Ferromagnetic insulators are uncommon

Magnetic interactions in oxides have been intensively studied by John B. Goodenough, one of the three winners of the 2019 Nobel Prize in Chemistry 2019. His models explaining the direct and indirect interactions have been applied in our R&D, examples of which are given in our recent paper published in the Annalen der Physik-journal. As pointed out by P. W. Anderson in New approach to the theory of superexchange interactions. Phys. Rev. 115, 2-13 (1959), the dominant coupling between magnetic cations in most compounds is antiferromagnetic, including ferrimagnetic materials. This is due to the fact that ferromagnetic insulators are rare compounds, but those operating at room temperature are indeed unique. Accordingly, experiments and characterizations requiring true insulating ferromagnets are performed at cryogenic temperatures, requiring expensive techniques characteristic of well-equipped research laboratories.

The impact of room-temperature insulating ferromagnets on practical devices is significant

The 1977 Nobel Prize in Physics was jointly awarded to Philip Warren Anderson, Sir Nevill Francis Mott and John Hasbrouck van Vleck "for their fundamental theoretical investigations of the electronic structure of magnetic and disordered systems".