Multiple-Q Structures in Frustrated Antiferromagnets

The concepts relevant to frustrated antiferromagnets are briefly reviewed. Antiferromagnets are classified according to their symmetry groups, with non-trivial point groups leading to the possibility of multiple-Q antiferromagnetism. The role of residual degeneracy is highlighted and the manner in which this degeneracy is lifted is discussed. The physical phenomena in competition within frustrated magnets, and the states that they prefer, yield ongoing theoretical research, and the way neutron scattering can be used, in conjunction with the application of pressure and magnetic fields, to determine which of the possible magnetic structures is stabilised is under experimental scrutiny. Multiple-Q antiferromagnetism finds varied and often exotic spin states with similar energies, and as such is the setting in which phase transitions between different magnetic states can be studied.