Alternating-spin-chain compound AgVOAsO$_4$ probed by $^{75}$As NMR

TL;DR

This study uses $^{75}$As NMR to investigate the magnetic properties of the spin-1/2 alternating chain compound AgVOAsO$_4$, confirming its spin gap and estimating key exchange parameters.

Contribution

First NMR investigation of AgVOAsO$_4$ confirming its alternating spin chain model and quantifying magnetic exchange interactions and spin gap.

Findings

NMR shift matches the alternating-chain model

Estimated exchange parameters: J/k_B ≈ 38.4 K, α ≈ 0.68, Δ/k_B ≈ 15 K

Observed activated behavior in spin-lattice relaxation rate

Abstract

$^{75}$As NMR measurements were performed on a polycrystalline sample of spin-1/2 alternating-spin-chain Heisenberg antiferromagnet AgVOAsO$_4$. 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 $J/k_{\rm B} \simeq 38.4$ K, alternation ratio $\alpha = J'/J \simeq 0.68$, and spin gap $\Delta/k_{\rm B} \simeq 15$ K. The transferred hyperfine coupling between the $^{75}$As nucleus and V$^{4+}$ spins obtained by comparing the NMR shift with bulk susceptibility amounts to $A_{\rm hf} \simeq 3.3$ T/$\mu_{\rm B}$. Our temperature-dependent spin-lattice relaxation rate $1/T_1(T)$ also shows an activated behaviour at low temperatures, thus confirming the presence of a spin gap in AgVOAsO$_4$.