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  • 1.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Superdiffusive Spin Transport and Ultrafast Magnetization Dynamics: Femtosecond spin transport as the route to ultrafast spintronics2013Doctoral 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.

    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, p. 027203-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
    Identifiers
    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, p. 024404-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
    Identifiers
    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, p. 1037-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
    Identifiers
    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, p. 332-336Article 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
    Identifiers
    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, p. 256-260Article 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
    Identifiers
    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
  • 2.
    Battiato, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Barbalinardo, G.
    Carva, Karel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Beyond linear response theory for intensive light-matter interactions: Order formalism and ultrafast transient dynamics2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 4, p. 045117-Article in journal (Refereed)
    Abstract [en]

    Recently constructed radiation sources deliver brilliant, ultrashort coherent radiation fields with which the material's response can be investigated on the femtosecond to attosecond time scale. Here, we develop a theoretical framework for the interaction of the material's electrons with such intensive, short radiation pulses. Our theory is based on the time evolution of the electron density matrix, as defined through the Liouville-von Neumann equation. The latter equation is solved here within the framework of the response theory, incorporating the perturbing field in higher orders. An analytical tool, called the order notation, is developed, which permits the explicit calculation of the arising nth-order operatorial convolutions. As examples of the formalism, explicit expressions for several optical phenomena are worked out. Through the developed theory presented here, two fundamental results are achieved: first, the perturbing field to higher than linear orders is included in an elegant and compact way, allowing to treat highly brilliant light, and, second, the complete transient time response on the subfemtosecond scale is analytically provided, thus dropping the adiabatic approximation commonly made in standard linear response theory.

  • 3.
    Battiato, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Barbalinardo, G.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Quantum theory of the inverse Faraday effect2014In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 89, no 1, p. 014413-Article in journal (Refereed)
    Abstract [en]

    We provide a quantum theoretical description of the magnetic polarization induced by intense circularly polarized light in a material. Such effect-commonly referred to as the inverse Faraday effect-is treated using beyond-linear response theory, considering the applied electromagnetic field as external perturbation. An analytical time-dependent solution of the Liouville-von Neumann equation to second order is obtained for the density matrix and used to derive expressions for the optomagnetic polarization. Two distinct cases are treated, the long-time adiabatic limit of polarization imparted by continuous wave irradiation, and the full temporal shape of the transient magnetic polarization induced by a short laser pulse. We further derive expressions for the Verdet constants for the inverse, optomagnetic Faraday effect and for the conventional, magneto-optical Faraday effect and show that they are in general different. Additionally, we derive expressions for the Faraday and inverse Faraday effects within the Drude-Lorentz theory and demonstrate that their equality does not hold in general, but only for dissipationless media. As an example, we perform initial quantum mechanical calculations of the two Verdet constants for a hydrogenlike atom and we extract the trends. We observe that one reason for a large inverse Faraday effect in heavy atoms is the spatial extension of the wave functions rather than the spin-orbit interaction, which nonetheless contributes positively.

  • 4.
    Battiato, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Carva, Karel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
    Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization2010In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 2, p. 027203-Article in journal (Refereed)
    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.

  • 5.
    Battiato, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Carva, Karel
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Theory of laser-induced ultrafast superdiffusive spin transport in layered heterostructures2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 2, p. 024404-Article in journal (Refereed)
    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.

  • 6.
    Battiato, Marco
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Treating the effect of interface reflections on superdiffusive spin transport in multilayer samples (invited)2014In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 17, p. 172611-Article in journal (Refereed)
    Abstract [en]

    Femtosecond laser-induced magnetization dynamics has recently been related to superdiffusive spin transport. With the aim to accurately compute spin superdiffusion in the complex geometries of layered heterostructures and free standing layers, we develop here a dedicated numerical scheme. We introduce a discretization technique to solve the superdiffusive equation numerically on a time and space grid. The discretization scheme facilitates an explicit treatment of the total reflection at the vacuum-material surfaces as well as of partial reflections at the interfaces between two different materials. The advantages of the numerical technique are discussed. (C) 2014 AIP Publishing LLC.

