On-line SPICE-SPIN+X Seminars

On-line Seminar: 16.06.2021 - 15:00 German Time

Electrical Generation of Spin Currents

Andrew Kent, New York University

Spin currents in magnetic random access memory (MRAM) devices being developed by the semiconductor industry are generated by passing an electrical current perpendicular to layers that form a magnetic tunnel junction [1]. However, it is now widely appreciated that current flow in the plane of a layer can generate significant spin currents through spin-orbit coupling, as first reported in heavy non-magnetic metal layers (e.g. Pt, Ta & W). In this case, however, the spin polarization is generally confined to the plane of the layers. An important research goal is to create a spin current with an arbitrary polarization, including one with a significant out-of-plane spin polarization to enable efficient switching and displacement of domain walls in perpendicularly magnetized layers. In this talk we discuss spin-orbit induced charge-to-spin conversion in various materials and nanostructures [2] and with magnetic materials. Specifically, we will report our observation of spin torques with a planar Hall effect symmetry from CoNi, with a spin polarization in the magnetization direction of the layer [3]. We found the strength of this effect to be comparable to that of the spin Hall effect in Pt, indicating that the planar Hall effect in ferromagnetic metals holds great promise as a spin current source with a controllable spin polarization direction.

[1] A. D. Kent and D. C. Worledge, Nature Nanotechnology 10, 187 (2015)
[2] J-W. Xu and A. D. Kent, Physical Review Applied 14, 014012 (2020)
[3] C. Safranski, J. Z. Sun, J-W., Xu and A. D. Kent, Physical Review Letters 124, 197204 (2020)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 30.06.2021 - 15:00 German Time

Building and investigating magnetic adatom chains on superconductors atom by atom

Katharina Franke, Freie Universität Berlin

Magnetic adatom chains on superconducting substrates are promising platforms for topological superconductivity and Majorana zero modes. Signatures of these have been found in densely-packed chains with direct exchange interaction among the adatoms. Here, we investigate adatom chains in the “dilute” limit. This means that the atoms are sufficiently far spaced that direct hybridization of their d orbitals is negligible, but close enough for sizeable substrate-mediated interactions. We build these chains from individual Fe atoms on a 2H-NbSe2 substrate. Using scanning tunneling microscopy and spectroscopy we first characterize the exchange coupling between the magnetic adatoms and the superconductor by detecting their Yu-Shiba-Rusinov states within the superconducting energy gap. We then use the tip of the STM to assemble dimers, trimers and chains of these Fe atoms. In each step, we track the evolution of the Yu-Shiba-Rusinov states and identify magnetic interactions, hybridization and band formation.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 23.06.2021 - 15:00 German Time

Spin And Charge Transport In Antiferromagnets

Vincent Baltz, SPINTEC, Grenoble

Antiferromagnets have attracted interest for use of their spin-dependent transport properties in electronic devices [1,2]. Towards this end, determining the characteristic lengths promoting spin dependent transport as well as understanding how antiferromagnetic spin structures [3,4] and spin textures [5,6] influence transport are some of the basic points that deserve to be investigated.
In this talk, I will first discuss experiments of spin injection and propagation in antiferromagnets. I will show how we demonstrated experimentally [7] the theoretical prediction [8] of the interplay between linear spin fluctuations and the spin mixing conductance, therefore opening perspectives for studies of critical phenomena in thin films of antiferromagnets. In search of non-linear spin fluctuations [9], I will then detail how we found experimental evidence of an overlooked effect: self-induced spin-charge conversion in the ferromagnetic spin-injector, corroborating the results of first-principle calculations [10]. Beyond extrinsic scattering, recent experimental findings relating to the interplay between the antiferromagnetic order and crystal symmetries [4] will be briefly announced.
In a second part, I will introduce a stimulating example of how antiferromagnets and superconductors [11] may envision a common future by showing how we inferred essential information using Cooper pair transport through antiferromagnets [12].
Finally, in search of the nucleation of skyrmions in antiferromagnets to study the associated spintronic effects, I will show how we took advantage of the exchange bias interaction between an antiferromagnet and an adjacent ferromagnet to stabilize several types of spin textures at the interface of the antiferromagnet [6,13,14].

[1] T. Jungwirth et al, Nat. Nanotechnol. 11, 231 (2016)
[2] V. Baltz et al, Rev. Mod. Phys. 90, 015005 (2018)
[3] L. Šmejkal et al, Nat. Phys. 14, 242 (2018)
[4] H. Reichlová et al, arXiv:2012.15651 (2020)
[5] S.-W. Cheong, et al, npj Quantum Materials 5, 3 (2020)
[6] K. G. Rana et al, arXiv:2009.14796 (2020)
[7] L. Frangou et al, Phys. Rev. Lett. 116, 077203 (2016); Phys. Rev. B 98, 094422 (2018)
[8] Y. Ohnuma et al, Phys. Rev. B 89, 174417 (2014)
[9] D. H. Wei et al, Nat. Commun. 3, 1058 (2012)
[10] O. Gladii et al, Phys. Rev. B 100, 1174409 (2019)
[11] A. I. Buzdin, Rev. Mod. Phys. 77, 935 (2005)
[12] R. L. Seeger et al, arXiv:2102.03425 (2021)
[13] S. Brück et al, Adv. Mater. 17, 2978 (2005)
[14] G. Salazar-Alvarez et al, Appl. Phys. Lett. 95, 012510 (2009)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 02.06.2021 - 15:00 German Time

Spintronic microwave and THz detectors: state-of-the art and future!

