Speaker
Description
Realisation of quantum information technologies has
become a key incentive in material studies and photonics.
Orbital 4f states of rare-earth dopants in solids have been
identified as one of the most promising agents of quantum
computation. Here I discuss how to utilize light waves at THz
frequencies to selectively control the low-energy rare-earth
states split by the crystal field. As THz photons have meV
energy scale, this approach does not require the very low
temperatures. Moreover, when embedded into magnetically
ordered solids such as 3d metal (e.g. iron, cobalt, nickel) oxides,
the rare-earth electronic transitions can strongly hybridise with
magnon modes of the ordered 3d spins, leading to spin
reorientation phase-transitions, enhanced magneto-optical
properties and magneto-electricity. The coupling between the
ordered spins and the quantum states of the rare-earth ions
mimics the well-known problem of the interface between the
quantum and the (quasi)classical systems.
Rare-Earth orthoferrites represent a family of magnetic oxides
containing both 3d and 4f (i.e. rare-earth) magnetic ions. The
magnetization in these materials mainly arises from the spins of
the 3d ions, whereas their orientation is set by the interaction of
the 3d spins with the 4f electronic orbitals. While the 3d spins
can be considered to behave like quasiclassical (macroscopic)
magnetic sub-lattices, the quantum mechanical properties of 4f
orbitals cannot be ignored as their populations determine the
magnetic state. The interaction between magnons of the ordered
spins and the transitions between the rare-earth states, which
often form low energy (quasi)doublets, well separated from the
higher energy states, is a direct solid-state analogue to the cavity
quantum electrodynamics toy model for cooperative behaviour
and quantum phase transitions [1].
Using the orthoferrite TmFeO3, we discovered a novel
mechanism of nonlinear THz light-spin coupling mediated by
Tm3+ electronic orbitals. Particularly, resonant pumping
the low-energy orbitals changes magnetic anisotropy, which in
turn efficiently drives spin motion into nonlinear regime [2].
The strength of the THz-driven anisotropy torque exceeds the
Zeeman torque exerted by the magnetic field by at least one
order of magnitude [3]. By further increasing the strength of the
THz field with the help of custom-made plasmonic THz
antennas, we generated anisotropy torques sufficiently high to
induce switching between stable antiferromagnetic states [4].
The demonstrated all-coherent spin switching by a THz pulse,
involved record low losses of energy of only 1 µeV per spin.The
possibility to manipulate the magnetization by THz electric
fields opens new prospects for an unprecedented flexibility in
the design of recording devices.
Recently we highlighted the influence of the rare-earth ions in the THz
response, by comparing the spin dynamics in ErFeO3 with
Kramers Er ions and TmFeO3 with non-Kramers Tm ions
respectively [5]. Although these materials exhibit very similar
properties macroscopically, we observe a remarkable difference
in their dynamics across the spin reorientation transition. In
ErFeO3, we observe a drastic enhancement of the amplitude of
the spin dynamics across the spin reorientation temperature
interval, in contrast the response of TmFeO3. We explain this
difference by accounting for the strong dynamical coupling of
Er electronic transitions and Fe spins and the lack of this
coupling between Tm and Fe.
Finally, we present the most recent results on THz-driven
dynamics in YbFeO3, which exhibits a spin reorientation
transition of the Jahn-Teller type at temperatures below 9 K. In
contrast to TmFeO3 and ErFeO3, the orthoferrite YbFeO3
features cross-over of the Yb3+ electronic transition with the
antiferromagnetic resonance of the iron spins. We show that this
energy crossing over results in ultrastrong coupling between
quantum Yb3+ modes and antiferromagnetic magnons. The
origin and properties of this coupling will be discussed, along
with our theoretical description We believe that our findings
represent a major step in understanding cooperative quantum
effects and THz -driven spin dynamics in ultrafast magnetism.
REFERENCES
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W. Ren, G. Ma, S. Cao, D. Turchinovich, and J. Kono, “Observation of
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[2] 1N. R. Vovk, E. V. Ezerskaya, and R. V. Mikhaylovskiy, “Theory of terahertz-driven magnetic switching in rare-earth orthoferrites: The case of TmFeO3,” Phys. Rev. B vol. 111, 064411, 2025.
[3] S. Baierl, M. Hohenleutner, T. Kampfrath, A. K. Zvezdin, A. V. Kimel, R. Huber, and R. V. Mikhaylovskiy, “Nonlinear spin control by terahertz
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[4] S. Schleusener, C. Lange, S. Baierl, T. Ebnet, C. P. Schmid, D. C. Valovcin, A. K. Zvezdin, A. V. Kimel, R. V. Mikhaylovskiy and R. Huber, “Temporal and spectral fingerprints of ultrafast all-coherent spin switching,” Nature vol. 569, pp. 383-387, 2019.
[5] R. A. Leenders, O. Y. Kovalenko, Y. Saito, N. R. Vovk, A. V. Kimel, and
R. V. Mikhaylovskiy, “THz-driven spin dynamics in orthoferrites with
Kramers and Non-Kramers rare-earth ions,” Phys. Rev. Lett., vol. 135,
246703, 2025.