Probe of spin dynamics in superconducting NbN thin films via spin pumping

The emerging field of superconductor (SC) spintronics has attracted intensive attentions recently. Many fantastic spin-dependent properties in SCs have been discovered, including large magnetoresistance, long spin lifetimes, and the giant spin Hall effect, etc. Regarding the spin dynamics in superconducting thin films, few studies have been reported yet. Here, we report the investigation of the spin dynamics in an $s$-wave superconducting NbN film via spin pumping from an adjacent insulating ferromagnet GdN film. A profound coherence peak of the Gilbert damping of GdN is observed slightly below the superconducting critical temperature of the NbN, which agrees well with recent theoretical prediction for $s$-wave SCs in the presence of impurity spin-orbit scattering. This observation is also a manifestation of the dynamic spin injection into superconducting NbN thin film. Our results demonstrate that spin pumping could be used to probe the dynamic spin susceptibility of superconducting thin films, thus pave the way for future investigation of spin dynamics of interfacial and two-dimensional crystalline SCs.

Figure 1

Spin pumping into the superconducting NbN thin films. (a) Schematic of the interfacial s-d exchange interaction ($J_{\mathrm{sd}}$) between the spins in the NbN layer and the rotating magnetization of the GdN layer under the ferromagnetic resonance (FMR) conditions. (b) The magnetic moment as a function of the temperature. Inset: the magnetic hysteresis loop at $T=5\mathrm{K}$. (c) The four-probe resistance as a function of the temperature with in-plane magnetic field at 0 and 4000 Oe. (d) The typical FMR signal measured at $T=10\mathrm{K}$ and $f=15\mathrm{GHz}$. The red line indicates the Lorentz fitting curve to obtain the half linewidth based on Eq. (1). (e) The half linewidth as a function of the microwave frequency at $T=10\mathrm{K}$. The red solid line is the fitting curve based on spin-relaxation model. The results in (b)–(d) are obtained on the NbN (10)/GdN (5)/NbN (10) sample.

Figure 2

Spin dynamics of the superconducting NbN thin films probed via spin pumping. The normalized four-probe resistance (a) and Gilbert damping (b) as a function of the temperature for the samples of NbN (2)/GdN (5)/NbN (2) and NbN (10)/GdN (5)/NbN (10), respectively.

Figure 3

The GdN thickness effect on spin dynamics of the superconducting NbN thin films. [(a) and (b)]. The Gilbert damping as a function of the temperature for the samples of NbN (t)/GdN (10)/NbN (t), and NbN (t)/GdN (30)/NbN (t). (Insets) The normalized four-probe resistance as a function of the temperature for the samples of NbN (10)/GdN (10)/NbN (10) and NbN (10)/GdN (30)/NbN (10). (c) The ratio of the peak Gilbert damping over the value at $T\approx T_C$ [${\alpha }_{\mathrm{peak}}/\alpha (\sim T_C)$] as a function of the GdN layer thickness.

Figure 4

The interface-induced Gilbert damping at the NbN/GdN interface. (a) The Gilbert damping as a function of the GdN thickness for the samples of NbN (10)/GdN (d)/NbN (10) at $T=11$, 8.5, and 6 K, respectively. (b) The interface-induced Gilbert damping as a function of temperature for NbN (10)/GdN (d)/NbN (10).