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.

Fano effect and Kondo effect in quantum dots formed in strongly coupled quantum wells

We present lateral transport measurements on strongly, vertically coupled quantum dots formed in separate quantum wells in a GaAs/AlGaAs heterostructure. Coulomb oscillations are observed forming a honeycomb lattice consistent with two strongly coupled dots. When the tunnel barriers in the upper well are reduced we observe the Fano effect due to the interfering paths through a resonant state in the lower well and a continuum state in the upper well. In both regimes an in plane magnetic field reduces the coupling between the wells when the magnetic length is comparable to the center to center separation of the wells. We also observe the Kondo effect which allows the spin states of the double dot system to be probed.Comment: 4 pages, 5 figure

Lithographically and electrically controlled strain effects on anisotropic magnetoresistance in (Ga,Mn)As

It has been demonstrated that magnetocrystalline anisotropies in (Ga,Mn)As are sensitive to lattice strains as small as 10^-4 and that strain can be controlled by lattice parameter engineering during growth, through post growth lithography, and electrically by bonding the (Ga,Mn)As sample to a piezoelectric transducer. In this work we show that analogous effects are observed in crystalline components of the anisotropic magnetoresistance (AMR). Lithographically or electrically induced strain variations can produce crystalline AMR components which are larger than the crystalline AMR and a significant fraction of the total AMR of the unprocessed (Ga,Mn)As material. In these experiments we also observe new higher order terms in the phenomenological AMR expressions and find that strain variation effects can play important role in the micromagnetic and magnetotransport characteristics of (Ga,Mn)As lateral nanoconstrictions.Comment: 11 pages, 4 figures, references fixe

Anisotropic Magnetoresistance components in (Ga,Mn)As

Our experimental and theoretical study of the non-crystalline and crystalline components of the anisotropic magnetoresistance (AMR) in (Ga,Mn)As is aimed at exploring the basic physical aspects of this relativistic transport effect. The non-crystalline AMR reflects anisotropic lifetimes of the holes due to polarized Mn impurities while the crystalline AMR is associated with valence band warping. We find that the sign of the non-crystalline AMR is determined by the form of spin-orbit coupling in the host band and by the relative strengths of the non-magnetic and magnetic contributions to the impurity potential. We develop experimental methods directly yielding the non-crystalline and crystalline AMR components which are then independently analyzed. We report the observation of an AMR dominated by a large uniaxial crystalline component and show that AMR can be modified by local strain relaxation. We discuss generic implications of our experimental and theoretical findings including predictions for non-crystalline AMR sign reversals in dilute moment systems.Comment: 4 pages, 3 figures. Phys. Rev. Lett. in pres

An Error-Control Code with an Imbalance of Ones and Zeros to Provide a Residual Carrier Component

We consider in this paper a direct sequence spread-spectrum communication system employing an error-control code having an imbalance of ones and zeroes. The primary motivation for using such a code is to provide a carrier component for synchronization as an alternative to the transmisson of a separate pilot tone. We evaluate the performance of this system when a concatenated code whose inner code is a constant-weight subcode of the (24, 12) extended Golay code and whose outer code is a Reed-Solomon code. We consider the effects of both white Gaussian noise and burst jamming, and we evaluate several decoding algorithms with different complexities and different coding gains. Near-maximum-likelihood decoding can be realized at the lowest data rates of interest, while successively less complicated algorithms achieving corresponding smaller coding gains must be used as the data rate increases. The performance of this system compares favorably with that of a more conventional pilot-tone system

Control of Coercivities in (Ga,Mn)As Thin Films by Small Concentrations of MnAs Nanoclusters

We demonstrate that low concentrations of a secondary magnetic phase in (Ga,Mn)As thin films can enhance the coercivity by factors up to ~100 without significantly degrading the Curie temperature or saturation magnetisation. Magnetic measurements indicate that the secondary phase consists of MnAs nanoclusters, of average size ~7nm. This approach to controlling the coercivity while maintaining high Curie temperature, may be important for realizing ferromagnetic semiconductor based devices.Comment: 8 pages,4 figures. accepted for publication in Appl. Phys. Let

Voltage control of magnetocrystalline anisotropy in ferromagnetic - semiconductor/piezoelectric hybrid structures

We demonstrate dynamic voltage control of the magnetic anisotropy of a (Ga,Mn)As device bonded to a piezoelectric transducer. The application of a uniaxial strain leads to a large reorientation of the magnetic easy axis which is detected by measuring longitudinal and transverse anisotropic magnetoresistance coefficients. Calculations based on the mean-field kinetic-exchange model of (Ga,Mn)As provide microscopic understanding of the measured effect. Electrically induced magnetization switching and detection of unconventional crystalline components of the anisotropic magnetoresistance are presented, illustrating the generic utility of the piezo voltage control to provide new device functionalities and in the research of micromagnetic and magnetotransport phenomena in diluted magnetic semiconductors.Comment: Submitted to Physical Review Letters. Updates version 1 to include a more detailed discussion of the effect of strain on the anisotropic magnetoresistanc