NQR indications of unconventional magnetism in some bismuth-based diamagnets

Abstract

An evidence that local ordered magnetic fields from 30 to 200 G exist in diamagnets α-Bi₂O₃, Bi₃O₄Br, Bi₄Ge₃O₁₂, Bi₂Al₄O₉ which comprise neither d- nor f-elements was earlier given by the zerofield and Zeeman-perturbed²⁰⁹Bi nuclear quadrupole resonance (NQR) spectra. With a view to find similar spectroscopic effects in other compounds, we examined the²⁰⁹Bi NQR Zeeman-perturbed spectra of the Bi₃B₅O₁₂ oriented single crystal as well as the zero-field spin echo envelopes in Bi₃B₅O₁₂ and Bi₂Ge₃O₉. Distinctive modulations were displayed by the zero-field²⁰⁹Bi spin echo envelopes in Bi₂Ge₃O₉ powder. The modeling of the spin echo envelopes within the density matrix formalism explained the observed effect by the presence of local ordered magnetic field of the order of 65 G at the Bi atoms. The zero-field modulations of the²⁰⁹Bi spin echo envelopes were also observed in Bi₃B₅O₁₂ indicating the presence of the internal source of line splitting. This finding and considerable deviation of the resonance intensity ratios from that in a pure NQR, found in the zero-field²⁰⁹Bi spectrum of the single crystal, were understood as indirect evidences that a local ordered magnetic field exists also in Bi₃B₅O₁₂. The zero-field²⁰⁹Bi spin echo envelopes assigned to the lines split by the local magnetic fields in α-Bi₂O₃ and Bi₄Ge₃O₁₂ were observed to display modulations on the appropriate curves.