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  • 1. Backes, D.
    et al.
    Macià, F.
    Bonetti, Stefano
    SLAC National Accelerator Laboratory, USA; Stanford University, USA.
    Kukreja, R.
    Ohldag, H.
    Kent, A. D.
    Direct Observation of a Localized Magnetic Soliton in a Spin-Transfer Nanocontact2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 12, article id 127205Article in journal (Refereed)
    Abstract [en]

    We report the direct observation of a localized magnetic soliton in a spin-transfer nanocontact using scanning transmission x-ray microscopy. Experiments are conducted on a lithographically defined 150 nm diameter nanocontact to an ultrathin ferromagnetic multilayer with perpendicular magnetic anisotropy. Element-resolved x-ray magnetic circular dichroism images show an abrupt onset of a magnetic soliton excitation localized beneath the nanocontact at a threshold current. However, the amplitude of the excitation ≃25° at the contact center is far less than that predicted (⪅180°), showing that the spin dynamics is not described by existing models.

  • 2.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    X-ray imaging of spin currents and magnetisation dynamics at the nanoscale2017In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 29, no 13, article id 133004Article in journal (Refereed)
    Abstract [en]

    Understanding how spins move in time and space is the aim of both fundamental and applied research in modern magnetism. Over the past three decades, research in this field has led to technological advances that have had a major impact on our society, while improving the understanding of the fundamentals of spin physics. However, important questions still remain unanswered, because it is experimentally challenging to directly observe spins and their motion with a combined high spatial and temporal resolution. In this article, we present an overview of the recent advances in x-ray microscopy that allow researchers to directly watch spins move in time and space at the microscopically relevant scales. We discuss scanning x-ray transmission microscopy (STXM) at resonant soft x-ray edges, which is available at most modern synchrotron light sources. This technique measures magnetic contrast through the x-ray magnetic circular dichroism (XMCD) effect at the resonant absorption edges, while focusing the x-ray radiation at the nanometre scale, and using the intrinsic pulsed structure of synchrotron-generated x-rays to create time-resolved images of magnetism at the nanoscale. In particular, we discuss how the presence of spin currents can be detected by imaging spin accumulation, and how the magnetisation dynamics in thin ferromagnetic films can be directly imaged. We discuss how a direct look at the phenomena allows for a deeper understanding of the the physics at play, that is not accessible to other, more indirect techniques. Finally, we present an overview of the exciting opportunities that lie ahead to further understand the fundamentals of novel spin physics, opportunities offered by the appearance of diffraction limited storage rings and free electron lasers.

  • 3.
    Bonetti, Stefano
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Hoffmann, M. C.
    Sher, M. -J.
    Chen, Z.
    Yang, S. -H.
    Samant, M. G.
    Parkin, S. S. P.
    Durr, H. A.
    THz-Driven Ultrafast Spin-Lattice Scattering in Amorphous Metallic Ferromagnets2016In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 117, no 8, article id 087205Article in journal (Refereed)
    Abstract [en]

    We use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (similar to 30 fs). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering.

  • 4.
    Bonetti, Stefano
    et al.
    Stanford University, USA; SLAC National Accelerator Laboratory, USA.
    Kukreja, R.
    Chen, Z.
    Macià, F.
    Hernàndez, J. M.
    Eklund, A.
    Backes, D.
    Frisch, J.
    Katine, J.
    Malm, G.
    Urazhdin, S.
    Kent, A. D.
    Stöhr, J.
    Ohldag, H.
    Dürr, H. A.
    Direct observation and imaging of a spin-wave soliton with p-like symmetry2015In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 6, article id 8889Article in journal (Refereed)
    Abstract [en]

    Spin waves, the collective excitations of spins, can emerge as nonlinear solitons at the nanoscale when excited by an electrical current from a nanocontact. These solitons are expected to have essentially cylindrical symmetry (that is, s-like), but no direct experimental observation exists to confirm this picture. Using a high-sensitivity time-resolved magnetic X-ray microscopy with 50 ps temporal resolution and 35 nm spatial resolution, we are able to create a real-space spin-wave movie and observe the emergence of a localized soliton with a nodal line, that is, with p-like symmetry. Micromagnetic simulations explain the measurements and reveal that the symmetry of the soliton can be controlled by magnetic fields. Our results broaden the understanding of spin-wave dynamics at the nanoscale, with implications for the design of magnetic nanodevices.

