From Digital to Analogue Magnetoelectronics: Theory of Transport in Non-Collinear Magnetic Nanostructures

Magnetoelectronics is mainly digital, i.e. governed by up and down magnetizations. In contrast, analogue magnetoelectronics makes use of phenomena occuring for non-collinear magnetization configurations. Here we review theories which have recently been applied to the transport in non-collinear magnetic nanostructures in two and multiterminal structures, viz. random matrix and circuit theory. Both are not valid for highly transparent systems in a resistive environment like perpendicular metallic spin valves. The solution to this problem is a renormalization of the conventional and spin-mixing conductance parameters.Comment: To be published in "Advance in Solid State Physics", edited by B. Kramer, Springer Verlag, Berlin, 200

On-chip Magnetoresistive Sensors for Detection and Localization of Paramagnetic Particles

This paper presents the work towards miniaturized magnetic biosensor array based on the detection of paramagnetic particles using the giant magnetoresistance (GMR) effect. GMR sensors have been studied for many years, but its application for on-chip integration and in complex configurations, as well as effective localization for Lab-On-Chip and Tissue Engineering applications is not yet explored. This work demonstrates the development of initial prototypes of 5 and 9 sensor GMR arrays of varying geometries and corresponding calibration and localization algorithms to detect and localize paramagnetic materials in 2D. The generation of a uniform magnetic field using a 16 magnet Halbach cylinder was also analyzed and optimized using FEA for different sensor configurations. Results show excellent localization for the fully calibrated 5 sensor arrays, with a mean (SD) error of 2.45 (1.61) mm for the ferrofluid as compared to 1.48 (1.14) mm for a strong ferromagnet for a 25×25mm2 array surface. The 9sensor array similarly showed good results for full calibration

Itinerant G-type antiferromagnetism in D03_3-type V3_3Z (Z=Al, Ga, In) compounds: A first-principles study

Heusler compounds are widely studied due to their variety of magnetic properties making them ideal candidates for spintronic and magnetoelectronic applications. V$_3$Al in its metastable D0$_3$-type Heusler structure is a prototype for a rare antiferromagnetic gapless behavior. We provide an extensive study on the electronic and magnetic properties of V$_3$Al, V$_3$Ga and V$_3$In compounds based on state-of-the-art electronic structure calculations. We show that the ground state for all three is a G-type itinerant antiferromagnetic gapless semiconductor. The large antiferromagnetic exchange interactions lead to very high N\'eel temperatures, which are predicted to be around 1000 K. The coexistence of the gapless and antiferromagnetic behaviors in these compounds can be explained considering the simultaneous presence of three V atoms at the unit cell using arguments which have been employed for usual inverse Heusler compounds. We expect that our study on these compounds to enhance further the interest on them towards the optimization of their growth conditions and their eventual incorporation in devices.Comment: Submitted to Physical Review B, 8 pages, 9 figures, 1 tabl

Electromodulation of the Magnetoresistance in Diluted Magnetic Semiconductors Based Heterostructures

We study the properties of heterostructures formed by two layers of diluted magnetic semiconductor separated by a nonmagnetic semiconductor layer. We find that there is a RKKY-type exchange coupling between the magnetic layers that oscilles between ferromagnetic and antiferromagnetic as a function of the different parameters in the problem. The different transport properties of these phases make that this heterostructure presents strong magnetoresistive effects. The coupling can be also modified by an electric field. We propose that it is possible to alter dramatically the electrical resistance of the heterostructure by applying an electric field. Our results indicate that in a single gated sample the magnetoresistance could be modulated by with an electrical bias voltage.Comment: 4 pages, 5 figures include

Preparation and structural properties of thin films and multilayers of the Heusler compounds Cu2MnAl, Co2MnSn, Co2MnSi and Co2MnGe

We report on the preparation of thin films and multilayers of the intermetallic Heusler compound CuMnAl, Co2MnSn, Co2MnSi and Co2MnGe by rf-sputtering on MgO and Al2O3 substrates. Cu2MnAl can be grown epitaxially with (100)-orientation on MgO (100) and in (110)-orientation on Al2O3 a-plane. The Co based Heusler alloys need metallic seedlayers to induce high quality textured growth. We also have prepared multilayers with smooth interfaces by combining the Heusler compounds with Au and V. An analysis of the ferromagnetic saturation magnetization of the films indicates that the Cu2MnAl-compound tends to grow in the disordered B2-type structure whereas the Co-based Heusler alloy thin films grow in the ordered L21 structure. All multilayers with thin layers of the Heusler compounds exhibit a definitely reduced ferromagnetic magnetization indicating substantial disorder and intermixing at the interfaces.Comment: 21 pages, 8 figure

Spin-injection through an Fe/InAs Interface

The spin-dependence of the interface resistance between ferromagnetic Fe and InAs is calculated from first-principles for specular and disordered (001) interfaces. Because of the symmetry mismatch in the minority-spin channel, the specular interface acts as an efficient spin filter with a transmitted current polarisation between 98 an 89%. The resistance of a specular interface in the diffusive regime is comparable to the resistance of a few microns of bulk InAs. Symmetry-breaking arising from interface disorder reduces the spin asymmetry substantially and we conclude that efficient spin injection from Fe into InAs can only be realized using high quality epitaxial interfaces.Comment: 4 pages, 4 figure

Non-collinear Magnetoelectronics

The electron transport properties of hybrid ferromagnetic|normal metal structures such as multilayers and spin valves depend on the relative orientation of the magnetization direction of the ferromagnetic elements. Whereas the contrast in the resistance for parallel and antiparallel magnetizations, the so-called Giant Magnetoresistance, is relatively well understood for quite some time, a coherent picture for non-collinear magnetoelectronic circuits and devices has evolved only recently. We review here such a theory for electron charge and spin transport with general magnetization directions that is based on the semiclassical concept of a vector spin accumulation. In conjunction with first-principles calculations of scattering matrices many phenomena, e.g. the current-induced spin-transfer torque, can be understood and predicted quantitatively for different material combinations.Comment: 163 pages, to be published in Physics Report

Spin battery operated by ferromagnetic resonance

Precessing ferromagnets are predicted to inject a spin current into adjacent conductors via Ohmic contacts, irrespective of a conductance mismatch with, for example, doped semiconductors. This opens the way to create a pure spin source spin battery by the ferromagnetic resonance. We estimate the spin current and spin bias for different material combinations.Comment: The estimate for the magnitude of the spin bias is improved. We find that it is feasible to get a measurable signal of the order of the microwave frequency already for moderate rf intensitie

Spin-dependent Transparency of Ferromagnet/Superconductor Interfaces

Because the physical interpretation of the spin-polarization of a ferromagnet determined by point-contact Andreev reflection (PCAR) is non-trivial, we have carried out parameter-free calculations of PCAR spectra based upon a scattering-theory formulation of Andreev reflection generalized to spin-polarized systems and a tight-binding linear muffin tin orbital method for calculating the corresponding scattering matrices. PCAR is found to measure the spin-dependent interface transparency rather than the bulk polarization of the ferromagnet which is strongly overestimated by free electron model fitting.Comment: 4 pages, 1figure. submitte

Orientation-Dependent Transparency of Metallic Interfaces

As devices are reduced in size, interfaces start to dominate electrical transport making it essential to be able to describe reliably how they transmit and reflect electrons. For a number of nearly perfectly lattice-matched materials, we calculate from first-principles the dependence of the interface transparency on the crystal orientation. Quite remarkably, the largest anisotropy is predicted for interfaces between the prototype free-electron materials silver and aluminium for which a massive factor of two difference between (111) and (001) interfaces is found