  • 7. Carva, Karel
    et al.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Legut, D.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab initio theory of electron-phonon mediated ultrafast spin relaxation of laser-excited hot electrons in transition-metal ferromagnets2013In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 18, p. 184425-Article in journal (Refereed)
    Abstract [en]

    We report a computational theoretical investigation of electron spin-flip scattering induced by the electron-phonon interaction in the transition-metal ferromagnets bcc Fe, fcc Co, and fcc Ni. The Elliott-Yafet electron-phonon spin-flip scattering is computed from first principles, employing a generalized spin-flip Eliashberg function as well as ab initio computed phonon dispersions. Aiming at investigating the amount of electron-phonon mediated demagnetization in femtosecond laser-excited ferromagnets, the formalism is extended to treat laser-created thermalized as well as nonequilibrium, nonthermal hot electron distributions. Using the developed formalism we compute the phonon-induced spin lifetimes of hot electrons in Fe, Co, and Ni. The electron-phonon mediated demagnetization rate is evaluated for laser-created thermalized and nonequilibrium electron distributions. Nonthermal distributions are found to lead to a stronger demagnetization rate than hot, thermalized distributions, yet their demagnetizing effect is not enough to explain the experimentally observed demagnetization occurring in the subpicosecond regime.

  • 8.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Legut, Dominik
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Theory of femtosecond laser-induced demagnetization2015In: ULTRAFAST MAGNETISM I, 2015, p. 111-115Conference paper (Refereed)
    Abstract [en]

    Using ab initio calculations we computed the ultrafast demagnetization that can be achieved by Elliott-Yafet electron-phonon spin-flip scatterings in laser-excited ferromagnets. Our calculations show that nonequilibrium laser-created distributions contribute mostly to the ultrafast demagnetization. Nonetheless, the total Elliott-Yafet contribution is too small to account for the fs-demagnetization.

  • 9.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ab Initio Investigation of the Elliott-Yafet Electron-Phonon Mechanism in Laser-Induced Ultrafast Demagnetization2011In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 20, p. 207201-Article in journal (Refereed)
    Abstract [en]

    The spin-flip (SF) Eliashberg function is calculated from first principles for ferromagnetic Ni to accurately establish the contribution of Elliott-Yafet electron-phonon SF scattering to Ni's femtosecond laser-driven demagnetization. This is used to compute the SF probability and demagnetization rate for laser-created thermalized as well as nonequilibrium electron distributions. Increased SF probabilities are found for thermalized electrons, but the induced demagnetization rate is extremely small. A larger demagnetization rate is obtained for nonequilibrium electron distributions, but its contribution is too small to account for femtosecond demagnetization.

  • 10.
    Carva, Karel
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Is the controversy over femtosecond magneto-optics really solved?2011In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 7, no 9, p. 665-665Article in journal (Refereed)
  • 11. Eschenlohr, A.
    et al.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pontius, N.
    Kachel, T.
    Holldack, K.
    Mitzner, R.
    Foehlisch, A.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stamm, C.
    Optical excitation of thin magnetic layers in multilayer structures Reply2014In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 13, no 2, p. 102-103Article in journal (Refereed)
  • 12. Eschenlohr, A.
    et al.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Pontius, N.
    Kachel, T.
    Holldack, K.
    Mitzner, R.
    Foehlisch, A.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Stamm, C.
    Ultrafast spin transport as key to femtosecond demagnetization2013In: Nature Materials, ISSN 1476-1122, E-ISSN 1476-4660, Vol. 12, no 4, p. 332-336Article in journal (Refereed)
    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.

  • 13. Kampfrath, T.
    et al.
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eilers, G.
    Noetzold, J.
    Maehrlein, S.
    Zbarsky, V.
    Freimuth, F.
    Mokrousov, Y.
    Bluegel, S.
    Wolf, M.
    Radu, I.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Muenzenberg, M.
    Terahertz spin current pulses controlled by magnetic heterostructures2013In: Nature Nanotechnology, ISSN 1748-3387, E-ISSN 1748-3395, Vol. 8, no 4, p. 256-260Article in journal (Refereed)
    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).

