Metamagnetic Quantum Criticality Revealed by ${}^{17}\mathrm{O}\mathrm{\text{−}}\mathrm{NMR}$ in the Itinerant Metamagnet ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$

We have investigated the spin dynamics using ${}^{17}\mathrm{O}\mathrm{\text{−}}\mathrm{NMR}$ in the bilayered perovskite ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, which sits close to a metamagnetic quantum critical point. The nuclear spin-lattice relaxation rate divided by temperature $1/T_{1}T$ is enhanced on approaching the metamagnetic critical field of $\sim 7.9\mathrm{T}$, and at the critical field $1/T_{1}T$ continues to increase and does not show Fermi-liquid behavior down to 0.3 K. The temperature dependence of $T_{1}T$ in this region suggests the critical temperature $\Theta$ to be $\sim 0\mathrm{K}$, which is strong evidence that the spin dynamics possesses a quantum critical character. Comparison between uniform susceptibility and $1/T_{1}T$ reveals that antiferromagnetic fluctuations instead of two-dimensional ferromagnetic fluctuations dominate the spin fluctuation spectrum at the critical field, which is unexpected for itinerant metamagnetism.

Figure 1

(a) The crystal structure of ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$. (b) Comparison between $1/T_{1}T$ of O(2) site and Knight shifts 17 K of O(3) site below (top: 3.7 T), at (middle: 7.8 T), and above the metamagnetic field (bottom: 12.5 T). The solid line at 3.7 T is magnetization measured by a commercial SQUID magnetometer.

Figure 2

NMR relaxation rate $1/T_{1}T$ at various temperatures and the $T^2$ coefficient of the resistivity, $A$ [2], versus field.

Figure 3

$1/T_{1}T$ vs $T$ at various magnetic fields. The inset shows a linear-linear plot for the inverse of $1/T_{1}T$ for $H\sim H_{\mathrm{M}}$.

Figure 4

Comparison of in-plane magnetoresistance ($T=0.2\mathrm{K}$), $1/T_{1}T$, and uniform susceptibility $dM/dH$ ($T=0.36\mathrm{K}$) [25] as a function of magnetic field.