Abstract-Temporal and spectral fingerprints of ultrafast all-coherent spin switching

S. Schlauderer, C. Lange, S. Baierl, T. Ebnet, C. P. Schmid, D. C. Valovcin, A. K. Zvezdin, A. V. Kimel, R. V. Mikhaylovskiy,  R. Huber

https://www.nature.com/articles/s41586-019-1174-7

             Extended Data Fig. 1 Experimental set-up.

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter, ballistic acceleration of electrons and coherent flipping of the valley pseudospin. These dynamics leave unique ‘fingerprints’, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute—the spin—between two states separated by a potential barrier is to trigger an all-coherent precession. Experimental and theoretical studies with picosecond electric and magnetic fields have suggested this possibility, yet observing the actual spin dynamics has remained out of reach. Here we show that terahertz electromagnetic pulses allow coherent steering of spins over a potential barrier, and we report the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 (thulium orthoferrite) with the locally enhanced terahertz electric field of custom-tailored antennas. Within their duration of one picosecond, the intense terahertz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the collective spin resonance and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with numerical simulations. The switchable states can be selected by an external magnetic bias. The low dissipation and the antenna’s subwavelength spatial definition could facilitate scalable spin devices operating at terahertz rates. 

Abstract-Temporal and spectral fingerprints of ultrafast all-coherent spin switching

S. Schlauderer, C. Lange, S. Baierl, T. Ebnet, C. P. Schmid, D. C. Valovcin, A. K. Zvezdin, A. V. Kimel, R. V. Mikhaylovskiy,  R. Huber

Fig. 1: Antenna-enhanced THz spin dynamics.

https://www.nature.com/articles/s41586-019-1174-7

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter, ballistic acceleration of electrons and coherent flipping of the valley pseudospin. These dynamics leave unique ‘fingerprints’, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute—the spin—between two states separated by a potential barrier is to trigger an all-coherent precession. Experimental and theoretical studies with picosecond electric and magnetic fields have suggested this possibility, yet observing the actual spin dynamics has remained out of reach. Here we show that terahertz electromagnetic pulses allow coherent steering of spins over a potential barrier, and we report the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 (thulium orthoferrite) with the locally enhanced terahertz electric field of custom-tailored antennas. Within their duration of one picosecond, the intense terahertz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the collective spin resonance and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with numerical simulations. The switchable states can be selected by an external magnetic bias. The low dissipation and the antenna’s subwavelength spatial definition could facilitate scalable spin devices operating at terahertz rates.

Abstract-Terahertz Magnon-Polaritons in TmFeO3

Rasing, A. V. Kimel, Kailing Zhang, Zuanming Jin, Shixun Cao, Wei Ren, Guo-Hong Ma, Rostislav Mikhaylovskiy

http://pubs.acs.org/doi/abs/10.1021/acsphotonics.7b01402?mi=aayia761&af=R&AllField=nano&target=default&targetTab=std

Magnon-polaritons are shown to play a dominant role in the propagation of terahertz (THz) waves through TmFeO3 orthoferrite, if the frequencies of the waves are in the vicinity of the quasi-antiferromagnetic spin resonance mode. Both time-domain THz transmission and emission spectroscopies reveal clear beatings between two modes with frequencies slightly above and slightly below this resonance, respectively. Rigorous modelling of the interaction between the spins of TmFeO3 and the THz light shows that the frequencies correspond to the upper and lower magnon-polariton branches. Our findings reveal the previously ignored importance of propagation effects and polaritons in such heavily debated areas as THz magnonics and THz spectroscopy of electromagnons. It also shows that future progress in these areas calls for an interdisciplinary approach at the interface between magnetism and photonics.

Abstract-Sub-100-ps dynamics of the anomalous Hall effect at terahertz frequencies

T. J. Huisman, R. V. Mikhaylovskiy, Th. Rasing, A. V. Kimel, A. Tsukamoto, B. de Ronde, L. Ma, W. J. Fan, and S. M. Zhou

Phys. Rev. B 95, 094418 – Published 13 March 2017

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.094418

We report about the anomalous Hall effect in $4f3d$ metallic alloys measured using terahertz time-domain spectroscopy. In this way, we succeeded in revealing ultrafast dynamics of the anomalous Hall effect which accompanies the sub-100-ps optically induced magnetization reversal in a GdFeCo alloy. The experiments demonstrate the ability to control currents at terahertz frequencies in spintronic devices magnetically and ultrafast.

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Abstract-Selective Excitation of Terahertz Magnetic and Electric Dipoles in Er3+ Ions by Femtosecond Laser Pulses in ErFeO3

R. V. Mikhaylovskiy, T. J. Huisman, R. V. Pisarev, Th. Rasing, and A. V. Kimel

Phys. Rev. Lett. 118, 017205 – Published 6 January 2017

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.017205

We show that femtosecond laser pulse excitation of the orthoferrite ErFeO3 triggers pico- and subpicosecond dynamics of magnetic and electric dipoles associated with the low energy electronic states of the Er3+ ions. These dynamics are readily revealed by using polarization sensitive terahertz emission spectroscopy. It is shown that by changing the polarization of the femtosecond laser pulse one can excite either electric dipole-active or magnetic dipole-active transitions between the Kramers doublets of the I415/2 ground state of the Er3+ (4f11) ions. These observations serve as a proof of principle of polarization-selective control of both electric and magnetic degrees of freedom at terahertz frequencies, opening up new vistas for optical manipulation of magnetoelectric materials.

