An exploration of field-induced ground states and phase transitions in pyrochlores using torque magnetometry

Geometrically frustrated systems have an inherent incompatibility between the lattice geometry and the magnetic interactions, resulting in macroscopically degenerate ground-state manifolds. The single-ion anisotropy and magnetic interactions in these systems give rise to unusual non-collinear spin textures. In classical spin ices this leads to Pauling states and emergent quasiparticle excitations equivalent to magnetic monopoles. The effective spin correlation strength (J_eff) determines the relative energies of the different possible spin ice states. I will review our recent work in which we benchmarked capacitive torque magnetometry as a unique tool to characterize the transitions between field-induced noncollinear spin configurations in spin ice Ho₂Ti₂O₇ single crystals. Studying these magnetic-field-induced phase transitions allows extraction of J_eff and the magneto-chemical potential of monopole formation in single crystal samples [1]. Our work shows that the spin ice state that features antiferromagnetic alignment of spin chains, is more stable (i.e., has a higher J_eff) than one would predict from the dipolar spin ice model based on nearest neighbor exchange. I will also review experimental results on thin film counterparts of this spin ice, which show that strain and disorder in the films play important roles in determining their magnetic properties [2] effectively destabilizing the formation of these antiferromagnetically aligned spin chains for some, but not all, thin films. Lastly, I will briefly highlight some recent data on Yb₂Ti₂O₇, which lives on the cusp of competing magnetic phases; and on the quantum spin ice candidate Pr₂Hf₂O₇, which displays a lack of ordering. My group has successfully grown epitaxial films of both of these pyrochlores on various substrates. In short, my talk will highlight torque magnetometry as a unique tool for exploring field-induced ground states and phase transitions in (quantum) spin ices and other pyrochlores.