Field-Induced Transition on a Triangular Plane in the Spin-Ice Compound Dy₂Ti₂O₇

The origin of the lowest-temperature anomaly reported several years ago using a polycrystalline sample of the spin-ice compound Dy₂Ti₂O₇ has remained unresolved. Here we finally clarify its origin by susceptibility measurements down to 65 mK using single crystals under accurate control of the magnetic fields in two independent directions. We demonstrate that the transition is induced under a subtle field combination that precisely cancels the nearest-neighbor spin interactions acting on the spins on the triangular lattice within the pyrochlore structure. Contrary to the other two field-induced transitions, this transition is driven only by the interactions beyond the nearest neighbors. Our observation thus provides the first qualitative evidence for the essential importance of the dipolar interaction beyond the nearest neighbors in the spin ice.