Magnetic Order-out-of-Disorder in Frustrated Magnets
Prof. Amnon Aharony (Tel Aviv University and Ben-Gurion University, Israel)

Insulating rare-earth oxides often exhibit rich magnetic phase diagrams, with interesting competing magnetic structures. Often, these phase diagrams are due to structures which are close to cubic or tetragonal. In these highly symmetric structures, the magnetic ground states are highly degenerate, implying magnetic frustration.

This talk reviews two mechanisms to lift the frustration, and to yield a variety of magnetic ordered states. One mechanism involves quantum fluctuations. The zero point motion associated with these fluctuations creates "order out of disorder", and chooses some ground states over others. The second mechanism involves small magnetic anisotropies, usually due to spin-orbit interactions. Again, these anisotropies lift the degeneracy, and pick specific structures.

Examples will include several cuprates (the parents of high temperature superconductors) and nickel vanadate, whose structure is close to a planar Kagome lattice.