Ultrafast spin transport and magnetic order controlled by terahertz electromagnetic pulses

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 was to use ultrashort optical and terahertz (THz) electromagnetic pulses to probe and eventually realize (i) ultrafast transport of spins and magnons and (ii) ultrafast control over magnetic order. Our strategy relied on transferring successful concepts from the fields of spintronics (i.e. electronics) and femtomagnetism (i.e. optics) to the THz frequency gap, thereby combining the benefits of both worlds. Novel measurement schemes (of, e.g. spin currents and magnetoresistive effects), new insights (into, e.g. spin interactions) and applications (such as spintronic THz emission) emerged.

Central achievements of the TERAMAG project are as follows. First, new experimental schemes were developed to probe ultrafast spin transport [1-9], magnetoresistive effects [11, 12] at THz frequencies, as well as the coupling between spins and electron-orbital and crystal-lattice subsystems from femtoseconds to microseconds [13, 14].

Second, based on this methodology, new insights into ultrafast spin dynamics were obtained, in particular, into the microscopic steps leading to the spin Seebeck effect [3, 4] and superdiffusive spin transport [5], the temporal structure and intrinsic and extrinsic contributions to the anomalous Hall effect [11] and anisotropic magnetoresistance [12], as well as the energy and angular-momentum equilibration between highly nonthermal and thermal electrons with electron spins in a ferromagnetic metal [13], and between a hot phonon bath and electron spins in a ferrimagnetic insulator [14].

Finally, interesting applications of (1-3) emerged, for instance optically gated switching of magnetoresistive elements [15], THz Néel spin orbit torque [16] and optically driven spintronic THz emitters [1, 17, 18]. The latter have attractive properties, e.g. large bandwidth and efficiency exceeding that of state-of-the-art emitters such as ZnTe and GaP [1, 17, 18], independence of the pump wavelength from the infrared to vacuum-ultraviolet [1, 17, 19, 20], contact-free setting of the THz polarization plane at rates >10 kHz [21], scalability of the emitter, resulting in THz peak fields >1 MV/cm and fluences of ~1 mJ/cm2 [22-24], and amenability to microstructuring, resulting in on-chip applications [25]. The spintronic THz emitters are meanwhile commercially available.

Selected publications of the >40 TERAMAG-related papers, in particular from the last funding period:

[1] Seifert et al., Appl. Phys. Lett. 120, 180401 (2022)
[2] Lu et al., Nanophotonics 11, 2661-2691 (2022)
[3] Seifert et al., Nature Commun. 9, Article number: 2899 (2018)
[4] P. Jiménez-Cavero et al., Phys. Rev B 105, 184408 (2022)
[5] Rouzegar et al.,Phys. Rev. B 106, 144427 (2022)
[6] Wahada et al., Nano Lett. 9, 3539–3544 (2022)
[7] Gueckstock et al., Appl. Phys. Lett. 120, 062408 (2022)
[8] Bierhance et al., Appl. Phys. Lett. 120, 082401 (2022)
[9] Gueckstock et al.,Advanced Materials 2006281 (2021)
[10] Nádvorník et al., Adv. Mater. Interfaces 2201675 (2022)
[11] Seifert et al., Advanced Materials 2007398 (2021)
[12] Nadvorník et al., Phys. Rev. X 11, 021030 (2021)
[13] Chekhov et al., Phys. Rev. X 11, 041055 (2021)
[14] Maehrlein et al., Sci. Adv. 4, eaar5164 (2018)
[15] Heitz et al., Phys. Rev. Appl. 16, 064047 (2021)
[16] Behovits et al., arXiv.2305.03368 (2023)
[17] Fülöp et al., Adv. Opt. Mat. 1900681 (2019)
[18] Seifert et al., Nat. Photon. 10, 483 (2016)
[19] Herapath et al., Appl. Phys. Lett. 114, 041107 (2019)
[20] Ilyakov et al., Optica 9, 545 (2022)
[21] Gueckstock et al., Optica 8, 1013-1019 (2021)
[22] Seifert et al., Appl. Phys. Lett. 110, 252402 (2017)
[23] Vogel et al., Opt. Expr. 30, 20451 (2022)
[24] Rouzegar et al., Phys. Rev. Appl. (in print, 2023)
[25] Hoppe et al., ACS Appl. Nano Mater. 4, 7454-7460 (2021)
[26] Vedmedenko et al., J. Phys. D: Appl. Phys. 53, 453001 (2020)
[27] Leitenstorfer et al., J. Phys. D: Appl. Phys. 56, 223001(2023)

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.

Periodic Reporting for period 4 - TERAMAG (Ultrafast spin transport and magnetic order controlled by terahertz electromagnetic pulses)

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