Antiferromagnetic state in EuAl₄ studied by ²⁷Al zero-field NMR

The divalent Eu intermetallic compound EuAl ₄ with the BaAl 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 Al zero-field NMR spectra in the antiferromagnetic state of EuAl ₄ at 4.2 K are detected in the vicinity of 12.8, 19.7 and 41.6 MHz, corresponding to the ²⁷ Al internal magnetic fields of 1.2, 1.8 and 3.7 T, respectively. The 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 Al spectrum at 41.6 MHz shows a broad line-shape. From analysis of these spectra, the 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.