Antiferromagnetic Insulatronics: Spintronics without Magnetic Fields and Moving Electrons

Mathias Kläui, Johannes Gutenberg University Mainz

IEEE Magnetics Society Distinguished Lecturer 2020

April 7, 2020

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].

We recently realized switching in the metallic antiferromagnet Mn2Au [3] by intrinsic staggered spin-orbit torques [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 [5,6], CoO [7,8] and hematite [8,9]. 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 signal [5,8].

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 [6,7]. For hematite, however, we find in a non-local geometry that spin transport of tens of micrometers is possible [9,10]. 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 [9]. 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. From the power and distance dependence, we unambiguously distinguish long-distance transport based on diffusion [9,10] from predicted spin superfluidity that can potentially be used for logic [11].

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[2] J. Zelezny et al., Phys. Rev. Lett. 113, 157201 (2014); P. Wadley et al., Science 351, 587 (2016).
[3] M. Jourdan et al., J. Phys. D: Appl. Phys. 48, 385001 (2015).
[4] S. Bodnar et al., Nature Commun. 9, 348 (2018).
[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] J. Cramer et al., Nature Commun. 9, 1089 (2018).
[8] L. Baldrati et al., arxiv:2003.05923 (2020).
[9] R. Lebrun et al., Nature 561, 222 (2018).
[10] A. Ross et al., Nano Lett. 20, 306 (2020).
[11] Y. Tserkovnyak et al., Phys. Rev. Lett. 119, 187705 (2017). 

Spin current propagation through an antiferromagnet