Cascade of field-induced magnetic transitions in a frustrated antiferromagnetic metal

Frustrated magnets can exhibit many novel forms of order when exposed to high magnetic fields. Much less, however, is known about materials where frustration occurs in the presence of itinerant electrons. Here we report thermodynamic and transport measurements on micron-size 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 39 T 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 low-field experimental data are compared with theoretical calculations for a frustrated easy-axis Heisenberg model based on realistic parameters for the localized moments of AgNiO2. Deviations from this model's predictions are attributed to the role played by the itinerant electrons.