Change search
CiteExportLink to record
Permanent link

Direct link
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Superdiffusive Spin Transport and Ultrafast Magnetization Dynamics: Femtosecond spin transport as the route to ultrafast spintronics
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1061
Keyword [en]
Ultrafast magnetisation dynamics, anomalous diffusion, femtosecond dynamics, magnetism
National Category
Condensed Matter Physics
Research subject
Materials Science
URN: urn:nbn:se:uu:diva-205265ISBN: 978-91-554-8722-5 (print)OAI: diva2:641045
Public defence
2013-09-27, Siegbahnsalen, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Available from: 2013-09-06 Created: 2013-08-15 Last updated: 2014-01-08
List of papers
1. Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization
Open this publication in new window or tab >>Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization
2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 2, 027203- p.Article in journal (Refereed) Published
Abstract [en]

We propose a semiclassical model for femtosecond laser-induced demagnetization due to spin-polarized excited electron diffusion in the superdiffusive regime. Our approach treats the finite elapsed time and transport in space between multiple electronic collisions exactly, as well as the presence of several metal films in the sample. Solving the derived transport equation numerically we show that this mechanism accounts for the experimentally observed demagnetization within 200 fs in Ni, without the need to invoke any angular momentum dissipation channel.

National Category
Physical Sciences
urn:nbn:se:uu:diva-135801 (URN)10.1103/PhysRevLett.105.027203 (DOI)000279697900001 ()
Available from: 2010-12-09 Created: 2010-12-08 Last updated: 2017-12-11Bibliographically approved
2. Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures
Open this publication in new window or tab >>Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures
2012 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 2, 024404- p.Article in journal (Refereed) Published
Abstract [en]

Femtosecond laser excitation of a ferromagnetic material creates energetic spin-polarized electrons that have anomalous transport characteristics. We develop a semiclassical theory that is specifically dedicated to capture the transport of laser-excited nonequilibrium (NEQ) electrons. The randomly occurring multiple electronic collisions, which give rise to electron thermalization, are treated exactly and we include the generation of electron cascades due to inelastic electron-electron scatterings. The developed theory can, moreover, treat the presence of several different layers in the laser-irradiated material. The derived spin-dependent transport equation is solved numerically and it is shown that the hot NEQ electron spin transport occurs neither in the diffusive nor ballistic regime, it is superdiffusive. As the excited spin majority and minority electrons in typical transition-metal ferromagnets (e.g., Fe, Ni) have distinct, energy-dependent lifetimes, fast spin dynamics in the femtosecond (fs) regime is generated, causing effectively a spin current. As examples, we solve the resulting spin dynamics numerically for typical heterostructures, specifically, a ferromagnetic/nonmagnetic metallic layered junction (i.e., Fe/Al and Ni/Al) and a ferromagnetic/nonmagnetic insulator junction (Fe or Ni layer on a large band-gap insulator as, e.g., MgO). For the ferromagnetic/nonmagnetic metallic junction where the ferromagnetic layer is laser-excited, the computed spin dynamics shows that injection of a superdiffusive spin current in the nonmagnetic layer (Al) is achieved. The injected spin current consists of screened NEQ, mobile majority-spin electrons and is nearly 90% spin-polarized for Ni and about 65% for Fe. Concomitantly, a fast demagnetization of the ferromagnetic polarization in the femtosecond regime is driven. The analogy of the generated spin current to a superdiffusive spin Seebeck effect is surveyed.

National Category
Physical Sciences
urn:nbn:se:uu:diva-178084 (URN)10.1103/PhysRevB.86.024404 (DOI)000306088700004 ()
Available from: 2012-07-30 Created: 2012-07-27 Last updated: 2017-12-07Bibliographically approved
3. Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
Open this publication in new window or tab >>Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current
Show others...
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.

National Category
Physical Sciences
urn:nbn:se:uu:diva-184474 (URN)10.1038/ncomms2029 (DOI)000309338100004 ()
Available from: 2012-11-08 Created: 2012-11-07 Last updated: 2017-12-07Bibliographically approved
4. Ultrafast spin transport as key to femtosecond demagnetization
Open this publication in new window or tab >>Ultrafast spin transport as key to femtosecond demagnetization
Show others...
2013 (English)In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 12, no 4, 332-336 p.Article in journal (Refereed) Published
Abstract [en]

Irradiating a ferromagnet with a femtosecond laser pulse is known to induce an ultrafast demagnetization within a few hundred femtoseconds. Here we demonstrate that direct laser irradiation is in fact not essential for ultrafast demagnetization, and that electron cascades caused by hot electron currents accomplish it very efficiently. We optically excite a Au/Ni layered structure in which the 30 nm Au capping layer absorbs the incident laser pump pulse and subsequently use the X-ray magnetic circular dichroism technique to probe the femtosecond demagnetization of the adjacent 15 nm Ni layer. A demagnetization effect corresponding to the scenario in which the laser directly excites the Ni film is observed, but with a slight temporal delay. We explain this unexpected observation by means of the demagnetizing effect of a superdiffusive current of non-equilibrium, non-spin-polarized electrons generated in the Au layer.

National Category
Natural Sciences
urn:nbn:se:uu:diva-199718 (URN)10.1038/NMAT3546 (DOI)000317164900022 ()
Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved
5. Terahertz spin current pulses controlled by magnetic heterostructures
Open this publication in new window or tab >>Terahertz spin current pulses controlled by magnetic heterostructures
Show others...
2013 (English)In: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 8, no 4, 256-260 p.Article in journal (Refereed) Published
Abstract [en]

In spin-based electronics, information is encoded by the spin state of electron bunches(1-4). Processing this information requires the controlled transport of spin angular momentum through a solid(5,6), preferably at frequencies reaching the so far unexplored terahertz regime(7-9). Here, we demonstrate, by experiment and theory, that the temporal shape of femtosecond spin current bursts can be manipulated by using specifically designed magnetic heterostructures. A laser pulse is used to drive spins(10-12) from a ferromagnetic iron thin film into a non-magnetic cap layer that has either low (ruthenium) or high (gold) electron mobility. The resulting transient spin current is detected by means of an ultrafast, contactless amperemeter(13) based on the inverse spin Hall effect(14,15), which converts the spin flow into a terahertz electromagnetic pulse. We find that the ruthenium cap layer yields a considerably longer spin current pulse because electrons are injected into ruthenium d states, which have a much lower mobility than gold sp states(16). Thus, spin current pulses and the resulting terahertz transients can be shaped by tailoring magnetic heterostructures, which opens the door to engineering high-speed spintronic devices and, potentially, broadband terahertz emitters(7-9).

National Category
Natural Sciences
urn:nbn:se:uu:diva-199719 (URN)10.1038/NNANO.2013.43 (DOI)000317046800011 ()
Available from: 2013-05-13 Created: 2013-05-13 Last updated: 2017-12-06Bibliographically approved

Open Access in DiVA

fulltext(4143 kB)1208 downloads
File information
File name FULLTEXT01.pdfFile size 4143 kBChecksum SHA-512
Type fulltextMimetype application/pdf