NMR Measurements of Power-Law Behavior in the Spin-Wave and Critical Regions of Ferromagnetic EuO

TL;DR

This study uses precise NMR measurements to analyze EuO's magnetization, revealing unexpected T^2 behavior at low temperatures and challenging existing spin-wave theories and interpretations of NMR spectra.

Contribution

The paper provides detailed NMR data across the entire magnetization curve of EuO, highlighting deviations from conventional spin-wave theory and questioning previous quadrupole splitting interpretations.

Findings

Observed T^2 behavior at low temperatures contradicts conventional theory.

Critical exponent beta measurements align with previous studies.

No quadrupole splitting detected at 4.2K, challenging prior interpretations.

Abstract

Precision continuous-wave NMR measurements have been carried out over the entire magnetization curve of EuO and are presented in tabular form. Two very closely spaced resonances are observed and are attributed to domain and domain-wall signals. Both of the signals are useful for analysis in the spin-wave region. Only the domain signal is measurable above ~50K. The latter is used for fitting Tc and the critical exponent beta. The critical-region fits agree with previous measurements, within experimental error. The low-temperature data exhibit a clear-cut T^2 behavior, at variance with the expectations of conventional spin-wave theory. This result is discussed in relation to two semi-empirical spin-wave schemes, one formulated by N. Bykovetz, and one by U. Koebler. The NMR signal at 4.2K gives no indication of a quadrupole splitting, in contradiction to the interpretation of several previous spin-echo NMR spectra observed in EuO. This issue remains unresolved.