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Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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2012 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 3, 1037- p.Article in journal (Refereed) Published
Abstract [en]

Uncovering the physical mechanisms that govern ultrafast charge and spin dynamics is crucial for understanding correlated matter as well as the fundamental limits of ultrafast spin-based electronics. Spin dynamics in magnetic materials can be driven by ultrashort light pulses, resulting in a transient drop in magnetization within a few hundred femtoseconds. However, a full understanding of femtosecond spin dynamics remains elusive. Here we spatially separate the spin dynamics using Ni/Ru/Fe magnetic trilayers, where the Ni and Fe layers can be ferroor antiferromagnetically coupled. By exciting the layers with a laser pulse and probing the magnetization response simultaneously but separately in Ni and Fe, we surprisingly find that optically induced demagnetization of the Ni layer transiently enhances the magnetization of the Fe layer when the two layer magnetizations are initially aligned parallel. Our observations are explained by a laser-generated superdiffusive spin current between the layers.

Place, publisher, year, edition, pages
2012. Vol. 3, 1037- p.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-184474DOI: 10.1038/ncomms2029ISI: 000309338100004OAI: oai:DiVA.org:uu-184474DiVA: diva2:565768
Available from: 2012-11-08 Created: 2012-11-07 Last updated: 2017-12-07Bibliographically approved
In thesis
1. Superdiffusive Spin Transport and Ultrafast Magnetization Dynamics: Femtosecond spin transport as the route to ultrafast spintronics
Open this publication in new window or tab >>Superdiffusive Spin Transport and Ultrafast Magnetization Dynamics: Femtosecond spin transport as the route to ultrafast spintronics
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The debate over the origin of the ultrafast demagnetization has been intensively active for the past 16 years. Several microscopic mechanisms have been proposed but none has managed so far to provide direct and incontrovertible evidences of their validity. In this context I have proposed an approach based on spin dependent electron superdiffusion as the driver of the ultrafast demagnetization.

Excited electrons and holes in the ferromagnetic metal start diffusing after the absorption of the laser photons. Being the material ferromagnetic, the majority and minority spin channels occupy very different bands. It is then not surprising that transport properties are strongly spin dependent. In most of the ferromagnetic metals, majority spin excited electrons have better transport properties than minority ones. The effect is that majority carriers are more efficient in leaving the area irradiated by the laser, triggering a net spin transport.

Recent experimental findings are revolutionising the field by being incompatible with previously proposed models and showing uncontrovertibly the sign of spin superdiffusion.

We have shown that spin diffusing away from a layer undergoing ultrafast demagnetization can be used to create an ultrafast increase of magnetization in a neighboring magnetic layer. We have also shown that optical excitation is not a prerequisite for the ultrafast demagnetization and that excited electrons superdiffusing from a non-magnetic substrate can trigger the demagnetization. Finally we have shown that it is possible to control the time shape of the spin currents created and developed a technique to detect directly spin currents in a contact-less way. 

The impact of these new discoveries goes beyond the solution of the mystery of ultrafast demagnetization. It shows how spin information can be, not only manipulated, as shown 16 years ago, but most importantly transferred at unprecedented speeds. This new discovery lays the basis for a full femtosecond spintronics.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 64 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1061
Keyword
Ultrafast magnetisation dynamics, anomalous diffusion, femtosecond dynamics, magnetism
National Category
Condensed Matter Physics
Research subject
Materials Science
Identifiers
urn:nbn:se:uu:diva-205265 (URN)978-91-554-8722-5 (ISBN)
Public defence
2013-09-27, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
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Supervisors
Available from: 2013-09-06 Created: 2013-08-15 Last updated: 2014-01-08

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Battiato, MarcoMaldonado, PabloOppeneer, Peter M.

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