Persistence of Ising-like easy-axis spin correlations in the paramagnetic state of the spin-1 chain compound ${\mathrm{NiTe}}_2{\mathrm{O}}_5$

A ${}^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) study was carried out in the paramagnetic state of the recently discovered quasi-one-dimensional spin-1 chain compound ${\mathrm{NiTe}}_2{\mathrm{O}}_5$. We observed that the ${}^{125}\mathrm{Te}$ NMR spectrum splits into two in a magnetic field applied along the $c$ axis. Based on the strong temperature variation of the relative intensity ratio of the split lines, we infer that the line splitting arises from the two sublattice susceptibilities induced in opposite directions along the chains. In great support of this interpretation, a quantitative analysis of the spin-lattice relaxation rate $T_1^{-1}$ and the Knight shift data unravels dominant transverse spin fluctuations. We conclude that Ising-like uniaxial spin correlations persist up to surprisingly high temperatures compared to the exchange energy scales. Spin-charge coupling mechanism via a self-doping effect may be important.

Figure 1

Crystal structure of ${\mathrm{NiTe}}_2{\mathrm{O}}_5$. (a) Side view along the $b$ axis with the spin configuration in the ordered state. (b) Top view along the chain direction. The tilting directions of the moments shown in (a) are $[\pm 1\pm 10]$ or $[\pm 1\mp 10]$ depending on the local distortion of the ${\mathrm{NiO}}_6$ octahedra. The transverse components of the moments (blue thin arrows) generate a nonvanishing planar net moment along the $\pm a$ axis (red thick arrows), whose direction alternates every two layers.

Figure 2

(a) ${}^{125}\mathrm{Te}$ NMR spectra as a function of temperature measured at ${\mu }_{0}H=10$ T applied along [110] and [001], respectively. The line for $H\parallel c$ splits into two whose separation increases with decreasing temperature. The signal intensity of the lower peak may reflect a magnetization antiparallel to $H$ in the chain. (b) Knight shift $\mathcal{K}$ as a function of temperature and field direction. The closed and open circles represent the data for the upper and lower peaks, respectively. Solid lines are Curie-Weiss fits, which deviate below 100 K. Inset: temperature dependence of the difference in Knight shift for $H\parallel c$. (c) Knight shift $\mathcal{K}$ versus bulk uniform susceptibility $\chi$ plot, with temperature as the implicit parameter. The linear fits in the high-temperature region yield the hyperfine coupling constant $A_{\text{hf}}$ and the temperature-independent orbital shift ${\mathcal{K}}_0$ for the two field orientations (see text).

Figure 3

(a) Intensity ratio between the two peaks observed for $H\parallel c$ as a function of temperature, obtained by the double Gaussian fit to the spectra. A fit to the 140-K spectrum is shown in the inset. (b) Temperature dependence of the full width at half-maximum of the spectra for $H\parallel c$ (upper peak only) and $H\parallel \left[110\right]$. It shows that the spectra for the two field orientations are similarly broadened with lowering temperature.

Figure 4

(a) Temperature dependence of spin-lattice relaxation time $T_1$. The linear relation between $T_1$ and temperature down to $\sim 70$ K is observed for the two field orientations. (b) Spin-lattice relaxation rate $T_1^{-1}$ is described by $1/(T+T_{\theta })$ where $T_{\theta }=473$ and 630 K for $H\parallel c$ and $H\parallel \left[110\right]$, respectively. The critical slowing down of spin fluctuations causes the diverging behavior of $T_1^{-1}$ toward $T_{\text{N}}$ below $\sim 70$ K. (c) ${\left(T_{1}T\right)}^{-1}$ is inversely quadratic in temperature. Inset: the ratio between the dynamical and uniform spin susceptibility ${\sum }_q{\chi }^{″}\left(q\right)/\chi \left(0\right)$ as a function of temperature. It reveals that the dynamical susceptibility increases faster than the uniform one with decreasing temperature. The strong anisotropy of the dynamical susceptibility demonstrates the dominant planar spin fluctuations.