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Spin Transport

#2705


Spin Transport in Magnetic Nano-Structures

Tech Area / Field

  • PHY-SSP/Solid State Physics/Physics

Status
3 Approved without Funding

Registration date
13.02.2003

Leading Institute
Federal State Unitary Enterprise Zelenograd Research Institute for Physical Problems, Russia, Moscow reg., Zelenograd

Supporting institutes

  • Russian Academy of Sciences / Institute of Radioengineering and Electronics / Fryazino Branch, Russia, Moscow reg., Fryazino\nInstitute of Physics of Microstructures, Russia, N. Novgorod reg., N. Novgorod\nInstitute of General Physics named after A.M. Prokhorov RAS, Russia, Moscow

Collaborators

  • New York University / Department of Physics, USA, New York\nUniversity of Virginia / Departmernt of Chemistry and Physics / The Center for Interactive Micromagnetics, USA, VA, Petersburg\nUniversity of Oxford / Department of Physics, UK, Oxford\nUniversität Osnabrück, Germany, Osnabrück\nMassachusetts Institute of Technology (MIT) / Francis Bitter Magnet Laboratory, USA, MA, Cambridge\nQueen's University of Belfast / School of Mathematics and Physics, UK, Belfast

Project summary

Objectives of the project.

The objective of the project consists in working out the physical basis of a novel technology for nascent spin-electronics, namely (1) development of technology for producing and testing spin-valves, point contacts, nanowires, and other nanostructures; (2) experimental and theoretical study of spin transport in single- and multilayered nanostructures; (3) formulation of new principles of functioning of electronic devices employing the effect of spin-dependent transport; (4) search for the possibility to make experimental samples of the devices based on spin-dependent transport; (5) mathematical modeling of magnetic nanostructures and devices of spin electronics.

Present state of research and problems.

A new generation of non-volatile random access magnetic memory devices using the giant magnetoresistance effect can be both competitive and compatible with modern semiconductor microchips. Spin-valve and spin-tunneling diode multilayered structures investigated recently consist of at least two ferromagnetic metallic layers separated by an ultrathin nonmagnetic layer of either a dielectric or a metal. Primarily, the two modes of current flow in the structures are studied: the current flow parallel to the layers (CIP-mode) and perpendicular to them (CPP-mode, taking place, for instance, in the magnetic tunneling transitions). There appears a problem of improving a quality of the structures, which requires perfecting the technological processes of their fabrication as well as optimizing their chemical compositions and shapes. Moreover, many essential features concerning the influence of the spin polarized current upon magnetic states of the layers have not been made clear yet. Further improvement of such a memory elements depends strongly on the problem of increasing signal-to-noise ratio and searching for alternative methods of controllable switching between magnetic states of memory cells.

Reduction of sizes of spin valves, spin-tunneling structures, and spin transistors, which is necessary for increasing the speed and information capacity of the storage, entails a growing influence of both imperfections of the materials and a thermally activated noise on the threshold of switching and read-out signal. The multimode character of the magnetization reversal along with the considerable influence of thermal activation can lead to an enormous instability of switching thresholds. Under these circumstances, there arises an urgent problem to increase stability of magnetic states through choosing the geometry and magnetic parameters of the elements. In addition to this, it becomes necessary to increase the magneto-resistance response as size of the elements decreases. The requirement is especially essential to tunneling structures, whose input resistance is large enough to cause an increasing noise. One of the possible ways to solve this problem may consist in an employing the giant quantum magneto resistance effect in conducting magnetic nano contacts of 3d metals (Ni and Co) which has recently observed experimentally. Application of nano contacts may turn out to be more preferable, than that of tunneling structures, since a value of current in the opening channel is essentially higher for the metallic contact. The giant quantum magnetoresistance effect in thin contacting magnetic wires is exceptionally well-pronounced (more than 300%), and this feature may be ascribed to the formation of a quantum domain wall (QDW) in the nanocontact; a size of the QDW is compared with that of the nanocontact itself and comprises several atomic layers. According to the existing theory, the effect of conductivity quantization in a nanoconstriction is caused by the transversal size quantization of transport electrons. So far there is a lack of comprehension in the theory of nanocontacts about a quantitative explanation of the large magnetoresistance associated with QDW and size quantization. Other possible mechanisms of magnetoresistance, for instance, originating from a spatial accumulation of spin-polarized carriers near a QDW (spin-accumulation effect) have not been analyzed in detail.

