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Thermoelectrical manipulation of nanomagnets
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
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2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 12, 123706- p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2010. Vol. 107, no 12, 123706- p.
Keyword [en]
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-29461DOI: 10.1063/1.3437054ISI: 000279993900068ScopusID: 2-s2.0-77954182379OAI: diva2:395946
Swedish Research CouncilEU, FP7, Seventh Framework Programme, FP7-ICT-2007-C 225955
QC 20110208Available from: 2011-02-08 Created: 2011-02-02 Last updated: 2012-03-13Bibliographically approved
In thesis
1. Spin-diode effect and thermally controlled switching in magnetic spin-valves
Open this publication in new window or tab >>Spin-diode effect and thermally controlled switching in magnetic spin-valves
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit.

The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. 85 p.
Trita-FYS, ISSN 0280-316X ; 2012:11
National Category
Condensed Matter Physics
urn:nbn:se:kth:diva-91300 (URN)978-91-7501-287-2 (ISBN)
Public defence
2012-03-30, FB52, Roslagstullsbacken 21, Stockholm, 13:00 (English)
QC 20120313Available from: 2012-03-13 Created: 2012-03-12 Last updated: 2012-03-13Bibliographically approved

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