2022 Abstracts UAW

Reinoud LAVRIJSEN

All-optical switching (AOS) of ferrimagnetic rare earth-transition metal compounds with femtosecond laser pulses shows great promise for technological applications [1]. However, the possibly essential role of spin transport has scarcely been addressed. While it has been claimed that Gd can produce large spin currents [2], these are notoriously difficult to probe, impeding a full understanding of the physics at play. We demonstrate the use of spin waves to probe spin currents generated by ferromagnetic rare earth films. Upon fs laser pulse excitation, spin waves are excited in an in-plane Co layer via an out-of-plane spin current [3, 4] originating from a ferrimagnetic Co/Gd bilayer. Here, Co stabilizes the antiparallel Gd magnetization, and provides a spin current to compare the effect of Gd to. For increasing Gd thickness, the spin current is expected to shift from Coto Gd-dominated, reversing its polarization. Using time-resolved MOKE, we find that the homogeneous (FMR) mode experiences a phase rotation of nearly 180° over a small Gd thickness range, which confirms a large contribution of Gd to the overall spin current. Qualitative modeling supports this interpretation, with efforts underway to better quantify the Gd contribution. Substituting Tb for Gd strongly decreases the amplitude of the FMR mode, implying weaker spin current generation. This might also partly explain the apparent difficulty in achieving AOS in Tb-containing systems. The same spin currents can excite THz frequency standing spin waves in the in-plane layer [5, 6], which appear to be strongly suppressed with increasing rare earth thickness. This is consistent with the relatively slow magnetization dynamics in these materials [7] leading to longer lasting spin currents, which excite high frequency modes less efficiently. This approach for probing optically generated spin currents can elucidate the processes at work in AOS, giving valuable insight for implementation in data storage devices of the future.

[1] A. Kimel and M. Li, Nature Reviews Materials, 4, p189-200 (2019)
[2] S. Iihama et al., Advanced Materials, 30.51, 1804004 (2018)
[3] A. Schellekens et al., Nature Communications, 5, 4333 (2014)
[4] G-M. Choi et al., Nature Communications 5, 4334 (2014)
[5] I. Razdolski et al., Nature Communications 8, 15007 (2016)
[6] M. Lalieu et al., Physical Review B, 99.18, 184439 (2019)
[7] B. Frietsch et al., Science Advances, 6.39, eabb1601 (2020)

Johan MENTINK

Antiferromagnets host the fastest and smallest magnetic waves of all magnets. With their additional intrinsically small dissipation antiferromagnets are ideal candidates to challenge the limits for energy and speed in data storage and processing technologies. However, understanding the magnon spectrum at short wavelengths and high oscillation periods has been challenging even for the simplest model: the antiferromagnetic Heisenberg model in 2D [1]. Furthermore, studying the space-time dynamics of this model defines an intricate quantum many-body problem out of equilibrium, for which until recently no accurate methods were available.
Beyond the limitations of existing methods, we adopt a machine learning inspired ansatz [2] to simulate the dynamics of the 2D Heisenberg model [3-4]. By sudden perturbations of the exchange interaction, we directly trigger dynamics of short-range spin correlations that is often described as the dynamics of magnon-pairs. Interestingly, although the anisotropic pattern can be indeed qualitatively understood with magnon theory, the spreading at the smallest length and time scales is up to 40% faster than expected from the highest magnon velocity. We explain the enhanced propagation speed by magnon-magnon interactions, which become exceptionally strong in the two dimensions and in the deep quantum limit (S=1/2).
Beyond sudden perturbations of the exchange interaction, we consider parametric driving of magnon pairs and explore the potential for switching between two stable oscillation states [5]. Using a semi-classical theory, we predict that switching can occur at the femtosecond timescale with an energy dissipation down to a few zepto Joule. This result touches the thermodynamical bound of the Landauer principle and approaches the quantum speed limit up to 5 orders of magnitude closer than demonstrated with magnetic systems so far.

[1] H. Shao et al, Phys. Rev. X 7, 041072 (2017)
[2] G. Carleo and M. Troyer, Science 355, 602 (2017)
[3] G. Fabiani & J.H. Mentink, SciPost Phys 7, 004(2019)
[4] G. Fabiani, M.D. Bouman and J.H. Mentink, Phys. Rev. Lett. 127, 097202 (2021)
[5] G. Fabiani & J.H. Mentink, Appl. Phys. Lett. 120, 152402 (2022)

Oksana Chubykalo-Fesenko

Novel possibilities for ultrafast magnetisation switching have been presented recently using ultrafast phonon excitations in Terahertz (THz) regime [1]. These results also suggest that at the picosecond timescale and below spin-phonon dynamics occur simultaneously and one system can excite another. Here we investigate the magnetisation dynamics in a spin-lattice model [2]parameterised for Fe under the application of a THz phonon pulse. The modeling is done within the molecular dynamics approach in a self-consistent spin-lattice framework. We demonstrate the possibility of a very energy efficient switching in the conditions when phonons are excited with high k-values and THz frequencies, corresponding to a maximum in the density of states and no possibility of spinwave excitation. The mechanism of switching
is via local magneto-elastic fields created by atom’s displacements. In the conditions of the absence of spinwave excitations, practically all phonon angular momentum is transferred to a precessional magnetisation switching. The spin temperature calculated during the switching
process shows a minimum increase (in the order of mK), hence the switching process can be considered non-dissipative. Finally, I will also discuss some very recent results on antiferromagnets.

