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Magneto-optical constants and their transient changes in ultrafast XUV spectroscopy

Time: Friday, October 25th, 11:00
Speaker: Felix WILLEMS, Berlin

With the advent of new free-electron and laboratory-based high harmonic radiation sources ultrafast magnetic spectroscopy at 3p to 3d transitions (M-edges) has started to develop into a widely used experimental technique. The advantages of this technique are manifold: (a) it leads to the element-specific response of complex multi-component magnetic systems simultaneously in a single measurement [1, 2] (b) it enables ultrafast distinction between the physics of various energy scales; exchange and spin-orbit interaction as well as collective spin excitations and (c) the wavelength of XUV radiation allows to directly probe nanoscale length scales via small angle X-ray scattering [2] and coherent imaging methods [2, 3]. However, to quantitatively interpret increasingly complex experimental data from this technique, the scientific community relies on fundamental experimental and theoretical groundwork. The lack of such a systematic and thorough study has resulted in a number of controversies in literature stemming mostly from difficulties to separate possibly overlapping resonances and from a non-zero off-resonant magnetic signal at lower photon energies. In our work we address all these important questions by presenting a complete measurement of the complex dichroic index of refraction for Co, Fe and Ni (i.e. both the dispersive and absorptive contributions) and comparing it with state-of-the-art ab-initio density functional theory (DFT) calculations [4].
Furthermore, in a time resolved high harmonic experiment, we measure the transient changes of the absorptive refractive index displaying distinct femtosecond dynamics depending on the polarization state of the probing XUV light. Doing so we can distinguish the between excitation of the pure electronic and the spin system. A comparison with time dependent DFT simulations suggests an explanation based on ultrafast, spin-orbit mediated spin-flips [4].

 

[1] F. Willems et al., Physical Review B 92, 220405(R) (2015)
[2] F. Willems et al., Structural Dynamics 4, 014301 (2017)
[3] C. von Korff Schmising et al., Phys. Rev. Lett., 112, 217203 (2014)
[4] F. Willems et al., in preparation

Intersublattice exchange interaction probed with high order harmonics over multiscale dynamics

Time Friday, October 26th, 10:10
Speaker: Marie BARTHELEMY, Strasbourg

Over the past few years, optically induced ultrafast magnetization dynamics has been investi-gated in alloyed ferromagnets by probing core level to 3d band optical transitions of transition metals [1,2]. Those chemical selective measurements are sensitive to the sub-lattices interaction processes during the transient magnetization dynamics. For example, it has been found that the inter-sub-lattice exchange interaction has to be taken into account if the demagnetization times of each sub-lattice of the ferromagnet are considered [3,5]. In this work, permalloy sub-lattices magnetic momenta dynamics are measured simultaneously with a table top Transverse Magneto Optical Kerr Effect by probing with High order Harmonics (20-70 eV range) over a broad tempo- ral scale (Figure1). The Landau-Lifshitz-Bloch equation associated to Langevin formalism can be used to deduce the demagnetization time t ε attributed to each element ε, by taking exchange interaction into account [5]. From our measurements in permalloy, both elements demagnetize simultaneously and precess in phase with same period and Gilbert damping due to strong exchange interaction. It will be shown that the ratio between effective exchange interaction constants of Ni and Fe can be retrieved from those measurements. Moreover, the dependence of relaxation rate upon the pump density of excitation will be discussed considering a multiscale approach including tem-perature dependent exchange parameters.

[1] I. Radu, C. Stamm, A. Eschenlohr, F. Radu, R. Abrudan, K. Vahaplar, T. Kachel, N. Pontius, R. Mitzner, K. Holldack, et al., SPIN 5, 1550004 (2015).
[2] C. La-O-Vorakiat, M. Siemens, M. M. Murnane, H. C.Kapteyn, S. Mathias, M. Aeschlimann, P. Grychtol, R. Adam, C. M. Schneider, J. M. Shaw, et al., Phys.Rev. Lett. 103, 257402 (2009).
[3] S. Mathias, C. La-O-Vorakiat, P. Grychtol, P. Granitzka,E. Turgut, J. Shaw, R. Adam, H. Nembach, M. Siemens, S. Eich, et al., Proc. Natl. Acad. Sci. 109, 4792 (2012).
[4] A. J. Schellekens and B. Koopmans, Phys. Rev. B 87,020407 (2013).
[5] D. Hinzke, U. Atxitia, K. Carva, P. Nieves,O. Chubykalo-Fesenko, P. M. Oppeneer, and U. Nowak, Phys. Rev. B 92, 054412 (2015).