Giovanni Finocchio, University of Messina

Microwave detectors based on the spin-torque diode effect are among the key emerging spintronic devices. By utilizing the spin of electrons in addition to their charge, they have the potential to overcome the theoretical performance limits of their semiconductor (Schottky) counterparts. Those devices realized with magnetic tunnel junctions exhibit high-detection sensitivity >200kV/W at room temperature, without any external bias fields, and for low-input power (micro-Watts or lower). In the first part of the talk, I will discuss our recent results in the field of microwave detectors based on spin diodes and possible implementations of THz detectors based on antiferromagnets.
Another application of spintronic diodes, when they have a broadband frequency response, is as electromagnetic energy harvesting, which offers an attractive energy source for applications in self-powered portable electronics in the “internet of things” era. Here I will show the development of a bias-field-free spin-torque diodes based on a magnetic tunnel junction having a canted magnetization in the free layer, and demonstrate that those devices could be an efficient harvester of broadband ambient RF radiation, capable to efficiently harvest microwave powers of microWatt and below and to power a black phosphorous nanodevice. The frequency response of spin-torque diodes and their current tunability can be also used as building blocks of the hardware realization of neurons and synapses in neuromorphic applications. Finally, I will show how to implement hardware multiplication with spintronic diodes by using the concept of degree of rectification.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.05.2021 - 15:00 German Time

Antiferromagnetic Switching Driven by the Collective Dynamics of Correlated Spin Textures

James Analytis, University of California

The theory behind the electrical switching of antiferromagnets is premised on the existence of a well-defined broken symmetry state that can be rotated to encode information. A spin glass is in many ways the antithesis of this state, characterized by an ergodic landscape of nearly degenerate magnetic configurations, choosing to freeze into a distribution of these in a manner that is seemingly bereft of information. Here, we show that the coexistence of spin glass and antiferromagnetic order allows a novel mechanism to facilitate the switching of the antiferromagnet \Fex{1/3+\delta}, rooted in the electrically-stimulated collective winding of the spin glass. The local texture of the spin glass opens an anisotropic channel of interaction that can be used to rotate the equilibrium orientation of the antiferromagnetic state. The use of a spin glass' collective dynamics to electrically manipulate antiferromagnetic spin textures has never been applied before, opening the field of antiferromagnetic spintronics to many more material platforms with complex magnetic textures.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 09.06.2021 - 15:00 German Time

Ultrafast coupled charge, spin and nuclear dynamics: ab-initio description

Sangeeta Sharma, MBI-Berlin

Laser induced ultrafast dynamics is a burgeoning field of condensed matter physics promising the ultimate short time control of light over matter. From the outset of research into femtomagnetism, the field in which spins are manipulated by light on femtosecond or faster time scales, several questions have arisen and remain highly debated: How does the light interact with spin moments? How is the angular momentum conserved between the nuclei, spin, and angular momentum degrees of freedom during this interaction? What causes the ultrafast optical switching of magnetic structures from anti-ferromagnetic to ferromagnetic and back again? What is the ultimate time limit on the speed of spin manipulation? What is the impact of nuclear dynamics on the light-spin interaction?
In my talk I will advocate a parameter free ab-initio approach to treating ultrafast light-matter interactions, and discuss how this approach has led both to new answers to these old questions but also to the uncovering of novel and hitherto unsuspected early time spin dynamics phenomena. In particular I will demonstrate OISTR (optical inter-site spin transfer)[1,2] to be one of the fastest means of spin manipulation via light [4,7,8,9], with changes in magnetic structure occurring on attosecond time scales [8]. I will also discuss the impact of nuclear dynamics on laser induced spin dynamics and demonstrate how selective phonon modes can be used to enhance the OISTR effect.
The ability to measure and calculate the same physical quantity forms the cornerstone of the vital collaboration between theory and experiment, and I will discuss recent work where we have ab-initio calculated the real time response functions of L-edge and M-edge semi-core states during spin dynamics, demonstrating both good quantitative agreement with experiment [5,6] but also showing how theory can actually predict new phenomena and guide new experiments.
[1] Dewhurst et al. Nano Lett. 18, 1842, (2018)
[2] Elliott et al. Scientific Reports 6, 38911 (2016)
[3] Shokeen et al. Phys. Rev. Lett. 119, 107203 (2017)
[4] Chen et al. Phys. Rev. Lett. 122, 067202 (2019)
[5] Willems et al. Nat. Comm. 11, 1 (2020)
[6] Dewhurst et al. Phys. Rev. Lett. 124, 077203 (2020)
[7] Hofherr et al. Sci. Advs. 6, eaay8717 (2020)
[8] Siegrist et al. Nature 571, 240 (2019)
[9] Golias et al. Phys. Rev. Lett. 126, 107202 (2021)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.05.2021 - 15:00 German Time

