News and posts

Time: Wednesday, October 24th, 17:10
Speaker: Martijn HECK, Aarhus

Photonics has been the technology of choice for long-distance telecommunications for many decades. The key enablers here are the vast bandwidth of optical fibers and their low loss of, currently, below 0.2 dB/km. The loss of an optical fiber or waveguide is independent of the bandwidth of the optical signal. This is unlike electrical cables, like copper, where the losses tend to increase rapidly with increasing bandwidth, to multiple dBs per meter at 25‑GHz bandwidths. This means that there is a clear trade-off for interconnects: the higher the bandwidth, the shorter the length becomes where optical communications is the preferable solution.
Following this trend, in high-end datacenters, vertical-cavity-laser-based multimode fiber interconnects connect the servers, with silicon-photonic-based single-mode fiber interconnects as the highest bandwidth solutions, operating at 100/200 Gbps [1]. For on-board interconnects, i.e., between processors, memory and the edge of the server, new standards are currently being drawn up for so-called optical engines, making this a near-term reality [2]. And finally, pioneering work has shown that the optics can even be brought onto the processor, into an existing CMOS process, for the highest-bandwidth communications [3]. This technology is currently being commercialized [4].
Looking even further, optical networks-on-chip are being considered, albeit mostly theoretical. Initial design studies have shown the validity of replacing copper interconnects on the processor with an optical network, e.g., to connect cores in a multi-core processor [5,6]. Such approaches are based on energy-efficient silicon photonics, promising attojoule-per-bit communications [7].
Leveraging these technology developments and trends, in the H2020-funded project SPICE, we intend to push the boundaries even further, by trying to address single memory elements optically, e.g., to write magnetic tunnel junctions that have magneto-optic layers [8]. This eventually requires ultra-dense and ultra-low power networks, for overall memory energy consumption in the sub-20-fJ/bit range. I will present the opportunities for this technology.

This project has received funding from the European Union’s Horizon 2020 research
and innovation programme under grant agreement No 713481.

1. Mekis et al., "A grating-coupler-enabled CMOS photonics platform." IEEE Journal of Selected Topics in Quantum Electronics 17, no. 3 (2011): 597-608.
2. https://onboardoptics.org/, checked d.d. 11-10-2018
3. Sun et al., "Single-chip microprocessor that communicates directly using light." Nature 528, no. 7583 (2015): 534.
4. https://ayarlabs.com/, checked d.d. 11-10-2018
5. Batten et al., "Building many-core processor-to-DRAM networks with monolithic CMOS silicon photonics." IEEE Micro 29, no. 4 (2009).
6. J. R. Heck, and J. E. Bowers, "Energy efficient and energy proportional optical interconnects for multi-core processors: Driving the need for on-chip sources." IEEE Journal of Selected Topics in Quantum Electronics 20, no. 4 (2014): 332-343.
7. A. B. Miller, "Attojoule optoelectronics for low-energy information processing and communications." Journal of Lightwave Technology 35, no. 3 (2017): 346-396.
8. http://spice-fetopen.eu/, checked d.d. 11-10-2018

Time: Wednesday, October 24th, 16:30
Speaker: Dries VAN THOURHOUT, Ghent

Silicon Photonics uses existing CMOS technology to realise complex photonic integrated chips that allow to route, control, switch and filter light on a chip.  Silicon waveguides strongly confine light allowing to make these chips very compact and enhance light-matter interaction.  In this talk I will introduce the status of the field and the operation of most relevant basic devices (sources, modulators, detectors, passive guiding and routing of signals…).  Then I will show how these building blocks can be combined to build more complex chips that can route and switch optical pulses, as would be needed for optically switching magnetic tunnel junctions.

Time: Wednesday, October 24th, 15:40
Speaker: Mo LI, Minnesota

All-optical switching (AOS) has been extensively explored for its prospects of enabling ultrafast magnetic recording and operation of spintronic devices. Nevertheless, there has not been any demonstration of AOS in realistic spintronic devices, such as magnetic tunnel junctions (MTJs). Since previous studies of AOS have only been performed on a single magnetic layer, how additional magnetic layers affect the AOS phenomenon remains unknown. In this work, we develop a perpendicularly magnetized MTJ (p-MTJ) employing GdFeCo as the free layer. We demonstrate, for the first time, all-optical switching of an MTJ, without using any external magnetic field, but rather with single sub-picosecond infrared laser pulses. The switching is read out electrically through measuring the tunneling magnetoresistance (TMR). We further demonstrate MHz repetition rate of switching GdFeCo film, which is limited by our instruments, but the fundamental upper limit of this rate should be higher than tens of GHz as has been revealed by previous time-resolved studies28. The demonstrated picosecond switching time of an MTJ by AOS is two orders of magnitude faster than that of any other switching methods.

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)