Time: Wednesday, October 24th, 9:00
Speaker: Rostislav MIKHAILOVSKIY, Nijmegen
The magnetic field component of intense terahertz pulses has been identified as the most direct ultrafast interface to the spin [1]. The strength of the underlying Zeeman coupling, however, has been relatively weak and has been limited to the linear regime. At the same time the applications in the field of magnetic storage technologies and quantum computation require distinctly nonlinear spin response with large amplitude.
To achieve large spin deflection we propose to excite low-energy orbital states, which can in turn drive the magnetic order. To this end we demonstrate that in archetypical antiferromagnet TmFeO3 the terahertz electric field repopulates rare-earth orbitals and therefore triggers large amplitude precession of Fe3+ spins [2]. The amplitude of the spin deflection scales quadratically with the terahertz peak field. Our experimental technique allows for a direct comparison of the strength of this nonlinear excitation by the electric field and the linear torque exerted by the magnetic field. For the maximum peak THz magnetic field of 0.3 Tesla (the corresponding electric field 1 MV/cm), the strength of the nonlinear torque exceeds that of the magnetic field by nearly one order of magnitude. An enhancement of the THz peak fields by only three times could be sufficient to switch the spins, reducing so far predicted thresholds by an order of magnitude. To demonstrate this switching we fabricated the negative plasmonic antennas in the gold mask on top of TmFeO3. We observed that the enhancement of the electric field in the gap of the antennas does indeed lead to the spin reorientation in the layer just beneath the gold antennas.
Our work utilizes a new, general concept of electric field control of magnetic excitations by creating hidden states of matter that involve the spin degree of freedom. In the same spirit, one may now investigate the role of other low energy elementary excitations, such as excitons or phonons [3], which could change the orbital wavefunctions of nearby atoms and lead to the spin switching by a related mechanism.
[2]. S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V. Kimel, R. Huber, & R. V. Mikhaylovskiy. Nonlinear spin control by terahertz driven anisotropy fields. Nature Photonics 10, 715 (2016).
[3]. T. F. Nova, A. Cartella, A. Cantaluppi, M. Foerst, D. Bossini, R. V. Mikhaylovskiy, A. V. Kimel, R. Merlin, & A. Cavalleri. An effective magnetic field from optically driven phonons. Nature Physics, 13, 132 (2017).