On-line SPICE-SPIN+X Seminars

On-line Seminar: 19.08.2020 - 15:00 (CET)

Antiferromagnetic Insulatronics: Spintronics without magnetic fields

Mathias Kläui, JGU Mainz

While known for a long time, antiferromagnetically ordered systems have previously been considered, as expressed by Louis Néel in his Nobel Prize Lecture, to be “interesting but useless”. However, since antiferromagnets potentially promises faster operation, enhanced stability with respect to interfering magnetic fields and higher integration due to the absence of dipolar coupling, they could potentially become a game changer for new spintronic devices. The zero net moment makes manipulation using conventional magnetic fields challenging. However recently, these materials have received renewed attention due to possible manipulation based on new approaches such as photons [1] or spin-orbit torques [2].

In this talk, we will present an overview of the key features of antiferromagnets to potentially functionalize their unique properties. This includes writing, reading and transporting information using antiferromagnets.

We recently realized switching in the metallic antiferromagnet Mn₂Au by intrinsic staggered spin-orbit torques [3,4] and characterize the switching properties by direct imaging. While switching by staggered intrinsic spin-orbit torques in metallic AFMs requires special structural asymmetry, interfacial non-staggered spin-orbit torques can switch multilayers of many insulating AFMs capped with heavy metal layers.

We probe switching and spin transport in selected collinear insulating antiferromagnets, such as NiO [5-7], CoO [8,9] and hematite [10,11]. In NiO and CoO we find that there are multiple switching mechanisms that result in the reorientation of the Néel vector and additionally effects related to electromigration of the heavy metal layer can obscure the magnetic switching [5,7,9]. For the spin transport, spin currents are generated by heating as resulting from the spin Seebeck effect and by spin pumping measurements and we find in vertical transport short (few nm) spin diffusion lengths [6,8].

For hematite, however, we find in a non-local geometry that spin transport of tens of micrometers is possible [10,11]. We detect a first harmonic signal, related to the spin conductance, that exhibits a maximum at the spin-flop reorientation, while the second harmonic signal, related to the Spin Seebeck conductance, is linear in the amplitude of the applied magnetic field [10]. The first signal is dependent on the direction of the Néel vector and the second one depends on the induced magnetic moment due to the field. We identify the domain structure as the limiting factor for the spin transport [11]. We recently also achieved transport in the easy plane phase [12], which allows us to obtain long distance spin transport in hematite even at room temperature [12]. From the power and distance dependence, we unambiguously distinguish long-distance transport based on diffusion [10,11] from predicted spin superfluidity that can potentially be used for logic [13].
A number of excellent reviews are available for further information on recent developments in the field [14].

PDF file of the talk available here

References

[1] A. Kimel et al., Nature 429, 850 (2004).

[2] J. Zelezny et al., Phys. Rev. Lett. 113, 157201 (2014); P. Wadley et al., Science 351, 587 (2016).

[3] S. Bodnar et al., Nature Commun. 9, 348 (2018)

[4] S. Bodnar et al., Phys. Rev. B 99, 140409(R) (2019).

[5] L. Baldrati et al., Phys. Rev. Lett. 123, 177201 (2019)

[6] L. Baldrati et al., Phys. Rev. B 98, 024422 (2018); L. Baldrati et al. Phys. Rev. B 98, 014409 (2018)

[7] F. Schreiber et al., arxiv:2004.13374 (in press 2020)

[8] J. Cramer et al., Nature Commun. 9, 1089 (2018)

[9] L. Baldrati et al., Phys. Rev. Lett. 125, 077201 (2020)

[10] R. Lebrun et al., Nature 561, 222 (2018).

[11] A. Ross et al., Nano Lett. 20, 306 (2020).

[12] R. Lebrun et al., arxiv:2005.14414 (2020).

[13] Y. Tserkovnyak et al., Phys. Rev. Lett. 119, 187705 (2017).

[14] Rev. Mod. Phys. 90, 15005 (2018); Nat. Phys. 14, 200-242 (2018); Adv. Mater. 32, 1905603 (2020)

On-line SPICE-SPIN+X Seminars

On-line Seminar: 26.08.2020 - 15:00 (CET)

Magnetic Matchmaking: Hybrid Magnon Modes

Axel Hoffmann, University of Illinois

Hybrid dynamic excitations have gained increased interest due to their potential impact on coherent information processing. Towards this end, magnons, the fundamental excitation quant of magnetically ordered systems, are of particular interest, since they can be easily tuned by external magnetic fields and interact with a wide range of other excitations, such as microwave and optical photons, phonons, and other magnons.1 We have explored recently the integration of permalloy (Ni80Fe20) thin film structures into hybrid magnon systems. By combining permalloy structures with high-quality superconducting microwave resonators, we demonstrated strong magnon-photon coupling in co-planar, on-chip geometry, which is readily scalable to more complex devices.2 Furthermore, we demonstrated strong coupling of permalloy magnons to standing magnon modes in yttrium iron garnet films, which revealed the importance of dampin-like torques originating from spin pumping.3 Lastly, we demonstrated how the coupling between magnons in Ni and surface acoustic waves in LiNbO3 can be used to modulate phonon propagation.4
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

PDF file of the talk available here

References:
1. Y. Li, et al., arXiv:2006.16158.
2. Y. Li, et al., Phys. Rev. Lett. 123, 107701 (2019).
3. Y. Li, et al., Phys. Rev. Lett. 124, 117202 (2020).
4. C. Zhao, et al., Phys. Rev. Appl. 13, 054032 (2020).