Anomalous Temperature Evolution of the Internal Magnetic Field Distribution in the Charge-Ordered Triangular Antiferromagnet ${\mathrm{AgNiO}}_2$

Zero-field muon-spin relaxation measurements of the frustrated triangular quantum magnet ${\mathrm{AgNiO}}_2$ are consistent with a model of charge disproportionation that has been advanced to explain the structural and magnetic properties of this compound. Below an ordering temperature of $T_N=19.9(2)\mathrm{K}$ we observe six distinct muon precession frequencies, due to the magnetic order, which can be accounted for with a model describing the probable muon sites. The precession frequencies show an unusual temperature evolution which is suggestive of the separate evolution of two opposing magnetic sublattices.

Figure 1

(a) Structure of ${\mathrm{AgNiO}}_2$ viewed along the $c$ direction with proposed muon stopping sites shown. The magnetic structure involves ordered Ni1 sites with moments aligned parallel ($+$ sign) or antiparallel ($-$ sign) to the $c$ direction, surrounded by a honeycomb of Ni2 and Ni3 sites. The $+$ and $-$ signs on the Ni3 sites give the orientation of the Ni1 spins in the neighboring layer directly above and below. The thick lines show the chemical unit cell, while the dashed line indicates the mirror plane for the magnetic structure (see text). (b) Proposed muon stopping sites occur 1 Å from each oxygen ion. Taking the oxygen position as the origin of coordinates, muons are found at $\theta =65°$ and $\varphi =\pm 38°$, separated by 1.4 Å from the Ni planes along $c$. (c) As (b) but viewed along $c$.

Figure 2

(a) Time-domain ${\mu }^{+}\mathrm{SR}$ spectrum measured above and below the antiferromagnetic transition with time-domain fits as described in the main text. (b) Maximum entropy analysis of the spectrum measured at $T=1.6\mathrm{K}$ showing six distinct frequencies. (c) Temperature evolution of the frequency spectrum extracted from maximum entropy analysis.

Figure 3

(a) Temperature evolution of the frequencies ${\nu }_i$, extracted from the spectra measured below $T_N$, fitted to Eq. (1). (b) The normalized frequencies ${\nu }_i/2{\nu }_{\mathrm{av}}$ form three pairs with roughly equal and opposite gradients. (c) Adding the pairs of normalized frequencies as indicated results in approximately flat lines. (d) Relaxation rates ${\lambda }_i$ showing a steep increase as $T_N$ is approached from below.