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Depth-resolved amorphization and nonuniformity in square-planar nickelate films

Purnima P. Balakrishnan1,*, Maria Bambrick-Santoyo2,3, Lin Er Chow4, Dan Ferenc Segedin2, Mythili Surendran5,6, Ranjan K. Patel7, Paige E. Quarterman1, Shin Muramoto8, Grace A. Pan2 et al.

Zhaoyang Luo4, Michael R. Fitzsimmons9,10,11, Amanda Huon9, Timothy R. Charlton9, Christy J. Kinane12, Andrew J. Caruana12, Hui Wu1, Charles M. Brooks2, Qi Song2, Hanjong Paik13, Srimanta Middey7, Jayakanth Ravichandran14,5,6, A. Ariando4, Julia A. Mundy2,15, and Alexander J. Grutter1,†

  • *Contact author: purnima.balakrishnan@nist.gov
  • †Contact author: alexander.grutter@nist.gov

Phys. Rev. Materials 10, 034801 – Published 31 March, 2026

DOI: https://doi.org/10.1103/9ldc-sk6x

Abstract

Superconducting nickelate thin films with the layered square-planar structure are synthesized via the topotactic reduction of a more oxygen rich perovskite or Ruddlesden-Popper (RP) phase due to the instability of the target Ni1+δ valence. The topotactic reduction induces a dramatic structural transformation, which yields high defect densities in the resulting film and has introduced uncertainty regarding film uniformity, particularly as it relates to the calculation of physical parameters based on estimates of superconducting volume fraction. Here we use neutron reflectometry (NR) and secondary ion mass spectrometry (SIMS) to obtain a sub-nm resolved understanding of the structural depth profile across a wide range of layered square-planar nickelates. While the as-grown parent compounds are bulklike and of very high quality, the depth profiles of reduced films are generally nonuniform and indicative of partial amorphization. We identify careful selection of substrate and reduction conditions as promising directions toward mitigating amorphization and improving uniformity.

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