Antiferromagnetic state in EuAl\(_{4}\) studied by \(^{27}\)Al zero-field NMR

The divalent Eu intermetallic compound EuAl \(_{4}\) ${}_4$ with the BaAl \(_{4}\) ${}_4$ -type structure (space group: I4/mmm) orders antiferromagnetically at 16 K. Al atoms have two crystallographically inequivalent sites, denoted as Al(I) and Al(II). Three \(^{27}\) ${}^{27}$ Al zero-field NMR spectra in the antiferromagnetic state of EuAl \(_{4}\) ${}_4$ at 4.2 K are detected in the vicinity of 12.8, 19.7 and 41.6 MHz, corresponding to the \(^{27}\) ${}^{27}$ Al internal magnetic fields of 1.2, 1.8 and 3.7 T, respectively. The \(^{27}\) ${}^{27}$ Al spectra at 12.8 and 19.7 MHz split into five resonance-lines because of the nuclear quadrupole interaction by I = 5/2. However, the \(^{27}\) ${}^{27}$ Al spectrum at 41.6 MHz shows a broad line-shape. From analysis of these spectra, the \(^{27}\) ${}^{27}$ Al spectra at 12.8 and 19.7 MHz correspond to those of the Al(II) site and the spectrum at 41.6 MHz belongs to that of the Al(I) site.