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

This review summarizes more than 100 years of research on spinel compounds, mainly focusing the progress in understanding their magnetic, electronic, and polar properties during last two decades. Many compounds are magnetic insulators or semiconductors; however, a number spinel-type metals exists including superconductors some rare examples d-derived heavy-fermion compounds. In early days, they gained importance as ferrimagnetic even ferromagnetic with relatively high saturation magnetization ordering temperatures, magnetite being first mineral known to mankind. However, spinels played an outstanding role development concepts magnetism, testing verifying fundamentals exchange, orbital-ordering charge-ordering phenomena. addition, A- site well B-site cations structure form lattices prone strong frustration effects resulting exotic ground-state properties. case A-site cation is Jahn-Teller active, additional entanglements spin orbital degrees freedom appear, which can give rise spin-orbital liquid glass state. The pyrochlore lattice, one strongest contenders three dimensions. both carrying moments, competing exchange interactions become important, yielding ground states like time-honoured triangular Yafet-Kittel structure. Finally, yet importantly, there long-standing dispute about possibility state spinels, despite reported overall cubic symmetry. Indeed, over multiferroic were identified.