductors. We previously demonstrated a prototype of such a device using a II-VI dilute-magnetic semiconductor quantum well which, however, still required an external magnetic field to generate the level splitting. Recent theory suggests that spin selection may be achievable in II-VI paramagnetic semiconductors without external magnetic field through local carrier mediated ferromagnetic interactions. We present the first experimental observation of such an effect using non-magnetic CdSe self-assembled quantum dots in a paramagnetic (Zn,Be,Mn)Se barrier.

cond-mat.mes-hall↗

arXiv2004

Molecular-beam epitaxy of (Zn,Mn)Se on Si(100)

We have investigated the growth by molecular-beam epitaxy of the II-VI diluted magnetic semiconductor (Zn,Mn)Se on As-passivated Si(100) substrates. The growth start has been optimized by using low-temperature epitaxy. Surface properties were assessed by Nomarski and scanning electron microscopy. Optical properties of (Zn,Mn)Se have been studied by photoluminescence and a giant Zeeman splitting of up to 30 meV has been observed. Our observations indicate a high crystalline quality of the epitaxial films.

cond-mat.mtrl-sci↗

arXiv2004

1500-fold Tunneling Anisotropic Magnetoresistance in a (Ga,Mn)As stack

We report the discovery of a super-giant tunneling anisotropic magnetoresistance in an epitaxially grown (Ga,Mn)As/GaAs/(Ga,Mn)As structure. The effect arises from a strong dependence of the electronic structure of ferromagnetic semiconductors on the magnetization orientation rather than from a parallel or antiparallel alignment of the contacts. The key novel spintronics features of this effect are: (i) both normal and inverted spin-valve like signals; (ii) a large non-hysteretic magnetoresistance for magnetic fields perpendicular to the interfaces; (iii) magnetization orientations for extremal resistance are, in general, not aligned with the magnetic easy and hard axis. (iv) Enormous amplification of the effect at low bias and temperatures.

cond-mat.mes-hall↗

arXiv2004

Tunneling Anisotropic Magnetoresistance: A spin-valve like tunnel magnetoresistance using a single magnetic layer

We introduce a new class of spintronics devices in which a spin-valve like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normal-metal/insulator/ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction, and the two-step magnetization reversal process in this material.

cond-mat.mes-hall↗

arXiv2003

Very large magnetoresistance in lateral ferromagnetic (Ga,Mn)As wires with nanoconstrictions

We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples we find a magnetoresistance up to 8%, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to 2000 %.

cond-mat.mes-hall↗

arXiv2003

Voltage-Controlled Spin Selection in a Magnetic Resonant Tunnelling Diode

We have fabricated all II-VI semiconductor resonant tunneling diodes based on the (Zn,Mn,Be)Se material system, containing dilute magnetic material in the quantum well, and studied their current-voltage characteristics. When subjected to an external magnetic field the resulting spin splitting of the levels in the quantum well leads to a splitting of the transmission resonance into two separate peaks. This is interpreted as evidence of tunneling transport through spin polarized levels, and could be the first step towards a voltage controlled spin filter.

cond-mat.mes-hall↗

arXiv2002

Temperature Dependent Magnetic Anisotropy in (Ga,Ma)As Layers

It is demonstrated by SQUID magnetization measurements that (Ga,Mn)As films can exhibit rich characteristics of magnetic anisotropy depending not only to the epitaxial strain but being strongly influenced by the hole and Mn concentration, and temperature. This behavior reflects the spin anisotropy of the valence subbands and corroborates predictions of the mean field Zener model of the carrier mediated ferromagnetism in III-V diluted magnetic semiconductors with Mn. At the same time the existence of in-plane uniaxial anisotropy with [110] the easy axis is evidenced. This is related to the top/bottom symmetry breaking, resulting in the lowering of point symmetry of (Ga,Mn)As to the C_{2v} symmetry group. The latter mechanism coexists with the hole-induced cubic anisotropy, but takes over close to T_C.

