University of Waterloo

Title: Spin Crystal, Spin Glass, Spin Ice and Spin Liquid States in Frustrated Magnetic Pyrochlore Materials

Abstract: The pyrochlore oxides, with general formula A$_2$B$_2$O$_7$ (A is a trivalent rare-earth cation, such as Ho$^{3+}$, Dy$^{3+}$, Gd$^{3+}$, Y$^{3+}$ and B is a tetravalent transition metal cation such as Ti$^{4+}$, Sn$^{4+}$, Mo$^{4+}$, Mn$^{4+}$, etc), display a very large level of frustration arising from the combination of the lattice geometry of corner-sharing tetrahedra and nearest-neighbor antiferromagnetic exchange interactions. For this reason, pyrochlore oxides have attracted much attention in the past ten years as they would appear ideal candidates to search for exotic quantum magnetic ground states. In this talk, I will review some of the experimental and theoretical results obtained in the past few years where quantum effects appear to have largely (perturbatively) slipped away in most systems, and where classical Neel order ("spin crystal"), spin glass, and a macroscopically degenerate magnetic analogue of ice water, "spin ice", have been observed. To contrast with these semi-classical systems, I will discuss the paradoxical problem of the spin liquid state in Tb$_2$Ti$_2$O$_7$ remains paramagnetic down to 50 mK despite a Curie-Weiss temperature, $\theta_{\rm CW}$, of -20 K.