Danko Radić

Thermoelectrical manipulation of nanomagnets

We investigate the interplay between the thermodynamic properties and spin-dependent transport in a mesoscopic device based on a magnetic multilayer (F/f/F), in which two strongly ferromagnetic layers (F) are exchange-coupled through a weakly ferromagnetic spacer (f) with the Curie temperature in the vicinity of room temperature. We show theoretically that the Joule heating produced by the spin-dependent current allows a spin-thermoelectronic control of the ferromagnetic-to-paramagnetic (f/N) transition in the spacer and, thereby, of the relative orientation of the outer F-layers in the device (spin-thermoelectric manipulation of nanomagnets). Supporting experimental evidence of such thermally-controlled switching from parallel to antiparallel magnetization orientations in F/f(N)/F sandwiches is presented. Furthermore, we show theoretically that local Joule heating due to a high concentration of current in a magnetic point contact or a nanopillar can be used to reversibly drive the weakly ferromagnetic spacer through its Curie point and thereby exchange couple and decouple the two strongly ferromagnetic F-layers. For the devices designed to have an antiparallel ground state above the Curie point of the spacer, the associated spin-thermionic parallel to antiparallel switching causes magnetoresistance oscillations whose frequency can be controlled by proper biasing from essentially dc to GHz. We discuss in detail an experimental realization of a device that can operate as a thermomagnetoresistive switch or oscillator.

Spin controlled nanomechanics induced by single-electron tunneling

We consider dc-electronic transport through a nanowire suspended between normal- and spin-polarized metal leads in the presence of an external magnetic field. We show that magnetomotive coupling between the electrical current through the nanowire and vibrations of the wire may result in self-excitation of mechanical vibrations. The self-excitation mechanism is based on correlations between the occupancy of the quantized electronic energy levels inside the nanowire and the velocity of the nanowire. We derive conditions for the occurrence of the instability and find stable regimes of mechanical oscillations.

Self-excited oscillations of charge-spin accumulation due to single-electron tunneling

We theoretically study electronic transport through a layer of quantum dots connecting two metallic leads. By the inclusion of an inductor in series with the junction, we show that steady electronic transport in such a system may be unstable with respect to temporal oscillations caused by an interplay between the Coulomb blockade of tunneling and spin accumulation in the dots. When this instability occurs, a new stable regime is reached, where the average spin and charge in the dots oscillate periodically in time. The frequency of these oscillations is typically on the order of 1 GHz for realistic values of the junction parameters.

Thermal-magnetic-electric oscillator based on spin-valve effect

A. M. Kadigrobov, 2 S. Andersson, Hee Chul Park, 4 D. Radić, 5 R. I. Shekhter, M. Jonson, 6, 7 and V. Korenivski Department of Physics, University of Gothenburg, SE-412 96 Göteborg, Sweden Theoretische Physik III, Ruhr-Universität Bochum, D-44801 Bochum, Germany Nanostructure Physics, Royal Institute of Technology, SE-106 91 Stockholm, Sweden Department of Physics, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Physics, Faculty of Science, University of Zagreb, 1001 Zagreb, Croatia School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, Scotland, UK 7 Division of Quantum Phases and Devices, School of Physics, Konkuk University, Seoul 143-701, Republic of KoreaA thermal-magnetic-electric valve with the free layer of exchange-spring type and inverse magnetoresistance is investigated. The structure has S-shaped current-voltage characteristics and can exhibit spontaneous oscillations when integrated with a conventional capacitor within a resonator circuit. The frequency of the oscillations can be controlled from essentially dc to the GHz range by the circuit capacitance.

Peierls-type structural phase transition in a crystal induced by magnetic breakdown

Abstract We predict a new type of phase transition in a quasi-two dimensional system of electrons at high magnetic fields, namely the stabilization of a density wave which transforms a two dimensional open Fermi surface into a periodic chain of large pockets with small distances between them. The quantum tunneling of electrons between the neighboring closed orbits enveloping these pockets transforms the electron spectrum into a set of extremely narrow energy bands and gaps which decreases the total electron energy, thus leading to a magnetic breakdown induced density wave (MBIDW) ground state. We show that this DW instability has some qualitatively different properties in comparison to analogous DW instabilities of Peierls type; e.g. the critical temperature of the MBIDW phase transition arises and disappears in a peculiar way with a change of the inverse magnetic field.

Chaotic Properties of the Elliptical Stadium Billiard

The two-parameter family of elliptical stadium billiards is discussed. Special attention is paid to the one-parameter subfamily inscribed into the square which interpolates between the circle and the square. It is shown that the classical dynamics of such system is mixed for all values of the shape parameter. For the same system the quantal spectra and wave functions are calculated, and the classical and quantal chaotic fraction values are compared. Other types of elliptical stadium billiards are briefly discussed.

Thermoelectrical control of magnetic states on the nanoscale

Discovery of giant magnetoresistance1, 2 by Albert Fert and Peter Grünberg in 1988 initiated a new technological concept: spin-based electronics, or spintronics. The challenge of manipulating magnetic states on the nanometer scale is central to this new branch of physics. The spin torque (ST) effect—which modulates the magnetization direction of a magnetic particle under an injection of spin-polarized electrons—is one possible solution that has since become a subject of intense study.3–5 Large current densities needed to achieve sufficient ST are accompanied by Joule heating, imposing a limit on the applicability of the ST effect. However, the fact that such heating can be precisely controlled by applying voltage opens up new possibilities for thermoelectric manipulation of the magnetic state, as we have predicted and experimentally observed.6–8 This novel control method works for a broad range of scenarios, including one in which the ST effect is absent. Figure 1 shows the system we considered. We assumed that the Curie temperature T c of layer 1 is lower than T .0;2/ c of layers 0 and 2. The magnetostatic field H is weak enough that, at low temperatures T, the magnetization of layer 2 is kept parallel to the magnetization of layer 0 due to the ‘exchange spring’ interaction of region 1. The top right of Figure 1 shows how an increase in the tilt angle TM in such a structure increases the exchange spring energy and decreases the total magnetic energy. We proposed that an increase of T results in an orientational phase transition. Indeed, the exchange interaction depends not only on the exchange energy integral, but on the magnitude of the magnetic moment. Therefore, an increase in T decreases M1(T) and weakens the Figure 1. Orientation of the magnetic moments M (shown with short arrows) in a stack of three ferromagnetic layers (0, 1, 2) of separation L1 (between layers 0 and 1) and L2 (1 and 2). The magnetic field H is shown with a long arrow. The magnetization in layer 0 is fixed. : Tilt angle.