cond-mat.mtrl-sci↗

arXiv2002

Spin injection in the non-linear regime: band bending effects

Semiconductor spintronics will need to control spin injection phenomena in the non-linear regime. In order to study these effects we have performed spin injection measurements from a dilute magnetic semiconductor [(Zn,Be,Mn)Se] into nonmagnetic (Zn,Be)Se at elevated bias. When the applied voltage is increased to a few mV we find a strong decrease of the spin injection efficiency. The observed behavior is modelled by extending the charge-imbalance model for spin injection to include band bending and charge accumulation at the interface of the two compounds. We find that the observed effects can be attributed to repopulation of the minority spin level in the magnetic semiconductor.

cond-mat.mes-hall↗

arXiv2001

Spin filtering and magnetoresistance in ballistic tunnel junctions

We theoretically investigate magnetoresistance (MR) effects in connection with spin filtering in quantum-coherent transport through tunnel junctions based on non-magnetic/semimagnetic heterostructures. We find that spin filtering in conjunction with the suppression/enhancement of the spin-dependent Fermi seas in semimagnetic contacts gives rise to (i) spin-split kinks in the MR of single barriers and (ii) a robust beating pattern in the MR of double barriers with a semimagnetic well. We believe these are unique signatures for quantum filtering.

cond-mat.mes-hall↗

arXiv2001

Large magnetoresistance effect due to spin-injection into a non-magnetic semiconductor

A novel magnetoresistance effect, due to the injection of a spin-polarized electron current from a dilute magnetic into a non-magnetic semiconductor, is presented. The effect results from the suppression of a spin channel in the non-magnetic semiconductor and can theoretically yield a positive magnetoresistance of 100%, when the spin flip length in the non-magnetic semiconductor is sufficiently large. Experimentally, our devices exhibit up to 25% magnetoresistance.

cond-mat.mes-hall↗

arXiv2000

The Addition Spectrum of a Lateral Dot from Coulomb and Spin Blockade Spectroscopy

Transport measurements are presented on a class of electrostatically defined lateral dots within a high mobility two dimensional electron gas (2DEG). The new design allows Coulomb Blockade(CB) measurements to be performed on a single lateral dot containing 0, 1 to over 50 electrons. The CB measurements are enhanced by the spin polarized injection from and into 2DEG magnetic edge states. This combines the measurement of charge with the measurement of spin through spin blockade spectroscopy. The results of Coulomb and spin blockade spectroscopy for first 45 electrons enable us to construct the addition spectrum of a lateral device. We also demonstrate that a lateral dot containing a single electron is an effective local probe of a 2DEG edge.

cond-mat.mes-hall↗

arXiv1998

Solar Fusion Cross Sections

We review and analyze the available information for nuclear fusion cross sections that are most important for solar energy generation and solar neutrino production. We provide best values for the low-energy cross-section factors and, wherever possible, estimates of the uncertainties. We also describe the most important experiments and calculations that are required in order to improve our knowledge of solar fusion rates.

astro-ph↗

arXiv1997

Fractal Conductance Fluctuations in a Soft Wall Stadium and a Sinai Billiard

Conductance fluctuations have been studied in a soft wall stadium and a Sinai billiard defined by electrostatic gates on a high mobility semiconductor heterojunction. These reproducible magnetoconductance fluctuations are found to be fractal confirming recent theoretical predictions of quantum signatures in classically mixed (regular and chaotic) systems. The fractal character of the fluctuations provides direct evidence for a hierarchical phase space structure at the boundary between regular and chaotic motion.

cond-mat.mes-hall↗

arXiv1997

Resonance Patterns of an Antidot Cluster: From Classical to Quantum Ballistics

We explain the experimentally observed Aharonov-Bohm (AB) resonance patterns of an antidot cluster by means of quantum and classical simulations and Feynman path integral theory. We demonstrate that the observed behavior of the AB period signals the crossover from a low B regime which can be understood in terms of electrons following classical orbits to an inherently quantum high B regime where this classical picture and semiclassical theories based on it do not apply.

cond-mat.mes-hall↗