Tutorial: Ultrafast spin and magnetization dynamics in rare earth metals

Time. Friday, October 26th, 9:00
Speaker: Martin WEINELT, Berlin

On which timescale do the electronic band structure and spin polarization of a ferromagnet change after femtosecond laser excitation and how do they react to spin transport and spin-flip scattering?
To answer these questionswe perform time-, spin-, and angle-resolved photoemission experiments with optical laser pulses and higher-order harmonic radiation and investigate the transient magnetization by time-resolved X-ray magnetic circular dichroism in reflection.
For the local ferromagnets gadolinium and terbium and their combination in bilayers, we show that exchange splitting, spin polarization, and transient magnetic moment can respond on significantly different timescales.  This allows us to distinguish between contributions of spin-flip scattering, spin-lattice coupling, and spin transport to the ultrafast magnetization dynamics.

Photocurrents in magnetic bilayers for ultrafast spinorbitronics

Time: Thursday, October 25th, 16:00
Speaker: Frank FREIMUTH, Julich

Spin photocurrents excited by femtosecond laser pulses bring spinorbitronics to ultrafast timescales. Using the Keldysh formalism we systematically identify mechanisms behind the generation of spin photocurrents and charge photocurrents by femtosecond laser pulses. Noncentrosymmetric crystals and inversion asymmetric magnetic bilayers exhibit several mechanisms of photocurrent generation that are absent in centrosymmetric systems. First, there is the circular photogalvanic effect [1,2]. While previous works on the circular photogalvanic effect have focused on nonmagnetic semiconductors, we will discuss the magnetic photogalvanic effect in metallic magnetic bilayers [3]. Second, when magnetic solids are excited by femtosecond laser pulses, superdiffusive spin currents are generated, which are converted into charge currents by the inverse spin Hall effect [4,5]. Third, laser pulses excite magnetization dynamics due to the inverse Faraday effect and the optical spintransfer torque, which we will discuss for Fe, Co and FePt [6]. This magnetization dynamics generates photocurrents, because electrical currents are pumped by the inverse spin‐orbit torque [7,8]. Fourth, we find that also demagnetization drives photocurrents due to the collapse of the exchange field [9]. Based on density‐functional theory calculations we investigate these effects in Co/Pt and Mn/W bilayers.

[1] J. W. McIver et al., Nature Nanotechnology 7, 96 (2012)
[2] S. D. Ganichev and W. Prettl, JPCM 15, R935 (2003)
[3] F. Freimuth et al., arXiv:1710.10480 (2017)
[4] T. Kampfrath et al., Nature Nanotechnology 8, 256 (2013)
[5] T. Seifert et al., Nature Photonics 10, 483 (2016)
[6] F. Freimuth, S. Blügel and Y. Mokrousov, PRB 94, 144432 (2016)
[7] T. J. Huisman et al., Nature Nanotechnology 11, 455 (2016)
[8] F. Freimuth, S. Blügel and Y. Mokrousov, PRB 92, 064415 (2015)
[9] F. Freimuth, S. Blügel and Y. Mokrousov, PRB 95, 094434 (2017

First-principles approach to ultrafast logic functionalization of magnetic molecules