  • 5.
    Bonetti, Stefano
    et al.
    Stanford University, USA; SLAC National Accelerator Laboratory, USA.
    Kukreja, Roopali
    Chen, Zhao
    Spoddig, Detlef
    Ollefs, Katharina
    Schöppner, Christian
    Meckenstock, Ralf
    Ney, Andreas
    Pinto, Jude
    Houanche, Richard
    Frisch, Josef
    Stöhr, Joachim
    Dürr, Hermann A.
    Ohldag, Hendrik
    Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range2015In: Review of Scientific Instruments, ISSN 0034-6748, E-ISSN 1089-7623, Vol. 86, no 9, article id 093703Article in journal (Refereed)
    Abstract [en]

    We present a scanning transmission x-ray microscopy setup combined with a novel microwave synchronization scheme for studying high frequency magnetization dynamics at synchrotron light sources. The sensitivity necessary to detect small changes in the magnetization on short time scales and nanometer spatial dimensions is achieved by combining the excitation mechanism with single photon counting electronics that is locked to the synchrotron operation frequency. Our instrument is capable of creating direct images of dynamical phenomena in the 5-10 GHz range, with high spatial resolution. When used together with circularly polarized x-rays, the above capabilities can be combined to study magnetic phenomena at microwave frequencies, such as ferromagnetic resonance (FMR) and spin waves. We demonstrate the capabilities of our technique by presenting phase resolved images of a ∼6 GHz nanoscale spin wave generated by a spin torque oscillator, as well as the uniform ferromagnetic precession with ∼0.1° amplitude at ∼9 GHz in a micrometer-sized cobalt strip.

  • 6. Chen, Z.
    et al.
    Higley, D. J.
    Beye, M.
    Hantschmann, M.
    Mehta, V.
    Hellwig, O.
    Mitra, A.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Bucher, M.
    Carron, S.
    Chase, T.
    Jal, E.
    Kukreja, R.
    Liu, T.
    Reid, A. H.
    Dakovski, G. L.
    Föhlisch, A.
    Schlotter, W. F.
    Dürr, H. A.
    Stöhr, J.
    Ultrafast Self-Induced X-Ray Transparency and Loss of Magnetic Diffraction2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 13, article id 137403Article in journal (Refereed)
    Abstract [en]

    Using ultrafast similar or equal to 2.5 fs and similar or equal to 25 fs self-amplified spontaneous emission pulses of increasing intensity and a novel experimental scheme, we report the concurrent increase of stimulated emission in the forward direction and loss of out-of-beam diffraction contrast for a Co/Pd multilayer sample. The experimental results are quantitatively accounted for by a statistical description of the pulses in conjunction with the optical Bloch equations. The dependence of the stimulated sample response on the incident intensity, coherence time, and energy jitter of the employed pulses reveals the importance of increased control of x-ray free electron laser radiation.

  • 7. Iacocca, E.
    et al.
    Liu, T-M
    Reid, A. H.
    Fu, Z.
    Ruta, S.
    Granitzka, P. W.
    Jai, E.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA; Ca’ Foscari University of Venice, Italy.
    Gray, A. X.
    Graves, C. E.
    Kukreja, R.
    Chen, Z.
    Higley, D. J.
    Chase, T.
    Le Guyader, L.
    Hirsch, K.
    Ohldag, H.
    Schlotter, W. F.
    Dakovski, G. L.
    Coslovich, G.
    Hoffmann, M. C.
    Carron, S.
    Tsukamoto, A.
    Kirilyuk, A.
    Kime, A.
    Rasing, Th
    Stöhr, J.
    Evans, R. F. L.
    Ostler, T.
    Chantrell, R. W.
    Hoefer, M. A.
    Silva, T. J.
    Dürr, H. A.
    Spin-current-mediated rapid magnon localisation and coalescence after ultrafast optical pumping of ferrimagnetic alloys2019In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 10, article id 1756Article in journal (Refereed)
    Abstract [en]

