Pyrochlore spin ice Dy2Ti2O7: Investigating a platform for magnetic monopolar quasiparticles

[摘要] Spin ice is a frustrated magnetic state exhibiting a residual entropy in the magnetic degrees of freedom down to the lowest experimentally measured temperatures. The pyrochlore oxide dysprosium titanate (Dy2Ti2O7) supports a spin ice state on its tetrahedral sublattice of dysprosium spins at low temperatures. Such tetrahedral arrangements of Ising spins yield have drawn interest from the scientific community not only for its unusual persistent low temperature frustrated state but also for exotic magnetic ordered states and transitions when oriented along different crystal axes in an external magnetic field, including the transition from a three-dimensional spin ice state to a two-dimensional kagome ice state. More recently, interest has expanded upon the proposal that Dy2Ti2O7 supports emergent fractionalized magnetic monopole quasiparticles as a consequence of defects in the spin ice symmetry. This work sought to probe the existence, statics, and dynamics of these monopole quasiparticles, and what factors affect their density and mobility. At the beginning of study, there were two significant confounding issues in the field of study of monopole quasiparticles: competing effects of oxygen vacancy defect sites in bulk crystal, and effects of strains and surfaces in real spin ice systems. Investigation of these issues began with collaborating on the growth of Dy2Ti2O7 crystals in a range of oxygen environments, and of Dy2Ti2O7 thin films of varying thickness in a pulsed laser deposition system. After determining spin ice quality, a supeconducting quantum interference device ("SQUID")-based magnetometer in a dilution refrigerator was used to search for signatures of monopole excitations in spin ice.From this work, structural, thermal, and magnetic trends were measured in spin ice crystals grown under varying oxygen conditions, and the large effect of oxygen content on the statistical mechanics and electromagnetism of spin ice has been demonstrated. The structural and thermal trends of varying thicknesses of thin films of spin ice have been probed by similar methods to demonstrate the impact of reduced dimensionality on a three-dimensional magnetically ordered state. Finally, an experimental probe of the monopolar quasiparticle behavior using SQUID magnetometry has been created.

[效力级别] Dysprosium Titanate

[关键词] Spin Ice;Dysprosium Titanate;Magnetic Monopoles;Physics