Altermagnetism: The Hunt for Experimental Verification of Spin Polarization in Antiferromagnets*

Altermagnetism is an emerging phenomena of significantly high interest to the spintronics community. While spintronics has persisted for decades across a wide array of memory and computing paradigms, it has reached a threshold. Ferromagnetic spintronics have the clear downsides of low switching speeds, stray fields and low packing densities which have hindered any new novel spin related paradigms from wide adaptation by our modern nanoelectronics. While Antiferromagnetics has breathed new life by promising to increase the switching speed of spintronic devices by multiple orders of magnitude by Neel vector manipulation, their diminishingly low spin polarizations prohibit spin current enabled electric readout. Altermagnets however promise to be a new subset of antiferromagnetism described as combining the ultra-high switching speeds of an antiferromagnet, with the spin polarization of a ferromagnet, and the spin momentum locking reminiscent of a topological insulator (be it non-relativistic in nature).

Before we as a community should adopt large scale device research efforts on the Altermagnets, there are critical fundamental questions which need to be answered by experimental demonstration:

1 - Is the polarization from the spin polarized states in the Altermagnets large enough to produce useful spin currents?

2 - Can these states be used as more efficient spin filters than our current state of the art (i.e. tunnel barriers / pinned layers)?

3 - Can the spin polarization be isolated by epitaxial growth / reduced dimensionality & switched at the ultra-high speeds of antiferromagnets (GHz)?

At the Naval research laboratory in Washington D.C we are undergoing a multi-pronged effort to experimentally test these effects. In this talk I will summarize our recent data and findings from our efforts to directly measure spin polarization in epitaxial films of RuO2 and MnTe using lithographically fabricated Point Contact Andreev Reflection (PCAR) test-bed devices as well as magneto-optic and magnetotransport results.

[1] Smejkal, L., et al. Phys. Rev. X 12, 031042 (2022).

[2] Mazin, I. I. Phys. Rev. B 107, L100418 (2023)