Scanning SQUID study of ferromagnetism and superconductivity in infinite-layer nickelates

Infinite-layer nickelates $R_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ ($R$ = La, Pr, Nd) are a class of superconductors with structural similarities to cuprates. Although long-range antiferromagnetic order has not been observed for these materials, magnetic effects such as antiferromagnetic spin fluctuations and spin-glass behavior have been reported. Different experiments have drawn different conclusions about whether the pairing symmetry is $s$ or $d$ wave. In this paper, we applied a scanning superconducting quantum interference device (SQUID) to probe the magnetic behavior of film samples of three infinite-layer nickelates (${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{NiO}}_2$, ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$, and ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$) grown on ${\mathrm{SrTiO}}_3$ (STO), each with a nominal thickness of 20 unit cells. In all three films, we observed a ferromagnetic background. We also measured the magnetic susceptibility above the superconducting critical temperature in ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ and ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}{\mathrm{NiO}}_2$ and identified a non-Curie-Weiss dynamic susceptibility. Both magnetic features are likely due to ${\mathrm{NiO}}_x$ nanoparticles. Additionally, we investigated superconductivity in ${\mathrm{Pr}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ and ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$, which exhibited inhomogeneous diamagnetic screening. The superfluid density inferred from the diamagnetic susceptibility in relatively homogeneous regions shows $T$-linear behavior in both samples. Finally, we observed superconducting vortices in ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$. We determined a Pearl length of $330\mu m$ for ${\mathrm{Nd}}_{0.775}{\mathrm{Sr}}_{0.225}{\mathrm{NiO}}_2$ at 300 mK, both from the strength of the diamagnetism and from the size and shape of the vortices. These results highlight the importance of considering ${\mathrm{NiO}}_x$ particles when interpreting experimental results for these films.