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

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

Ruby A. Shi, Bai Yang Wang, Yusuke Iguchi, Motoki Osada, Kyuho Lee, Berit H. Goodge, Lena F. Kourkoutis, Harold Y. Hwang, and Kathryn A. Moler

Phys. Rev. Materials 8, 024802 – Published 21 February 2024

Abstract

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 $330 µ m$ 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.

Received 22 August 2023
Revised 14 December 2023
Accepted 19 January 2024
Corrected 19 March 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.024802

Physics Subject Headings (PhySH)

Ferromagnetism Flux pinning Impurities in superconductors Magnetic susceptibility Meissner effect Penetration depth Superconducting gap Superconductivity Superfluid density Vortices in superconductors

Condensed Matter, Materials & Applied Physics

Corrections

19 March 2024

Correction: The density units in the last sentence of the caption to Figure 1 and in the 10th paragraph contained errors and have been fixed. The fifth sentence of the caption to Figure 2 contained an error in wording and has been fixed.

Authors & Affiliations

Ruby A. Shi ^1,2,3,*, Bai Yang Wang^1,2, Yusuke Iguchi ^1,3, Motoki Osada ^1,4, Kyuho Lee^1,2, Berit H. Goodge ^5,6, Lena F. Kourkoutis^5,6, Harold Y. Hwang^1,3,7, and Kathryn A. Moler^1,2,3,7

¹Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025-7015, USA
²Department of Physics, Stanford University, California 94305-4045, USA
³Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305-4045, USA
⁴Department of Material Science and Engineering, Stanford University, Stanford, California 94305-4045, USA
⁵School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853-3501, USA
⁶Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853-3501, USA
⁷Department of Applied Physics, Stanford University, California 94305-4045, USA

^*rubyshi@stanford.edu

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Issue

Vol. 8, Iss. 2 — February 2024

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Images

Figure 1
Diamagnetic screening in the presence of a ferromagnetic background. Susceptometry images of (a) ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ ( $T = 0.3 K$ ), (b) $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ ( $T = 0.3 K$ ), and (c) ${La}_{0.85} {Sr}_{0.15} {NiO}_{2}$ ( $T = 3 K$ ). The images of ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ and $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ , which are below their respective critical temperatures, show diamagnetic screening, while the image of ${La}_{0.85} {Sr}_{0.15} {NiO}_{2}$ , which is above its critical temperature, shows paramagnetism. (d)–(f) Magnetometry images taken simultaneously with the susceptometry images. A planar background fit is subtracted from each scan. The variation in the sharpness of the image from left to right in (a) is due to an imperfect scan plane. The estimated scan heights are $4.5 µ m$ ( ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ ), $3.1 µ m$ ( $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ ), and $3.1 µ m$ ( ${La}_{0.85} {Sr}_{0.15} {NiO}_{2}$ ) with the bender voltage of 1 V (see the Supplemental Material [43]). (g) Simulated magnetometry image of randomly positioned in-plane magnetic dipoles, each carrying a magnetic moment of $m_{0} = 240 000 µ_{B}$ , with a density of $37.5 nanoparticle / µ m^{2}$ , at a scan height of $3.1 µ m$ .
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Figure 2
ac susceptibility and dc magnetometry consistent with superparamagnetism in extrinsic nanomagnetic particles. (a) Susceptometry vs bender voltage at 5, 10, 15, 20, 25, 30, and 35 K, fitted to the expected functional for a thin-film paramagnet. The bender voltage is proportional to the SQUID-sample separation. (b) Temperature-dependent in-phase susceptibility of ${La}_{0.85} {Sr}_{0.15} {NiO}_{2}$ and $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ , obtained at the locations marked in (d) and (j), respectively. Each marker plotted is determined from a fit such as those shown in (a). (c) Temperature history for the magnetometry scans of ${La}_{0.85} {Sr}_{0.15} {NiO}_{2}$ shown in (d)–(h). (i) Temperature history for the magnetometry scans of $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ shown in (j) –(n).
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Figure 3
Temperature dependence of the superfluid density. (a) Susceptometry vs bender voltage measured over $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ at 2–8 K, shown with fits to a model assuming a diamagnetic thin film as described in the Supplemental Material [43]. These fits determine the Pearl length $Λ (T)$ . Fitted values of $Λ (T = 0) / Λ (T)$ vs normalized temperature determined at (b) the point in ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ marked with a diamond in the inset, (c) the point in $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ marked with a star in the inset, and (d) the point in $\Pr_{0.8} {Sr}_{0.2} {NiO}_{2}$ marked with a triangle in the inset. The solid markers with error bars denote $1 / Λ$ fitted from susceptibility approaches. The temperature dependence of $1 / Λ$ was fitted to nodal (dashed lines) and nonnodal (solid lines) gap function models, with the best fits plotted. The zero-temperature Pearl length $Λ (T = 0)$ from the nodal fit was chosen as a normalization factor.
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Figure 4
Superconducting vortices. Temperature and applied field history of two regions in ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ . The red dots are background scans, taken at (a) 6 K and (b) 4 K. The dashed lines indicate field cooling. (c)–(k) Background-subtracted magnetometry scans. The estimated scanning height is $4.6 µ m$ . The vortices appear as yellow regions representing a local peak in magnetic flux. (l) Number of vortices as a function of the cooling field, with corresponding scans labeled. The dashed line shows the best linear fit.
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Figure 5
Close-up view of Pearl vortices in ${Nd}_{0.775} {Sr}_{0.225} {NiO}_{2}$ fitted to simulations. (a) Background-subtracted magnetometry scan from Fig. 4. Five individual vortices (one of which is an antivortex) are boxed by dashed lines and labeled 1–5. A random background acquired from a location away from the vortices is boxed by solid lines and labeled as BG. (b) Simulated Pearl vortex with a Pearl length of $Λ = 329 µ m$ at a scan height of $h = 4.6 µ m$ . (c) A region away from the vortices boxed by the red lines in (a) is taken as a random background. (d)–(g) Individual views of each vortex labeled in (a), with the fitted Pearl length $Λ_{f}$ indicated. The fitted scanning height $h_{f}$ from (d) to (g) is 4.8, 5, 5.2, and $5 µ m$ . (h)–(k) Residuals of individual vortex data subtracted from the simulation at the best fit. These residuals are similar in shape and strength to the random background in (c).
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