Time: Thursday, October 25th, 15:10
Speaker: Wolfgang HÜBNER, Kaiserslautern

 Following the historic discovery of Beaurepaire et al. [1] five main time scales of ultrafast spin dynamics have been established: coherent electron-photon interaction, magnetic dephasing [2], electron-spin correlation, electron-phonon interaction, and spin-lattice interaction. The main focus of these demagnetization investigations was on extended ferromagnetic systems, corresponding to one point in the reciprocal space. Controlled switching additionally requires spin localization and thus two or more distinguishable active magnetic centers, thus leading to the investigation of antiferromagnets [3] or ferrimagnets [4].
Technological application, however, requires both spin localization and spin transfer. While reciprocal space approaches correspond to top-down patterning of nanostructures, we follow the bottom-up approach of molecular nanostructures. Pursuant to this approach, we use quantum chemical many-body methods to describe the electronic structure and ultrafast spin dynamics of various molecular nanostructures.
 In this way we are able to establish the following logic functionalities on small magnetic molecules: ERASE functionality, which can be realized in two-center molecules with chirp [5] or by exploiting quantum interference [6], OR gate in molecules with three active magnetic centers in the presence of an external actively participating magnetic field [7], and OR gate in in molecules with four active magnetic centers, and no active participation of the field. Exploiting quantum interference effects in four-center molecules we can also achieve spin bifurcation and merging [8]. Furthermore, we propose a cyclic SHIFT register (Fig. 1) using three active centers of a recently synthesized four-center molecule [9]. Quantum interference effects even allow us to implement non-Boolean logic functionalities in the very same molecules. Finally, we discuss over which distances spin can be transferred by femtosecond laser pulses.

[1] E. Beaurepaire, J.-C. Merle. A. Daunois, and J.-Y. Bigot, Phys. Rev. Lett. 76, 4250 (1996)
[2] W. Hübner and G. P. Zhang, Phys. Rev. B 58, R5920 (1998)
[3] R. Gómez-Abal, O. Ney, K. Satitkovitchai, and W. Hübner, Phys. Rev. Lett. 92, 227402 (2004)
[4] C. D. Stanciu, A. Tsukamoto, A. V. Kimel, F. Hansteen, A. Kirilyuk, A. Itoh, and Th. Rasing, Phys. Rev. Lett. 99, 217204 (2007)
[5] G. P. Zhang, G. Lefkidis, W. Hübner, and Y. Bai, J. Appl. Phys. 111, 07C508 (2012)
[6] C. Li, S. Zhang, W. Jin, G. Lefkidis, and W. Hübner, Phys. Rev. B 89, 184404 (2014)
[7] W. Hübner, S. Kersten, and G. Lefkidis, Phys. Rev. B 79, 184431 (2009)
[8] D. Chaudhuri, G. Lefkidis, and W. Hübner, Phys. Rev. B 96, 184413 (2017)
[9] D. Dutta, M. Becherer, D. Bellaire, F. Dietrich, M. Gerhards, G. Lefkidis, and W. Hübner, Phys. Rev. B 97, 224404 (2018) 

 

Tutorial: Femto-Second Light and Electron Pulses to switch magnetisation

Time: Thursday, October 25th, 14:00
Speaker: Stephane MANGIN, Nancy

Since the first observation of magnetization switching in ferrimagnetic GdFeCo alloy films using femtosecond laser pulses in 2007 [1],understanding the mechanism behind all-optical switching (AOS) is becoming a topic of huge interest in the magnetism community. Moreover ultrafast magnetization switching in magnetic material thin film without any applied external magnetic field is drawing a lot of attention for the development of future ultrafast and energy efficient magnetic data storage and memories.
Two type of all optical switching have then ben distinguished: Helicity Independent – All Optical Switching (HI- AOS) and Helicity Dependent – All Optical Switching (HD- AOS). HI-AOS has only been demonstrated for GdFeCo based material and is observed after a singlelaser pulse [2]. After one pulse the magnetization is reversed in the opposite direction independently of the light helicity. On the other hand, HD- AOS has been observed for a large variety of magnetic material such as ferrimagnetic alloy, ferrimagnetic multilayer, ferromagnet, and granular media [3-5].  However several studies shows that HD-AOS is only observed after multiple pulses [6].
During the presentation I will present experimental results showing that the number of pulses can be reduced significantly in order to switch ferromagnetic [Co/Pt] multilayers using only several light pulses. Those results can be explained by considering the transfer of heat and angular from light to the sample’s electron bath [7].
In all the previously reported experiments light is used to manipulate magnetization. However, recently we have engineered multilayer structures in order to create hot electrons femto second pulses. We have demonstrate that the magnetization of GdFeCo can be switched using a femto-second hot electron pulse with no direct light interaction [8]  which confirm the work from Wilson et al [9]. Indeed they reported the switching of GdFeCo/Au bilayer via hot electrons generated by single pulse femtosecond laser. Moreover we have studied the magnetization reversal in a GdFeCo / Cu / [Co/Pt] spin valve structure. We observed single shot switching of both the ferrimagnetic GdFeCo and  the ferromagnetic [Co/Pt] layer. The magnetisation switching is found to be mediated by spin polarized hot electron transport [10].

