News and posts

Time: Wednesday, October 24th, 14:30
Speaker: Bernard DIENY, INAC

Heat assisted magnetic recording (HAMR)  uses temporary near field heating of the media during write to increase hard disk drive storage density. By using a plasmonic antenna embedded in the write head, an extremely high thermal gradient iscreated in the recording media (up to 10K/nm). State of the art HAMR media consists of grains of L1₀-FePt exhibiting high perpendicular anisotropy separated by 1 to 2 nm thick
carbon segregant. Next to the plasmonic antenna, the difference of temperature between two nanosized FePt grains in the media can reach 80K across the 2 nm thick grain boundary. This represents a gigantic local thermal gradient of 40K/nm across a carbon tunnel barrier. In the field of spincaloritronics, much weaker thermal gradients of ~1K/nm were shown to cause a thermal spin-transfer torque capable of inducing magnetization switching in magnetic tunnel junctions. Considering on one hand that two neighboring grains separated by an insulating grain boundary in a HAMR media can be viewed as a magnetic tunnel junction, and on the other hand, that the thermal gradients in HAMR are one to two orders of magnitude larger than those used in conventional spincaloritronic experiments, one may expect a strong impact from these thermal spin-transfer torques on magnetization switching dynamics in HAMR recording. This issue has been totally overlooked in earlier development of HAMR technology. It is shown that the thermal in-plane torque can have a detrimental impact on the recording performances by favoring antiparallel magnetic alignment between neighboring grains during the media cooling. Implications on media design are discussed in order to overcome the influence of these thermal torques. Suggestions of spincaloritronic experiments taking advantage of these huge thermal gradients produced by plasmonic antenna are also given.

Time: Wednesday, October 24th, 12:20
Speaker: Enrique DEL BARCO, Florida

Emerging phenomena, such as the spin-Hall effect (SHE), spin pumping, and spin-transfer torque (STT), allow for interconversion between charge and spin currents and the generation of magnetization dynamics that could potentially lead to faster, denser, and more energy efficient, non-volatile memory and logic devices. Present STT-based devices rely on ferromagnetic (FM) materials as their active constituents. However, the flexibility offered by the intrinsic net magnetization and anisotropy for detecting and manipulating the magnetic state of ferromagnets also translates into limitations in terms of density (e.g., because neighboring units can couple through stray fields) and speed (frequencies are limited to the GHz range). A new direction in the field of spintronics is to employ antiferromagnetic (AF) materials, particularly antiferromagnetic insulators. In contrast to ferromagnets, where magnetic anisotropy dominates spin dynamics, in antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range. We will present studies of spin pumping at Manganese Difluoride(MnF₂) / platinum (Pt) interfaces at temperatures below the MnF₂Néel temperature (T_N =  67.34K). In particular, measurements of the inverse spin Hall effect (ISHE) voltage arising from the interconversion of the dynamically injected spin currents into Pt will be reported. We observe a clear electrostatic potential signal coinciding with the MnF²spin flip-flop transition (H_SF= 9T). The signal reverses switching the polarity of the magnetic field, and displays a marked dependence on the power of the microwave stimuli, as expected from the ISHE.

Time: Wednesday, October 24th, 11:40
Speaker: Anatoly ZVEZDIN, Moscow

We study ultrafast spin dynamics and all-optical photo-magnetic switching in transparent magnetic films with an example of the ferromagnetic  films with compensation temperature.  The corresponding H-T phase diagrams of the GdFeCo , TbFeCo  and epitaxial ferrite-garnet films depending on the parameters of the materials   were constructed.A possibility of ultrafast non-thermal magnetization switching in garnet films was shown in the recent experimental work [1].  Understanding all-optical switching in magnetic dielectrics is an important step towards realization of ultrafast magnetic memory with record low energy per bit writing.
The theoretical model is based on the nonlinear Landau-Lifshitz-Gilbert equations fora [001] iron garnet film with uniform magnetization.The effect of the femtosecond laser pulse is described as an additional optical spin-torque, which is quadratic in laser electric field. We conducted simulations of the magnetization dynamics after the impact of a single femtosecond linearly polarized laser pulse in cases of different polarizations of the latter: along <100> and <110> directions.  As a result, different switching patterns were obtained in agreement with the recent experimental work [1]. By changing the polarization of the laser pulse different outcomes of the magnetization dynamics were studied, which include repeatable magnetization reversal under alternation of different laser pulse polarizations.

