New manifestations of classical ground state degeneracy in frustrated magnetic materials
Magnetic frustration, which refers to competing interactions between spins on a lattice, often results in a high degeneracy of the classical ground state. In real materials that display such classical degeneracies, one possibility is that they become “trapped” kinetically, and the signatures of classical degeneracy persist down to the lowest measurable temperatures. Another possibility is that they are lifted by quantum or thermal fluctuations and form a well behaved ordered state (aka, “order by disorder”). Most interestingly, they might be lifted by forming a quantum spin liquid ground state, which involves developing long range quantum entanglement between spins. In this talk I will show new results on two material examples that exhibit evidence of large classical degeneracy. BaCo2(PO4)2 is a S=1/2 honeycomb lattice material that suffers from phase competition and a resulting lack of long range order. Inelastic neutron scattering results suggest that it is located in proximity to highly degenerate regions of the J1-J2-J3 XXZ Honeycomb lattice model, raising the prospect of a nearby quantum spin liquid phase. A second material, Fe3PO7, undergoes a antiferromagnetic transition at 163 K that involves a degenerate ring of helical ordering wavevectors. The resulting magnetic state remains only partially ordered down to at least 4 K, with the domain size restricted to 7nm, implying a high density of topological defects (domain walls, or possibly vortices or skyrmions) exists in this material. Perplexingly, the domain size is directly correlated with the helical pitch length, as we have shown by diluting the Fe3+ sites with non-magnetic Ga3+. This correlation suggests an intrinsic mechanism for the disruption of long range order, likely resulting from the observed degenerate manifold of ordering wavevectors. These two materials represent two very different realizations of classical degeneracy resulting from frustration, and exemplify the diversity of interesting phenomena that can occur as a result.