[1] A. Stupakiewicz et al Nat. Phys. 17 (2021) 489.
[2] M.Strungary et al Phys Rev B 103 (2021) 024429

Joerg Wunderlich

Magnetic data storage is based on the switching and detection of energetically degenerate ferromagnetic ground states with reversed magnetization separated by a sufficiently high energy barrier to maintain long-term non-volatility of the stored data. Therefore, exploiting the many advantages of zero net moment antiferromagnets for fast and energy-efficient magnetic storage will also rely on the realization of switching and detecting stable antiferromagnetic states with reversed magnetic order.
In this talk, we discuss that switching between nonvolatile stable states with opposite Néel vector orientations and their detection in collinear antiferromagnetic systems with combined spatial inversion and time-reversal (PT) symmetry can be realized by generating relativistic effective spin-orbit fields and by detecting higher-order magneto-transport responses. As a model system, we consider a fully compensated synthetic antiferromagnet (SAF) with engineered PT symmetry and an natural equivalent, the antiferromagnet CuMnAs.
Besides just storing "0"-s or "1"-s corresponding to two fully polarized magnetic states with reversed Néel vectors, we also show that partial switching enables the realization of nonvolatile memristor type of devices.

Rostislav Mikhaylovskiy

The antiferromagnetic materials appeal to spintronics and magnonics because of their very high terahertz (THz) frequencies of spin dynamics and unique functionalities in comparison to conventional ferromagnets. Due to the strong coupling of the propagating THz magnetic fields with magnons, the hybrid magnon-polariton modes are formed. The physics of the magnon-porations calls for an interdisciplinary approach at the merge of magnetism and photonics.
For instance, magnon-polaritons are shown to play a dominant role in propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in vicinity of the quasi-antiferromagnetic mode of spin resonance [1]. This leads to beating between magnon-polaritons due to the energy exchange between the higher and lower polariton branches formed in vicinity of the antiferromagnetic magnon frequency.
Polaritonic nature of spin modes in antiferromagnets has important implications for THz-driven spin control [2]. In DyFeO3 orthoferrite the lattice-mediated coupling of the electric fields produced by otherwise orthogonal magnon modes leads to internal resonance, when the frequencies of the modes are close to each other. This resonance results in a dramatic enhancement of spin oscillations excited by THz magnetic field.

[1] K. Grishunin , T. Huisman, G. Li, E. Mishina, Th. Rasing, A. V. Kimel, K. Zhang, Z. Jin, S. Cao, W. Ren , G.-H. Ma and R. V. Mikhaylovskiy. ACS Photonics 5, 1375 (2018)
[2] S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V. Kimel, R. Huber, and R. V. Mikhaylovskiy. Nature Photonics 10, 715–718 (2016)

Yannic Behovits

In antiferromagnets, strong exchange coupling leads to intrinsic terahertz (THz) magnon resonances, which have large potential for high-speed spin information processing. For CuMnAs and Mn2Au, switching of the Néel vector has been demonstrated by using pulsed electrical currents and free-space THz pulses [1-3]. The switching was attributed to the Néel spin-orbit torque (NSOT), which is proportional to the current [4]. However, the underlying spin dynamics have not been observed on ultrafast timescales.
Here, we employ a THz-pump magneto-optic-probe setup to investigate ultrafast dynamics of antiferromagnetic order induced by THz electromagnetic fields in Mn2Au. In our samples, the direction of the Néel vector was prealigned via a spin-flop transition in a high magnetic field (60 T) [5]. We observe a strongly damped oscillatory signal at 0.6 THz, whose amplitude is proportional to the driving THz electric field. Our observations are consistent with an NSOT-driven magnon mode.
Upon increasing the THz field strength to 0.65 MV/cm, a non-linear response emerges. By using a simple model, the signal can be related to a substantial deflection of the Néel vector from its equilibrium position. Based on our results, we can estimate important material-specific parameters and calculate THz pulse field strengths at which switching of the antiferromagnetic order of Mn2Au on picosecond timescales is achieved.

