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

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 17.03.2021 - 15:00 German Time

Charge and Spin transport physics of organic semiconductors

Henning Sirringhaus, University of Cambridge

Organic semiconductors are characterised by weak intermolecular van der Waals bonding. Many vibrational modes are soft and strongly anharmonic and any electronic processes occur in a strongly fluctuating structural landscape. This gives rise to a unique and interesting transport regime not found in inorganic semiconductors in which electronic excitations are effectively able to surf on the waves of molecular lattice distortion. A key requirement is that the energetic site disorder is sufficiently small that it becomes possible for vibrational modes to couple localized states near the band edges to highly delocalised states within the bands that can then transport excitations over long length scales. In this talk we will provide an overview over how we currently understand the charge and spin transport physics of organic semiconductors in this regime.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 24.02.2021 - 15:00 German Time

Spatio-Temporal Dynamics of Magnon Bose-Einstein Condensates

Sergej O. Demokritov, Münster University

Recent advances in the studies of magnon gases have opened new horizons for the deep understanding of physics of room-temperature macroscopic coherent states including Bose-Einstein condensates of magnons. Although this phenomenon was discovered almost 15 years ago, a lot of important issues associated with the magnon Bose-Einstein condensation still remain unclear. Here I review the recent experimental achievements in the investigations of this phenomenon. I show that magnon condensates are characterized by high degree of temporal and spatial coherency, which enables, for instance, observation of the interference of two condensates in the real space. Discovery of the second sound in magnon condensates is also discussed. I also address the dynamics of the condensate in inhomogeneous time-varying external fields. I show that this approach allows one to implement a magnon laser, which can generate a freely propagating cloud of coherent magnons, as well as to separate in the real space degenerate condensates corresponding to two opposite wavevectors. These studies also allowed us to answer the long-standing question concerning the physical origin of the spatial stability of the condensate. Finally, I examine the applicability of the spin-current paradigm for description of magnon condensates.

PDF file of the talk available here

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 10.03.2021 - 15:00 German Time

All-optical control of magnetism: from fundamentals to brain-inspired computing concepts

Theo Rasing, Radboud University

The ability to switch magnets between two stable bit states is the main principle of digital data storage technologies since the early days of the computer. Due to many new ideas, originating from fundamental research during the last 50 years, this technology has developed in a breath-taking fashion. However, the explosive growth of digital data and its related energy consumption is pushing the need to develop fundamentally new physical principles and materials for faster, smaller and more energy-efficient processing and storage of data.
Since our demonstration of magnetization reversal by a single 40 femtosecond laser pulse, the manipulation of magnetism by ultra-short laser pulses has developed into an alternative and energy efficient approach to magnetic recording. Plasmonic antennas have allowed to push this even down to nanometer length scales while photonic networks allow the development of an opticaly swichable MRAM. However, new ICT technologies, such as Artificial Intelligence push the exponentially increasing energy requirement of data manipulation even more. Therefore, the development of radically new physical principles that combine energy-efficiency with high speeds and high densities is crucial for a sustainable future. One of those is neuromorphic computing, that is inspired by the notion that our brain uses a million times less energy than a supercomputer while, at least for some tasks, it even outperforms the latter.
In this talk, I will discuss the state of the art in ultrafast optical manipulation of magnetic bits and present some first results and the potential of optical control of magnetism to implement brain-inspired computing concepts in magnetic materials.

PDF file of the talk available here