Alternating-spin-chain compound AgVOAsO4 probed by ⁷⁵As NMR

⁷⁵As NMR measurements were performed on a polycrystalline sample of spin-1/2 alternating-spin chain Heisenberg antiferromagnet AgVOAsO₄. Temperature-dependent NMR shift K(T), which is a direct measure of the intrinsic spin susceptibility, agrees very well with the spin-1/2 alternating-chain model, justifying the assignment of the spin lattice. From the analysis of K(T), magnetic exchange parameters were estimated as follows: the leading exchange J/k_B = 38.4 K and alternation ratio α = J′/J = 0.69. The transferred hyperfine coupling between the 75As nucleus and V4+ spins obtained by comparing the NMR shift with the bulk susceptibility amounts to A_hf = 3.3 T/μ_B. By fitting the low temperature region of K(T) and 1/T₁ to a 3D activated behaviour, a zero-field spin gap of around 15 K was obtained. This matches very well with the results and theoretical calculations of previous paper on the same compound by Tsirlin et. al. The effect of interchain couplings on the low-temperature activated behavior of K(T) and spin-lattice relaxation rate 1/T₁ is identified.