Nuclear magnetic resonance study of the magnetic line phases in SrCoOx (2.5 ≤ × ≤ 3)

Abstract

A 59Co spin-echo nuclear magnetic resonance (NMR) study, along with complimentary magnetization and x-ray diffraction characterization, is reported for three of the four magnetic line phases which occur in the hole-doped perovskite system SrCoOx, 2.5 ≤ x ≤ 3. The magnetic line phases are the single-valence state end-member compounds SrCoO2.5 and SrCoO3, as well as the mixed-valence state compound SrCoO2.88. In this work, the Co valence was varied by changing the oxygen stoichiometry through electrochemical oxidation/reduction. Antiferromagnetic SrCoO2.5, with trivalent Co3+ (3d6), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 314 MHz. The 314 MHz peak frequency can be explained using a typical 3d hyperfine coupling constant and the Co moment value ≈3 μB obtained from neutron diffraction measurements. Ferromagnetic SrCoO3 with tetravalent Co4+ (3d5), is characterized by a single strong, relatively narrow, 59Co NMR peak centered at 93 MHz. However, the 93 MHz peak frequency is considerably smaller than expected from the 2.1 μB Co moment values obtained from the saturation magnetization indicating a significant orbital contribution. Ferromagnetic and mixed-valent (Co3+/Co4+) SrCoO2.88 is characterized by three broad 59Co peaks, centered at 47, 134, and 280 MHz, consistent with previous reports. Based on the NMR results from the end-member compounds, the 280 MHz peak is most likely due to Co3+ and not Co4+ as previously reported.