Kavli Affiliate: Lena F. Kourkoutis
| First 5 Authors: Ruby A. Shi, Bai Yang Wang, Yusuke Iguchi, Motoki Osada, Kyuho Lee
| Summary:
Infinite-layer nickelates $R_{1-x}$Sr$_{x}$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 (La$_{0.85}$Sr$_{0.15}$NiO$_2$, Pr$_{0.8}$Sr$_{0.2}$NiO$_2$, and
Nd$_{0.775}$Sr$_{0.225}$NiO$_2$) grown on 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 Pr$_{0.8}$Sr$_{0.2}$NiO$_2$ and
La$_{0.85}$Sr$_{0.15}$NiO$_2$ and identified a non-Curie-Weiss dynamic
susceptibility. Both magnetic features are likely due to NiO$_x$ nanoparticles.
Additionally, we investigated superconductivity in Pr$_{0.8}$Sr$_{0.2}$NiO$_2$
and Nd$_{0.775}$Sr$_{0.225}$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
Nd$_{0.775}$Sr$_{0.225}$NiO$_2$. We determined a Pearl length of 330 $upmu$m
for Nd$_{0.775}$Sr$_{0.225}$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 NiO$_x$ particles when interpreting
experimental results for these films.
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