The work was supported by RSF grant No. 17-12-01333.

[1] A. Stupakiewicz et al. Nature, 542 (2017), 71–74.

Time: Wednesday, October 24th, 11:00
Speaker: Chiara CICCARELLI, Cambridge

The possibility to read the magnetisation at picosecond timescales is intriguing, being able to read the Neel vector of an antiferromagnet at these time scales is even more intriguing. In this talk I will discuss three different approaches to achieve this: charge pumping[1,2], the spin Seebeck effect [3] and spin pumping [4].

1. Ciccarelli et al., Nature Nanotechnology 10, 50 (2015)
2. Zhou et al., PRL 121, 086801 (2018)
3. Seifert et al., Nature Communications,2899 (2018)
4. Huisman et al., APL 110, 072402 (2017)

Time: Wednesday, October 24th, 10:10
Speaker: Olena GOMONAY, Mainz

In the present work we focus on the high-frequency dynamics of Heisenberg antiferromagnet related with the impulsively generated coherent magnons with the wavevector near the edges of the Brillouin zone -- femto-nanomagnons. We present experimental and theoretical investigation of the macrospin dynamics induced by femto-nanomagnons and propose a simple quantum mechanical description of the two-magnon excitation mechanism, based on a light-induced modification of the exchange. We demonstrate that femto-nanomagnonics dynamical regime has purely quantum mechanical nature and can be explained in terms of two-magnon entangled states. Since the conventional decription of spin dynamics relies on classical torque equation of motions, which are not applicable in the present case, we fill this gap by formulating an effective phenomenological theory of the femtosecond longitudinal spin dynamics. We believe that our results pave a way to manipulation and control of squeezed states in the magnetic systems.

Time: Wednesday, October 24th, 9:00
Speaker: Rostislav MIKHAILOVSKIY, Nijmegen

 The magnetic field component of intense terahertz pulses has been identified as the most direct ultrafast interface to the spin [1]. The strength of the underlying Zeeman coupling, however, has been relatively weak and has been limited to the linear regime. At the same time the applications in the field of magnetic storage technologies and quantum computation require distinctly nonlinear spin response with large amplitude.
To achieve large spin deflection we propose to excite low-energy orbital states, which can in turn drive the magnetic order. To this end we demonstrate that in archetypical antiferromagnet TmFeO3 the terahertz electric field repopulates rare-earth orbitals and therefore triggers large amplitude precession of Fe3+ spins [2]. The amplitude of the spin deflection scales quadratically with the terahertz peak field. Our experimental technique allows for a direct comparison of the strength of this nonlinear excitation by the electric field and the linear torque exerted by the magnetic field. For the maximum peak THz magnetic field of 0.3 Tesla (the corresponding electric field 1 MV/cm), the strength of the nonlinear torque exceeds that of the magnetic field by nearly one order of magnitude. An enhancement of the THz peak fields by only three times could be sufficient to switch the spins, reducing so far predicted thresholds by an order of magnitude. To demonstrate this switching we fabricated the negative plasmonic antennas in the gold mask on top of TmFeO3. We observed that the enhancement of the electric field in the gap of the antennas does indeed lead to the spin reorientation in the layer just beneath the gold antennas.
Our work utilizes a new, general concept of electric field control of magnetic excitations by creating hidden states of matter that involve the spin degree of freedom. In the same spirit, one may now investigate the role of other low energy elementary excitations, such as excitons or phonons [3], which could change the orbital wavefunctions of nearby atoms and lead to the spin switching by a related mechanism. 

[1]. T. Kampfrath, et al. Coherent terahertz control of antiferromagnetic spin waves. Nature Photonics 5, 31(2011).
[2]. S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V. Kimel, R. Huber, & R. V. Mikhaylovskiy. Nonlinear spin control by terahertz driven anisotropy fields. Nature Photonics 10, 715 (2016).
[3]. T. F. Nova, A. Cartella, A. Cantaluppi, M. Foerst, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel, R. Merlin, & A. Cavalleri. An effective magnetic field from optically driven phonons. Nature Physics, 13, 132 (2017). 