Apart from the problems of spin transport in the above-mentioned structures there have appeared numerous works on the properties of triode multilayered structures called spin transistors. A spin transistor is essentially analogous to a common bipolar transistor, except its properties crucially depend on magnetic states of its elements and the spin polarization of current carriers. Many versions of spin transistors have been put forward, particularly based on the so-called ‘half-metals’ along with the Heusler alloys, the feature of which consists in that a partial conductivity of one of the spin sub-bands exhibits the semiconductor character, while in the other the conductivity is similar to that of metals. This structure of the sub-bands determines the main features of conductivity in the materials of this kind, and also suggests some ways to amplify the current. Another advantage of the spin transistor over the common one is a possibility to reduce the base size down to a nano-meter scale. The processes in such a device obey quantum mechanics laws. In the limiting case, a quantum dot may play the role of the base. These ideas seem very attractive, but only a few experimental research works in this direction were made while and no detailed theory of such a device was elaborated yet. One of the problems in spin transistors is to find out how to switch elements of the transistor independently, i.e. separately one from the other. This problem is traditionally solved by means of choosing appropriate materials for the base, emitter, and collector so that their coercive fields are different; however magnetization reversal in the base via an injection of the spin-polarized current into it seems to be an interesting and promising alternative to the traditional method.

New kinds of nanostructures can be fabricated on the basis of quantum point contacts, quantum dots and wires. Quite unusual transport properties of such low-dimensional structures could be revealed when analyzing the processes in them. Explanation of these properties seems impossible without taking into account complex processes of interaction between current carriers themselves (electronic correlation).

Brief description of research.

To implement the project it is necessary to solve the following five main tasks:

Task 1 consists in development of the new technological methods for making and testing spin-valve, spin-tunneling, spin-transistor nanostructures and spin-injection structures on the basis of two-dimensional electron gas. Electron beam sputtering, AC and DC magnetron sputtering, thermal evaporation, and pulse laser sputtering will be used for vacuum deposition of layers.

Task 2 consists in development of new technology methods for making low-dimensional and quantum nanostructures. A series of technological works will be carried out in order to deposit thin Co, Ni, Fe, Pt, etc. metal films and to form topological patterns of nanostructures by means of ultraviolet photolithography, electron beam lithography, local electrochemical and mechanical film surface modification by the probe of a vacuum scanning microscope.

Task 3 consists in control of quality and measurement of parameters of synthesized nanoparticles, nanocontacts, and nanowires. For this purpose a number of methods will be used: electron microscopy, tunneling microscopy, atomic and magnetic-force microscopy, X-ray and electronic diffractometry, ellipsometry, induction magnetometry, and measurement of capacity-voltage and current-voltage characteristics in a range of temperatures.

Task 4 includes experimental and theoretical research on new physical phenomena in various nano-structures. Injection of spin in metal films and in a two-dimensional electronic gas (injection level and kinetics), influence of the current on a magnetic domain structure including the possibility to switch the magnetic states by the current, high-frequency impedance, magnetic and elastic resonance, transport of spin-polarized electrons in quasi-one-dimensional channels and quantum size effects in nanostructures will be studied. Adequate theoretical models and computer programs are to be elaborated for modeling micromagnetism of single- and multi-layered patterns, nanocontacts, and nanowires taking into consideration thermal spin fluctuations and imperfections of the materials. Linear and nonlinear magnetooptical properties of structures will be studied including their application to examination of parameters of the structures.

Task 5 concerns the development of functioning principles and constructions of the devices based on investigated nano-structures and the theoretical analysis of their operation. Possible ways to increase the current amplification coefficient in a spin transistor will be discussed by matching materials and architecture of the devices. An outcome in decreasing a size of the base in the transistor down right to a quantum dot will be analyzed. The prospects to produce experimental samples of the devices consisting of spin-tunneling, magneto-metallic and magneto-semiconductor spin valve structures as such as elements of memory, magnetic field sensors, and transistors will be considered.

Prospective application:

(1) an array of sub-micron particles – a medium for magnetic storage of large capacity,


(2) spin valves, spin transistors, and other spin injection devices – a base of components for memory devices, logical devices, and magnetoresistance sensors.

Key researchers:

It is supposed that join efforts of several leading groups from Russia, who specialize in the field of magnetoelectronics, have experience and enough equipment possibilities for carrying out research on spin-transport, micromagnetic, and magnetodynamic properties of submicron and nanometer magnetic structures, which are currently considered as the promising ones for information technology. All the key members of the team have received recognition as a specialists in a theory, experiment and technology of magnetic structures. They have published in total 1000 papers in Russian and international refereed journals. The project will allow this large group of scientists to be involved long time into fundamental research work with civilian application.


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