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

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 12.05.2021 - 15:00 German Time

Spin dynamics: the Landau-Lifshitz equation and beyond

Ulrich Nowak, Konstanz University

Our understanding of spin dynamics rests on equations of motion, the most famous one being the Landau-Lifshitz equation. In this talk I will give a short introduction in modern, microscopic interpretations of this equation and how it is linked to other approaches in the spirit of multi-scale modelling. Then I will discuss applications of the Landau-Lifshitz equation in the context of ultrafast spin dynamics, extensions of this equation to include new types of dynamics as, e.g., nutation and field-derivative torques, and finally, present an outlook towards spin-phonon coupling and the ultrafast transfer of spin angular momentum into the lattice.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.03.2021 - 15:00 German Time

Optical and Electrical Detection of Spin-Orbit Fields

Christian Back, TU Munich

Interfacial spin-orbit fields enable the manipulation of the magnetization through in-plane charge currents in e.g. bilayers of ferromagnets and heavy metals. To obtain a detailed understanding of the origin of the acting spin orbit fields – i.e. the spin Hall effect vs. the inverse spin galvanic effect – one needs to be able to unambiguously separate field-like and damping like torques. We use a ferromagnet/semiconductor model system to determine spin orbit fields using optical and electrical detection techniques. In this talk I will review the mechanisms we have identified to be responsible for exciting magnetization dynamics in this model system and show how the measurement of the time resolved magnetization trajectory may further provide information concerning the underlying microscopic mechanisms.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 31.03.2021 - 15:00 German Time

Dynamic generation of scalar chirality and topological Hall effect in spiral magnets

Igor Mazin, George Mason University

The concept of scalar chirality (SS) was introduced in the 1990s by Kubo
and others, as a measure of noncoplanarity of a spin texture. In a triangle, it is defined simply as the triple product of the three spins.
In a continuous medium, it can be generalized as a triple product of the magnetization and its two spatial derivatives. It has generated considerable interest in the last few years, after it was shown that SS generates a nontrivial contribution to the Hall effect, dubbed
Topological Hall Effect (THE). Intriguingly, a THE was discovered in a few materials where the known crystal and magnetic structure could not afford SS. Hidden phases of unknown nature, not detected by usual means, were hypothesized to explain these observations.
In this talk, I will present a theory that explains how SS can emerge in an external field in a particular class of non-chiral textures, in a manner similar to emergence of nematic order without underlying Magnetic order in Fe-based superconductors. The essence of the theory is extremely basic: we show that while single-spiral states do not support SS, in some cases a single magnon can generates SS, which then couples with the external magnetic field. While both positive and negative SS
are generated, thermodynamically one sign is preferred. The resulting THE amplitude has a simple temperature and magnetic field dependence, confirmed by the experiment. I will discuss three examples, of which two are predicted to have such fluctuation-driven THE (and they do), and
the third is not (and it doesn’t).

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 03.03.2021 - 15:00 German Time

Improper Dyzaloshinskii spirals and metamagnetic textures - and where to look for them

Ulrich K. Rößler, IFW Dresden

Chiral magnetic textures have been theoretically predicted for systems violating criteria of the Landau theory for continuous phase transitions [1,2]. Their experimental discovery followed with a characteristic time-lag [3,4]. Proper Dzyaloshinskii spiral states, kinks or soliton lattices, and skyrmions as twisted and localized textures of a (axial) vector order-parameter-field in non-centrosymmetric magnets now are best known examples of such textures. Theoretically, more complex states may be expected in magnetism and beyond by coupling several ordering modes. I will discuss the corresponding concept of improper Dzyaloshinskii textures that can be realized near multi-criticality. The concept has led to the first observation of a metamagnetic texture in the non-centrosymmetric antiferromagnet Ca3Ru2O7 [5]. I will sketch the phenomenology of such states and present a selection of further candidate systems. Metamagnetic textures, or more generally improper textures, are mixtures of different ordering modes. They may form either extended modulated states or localized lumps, i.e. static, mostly non-topological solitons, that can condense into mesophases. In a wide class of clean magnetic crystals, where inversion symmetry is broken, a conventional magnetic long-range order could even be avoided, owing to a specific geometric frustration enforced by the twisting influence of a static background gauge field. I will conclude with some conjectures about the existence of classical chiral spin-liquid states in acentric magnets and the question of amorphous ground-states in systems without quenched disorder.

[1] I.E. Dzyaloshinskii, Sov. Phys. JETP 19, 960 (1964)

[2] A.N. Bogdanov, D.A. Yablonskii, JETP 95, 178 (1989)

[3] P. Bak, M.H. Jensen, J.Phys. C 13, L881 (1980); O.Nakanishi et al., Solid State Comm. 35, 995 (1980)

[4] S. Mühlbauer et al., Science 323, 915 (2009); X.Z. Yu et al. Nature 465, 901 (2010)

[5] D. A. Sokolov et al., Nat. Phys. 15, 671 (2019)

PDF file of the talk available here