Incommensurate magnetic order in Ag₂NiO₂

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The nature of the magnetic transition of the half-filled triangular antiferromagnet Ag₂NiO₂ with T N=56K was studied with positive muon-spin-rotation and relaxation (μ+SR) spectroscopy. Zero field μ+SR measurements indicate the existence of a static internal magnetic field at temperatures below T N. Two components with slightly different precession frequencies and wide internal-field distributions suggest the formation of an incommensurate antiferromagnetic order below 56 K. This implies that the antifrerromagnetic interaction is predominant in the NiO₂ plane in contrast to the case of the related compound NaNiO₂. An additional transition was found at 22 K by both μ+SR and susceptibility measurements. It was also clarified that the transition at 260 K observed in the susceptibility of Ag₂NiO₂ is induced by a purely structural transition.