SPICE Workshop on Hybrid Correlated States and Dynamics in Quantum Materials, May 14th - 16th 2024
Matthias Althammer
M. Althammer1,2
1Walther-Meißner-Institut, Bavarian Academy of Sciences and Humanities, Garching, Germany
2TUM School of Natural Sciences, Physics Department, Technical University of Munich, Garching, Germany
Pure spin currents, i.e., the flow of angular momentum without an accompanying charge current, represent a new paradigm in spintronics. Most importantly, pure spin currents can be transported by fermions, i.e., electrons, in electrical conductors, and bosons, i.e., by magnons, the quantized spin excitations in magnetically ordered systems. This talk will present our recent progress in probing angular momentum transport via all-electrical measurements in antiferromagnetic insulators [1,2,3,4] and isolated metallic ferromagnet strips [5].
In the first part, I will show that the quantized spin excitations of an ordered antiferromagnet with opposite chirality represent pairs of spin-up and -down magnons. A magnonic pseudospin can characterize this two-level nature. Over the last years, we studied the associated dynamics of antiferromagnetic pseudospin and observed the magnon Hanle effect in hematite thin films [1,2,3,4]. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet [2,3]. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector/ detector [4]. Interchanging their roles was found to alter the detected magnon spin signal. We explain these observations in terms of a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, controllable via the applied magnetic field. The observed nonreciprocal response in the readily available hematite films opens exciting opportunities for realizing exotic physics predicted so far only for antiferromagnets with unique crystal structures.
The second part will present our recent progress toward separating electronic and magnonic contributions to angular momentum transport in metallic ferromagnets [5]. To this end, we electrically excite and detect spin transport between two parallel and electrically insulated ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin current conversion within the ferromagnetic strip generates electronic spin angular momentum transferred to magnons via electron-magnon coupling. We observe a finite angular momentum flow to the second ferromagnetic strip across a diamagnetic substrate over micron distances, which is electrically detected in the second strip by the inverse charge-to-spin current conversion process. We discuss phononic and dipolar interactions as the likely cause of angular momentum transfer between the two strips.
[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, 247204 (2023).
[5] R. Schlitz et al., arXiv:2311.05290 (2023)