On the complexity of spinels: Magnetic, electronic, and polar ground\n states

Abstract

This review summarizes more than 100 years of research on spinel compounds,\nmainly focusing on the progress in understanding their magnetic, electronic,\nand polar properties during the last two decades. Many spinel compounds are\nmagnetic insulators or semiconductors; however, a number of spinel-type metals\nexists including superconductors and some rare examples of d-derived\nheavy-fermion compounds. In the early days, they gained importance as\nferrimagnetic or even ferromagnetic insulators with relatively high saturation\nmagnetization and high ordering temperatures, with magnetite being the first\nmagnetic mineral known to mankind. However, spinels played an outstanding role\nin the development of concepts of magnetism, in testing and verifying the\nfundamentals of magnetic exchange, in understanding orbital-ordering and\ncharge-ordering phenomena. In addition, the A- site as well as the B-site\ncations in the spinel structure form lattices prone to strong frustration\neffects resulting in exotic ground-state properties. In case the A-site cation\nis Jahn-Teller active, additional entanglements of spin and orbital degrees of\nfreedom appear, which can give rise to a spin-orbital liquid or an orbital\nglass state. The B-site cations form a pyrochlore lattice, one of the strongest\ncontenders of frustration in three dimensions. In addition, in spinels with\nboth cation lattices carrying magnetic moments, competing magnetic exchange\ninteractions become important, yielding ground states like the time-honoured\ntriangular Yafet-Kittel structure. Finally, yet importantly, there exists a\nlong-standing dispute about the possibility of a polar ground state in spinels,\ndespite their reported overall cubic symmetry. Indeed, over the years number of\nmultiferroic spinels were identified.\n