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Abstract-Nonlinear spin control by terahertz-driven anisotropy fields

• S. Baierl,
• M. Hohenleutner,
• T. Kampfrath,
• A. K. Zvezdin,
• A. V. Kimel,
• R. Huber
• & R. V. Mikhaylovskiy

http://www.nature.com/nphoton/journal/v10/n11/full/nphoton.2016.181.html

Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light^{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}. Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations^5, 11. Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field^5, 10. Using the prototypical antiferromagnet thulium orthoferrite (TmFeO₃), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe³⁺ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials.

Abstract-Selective excitation of terahertz magnetic and electric dipoles in Er3+ ions by femtosecond laser pulses in ErFeO3

R. V. Mikhaylovskiy, T. J. Huisman, R. V. Pisarev, Th. Rasing, and A. V. Kimel

https://journals.aps.org/prl/accepted/b3076Ya2D7914e5035ed35a67890ca74eb7f0f2a6

We show that femtosecond laser pulse excitation of the orthoferrite 1 ErFeO3 triggers pico- and subpicosecond dynamics of magnetic and electric dipoles associated with the low energy electronic states of the Er3+ ions. These dynamics are readily revealed by using polarization sensitive terahertz emission spectroscopy. It is shown that by changing the polarization of the femtosecond laser pulse one can excite either electric dipole-active or magnetic dipole-active transitions between the Kramers doublets of the 4I15/2 ground state of the Er3+ (4f11) ions. These observations serve as a proof-of7 principle of polarization-selective control of both electric and magnetic degrees of freedom at terahertz frequencies, opening new doors for optical manipulation of magneto-electric materials

Abstract-Simultaneous measurements of terahertz emission spectroscopy and magneto-optical Kerr effect for resolving ultrafast laser-induced demagnetization dynamics

Phys. Rev. B

T. J. Huisman, R. V. Mikhaylovskiy, A. Tsukamoto, Th. Rasing, and A. V. Kimel

http://journals.aps.org/prb/accepted/30073YdcM281a444f2e17ea6e5e5e22915021bb26

Simultaneous detection of THz emission and transient magneto-optical response is employed to study ultrafast laser-induced magnetization dynamics in three different types of amorphous metallic alloys: Co, GdFeCo and NdFeCo. A satisfactory agreement between the dynamics revealed with the help of these two techniques is obtained for Co and GdFeCo. For NdFeCo the THz emission indicates faster dynamics than the magneto-optical response. This observation indicates that in addition to spin dynamics of Fe, ultrafast laser excitation of NdFeCo triggers faster magnetization dynamics of Nd originating from its orbital momentum.

Abstract-Terahertz magneto-optics in the ferromagnetic semiconductor HgCdCr2Se4

T. J. Huisman¹, R. V. Mikhaylovskiy¹, A. V. Telegin², Yu. P. Sukhorukov², A. B. Granovsky³, S. V. Naumov², Th. Rasing¹ and A. V. Kimel¹
http://scitation.aip.org/content/aip/journal/apl/106/13/10.1063/1.4916884
1 Institute for Molecules and Materials, Radboud University Nijmegen, 6525 AJ Nijmegen, The Netherlands
2 M. N. Miheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences, 620990 Ekaterinburg, Russia
3 Department of Physics, M. V. Lomonosov Moscow State University, Moscow 119991, Russia
Appl. Phys. Lett. 106, 132411 (2015); http://dx.doi.org/10.1063/1.4916884

The magneto-optical response of the ferromagnetic semiconductor HgCdCr2Se4 at terahertz (THz) frequencies is studied using polarization sensitive THz time-domain spectroscopy. It is shown that the polarization state of broadband terahertz pulses, with a spectrum spanning from 0.2 THz to 2.2 THz, changes as an even function of the magnetization of the medium. Analysing the ellipticity and the rotation of the polarization of the THz radiation, we show that these effectsoriginate from linear birefringence and dichroism, respectively, induced by the magnetic ordering. These effects are rather strong and reach 10²rad/m at an applied field of 1 kG which saturates the magnetization of the sample. Our observation serves as a proof-of-principle showing strong effects of the magnetic order on the response of a medium to electric fields at THz frequencies. These experiments also suggest the feasibility of spin-dependent transport measurements on a sub-picosecond timescale.

Abstract-Terahertz dynamics of spins and charges in CoFe/Al2O3 multilayers

J. D. Costa, T. J. Huisman, R. V. Mikhaylovskiy, I. Razdolski, J. Ventura, J. M. Teixeira, D. S. Schmool, G. N. Kakazei, S. Cardoso, P. P. Freitas, Th. Rasing, and A. V. Kimel

Phys. Rev. B 91, 104407 – Published 12 March 2015

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.104407

The ultrafast laser-induced response of spins and charges in CoFe/Al2O3 multilayers are studied using THz and optical pump-probe spectroscopies. We demonstrate the possibility of ultrafast manipulation of the transport and magnetic properties of the multilayers with femtosecond laser excitation. In particular, using time-resolved THz transmission experiments we found that such an excitation leads to a rapid increase of the THz transmission (i.e., electric resistivity). Our experiments also reveal that femtosecond laser excitation results in the emission of broadband THz radiation. To reveal the origin of the emitted THz radiation, we performed magnetic-dependent measurements of the THz emission. We also compared the observed electric field of the THz radiation to calculations performed using subpicosecond laser-induced demagnetization measurements. The good agreement between the experimentally obtained spectra and the calculations corroborates that the measured THz emission originates from the demagnetization process.

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