 

[1] C. D. Stanciu, et al, Phys. Rev. Lett. 99, 047601 (2007).
[2] T. A. Ostler, etal  Nat. Commun. 3, 666 (2012).
[3] S. Alebrand,et al   Appl. Phys. Lett. 101, 162408 (2012)
[4] S. Mangin, et al Nat. Mater. 13, 286 (2014).
[5] C.-H. Lambert, et al , Science 1253493 (2014).
[6 ] M. S. El Hadri et al. Phys. Rev. B 94, 064412 (2016)
[7] G. Kichin et al in Preparation
[8] Y. Xu, et al l Adv Matter 2942 1703474 (2017)
[9]  R. B. Wilson, et al  Physical Review B 95 (18), 180409
[10] S. Iihama et al. Adv Mat (2018)

 

Ultrafast MRAM strategies for cache applications and beyond

Time: Thursday, October 25th, 11:40
Speaker: Lucian PREJBEANU, INAC

Spin transfer torque MRAM (STT-MRAM) are considered as one of the most promising technology for non-volatile RAM due to their non-volatility, quasi-infinite endurance, speed and high-density. In standard STT-MRAM, particularly the in-plane magnetized ones, the switching dynamics of the storage layer magnetization is characterized by an incubation delay that can take up to a few nanoseconds. This is because the spin transfer torque is initially zero at the onset of the write current, since at equilibrium the storage layer magnetization and spin-current polarization are parallel or antiparallel. The magnetization reversal is actually triggered by thermal fluctuations which makes the switching stochastic. Consequently, it is necessary to increase the write pulse duration and/or its amplitude to reach sufficiently low write error rates (for instance lower than 10-11). This is detrimental for the realization of short access time memory such as SRAM-like used for Cache applications, which means that addressing fast switching memories, as SRAM replacement in cache or beyond, up to the processor, it is mandatory to deploy new ultrafast writing strategies. Therefore, we present in this study three MRAM writing strategies allowing to greatly improve the write speed down to the sub-nanosecond range.
First, we demonstrate that sub-ns switching with final state determined by the current polarity through the stack can be achieved in STT-MRAM cells comprising two spin-polarizing layers having orthogonal magnetic anisotropies. Two solutions were found and demonstrated : one consists in increasing the cell aspect ratio, the other consists in applying a static transverse field on the cells, the second one being preferred since it does not require to increase the footprint of the cell.
Second, we have shown that the write stochasticity can be almost completely suppressed and the writing speed greatly increased by inducing an oblique anisotropy (also called easy-cone anisotropy) in the storage layer [7]. This means that, at equilibrium, the storage layer magnetization, instead of being aligned along the normal to the plane of the layer, lies along any direction on a cone of axis normal to the plane. In contrast, the reference layer is designed to keep its perpendicular anisotropy. Thanks to this easy cone anisotropy, the storage and reference layer magnetizations always keep a relative angle so that upon write, the storage layer magnetization reversal can be triggered at the very onset of the write current pulse. This quadratic anisotropy itself results from spatial fluctuations of uniaxial anisotropy which can be induced during deposition and annealing of the magnetic tunnel junction stacks. Thanks to this easy cone anisotropy, the writing is much more reproducible and can be faster and/or realized at lower write voltage thereby reducing the write energy consumption).
Third, we recently demonstrated the first proof of concept of a perpendicular Spin orbit torque (SOT)-MRAM. SOT-RAM combining high speed, non-volatility and potentially infinite endurance while being compatible with technological nodes below 22nm and as such appears as a strong candidate for future non-volatile cache memory. In SOT-MRAM, whose write can be achieved with sub-ns pulses, we have shown that the inclusion of SOT-MRAM at the L1-instruction cache and L2-cache can reduce the energy consumption of processors by 60% while solving radiation induced soft errors of SRAM-only configuration.