    Sub-picosecond magnetisation manipulation via femtosecond optical pumping has attracted wide attention ever since its original discovery in 1996. However, the spatial evolution of the magnetisation is not yet well understood, in part due to the difficulty in experimentally probing such rapid dynamics. Here, we find evidence of a universal rapid magnetic order recovery in ferrimagnets with perpendicular magnetic anisotropy via nonlinear magnon processes. We identify magnon localisation and coalescence processes, whereby localised magnetic textures nucleate and subsequently interact and grow in accordance with a power law formalism. A hydrodynamic representation of the numerical simulations indicates that the appearance of noncollinear magnetisation via optical pumping establishes exchange-mediated spin currents with an equivalent 100% spin polarised charge current density of 10(7) A cm(-2). Such large spin currents precipitate rapid recovery of magnetic order after optical pumping. The magnon processes discussed here provide new insights for the stabilization of desired meta-stable states.

  • 8. Kovalev, S.
    et al.
    Wang, Zhe
    Deinert, J-C
    Awari, N.
    Chen, M.
    Green, B.
    Germanskiy, S.
    de Oliveira, T. V. A. G.
    Lee, J. S.
    Deac, A.
    Turchinovich, D.
    Stojanovic, N.
    Eisebitt, S.
    Radu, I.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Kampfrath, T.
    Gensch, M.
    Selective THz control of magnetic order: new opportunities from superradiant undulator sources2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 11, article id 114007Article in journal (Refereed)
    Abstract [en]

    Recent advancements of accelerator technology enable the generation of carrier-envelope-phase stable THz pulses with high fields at adjustable high repetition rates. The appropriate choice of THz radiator allows generation of narrow-band, spectrally dense, multicycle THz transients of tunable THz frequency which are ideally suited to selectively excite low-energy excitations such as magnons or phonons. They also allow one to study the frequency dependence of nonresonant THz-field interactions with various order parameters with high dynamic range. In this paper, we discuss the future prospects of this new type of THz light source for studying the coherent control of magnetic order based on recent results.

  • 9. Kozina, M.
    et al.
    Fechner, M.
    Marsik, P.
    van Driel, T.
    Glownia, J. M.
    Bernhard, C.
    Radovic, M.
    Zhu, D.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Staub, U.
    Hoffmann, M. C.
    Terahertz-driven phonon upconversion in SrTiO32019In: Nature Physics, ISSN 1745-2473, E-ISSN 1745-2481, Vol. 15, no 4, p. 387-+Article in journal (Refereed)
    Abstract [en]

    Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high-frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower-frequency lattice modulations. Here, we drive the lowest-frequency optical phonon in the prototypical transition metal oxide SrTiO3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher-frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond X-ray pulses, enabling direct visualization of the atomic displacements.

  • 10. Kozina, M.
    et al.
    Pancaldi, Matteo
    Stockholm University, Faculty of Science, Department of Physics. University of Fribourg, Switzerland.
    Bernhard, C.
    van Driel, T.
    Glownia, J. M.
    Marsik, P.
    Radovic, M.
    Vaz, C. A. F.
    Zhu, D.
    Bonetti, Stefani
    Stockholm University, Faculty of Science, Department of Physics.
    Staub, U.
    Hoffmann, M. C.
    Local terahertz field enhancement for time-resolved x-ray diffraction2017In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 8, article id 081106Article in journal (Refereed)
    Abstract [en]

    We report local field strength enhancement of single-cycle terahertz (THz) pulses in an ultrafast time-resolved x-ray diffraction experiment. We show that patterning the sample with gold microstructures increases the THz field without changing the THz pulse shape or drastically affecting the quality of the x-ray diffraction pattern. We find a five-fold increase in THz-induced x-ray diffraction intensity change in the presence of microstructures on a SrTiO3 thin-film sample.

