Seminar Dr. Matthias Althammer - Observation of the nonreciprocal magnon Hanle effect

The spin-1/2 of an electron makes it an archetypal two-level system and inspires the description
of other two-level systems using an analogous pseudospin. The quantized spin excitations of an
ordered antiferromagnet with opposite chirality represent pairs of spin-up and -down magnons and
thus can be characterized by a magnonic pseudospin. In the last years, first descriptions and
observations of the associated dynamics of antiferromagnetic pseudospin have been reported
[1,2,3]. I will introduce these magnon pseudospin dynamics and describe how they lead to the
manifestation of the magnon Hanle effect. In our experiments, we use the antiferromagnetic
insulator hematite ( α -Fe2O3) utilizing two Pt strips for all-electrical magnon injection and
detection [1,3]. We observe an oscillation in polarity of the magnon spin signal at the detector as a
function of the applied magnetic field, which we quantitatively explain in terms of diffusive
magnon transport. In particular, we observe a coherent precession of the magnon pseudospin
caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. In a next series of
experiments, we observe a nonreciprocity in the Hanle signal [4]. Interchanging the roles of the Pt
injector and detector strips was found to alter the detected magnon spin signal. The recorded
difference depends on the applied magnetic field and reverses sign when the signal passes its
nominal maximum at the so-called compensation field. We explain these observations in terms of
a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, which is
found to be controllable via the applied magnetic field. The observed nonreciprocal response in
the readily available hematite films opens interesting opportunities for realizing exotic physics
predicted so far only for antiferromagnets with special crystal structures.
Financial support from the Deutsche Forschungsgemeinschaft (DFG) under Germany’s
Excellence Strategy– EXC-2111 – 390814868 is gratefully acknowledged.
[1] T. Wimmer et al., Phys. Rev. Lett. 125, 247204 (2020).
[2] A. Kamra et al., Phys. Rev. B 102, 174445 (2020).
[3] J. Gückelhorn et al., Phys. Rev. B 105, 094440 (2022).
[4] J. Gückelhorn et al., Phys. Rev. Lett. 130, 216703 (2023).