Magnetic Excitations in the Stacked Quantum Magnets NaNiO$_2$ and LiNiO$_2$

NaNiO$_2$ and LiNiO$_2$ are isostructural stacked triangular lattice quantum magnets, in which magnetism is conventionally thought to arise due to spin 1/2 moments carried by Ni$^{3+}$ ions. Surprisingly, while NaNiO$_2$ undergoes a cooperative Jahn-Teller transition at 480K and magnetically orders below T$_N$ $\sim$ 23K, LiNiO$_2$ undergoes a glass transition at T$_g$ $\sim$ 9K and remains disordered down to the lowest measured temperatures. The absence of long-range magnetic order in LiNiO$_2$ has been attributed to either geometric frustration caused by mixing of the Li and Ni sublattices, or orbital degeneracy due to the absence of a coherent Jahn-Teller distortion. We have performed time of flight neutron scattering measurements on polycrystalline samples of NaNiO$_2$ and LiNiO$_2$ using the wide Angular-Range Chopper Spectrometer (ARCS) at the SNS. Our measurements reveal previously unobserved magnetic excitations at relatively high energy transfers, which we associate with ferromagnetic spin waves mediated by in-plane interactions. We also find evidence of critical scattering in NaNiO$_2$ near the magnetic phase transition at T$_N$. These results will be compared with previous measurements collected using the DCS at NIST.