2022 Abstracts UAW

Kamil Olejnik

The promise of ultrafast dynamics of antiferromagnets motivates a broad effort to develop new materials and techniques suitable for the study and manipulation of the magnetic state of antiferromagnets. As a result of this activity and thanks to the spintronic perspective, many new interesting phenomena and properties of antiferromagnets were discovered recently [1].
The research of CuMnAs antiferromagnet was initiated by theoretical prediction and experimental demonstration of manipulation of the L-vector using staggered spin-orbit field [2]. Besides this effect, a new type of switching was discovered in CuMnAs which is based on the quenching of nanometer-scale multi-domain state. This effect is accompanied by a significant change of resistivity (reaching 100% at low temperatures) making it interesting for magnetic memory applications [3].
The functional characteristics of the quench-switching of CuMnAs will be discussed in detail. The dynamics of the effect will be assessed from the comparison of switching induced by electrical, THz radiation, and optical pulses with lengths ranging from milliseconds to 100 femtoseconds [4], and from analysis of temperature dependent relaxation [3], both indicating its ultrafast antiferromagnetic nature.

[1] Jungwirth, T. et. al, Nature Physics 14,200–203 (2018)
[2] Wadley, P. et al., Science 351, 587-590 (2016)
[3] Kašpar, Z. et al., Nature Electronics 4, 30–37 (2021)
[4] Olejník, K. et al., Science Advances 4, eaar3566 (2018)

Ulrich Nowak

On the basis of spin model calculations, the dynamics of antiferromagnets is discussed and compared to that of ferromagnets. The magnetic field component of a THz laser excitation can excite antiferromagnetic magnon modes, for very large fields eventually leading even to switching [1]. Relativistic extensions of the Landau-Lifshitz-Gilbert equation like field-derivative torques [2] and inertial spin dynamics [3] lead to additional dynamic effects which can facilitate switching in the THz regime.
Furthermore, we explore the possibility of ultrafast, coherent all-optical magnetization switching by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density-functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect, leading to reliable switching between two perpendicular magnetic states [4].

[1] S. Wienholdt et al., Phys. Rev. Lett. 108, 247207 (2012)
[2] R. Mondal et al., Phys. Rev. B 100, 060409 (R) (2019)
[3] R. Mondal et al., Phys. Rev. B 103, 104404 (2021)
[4] T. Dannegger et al., Phys. Rev. B 104, L060413 (2021)

Olena Gomonay

Antiferromagnets show ultrafast magnetic dynamics that can be effectively induced by optical or current pulses. Among plethora of materials the insulating antiferromagnets are of special interest due to low magnetic damping and reduced energy losses. However, optically generated spin torques induce precession of the Néel vector rather than switching into desirable final state. Here we discuss different scenario of ultrafast switching in noncollinear [1] and collinear [2] antiferromagnets focusing on tailoring of the pulse shapes and combination of different torques. We show that in antiferromagnets the femtosecond optical pulses generate simultaneously two types of spin torques: one scaling with spin current and one scaling with time derivative. Competition between these two opens a way to combine fast rotation of the Néel vector with the effective dynamical damping that enables effective switching into the desired final state. We further discuss the role of magnetoelastic interactions in switching which are relevant for antiferromagnets with strong magnetoelastic coupling like NiO and CoO [3]. We show that optical pulses can induce oscillations of the domain walls pinned by spontaneous strains. Strong enough pulses depin the domain walls and thus induce switching via domain wall motion. Moreover, magnetoelastic domain walls work as convertors between the different magnon modes. This opens a way of a coherent switching using the magnon modes that are most suitable for optical excitations. To conclude, we consider different ultrafast switching mechanisms that could be realised in NiO and Mn3Si antiferromagnets.

[1] O. V. Gomonay and V. M. Loktev. Low Temperature Physics, 41, 698 (2015)
[2] Th. Chirac, J.-Y. Chauleau, P. Thibaudeau, O. Gomonay, and M. Viret. Phys. Rev. B 102, 134415 (2020)
[3] O. Gomonay and D. Bossini. J. Phys. D, 54, 374004 (2021)

Davide Bossini

The wildly growing field of antiferromagnetic spintronics mainly deals with single-domain states materials, even if generating this configuration requires magnetic fields available only in a handful of dedicated facilities in the world. Optical experiments are usually performed focussing the beams into a single domain, whose size can be increased in several materials by annealing. Domains are thus perceived as a nuisance, occurring in the ground state of antiferromagnets, to be avoided for an efficient control of spins. In my talk paper I will discuss recent results, which experimentally disprove this commonly accepted wisdom. Relying on a spectroscopic opto-magnetic investigation of the femtosecond spin dynamics in the archetypal antiferromagnet NiO in a multidomain state I will demonstrate: i) the excitation and a novel mechanism to arbitrary amplify a THz magnon mode via the exciton-magnon transition[1]; ii) nonlinear femtosecond spin dynamics, in the form of coupling between the different magnon modes, typically orthogonal in a single-domain state; iii) the microscopic nature of the coupling between modes, which is due to the presence of domain walls. This last point was supported by a phenomenological model[2] and, most importantly, by means of a control experiment performed in a single domain of the material. This experiment confirms that the coupling between the modes requires domain walls, as it is not observed in a single domain[3].

[1] Nat. Phys.14, 370 (2018)
[2] J. Phys. D: Appl. Phys. 54, 374004 (2021)
[3] Physical Review Letters 127, 077202 (2021)

Dmytro Afanasiev

Magnonics is a research field complementary to spintronics, in which quanta of spin waves (magnons) replace electrons as information carriers, promising lower dissipation [1,2]. The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible [3]. Antiferromagnets can host spin waves at terahertz (THz) frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information [4]. However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. We report the efficient emission and detection of a nanometer-scale wavepacket of coherent propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of light [5]. The subwavelength confinement of the laser field due to large absorption creates a strongly non-uniform spin excitation profile, enabling the propagation of a broadband continuum of coherent THz spin waves (see Fig. 1). The wavepacket features magnons with detected wavelengths down to 125 nm that propagate with supersonic velocities V0 of more than 13 km/s into the material. This long-sought source of coherent short-wavelength spin carriers opens up new prospects for THz antiferromagnetic magnonics and nanoscale coherence-mediated logic devices at THz frequencies.

[1] V.V. Kruglyak et al., J. Phys. D 43, 264001 (2010)
[2] B. Lenk et al., Phys. Rep. 507, 107 (2011)
[3] A.V. Chumak et al. Nat. Phys. 11, 453 (2015)
[4] T. Jungwirth et al. Nat. Nano. 11, 231 (2016)
[5] J.R. Hortensius et al. Nat. Phys. 17, 1001 (2021)

Sangeeta Sharma

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)