Frustrated magnetism and cooperative phase transitions in spinels

The spinel crystal system, AB ₂O ₄, has been fertile ground in studying the effects of magnetic and orbital frustration. The experimental findings, with primary focus on neutron and synchrotron x-ray scattering techniques, are hereby reviewed in spinels with magnetic B ions. Highlighted are novel collective phenomena, such as the zero-energy excitation mode in the spin liquid phase, zero-field and field-induced novel phase transitions, the emergence of complex local spin entities, and heavy fermionic behaviors. Such a diversity in the exotic properties stems from their close proximity to critical points among degenerate states and a delicate balance among different degrees of freedom such as spin, orbital, and lattice.