  • 11. Kukreja, R.
    et al.
    Bonetti, Stefano
    SLAC National Accelerator Laboratory, USA; Stanford University, USA.
    Chen, Z.
    Backes, D.
    Acremann, Y.
    Katine, J. A.
    Kent, A. D.
    Dürr, H. A.
    Ohldag, H.
    Stöhr, J.
    X-ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu.2015In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 115, no 9, article id 096601Article in journal (Refereed)
    Abstract [en]

    We have used a MHz lock-in x-ray spectromicroscopy technique to directly detect changes in magnetic moment of Cu due to spin injection from an adjacent Co layer. The elemental and chemical specificity of x rays allows us to distinguish two spin current induced effects. We detect the creation of transient magnetic moments of 3×10^{-5}μ_{B} on Cu atoms within the bulk of the 28 nm thick Cu film due to spin accumulation. The moment value is compared to predictions by Mott's two current model. We also observe that the hybridization induced existing magnetic moments at the Cu interface atoms are transiently increased by about 10% or 4×10^{-3}μ_{B} per atom. This reveals the dominance of spin-torque alignment over Joule heat induced disorder of the interfacial Cu moments during current flow.

  • 12.
    Pancaldi, Matteo
    et al.
    Stockholm University, Faculty of Science, Department of Physics. CIC nanoGUNE, Spain.
    Freeman, Ryan
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Hoffmann, Matthias C.
    Urazhdin, Sergei
    Vavassori, Paolo
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Anti-reflection coating design for metallic terahertz meta-materials2018In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 3, p. 2917-2927Article in journal (Refereed)
    Abstract [en]

    We demonstrate a silicon-based, single-layer anti-reflection coating that suppresses the reflectivity of metals at near-infrared frequencies, enabling optical probing of nano-scale structures embedded in highly reflective surroundings. Our design does not affect the interaction of terahertz radiation with metallic structures that can be used to achieve terahertz near-field enhancement. We have verified the functionality of the design by calculating and measuring the reflectivity of both infrared and terahertz radiation from a silicon/gold double layer as a function of the silicon thickness. We have also fabricated the unit cell of a terahertz meta-material, a dipole antenna comprising two 20-nm thick extended gold plates separated by a 2 mu m gap, where the terahertz field is locally enhanced. We used the time-domain finite element method to demonstrate that such near-field enhancement is preserved in the presence of the anti-reflection coating. Finally, we performed magneto-optical Kerr effect measurements on a single 3-nm thick, 1-mu m wide magnetic wire placed in the gap of such a dipole antenna. The wire only occupies 2% of the area probed by the laser beam, but its magneto-optical response can be clearly detected. Our design paves the way for ultrafast time-resolved studies, using table-top femtosecond near-infrared lasers, of dynamics in nano-structures driven by strong terahertz radiation.

  • 13.
    Polley, Debanjan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Pancaldi, Matteo
    Hudl, Matthias
    Stockholm University, Faculty of Science, Department of Physics.
    Vavassori, Paolo
    Urazhdin, Sergei
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    THz-driven demagnetization with perpendicular magnetic anisotropy: towards ultrafast ballistic switching2018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, no 8, article id 084001Article in journal (Refereed)
    Abstract [en]

    We study THz-driven spin dynamics in thin CoPt films with perpendicular magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show that demagnetization amplitude of about 1% can be achieved with a peak THz electric field of 300 kV cm(-1), and a corresponding peak magnetic field of 0.1 T. The effect is more than an order of magnitude larger than observed in samples with easy-plane anisotropy irradiated with the same field strength. We also utilize finite-element simulations to design a meta-material structure that can enhance the THz magnetic field by more than an order of magnitude, over an area of several tens of square micrometers. Magnetic fields exceeding 1 Tesla, generated in such meta-materials with the available laser-based THz sources, are expected to produce full magnetization reversal via ultrafast ballistic precession driven by the THz radiation. Our results demonstrate the possibility of table-top ultrafast magnetization reversal induced by THz radiation.

