Magnetic Monopoles in Spin Ices

The simplest excitations out of the lowest energy 2-in-2-out configuration of the Dy spins in Dy2Ti2O7 are not the typically expected spin waves. In fact, individual spin flips breaking the ice rules are the elementary excitations in this compound. These individual flips may be generated by temperature fluctuations, or applied magnetic fields of the order of a few Tesla. Since the pyrochlore lattice is comprised of interconnected tetrahedra, a single spin flip would result in a 3-out-1-in and 3-in-1-out spin configuration on two adjoining tetrahedra (Fig. 2.1 left). The center of a tetrahedron for these positive (negative) defects can be thought of as a source(sink) of magnetic flux and was thus termed as magnetic monopole (anti-monopole) for this solid state system. In this chapter, I discuss the energetics of these monopoles, how they are predicted to interact with each other, as well as some of the past searches for these elusive particles in Dysprosium Titanate. Towards the end of the chapter, a new technique employing a Superconducting QUantum Interference Device is proposed to look for magnetic monopoles.