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High microwave frequency EPR study of the axial Fe3+−VO centers in the quantum paraelectrics SrTiO3 and KTaO3

Ikram Zdeg, Oleksii Laguta*, and Petr Neugebauer†

Valentyn Laguta

  • *Contact author: oleksii.laguta@ceitec.vutbr.cz
  • †Contact author: petr.neugebauer@ceitec.vutbr.cz
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Phys. Rev. B 112, 214117 – Published 29 December, 2025

DOI: https://doi.org/10.1103/7nzh-gq85

Abstract

Oxygen vacancies are the most common defects in oxide materials. They frequently couple with transition-metal ions to form charge-compensated paramagnetic Me-VO pair centers, which exhibit large zero-field splitting of 2.6−6cm−1. Such splitting requires the use of high microwave frequencies in electron paramagnetic resonance (EPR) spectroscopy. Using high-frequency EPR (up to 210 GHz), we investigated Fe3+−VO centers in the quantum paraelectrics SrTiO3 and KTaO3 over the temperature range 5–296 K. These materials have attracted significant interest due to their unique dielectric properties and recently discovered spin-electric phenomena, including two-dimensional superconductivity and the anomalous Rashba effect in spin-charge conversion. To probe spin-electric coupling, we also performed pulse EPR measurements at 9.79 GHz under applied electric fields. Our results show that the axial crystal-field parameter increases nearly linearly from 13.1 GHz in KTaO3 and 13.55 GHz in SrTiO3 at room temperature to ≃14.6GHz at 20 K, below which it saturates. This temperature dependence is attributed to anomalous displacements of four lateral oxygen ions relative to Fe3+, likely governed by the soft optical mode. Application of electric fields up to 30 kV/cm revealed not only splitting of the EPR lines but also redistribution of domain populations, observable at fields as low as 10 kV/cm in the nonpolar tetragonal phase of SrTiO3.

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Phys. Rev. B 112, 214117– Published 29 December, 2025
  • Received 6 October 2025
  • Revised 29 November 2025
  • Accepted 3 December 2025
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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