  • 14.
    Polley, Debanjan
    et al.
    Stockholm University, Faculty of Science, Department of Physics.
    Zhou Hagström, Nanna
    Stockholm University, Faculty of Science, Department of Physics. MBI Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, Germany.
    von Korff Schmising, Clemens
    Eisebitt, Stefan
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics.
    Terahertz magnetic field enhancement in an asymmetric spiral metamaterial2018In: Journal of Physics B: Atomic, Molecular and Optical Physics, ISSN 0953-4075, E-ISSN 1361-6455, Vol. 51, no 22, article id 224001Article in journal (Refereed)
    Abstract [en]

    We use finite element simulations in both the frequency and the time-domain to study the terahertz resonance characteristics of a metamaterial (MM) comprising a spiral connected to a straight arm. The MM acts as a RLC circuit whose resonance frequency can be precisely tuned by varying the characteristic geometrical parameters of the spiral: inner and outer radius, width and number of turns. We provide a simple analytical model that uses these geometrical parameters as input to give accurate estimates of the resonance frequency. Finite element simulations show that linearly polarized terahertz radiation efficiently couples to the MM thanks to the straight arm, inducing a current in the spiral, which in turn induces a resonant magnetic field enhancement at the center of the spiral. We observe a large (approximately 40 times) and uniform (over an area of similar to 10 mu m(2)) enhancement of the magnetic field for narrowband terahertz radiation with frequency matching the resonance frequency of the MM. When a broadband, single-cycle terahertz pulse propagates towards the MM, the peak magnetic field of the resulting band-passed waveform still maintains a six-fold enhancement compared to the peak impinging field. Using existing laser-based terahertz sources, our MM design allows to generate magnetic fields of the order of 2 T over a time scale of several picoseconds, enabling the investigation of nonlinear ultrafast spin dynamics in table-top experiments. Furthermore, our MM can be implemented to generate intense near-field narrowband, multi-cycle electromagnetic fields to study generic ultrafast resonant terahertz dynamics in condensed matter.

  • 15. Salén, Peter
    et al.
    Basini, Martina
    Stockholm University, Faculty of Science, Department of Physics.
    Bonetti, Stefano
    Stockholm University, Faculty of Science, Department of Physics. Ca’ Foscari University of Venice, Italy.
    Hebling, János
    Krasilnikov, Mikhail
    Nikitin, Alexey Y.
    Shamuilov, Georgii
    Tibai, Zoltán
    Zhaunerchyk, Vitali
    Goryashko, Vitaliy
    Matter manipulation with extreme terahertz light: Progress in the enabling THz technology2019In: Physics reports, ISSN 0370-1573, E-ISSN 1873-6270, Vol. 836-837, p. 1-74Article, review/survey (Refereed)
    Abstract [en]

    Terahertz (THz) light has proven to be a fine tool to probe and control quasi-particles and collective excitations in solids, to drive phase transitions and associated changes in material properties, and to study rotations and vibrations in molecular systems. In contrast to visible light, which usually carries excessive photon energy for collective excitations in condensed matter systems, THz light allows for direct coupling to low-energy (meV scale) excitations of interest, The development of light sources of strong-field few-cycle THz pulses in the 2000s opened the door to controlled manipulation of reactions and processes. Such THz pulses can drive new dynamic states of matter, in which materials exhibit properties entirely different from that of the equilibrium. In this review, we first systematically analyze known studies on matter manipulation with strong-field few-cycle THz light and outline some anticipated new results. We focus on how properties of materials can be manipulated by driving the dynamics of different excitations and how molecules and particles can be controlled in useful ways by extreme THz light. Around 200 studies are examined, most of which were done during the last five years. Secondly, we discuss available and proposed sources of strong-field few-cycle THz pulses and their state-of-the-art operation parameters. Finally, we review current approaches to guiding, focusing, reshaping and diagnostics of THz pulses.

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