Advances in Science and Technology Vol. 52

Abstract: Transport properties of systems based on diluted magnetic semiconductors (Ga,Mn)As are investigated theoretically by means of the Kubo linear response theory. The underlying electronic structure is obtained within the local spin-density approximation using the scalarrelativistic tight-binding linear muffin-tin orbital (TB-LMTO) method. The effect of substitutional randomness on the electronic structure and on the transport properties is systematically described in the coherent potential approximation (CPA). The quantities studied include the residual resistivities of the bulk alloys as well as conductances of epitaxial trilayers Cr/(Ga,Mn)As/Cr (001) in the current-perpendicular-to-plane (CPP) orientation. The results witness that various compensating defects such as As antisite atoms and Mn interstitials have much stronger detrimental effect on the spin polarization of the CPP conductances as compared to their influence on the spin polarization of the bulk conductivities.

Abstract: We present an extensive theoretical and experimental study of the Mn-Ge dilute magnetic semiconductor, a material which - due to its high integrability with mainstream Si technology - may hold good promises for spintronic applications. Ab-initio calculations on several different systems containing isolated Mn impurities as well as small clusters (up to three Mn impurities) show that Mn has a tendency to segregate into the Ge matrix and to stabilize occupation of interstitial sites if these are coordinated with other Mn occupying substitutional sites nearby. Several different experimental characterizations (HRTEM, XRD, UPS, MOKE) performed on Mn ion-implanted systems are analyzed and discussed: a close comparison betweeen experimental evidences and density functional calculations allows a full understanding of the sample properties and to disentagle the contributions coming from the diluted and segregated phases. The complexity of this system shows that much has to be done still to understand the physics of these materials and to undisclose all their possible applications.

Abstract: It is shown that in colossal magnetoresistance materials an inhomogeneous alternating magnetic field generates a strong electric field of non-inductive nature. This magnetoelectric effect is an analog of acoustoelectric effect in conventional semiconductors. Due to the above electric field spin waves in the former materials, like acoustic waves in the latter ones, acquire an additional attenuation at equilibrium. This attenuation may be converted to amplification by applying strong enough dc electric field drifting the carriers (solid-state Cherenkov’s effect). The experiments, which probed this phenomenon in HgCr2Se4 using spin wave pumping, are discussed.

Abstract: Polycrystalline stoichiometric Co-substituted ZnO oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in air. The precise nature of magnetic coupling is determined by studying carefully structural and magnetic properties. The magnetization as a function of temperature for Zn1-xCoxO (x = 0.02, 0.05, 0.0625 and 0.10) can be fitted well to a model with a paramagnetic Curie term, an antiferromagnetic Curie-Weiss term and a diamagnetic constant, which could arise from spins of isolated Co ions, grouped Co ions that are in nearest neighbor positions and a diamagnetic background, respectively. The substitution of Co at the Zn sites does not occur in a completely random manner but the Co ions appear to have a tendency for grouping. It is interesting to note in Raman measurements that host lattice defects with 2 distinct impurity modes may be induced by isolated and grouped Co spins.

Abstract: The present paper describes the contacting technology to the diluted magnetic semiconductor Cd1-xMnxTe having potential applications in optoelectronic and spintronic devices. For efficient spin injection into a spintronic material, a matching ohmic contact is the demand of the time. Since cadmium telluride has a well-known contact problem, its manganese-doped counterpart is also facing a similar difficulty. In the present case Cd1-xMnxTe was fabricated using thermally assisted interdiffusion and compound formation between repeated stacked elemental layers of manganese, cadmium and tellurium. A wet electroless deposition technique was employed to deposit manganese doped nickel phosphide as a magnetic contact onto Cd1-xMnxTe. It appeared that the contact resistivity improved compared to the case of gold contact. The details of the contacting technology and the results have been described in the text of the paper.

Abstract: Incorporating diluted magnetic semiconductor (DMS) layers within barrier devices offers new device design potential. To study these devices we have generalized an existing time dependent transient algorithm that couples the Wigner transport equation to Poisson’s equation and an external circuit. For electron transport we have studied alterations in the dc and time dependent behavior of resonant tunneling diodes with DMS barriers and wells, have transformed a single barrier structure into a double barrier structure and examined the increased functionality of the devices. We present new results, including some preliminary calculations incorporating holes, discuss transients and the potential role that DMS layers will play in controlling the transient operation of superlattice structures.

Abstract: The results Co and Fe implanted ZnO thin films were studied before and after 200 MeV Ag ion irradiation. The as-implanted films shows the presence of nano sized Co and Fe clusters as seen through XRD patterns and exhibited high resistivity compared to un-implanted films. After Ag ion irradiation the Co and Fe clusters get dissolved in ZnO lattice and the films resistivity reduced to half of the as implanted values. The magnetic properties of Ag irradiated films were confirmed through magnetization hysteresis and Co implanted films exhibit higher magnetization compared to Fe implanted films.

Abstract: The new hybrid device consisting of patterned array of Co nanodots on GaMnAs channel was fabricated and demonstrated to give a new functionality to control the transport properties of GaMnAs. Magnetic state of array of Co nanodots was observed with magnetic force microscopy (MFM) observation. The magnetic state of individual Co dots can be controlled by using a MFM tip. Different distribution of magnetic state of Co nanodots in an array structure resulted in an inhomogeneous magnetic field. Magnetic field induced by the array of nanodots leads to change in the transport properties of GaMnAs. This inhomogeneous magnetic field was regarded to act as an effective potential that can trap the spin polarized carriers in the GaMnAs system with giant Zeeman splitting.