  • 14.
    Kampfrath, T.
    et al.
    Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany..
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sell, A.
    Univ Konstanz, D-78464 Constance, Germany..
    Freimuth, F.
    Peter Grunberg Inst, D-52425 Julich, Germany.;Inst Adv Simulat, D-52425 Julich, Germany..
    Leitenstorfer, A.
    Univ Konstanz, D-78464 Constance, Germany..
    Wolf, M.
    Max Planck Gesell, Fritz Haber Inst, D-14195 Berlin, Germany..
    Huber, R.
    Univ Regensburg, D-93053 Regensburg, Germany..
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Muenzenberg, M.
    Univ Gottingen, D-37077 Gottingen, Germany..
    Ultrafast spin precession and transport controlled and probed with terahertz radiation2015In: Ultrafast Magnetism I / [ed] Bigot, JY; Hubner, W; Rasing, T; Chantrell, R, 2015, p. 324-326Conference paper (Refereed)
    Abstract [en]

    We present examples of how terahertz (THz) electromagnetic transients can be used to control spin precession in antiferromagnets (through the THz Zeeman torque) and to probe spin transport in magnetic heterostructures (through the THz inverse spin Hall effect), on femtosecond time scales.

  • 15.
    Rudolf, Dennis
    et al.
    Res Ctr Julich, Peter Grnberg Inst PGI 6, D-52425 Julich, Germany..
    La-O-Vorakiat, Chan
    Univ Colorado, Dept Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA..
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Adam, Roman
    Res Ctr Julich, Peter Grnberg Inst PGI 6, D-52425 Julich, Germany..
    Grychtol, Patrik
    Univ Colorado, Dept Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA..
    Shaw, Justin M.
    NIST, Electromagnet Div, Boulder, CO 80305 USA..
    Turgut, Emrah
    Univ Colorado, Dept Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA..
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mathias, Stefan
    Univ Kaiserslautern, D-67663 Kaiserslautern, Germany.;Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Nembach, Hans T.
    NIST, Electromagnet Div, Boulder, CO 80305 USA..
    Silva, Thomas J.
    NIST, Electromagnet Div, Boulder, CO 80305 USA..
    Aeschlimann, Martin
    Univ Kaiserslautern, D-67663 Kaiserslautern, Germany.;Res Ctr OPTIMAS, D-67663 Kaiserslautern, Germany..
    Kapteyn, Henry C.
    Univ Colorado, Dept Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA..
    Murnane, Margaret M.
    Univ Colorado, Dept Phys, Boulder, CO 80309 USA.;Univ Colorado, JILA, Boulder, CO 80309 USA..
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Schneider, Claus M.
    Res Ctr Julich, Peter Grnberg Inst PGI 6, D-52425 Julich, Germany..
    Element Selective Investigation of Spin Dynamics in Magnetic Multilayers2015In: Ultrafast Magnetism I / [ed] Bigot, JY; Hubner, W; Rasing, T; Chantrell, R, 2015, p. 307-309Conference paper (Refereed)
    Abstract [en]

    Our understanding of ultrafast switching processes in novel spin-based electronics depends on our detailed knowledge of interactions between spin, charge and phonons in magnetic structures. We present element-selective studies, using extreme ultraviolet (XUV) light, to gain insight into spin dynamics in exchange coupled magnetic multilayers on the femtosecond time scale.

  • 16. Rudolf, Dennis
    et al.
    La-O-Vorakiat, Chan
    Battiato, Marco
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Adam, Roman
    Shaw, Justin M.
    Turgut, Emrah
    Maldonado, Pablo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Mathias, Stefan
    Grychtol, Patrik
    Nembach, Hans T.
    Silva, Thomas J.
    Aeschlimann, Martin
    Kapteyn, Henry C.
    Murnane, Margaret M.
    Schneider, Claus M.
    Oppeneer, Peter M.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current2012In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 3, p. 1037-Article in journal (Refereed)
    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.

1 - 16 of 16
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