Topological Magneto-Optical Effect and Its Quantization in Noncoplanar Antiferromagnets

Time: Thursday, October 25th, 11:00
Speaker: Wangxiang FENG, Beijing

Magneto-optical (MO) effects have been known for more than a century as they reflect the basic interactions between light and magnetism.  The origin of MO effects is normally believed by the simultaneous presence of band exchange splitting and spin-orbit coupling.  Using a tight-binding model and first-principles calculations, we show that topological MO effects, in analogy to the topological Hall effect, can arise in noncoplanar antiferromagnets caused entirely by scalar spin chirality instead of spin-orbit coupling.  The band exchange splitting is not indispensable to topological MO effects.  Moreover, the Kerr and Faraday rotation angles in two-dimensional or layered noncoplanar antiferromagnets are found to be quantized in the low-frequency limit, implying the appearance of quantum topological MO effects, which can be measured by time-domain THz spectroscopy.

Anitferromagnetic memory with ultrafast writing speed

Time: Thursday, October 25th, 10:10
Speaker: Kamil OLEJNIK, Prague

The electrical control of magnetic moments of antiferromagnets (AFMs) using Néel-ordered current induced spin-orbit fields [1] opened possibility to use AFMs for practical memory applications [2]. Compared to ferromagnets (FMs) the AFMs theoretically promise up to three orders of magnitude faster magnetization dynamics.
Experimental results investigating the functionality of AFM memory cells fabricated from epitaxial CuMnAs films will be presented. First we investigate the electrical writing with pulse lengths ranging from milliseconds to hundreds of picoseconds [3]. Since shorter pulses cannot be applied using standard electrical circuitry, we use THz radiation pulses to investigate the writing in THz range. With the THz radiation pulses we observe analogous memory functionality demonstrating that the AFM memory cells can be written using picosecond electrical pulses [4].

 

[1] J. Železný et al. Phys. Rev. Lett. 113 (2014).
[2] P. Wadley et al., Science 351, 587–590 (2016).
[3] K. Olejník et al., Nat. Commun. 8, 15434 (2017).
[4] K. Olejník et al., Sci. Adv. 4, eaar3566 (2018).

Tutorial: Ultrafast optical manipulation of magnetic materials

Time: Thursday, October 25th, 9:00
Speaker: Davide BOSSINI, Dortmund

Magnetism in solid state materials is one of the most widely investigated phenomena in condensed matter physics. The conventional description of a magnetic material is formulated in the framework of thermodynamics, since it relies on the concept of equilibrium. While this approach is effective for the ground state properties, its application to the dynamical regime is limited to to spin dynamics with characteristic timescales in which the adiabatic approximation can still be invoked. The technical progresses of pulsed laser sources have provided the possibility to generate intense laser pulses with duration in the 10-100 femtosecond range. Such laser pulses are among the shortest stimuli in contemporary solid state physics. They provide the groundbreaking possibility to drive and detect spin dynamics in magnetic materials in real-time experiments, whose time-resolution is comparable to or even shorter than the two main magnetic interactions, i.e. the spin-orbit coupling and the exchange interaction. Note that aside from the clear academic interest, investigating the optical control of spins on ever-shorter timescales may have relevant implications for possible future developments of the magnetic recording industry. In this talk I will present the basic concepts, methods and goals of the field called “ultrafast magnetism”[1]. In particular, I aim at demonstrating the potentiality and wide applicability of the optical methods, by describing the major breakthroughs reported in this research area. Spectacular phenomena have already been observed, such as the ultrafast demagnetisation[2], the picosecond-deterministic reversal of the magnetisation[3], the coherent control collective spin excitations[4] and even the photo-induced magnetic phase transitions on the picosecond timescale[5]. In the last part of my talk, I plan to discuss the most recent trends[6,7] and some possible future directions.

[1] A. Kirilyuk et al. Rev. Mod. Phys. 82, 2731 (2010)
[2] E. Beaurepaire et al. PRL 76, 4250 (1996)
[3] C. Stanciu et al. PRL 99, 047601 (2007)
[4] A.V. Kimel et al Nature 435, 655 (2005)
[5] D. Afanasiev et al. PRL 116, 097401 (2016)
[6] D. Bossini et al. Nat. Comm. 7, 10645 (2016)
[7] D. Bossini et a. Nat. Phys. 14, 370 (2018)