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Exchange coupling and magnetoresistance in CoFe/NiCu/CoFe spin valves near the Curie point of the spacer
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.ORCID iD: 0000-0003-2339-1692
2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 107, no 9, 09D711- p.Article in journal (Refereed) Published
Abstract [en]

Thermal control of exchange coupling between two strongly ferromagnetic layers through a weakly ferromagnetic Ni-Cu spacer and the associated magnetoresistance is investigated. The spacer, having a Curie point slightly above room temperature, can be cycled between its paramagnetic and ferromagnetic states by varying the temperature externally or using joule heating. It is shown that the giant magnetoresistance vanishes due to a strong reduction in the mean free path in the spacer at above similar to 30% Ni concentration-before the onset of ferromagnetism. Finally, a device is proposed which combines thermally controlled exchange coupling and large magnetoresistance by separating the switching and the readout elements.

Place, publisher, year, edition, pages
2010. Vol. 107, no 9, 09D711- p.
Keyword [en]
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-29840DOI: 10.1063/1.3340509ISI: 000277834300288ScopusID: 2-s2.0-77951697796OAI: diva2:399826
QC 20110222 11th Joint MMM-Intermag Conference, Washington, DC, JAN 18-22, 2010Available from: 2011-02-23 Created: 2011-02-17 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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