Time: Tuesday, October 23rd, 17:40
Speaker: Konstantin ZVEZDIN, Moscow

Spin-orbitronics is considered as one of the most promising directions of spintronics due to the high excitation eciency of magnetization dynamics using of spin-orbit coupling (SOC) effects. Among many SOC materials the topological insulators stand out due to their high spin-to-charge conversion factors as well as the potential of utilization of the surface states. In the recent experimental works [1,2,3,4] ]the 3D topological insulator Bi2Se3 was shown to possess the outstanding level of the spin-charge conversion, which leads to ample opportunities of magnetization control.
We present the ferromagnetic resonance (FMR) spin pumping experiment along with the theoretical study of spin-to-charge conversion in the periodic arrays of permalloy nanodots deposited onto 3D topological insulator Bi2Se3. Two resonance peaks are observed and related to the Kittel and the inhomogeneous edge modes respectively. The features of the magnetization dynamics under FMR excitation in a nanodot are successfully simulated with micromagnetic modelling. A numerical approach for calculating the spin-pumping voltage is proposed; the corresponding voltages are simulated using micromagnetic modeling data. The efficiency of spin-to-charge conversion is estimated for two nanostructured systems with di
erent dot-size. In the report the results of the following experiments with Bi₃Se₃- based topological insulator
will be presented:
Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature in memory and logic applications, supersensitive spin diodes, neuromorphic structures and other advanced spintronic applications.

This research has been supported by RSF grant No. 17-12-01333, MOST 105-2112-M-002-010-MY3 and MOST 104-2112-M-018-001-MY3.

1. Mellnik, J. Lee, A. Richardella, J. Grab, P. Mintun, M. H.Fischer, A. Vaezi, A. Manchon, E.-A. Kim, N. Samarth, et al.,Nature 511, 449 (2014).
2. Figueroa, A. Baker, L. Collins-McIntyre, T. Hesjedal, and G. van der Laan, Journal of Magnetism and Magnetic Materials 400, 178 (2016), proceedings of the 20th International Conference on Magnetism (Barcelona) 5-10 July 2015.
3. B. Abdulahad, J.-H. Lin, Y. Liou, W.-K. Chiu, L.-J. Chang, M.-Y. Kao, J.-Z. Liang, D.-S. Hung, and S.-F. Lee, Phys. Rev. B 92, 241304 (2015).
4. C. Han, Y. S. Chen, M. D. Davydova, P. N. Petrov, P. N. Skirdkov, J.G. Lin, J.C. Wu, J.C.A. Huang, K. A. Zvezdin, and A. K. Zvezdin, Appl.Phys.Lett. 111, 182411 (2017)

Time: Tuesday, October 23rd, 17:00
Speaker: Carl DAVIES, Nijmegen

Ever-increasing demand for faster and more energy-efficient information processing and storage has fuelled intense fundamental studies on the magnetization dynamics displayed by ferroics.  Here, we experimentally and numerically explore how magnetization dynamics within the dielectric bismuth-substituted yttrium iron garnet (Bi:YIG) can be controlled using the thermal load associated with a laser pulse.  In particular, we focus on how a laser-induced diminishment of the growth-induced crystalline anisotropy can affect magnetization reversal and domain-wall motion, thus revealing a new degree of freedom for the optical control of magnetism.
In the first set of experiments and calculations, we show that non-linear precessional magnetization dynamics can be triggered in Bi:YIG via a laser-induced modification of the effective magnetic field. Indeed, the amplitude of the resulting precession can be large enough to allow for magnetic recording at sub-nanosecond timescales, within just one-half of a precessional period. Surprisingly, we observe that the magnetic damping becomes anomalously large during the switching process, rendering the switching route robust.
In the second set of experiments, we explore how the velocity of moving domain walls (DWs) can be affected by a laser pulse.  Using the technique of double photography, we show that the impact of light on the DW velocity is a function of the optical intensity and the velocity itself. The results are explained in terms of interplay between photo-induced localization of Bloch lines, due to local intensity-dependent change of the magnetic anisotropy, and the velocity-dependent Magnus force which inhibits localization of Bloch-lines.

Time: Tuesday, October 23rd, 16:10
Speaker: Sangeeta SHARMA, Berlin

The type of magnetic coupling between the constituent atoms of a solid, i.e. ferromagnetic, anti-ferromagnetic or non-collinear, is one of the most fundamental properties of any magnetic material. This magnetic order is governed by the exchange interaction, which is associated with a characteristic time scale at which spin-flip scattering processes occur and change the intrinsic magnetic structure. These time scales can be determined from the exchange parameters and are of the order of 40-400 fs for transition metal atoms. It is a challenge to manipulate this magnetic order at sub-exchange time scales.
We demonstrate[1,2,3,4,5] that spin transfer driven by inter-site spin-selective charge transfer is one of the key mechanisms that underpins spin manipulation at sub-exchange time scales. This charge flow is induced by optical excitations and represents both the fastest possible response of an electronic system to a laser pulse, as well as a response highly sensitive to pulse intensity and structure. By investigating a wide range of interfaces and multi-sub-lattice magnetic materials we demonstrate a rich phenomena of sub-exchange spin manipulation, including even changing the magnetic order of a material from AFM to FM on femtosecond time scales. We furthermore are able to formulate three simple rules that predict the early time qualitative dynamics of magnetization for ferromagnetic, anti-ferromagnetic and ferri-magnetic materials.

