Igor Mazin, George Mason University

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 Luttinger theorem, and therefore truly 
belonging to the FM class.
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.