Magnetic skyrmions for unconventional computing and revealing latent information

Karin Everschor-Sitte, University of Duisburg-Essen

Novel computational paradigms in combination with proper hardware solutions are required to overcome the limitations of our state-of-the-art computer technology. [1-3] In this talk, I will focus on the potential of topologically stabilized magnetic whirls – so-called skyrmions for reservoir computing. Reservoir computing is a computational scheme that allows to drastically simplify spatial-temporal recognition tasks. We have shown that random skyrmion fabrics provide a suitable physical implementation of the reservoir. [4,5] They allow to classify patterns via their complex resistance responses either by tracing a signal over time or by a single spatially resolved measurement. [6] In a second part of the talk, I will introduce two recently developed data analysis tools. [7, 8] While often a significant effort is made in enhancing the resolution of an experimental technique to obtain further insight into the sample and its physical properties, an advantageous data analysis has the potential to provide deep insights into given data set.

[1] J. Grollier, D. Querlioz, K.Y. Camsari, KES, S. Fukami, M.D. Stiles, Nat. Elect. 3, 360 (2020)
[2] E. Vedmedenko, R. Kawakami, D. Sheka, ..., KES, et al., J. of Phys. D 53, 453001 (2020)
[3] G. Finocchio, M. Di Ventra, K.Y. Camsari, KES, P. K. Amiri, Z. Zeng, JMMM 521, 167506 (2021)
[4] D. Prychynenko, M. Sitte, et al, KES, Phys. Rev. Appl. 9, 014034 (2018)
[5] G. Bourianoff, D. Pinna, M. Sitte and KES, AIP Adv. 8, 055602 (2018)
[6] D. Pinna, G. Bourianoff and KES, Phys. Rev. Appl. 14, 054020 (2020)
[7] I. Horenko, D. Rodrigues, T. O’Kane and KES, arXiv:1907.04601
[8] D. Rodrigues, KES, S. Gerber, I. Horenko, iScience 24, 102171 (2021)

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 21.04.2021 - 15:00 German Time

Spin transport in a conventional superconductor

Chiara Ciccarelli, University of Cambridge

I will give an overview of our work in collaboration with the Department of Materials Science and Metallurgy in Cambridge [1-5] on the spin pumping into a Nb thin film. Unlike conventional spin-singlet Cooper pairs, spin-triplet pairs can carry spin. Triplet supercurrents were discovered in Josephson junctions with metallic ferromagnet spacers, where spin transport can occur only within the ferromagnet and in conjunction with a charge current. Ferromagnetic resonance injects a pure spin current from a precessing ferromagnet into adjacent non-magnetic materials. For spin-singlet pairing, the ferromagnetic resonance spin pumping efficiency decreases below the critical temperature (Tc) of a coupled superconductor. Here we present ferromagnetic resonance experiments in which spin sink layers with strong spin–orbit coupling are added to the superconductor. We show that the induced spin currents, rather than being suppressed, are substantially larger in the superconducting state compared with the normal state and show that this cannot be mediated by quasiparticles and is most likely a triplet pure spin supercurrent. By carrying angular dependence studies of the Gilbert damping we are able to link the emergence of the triplet condensate to the Rashba spin-orbit coupling.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 14.04.2021 - 15:00 German Time

Magneto-Seebeck microscopy of spin-orbit-torque driven domain wall motion in a collinear antiferromagnet

Jörg Wunderlich, Regensburg University

We introduce a novel microscopy for antiferromagnetic nanostructures based on the local generation and detection of photo-currents. We apply this method to the collinear and fully compensated antiferromagnet CuMnAs where the photocurrents result from the local variation of the magneto-Seebeck effect (MSE). Using a scattering near-field microscope, we display narrow 180-degree domain walls (DWs) and provide experimental evidence for reversible spin-orbit torque-driven domain wall motion of 180-degree domain walls. MSE-based microscopy can be applied in principle to the large class of conductive antiferromagnets. Unlike the established X-ray linear dichroism microscopy based on large-area synchrotrons, photocurrent-based microscopy can be easily performed with ordinary laboratory equipment.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 07.04.2021 - 15:00 German Time

Quantum magnonics: Quantum optics with magnons

Silvia Viola-Kusminskiy, MPL

In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonics”. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a single-magnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity magnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field and present some theoretical results from our group which aim to push the boundaries of the current state of the art.

PDF file of the talk available here