 

[1]   V. Shokeen et al., Phys. Rev. Lett. 119, 107203 (2017)
[2]   K. Krieger et al. J. Phys. Condens. Matter 29, 224001 (2017)
[3]   P. Elliott et al., Scientific Reports 6, 38911 (2016)
[4]   J. K. Dewhurst et al., Computer Phys. Comm. 209, 92 (2016)
[5]   K. Krieger et al., J. Chem. Theory Comput. 11, 4870 (2015)

Time: Tuesday, October 23rd, 15:00
Speaker: Peter OPPENEER, Uppsala

Essential spin operations such as spin-polarized current-induced switching or all-optical helicity-dependent spin switching have already been shown to be realizable, however, the underlying fundamental mechanisms are still poorly understood. In this tutorial I aim to address theoretical advancements in three areas, 1) ultrafast laser-induced demagnetization, 2) all-optical helicity-dependent magnetization switching (AOS), and 3) spin-polarized current or electric-field induced spin switching. Ultrafast laser-induced demagnetization was discovered two decades ago [1], however, the underlying fundamental processes of the fast magnetization decay continue to be debated. I shall discuss various proposed microscopic mechanisms [2-4] as well as ab initiocalculations that might lead to identification of the responsible mechanism(s) and compare to recent experiments.
All-optical switching is a very different, interesting process in which the magnetization of a material is manipulated with ultrashort circularly polarized laser pulses [5,6]. Its origin is still not well understood. In order to progress in this technologically relevant area, ab initiomaterials’ specific theory is needed to know precisely how much spin and orbital magnetization can be induced by a circularly polarized pulse by the inverse Faraday effect in absorbing metals [7]. To understand the process of AOS, multiscale modeling is required, which combines the effects of thermal laser heating with laser-induced magnetization to reach a predictive description of AOS in real materials [8].
Switching the magnetization of a thin layer by a spin-polarized current is the underlying principle of STT-MRAM technology. This process has often been modeled in micromagnetics by adding ad hoca spin-transfer torque (STT) to the phenomenological Landau-Lifshitz-Gilbert (LLG) of magnetization dynamics. The LLG equation has been extended further in recent years, by adding other terms, such as spin-orbit torques (SOTs) [9,10]. It is our aim is to achieve a unified theoretical framework to describe the ultrafast spin dynamics in magnetic systems and quantify the spin dynamics’ quantities on the basis of materials’ specific abinitiocalculations. To this end I discuss recent theory that rigorously shows that the LLG equation can be derived from the fundamental Dirac equation [11], and in addition, leads to the nonrelativistic adiabatic and nonadiabatic STT terms as well as relativistic SOT terms.
The presented theory work has been done together with R. Mondal, M. Berritta, A.K. Nandy, L. Salemi, K. Carva, U. Ritzmann, P. Maldonado, and J. Hurst, and with D. Hinzke and U. Nowak.

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[2] B. Koopmans et al, Nat. Mater. 9, 259 (2010).
[3] E. Carpene et al, Phys. Rev. B 78, 174422 (2008).
[4] M. Battiato, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 105, 027203 (2010).
[5] C.D. Stanciu et al, Phys. Rev. Lett. 99, 047601 (2007).
[6] C.-H. Lambert, S. Mangin et al, Science 345, 1337 (2014).
[7] M. Berritta, R. Mondal, K. Carva, and P.M. Oppeneer, Phys. Rev. Lett. 117, 137203 (2016).
[8] R. John et al, Sci. Rep. 7, 4114 (2017).
[9] I.M. Miron et al. Nature 476, 189 (2011)
[10] H. Kurebayashi et al, Nat. Nanotechn. 9, 211 (2014).
[11] R. Mondal, M. Berritta, and P.M. Oppeneer, Phys. Rev. B 94, 144419 (2016).