Magnetothermodynamics of spin ice and related compounds

Geometrically magnetic frustration, which results from the competition of spin-spin interactions of magnetic ions on a regular magnetic lattice, leads to a variety of exotic low temperature states including ``spin ice.'' ``Spin ice'' refers to a magnetic state wherein the two-in/two-out spin configurations of rare earth pyrochlore compounds mimic the proton positions in the water ice, characterized by the ``zero point entropy'' of (R/2) ln(3/2). In this study, we examine how structural disorder affects spin dynamics and the magnetic ``zero point entropy.'' By diluting the ``spin ice'' materials with nonmagnetic ions on the rare earth sites, we have found that the entropy of the diluted species depends non-monotonically on the dilution concentration, and we explain this behavior using a generalized Pauling approximation. Nonmagnetic doping on B sites leads to only a small decrease of the ``zero point entropy,'' indicating the robust nature of ``spin-ice.'' We have also studied Dy2Ge2O7, which has the same chemical formula as ``spin ice'' materials and Ising-like spins but a tetragonal structure. Dy2Ge2O7 undergoes a long range antiferromagnetic ordering transition, but the spin dynamics at temperatures above the order transition is similar to that observed in the canonical ``spin ice'' systems, suggesting that such dynamics are generic to a broader class of Ising-like rare earth systems. \\[4pt] References: \\[0pt] [1] X. Ke \textit{et al}., Phys. Rev. Lett. \textbf{99}, 137203 (2007).\\[0pt] [2] X. Ke \textit{et al}., Phys. Rev. B. \textbf{76}, 214413 (2007).\\[0pt] [3] X. Ke \textit{et al}., Phys. Rev. B. 7\textbf{8}, 104411 (2008).