  Frustrated magnets can exhibit many novel forms of order when exposed to high
magnetic fields, however, much less is known about materials where frustration
occurs in the presence of itinerant electrons. Here we report thermodynamic and
transport measurements on micron-sized single crystals of the
triangular-lattice metallic antiferromagnet 2H-AgNiO2, in magnetic fields of up
to 90 T and temperatures down to 0.35 K. We observe a cascade of magnetic phase
transitions at 13.5 20, 28 and 39T in fields applied along the easy axis, and
we combine magnetic torque, specific heat and transport data to construct the
field-temperature phase diagram. The results are discussed in the context of a
frustrated easy-axis Heisenberg model for the localized moments where
intermediate applied magnetic fields are predicted to stabilize a magnetic
supersolid phase. Deviations in the measured phase diagram from this model
predictions are attributed to the role played by the itinerant electrons.
