The Pauli exclusion principle combined with interactions between fermions is a unifying basic mechanism that can give rise to quantum phases with spin order in diverse physical systems. Transition-metal ferromagnets, with isotropic ordering respecting crystallographic rotation symmetries and with a net magnetization, are a relatively common manifestation of this mechanism, leading to numerous practical applications, e.g., in spintronic information technologies. In contrast, superfluid ³He has been a unique and fragile manifestation, in which the spin-ordered phase is anisotropic, breaking the real-space rotation symmetries, and has vanishing net magnetization. The recently discovered altermagnets share the spin-ordered anisotropic vanishing-magnetization nature of superfluid ³He. Yet, altermagnets appear to be even more abundant than ferromagnets, can be robust, and are projected to offer superior scalability for spintronics compared to ferromagnets. The talk revisits the decades of research of the spin-ordered anisotropic phases with vanishing net magnetization including, besides superfluid ³He, also theoretically conceived Pomeranchuk instabilities of Fermi liquids [1,2]. While all sharing the same extraordinary character of symmetry breaking, we highlight the distinctions in microscopic physics which set altermagnets apart and enable their robust and abundant material realizations. We show coordinate-space and momentum-space microscopies, experimentally demonstrating and exploring the altermagnetic ordering in MnTe [3,4].

References
[1] L. Smejkal, J. Sinova, T. Jungwirth, Physical Review X (Perspective) 12, 040501 (2022).
[2] T. Jungwirth, R. M. Fernandes, E. Fradkin, A. H. MacDonald, J. Sinova, L. Smejkal, (Perspective) arXiv:2411.00717
[3] O. J. Amin et al., Nature in press, arXiv:2405.02409.
[4] J. Krempasky et al., Nature 626, 517 (2024).