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Ultrafast spin transport and magnetic order controlled by terahertz electromagnetic pulses

Periodic Reporting for period 3 - TERAMAG (Ultrafast spin transport and magnetic order controlled by terahertz electromagnetic pulses)

Reporting period: 2019-01-01 to 2020-06-30

Information processing based on conventional electronics makes use of the charge of the electron to encode the value of a bit (0 or 1). The research field of spintronics, on the other hand, aims at extending normal electronics by using the spin of the electron as information carrier. To fully make spintronics compatible and competitive with electronics, spins need to be transported and flipped as fast as possible.

The goal of the TERAMAG project is to use terahertz (THz) electromagnetic pulses and femtosecond laser pulses to probe and eventually realize
A) Ultrafast transport of spins and magnons, and
B) Ultrafast control over magnetic order.

The strategy relies on extending successful concepts from the fields of spintronics (electronics) and femtomagnetism (optics) to the elusive THz frequency gap (0.3 to 30 THz), thereby combining the benefits of both worlds. Novel measurement schemes (e.g. of spin-to-charge-current conversion from ~0.3 to 30 THz) and applications (such as spintronic THz emitters) will emerge.
Concerning objective (A), significant progress was made in the understanding of laser-triggered spin transport in a model spintronic structure: a stack consisting of an insulating or conducting magnetic layer and a nonmagnetic metal layer. In the nonmagnetic layer, the spin current ejected from the magnetic layer is converted into a transverse charge current, leading to the emission of an ultrashort terahertz (THz) electromagnetic pulse that is measured by optical means. We have been exploring three relevant applications of this model-type process: (i) A novel emitter of THz pulses [1], (ii) the straightforward characterization of the spin-to-charge conversion capability of materials [2,3] and (iii) the measurement of laser-triggered charge currents and spin currents and the identification of their microscopic origin [4,5]. To a large extent, this progress relies on the development of sensitive measurement techniques, robust data-analysis methods and suitable models [4,5].

Regarding objective (B), we successfully set up a THz-pump optical-probe experiment which led to two very important achievements. First, by applying intense THz pulses, we demonstrated reversible switching of the magnetic order of the antiferromagnet CuMnAs (one of the model systems of antiferromagnetic spintronics) between two states [6]. Second, we revealed the physics of a fundamental dynamic process in magnets: The equilibration of energy and angular momentum between the crystal lattice and the spins of the model ferrimagnet YIG. In this work, a new transient state of matter was found: A heated magnet whose magnetic moment remains, however, unchanged [7].
Further results from our THz-pump optical-probe setup are the observation of a new mechanism to excite lattice vibrations (phonons) [8] and of the alignment of solvent molecules by intense THz fields [9].

Selected publications:
[1] T. Seifert et al., Efficient metallic spintronic emitters of ultrabroadband terahertz radiation, Nature Photonics 10, 483 (2016)
[2] T. Seifert et al., Terahertz spectroscopy for all-optical spintronic characterization of the spin-Hall-effect metals Pt, W and Cu80Ir20, J. Phys. D: Appl. Phys. 51, 364003 (2018)
[3] J. Cramer et al., Complex Terahertz and Direct Current Inverse Spin Hall Effect in YIG/Cu1−xIrx Bilayers Across a Wide Concentration Range, Nano Letters 18, 1064-1069 (2018)
[4] L. Braun et al., Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3, Nature Communications 7, Article number: 13259 (2016)
[5] T. Seifert et al., Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy, Nature Communications 9, Article number: 2899 (2018)
[6] K. Olejnik et al., Terahertz electrical writing speed in an antiferromagnetic memory, Science Advances 4, eaar3566 (2018)
[7] S. Maehrlein et al., Dissecting spin-phonon equilibration in ferrimagnetic insulators by ultrafast lattice excitation, Science Advances 4, eaar5164 (2018)
[8] S. Maehrlein et al., Terahertz sum-frequency excitation of a Raman-active phonon, Physical Review Letters 119, 127402 (2017)
[9] M. Sajadi et al., Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles, Nature Communications 8, Article number: 14963 (2017)
Based on our current results, future work will in particular focus on using intense THz pulses to control magnetic order of solids on ultrafast time scales, in particular in terms of antiferromagnets.