  Recently several triangular-lattice magnets with delafossite structure have
been found to display spin-liquid behavior down to the lowest temperatures.
Remarkably, applying a magnetic field destroys the spin liquid which then gives
way to symmetry-breaking states, identified as semiclassical coplanar states
including a magnetization plateau at 1/3 total magnetization. Here we provide a
theoretical approach rationalizing this dichotomy, utilizing a Schwinger-boson
theory that captures both ordered and disordered magnetic phases. We show that
a zero-field spin liquid, driven by strong frustration, is naturally
destabilized in a magnetic field via spinon condensation. Symmetry-breaking
order akin to the standard triangular-lattice Heisenberg model then arises via
an order-by-disorder mechanism. We discuss implications for pertinent
experiments.