[1] Wadley, P., et al., Electrical switching of an antiferromagnet. Science, 2016. 351(6273): p. 587-590.
[2] Olejník, K., et al., Terahertz electrical writing speed in an antiferromagnetic memory. Science Advances, 2018. 4(3): p. eaar3566.
[3] Bodnar, S.Y., et al., Writing and reading antiferromagnetic Mn2Au by Néel spin-orbit torques and large anisotropic magnetoresistance. Nature Communications, 2018. 9(1): p. 348.
[4] Železný, J., et al., Spin-orbit torques in locally and globally noncentrosymmetric crystals: Antiferromagnets and ferromagnets. Physical Review B, 2017. 95(1): p. 014403.
[5] Sapozhnik, A.A., et al., Direct imaging of antiferromagnetic domains in Mn2Au manipulated by high magnetic fields. Physical Review B, 2018. 97(13): p. 134429.

Yoav William Windsor

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum, materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4f magnetic exchange is mediated indirectly through the conduction electrons (the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4f antiferromagnets and systematically vary the 4f occupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices. If time permits, we will further discuss results on deterministic ultrafast light-induced rotation of the antiferromagnetic spin arrangement by means of a coherent displacive excitation of the 4f moments’ local magnetic anisotropy.
Further info:

[1] Windsor et al., Communications Physics 3, 139 (2020) https://doi.org/10.1038/s42005-020-00407-0
[2] Windsor et al., Nat. Mater. (2022). https://doi.org/10.1038/s41563-022-01206-4

Peter M. Oppeneer

Magnetization switching processes in magnetic materials can generally be divided in coherent and incoherent processes. Whereas the latter involve thermal quenching and then rebuilding of the magnetic order, the former are particularly interesting for fast and energy-efficient switching. I will focus on several mechanisms that could provide coherent torques for magnetization switching, in particular, the spin and orbital Hall and Rashba-Edelstein effects and inverse Faraday effect. To understand the possible achievable magnitudes of these mechanisms, we perform ab initio calculations of the current-induced or light-induced magnetizations in selected materials, including ferromagnets and antiferromagnets (AFMs) [1-3].
We show that both the Rashba-Edelstein effect and the inverse Faraday effect can lead to staggered torques in AFM materials [2,3]. Those induced by the inverse Faraday effect are particularly large. For the selected case of AFM CrPt we find, in collaboration with the group of U. Nowak, that coherent ultrafast single-shot switching of the AFM order is possible within less than 200 fs [4]. This is due to the coherent action of the staggered induced moments as well as the speed of exchange enhanced dynamics in AFMs.

[1] M. Berritta et al., Phys. Rev. Lett. 117, 137203 (2016)
[2] L. Salemi et al., Nat. Commun. 10, 5381 (2019)
[3] L. Salemi et al., Phys. Rev. Mater. 5, 074407 (2021)
[4] T. Dannegger et al., Phys. Rev. B 104, L060413 (2021)

Christian Tzschaschel

Spin damping effects form the core of many emerging concepts for high-speed spintronic applications. Important characteristics such as device switching times and magnetic domain-wall velocities depend critically on the damping rate. Although the implications of spin damping for relaxation processes are intensively studied, damping effects during impulsive spin excitations are assumed to be negligible because of the shortness of the excitation process. Herein we show that, unlike in ferromagnets, ultrafast damping plays a crucial role in antiferromagnetic dynamics because of their strongly elliptical spin precession. In time-resolved measurements, we find that ultrafast damping results in an immediate spin canting along the short precession axis. The interplay between antiferromagnetic exchange and magnetic anisotropy amplifies this canting by several orders of magnitude towards large-amplitude modulations of the antiferromagnetic order parameter. This leverage effect discloses a highly efficient route towards ultrafast manipulation of magnetism in antiferromagnetic spintronics.

Daria Gorelova

We developed a quantum-mechanical approach to derive equations of motion for magnetic vectors under the influence of an ultrashort light pulse [1]. Within our approach, the opto-magnetic effect caused by a light pulse on a magnetic system is described by a time-dependent magnetic operator that separates the effect of the laser pulse on the magnetic system from other magnetic interactions. We model and compare laser-induced precessions of magnetic sublattices of easy-plane and easy-axis antiferromagnetic systems. Using these models, we show how the ultrafast inverse Faraday effect induces a net magnetic moment in antiferromagnets and demonstrate that a crystal field environment and the exchange interaction play essential roles for laser-induced magnetization dynamics even during the action of a pump pulse. Suprisingly, light-induced precessions can start even during the action of the pump pulse with a duration several tens times shorter than the period of induced precessions and affect the position of magnetic vectors after the action of the pump pulse.

[1] Daria Popova-Gorelova, Andreas Bringer, and Stefan Blügel, Phys. Rev. B 104, 224418 (2021)