Since many years, the canonincal classification of ordered magnets included noncollinear (with many further subdivisions) and two collinear
types: antiferromagnets (AF), which have net magnetization zero by symmetry, and ferro/ferrimagnets (FM), which do not have this property.
The two have distinctly different micro- and macroscopic properties. It was supposed, for instance, that only FM can exhibit spin-splitting of the electronic bands in absence of spin-orbit coupling AND lack of inversion symmetry, have anomalous Hall effect (i.e., Hall effect driven by variation of the Berry phase), magnetooptical effects, suppressed Andreev scattering in contact with a singlet superconductor etc.
A surprisingly recent development (~2019) is that this classification is
incomplete: there are collinear magnets that would belong to AF by this classification, but show all characteristics of FM, *except the net spin polarization*! They were recently dubbed by Mainz group "altermagnets", AM. Incidentally, what has also not been fully appreciated was that there are also materials that have strictly zero net magnetization, but enforced not by symmetry, but by the Luttinger's theorem, and therefore truly belonging to the FM class ("Luttinger-compensated ferrimagnets").
In this talk I will present the new classification and explain, in specific examples, what are the symmetry conditions for AM, why these are a truly new class deserving a new name, and how their unusual properties appear.