\(^{27}\)Al-NMR study on a square-kagome lattice antiferromagnet

NMR study has been performed on an \(S=1/2\) $S=1/2$ antiferromagnet KCu \(_6\) ${}_6$ AlBiO \(_4\) ${}_4$ (SO \(_4\) ${}_4$ ) \(_5\) ${}_5$ Cl on the square-Kagomé lattice, which has three slightly inequivalent nearest-neighbor interactions. Because of the geometrical frustration inherited from triangles within the square kagomé lattice and of the low dimensionality, a long range magnetic order is strongly suppressed; its absence has so far been confirmed in low temperatures down to dilution refrigerator region. \(^{27}\) ${}^{27}$ Al-NMR spectra and the longitudinal relaxation time \(T_1\) $T_1$ were measured by a conventional pulsed spectrometer on powder sample under several magnetic fields between 3 and 10 T and in low temperatures down to 0.35 K. The NMR line width due to the inhomogeneous broadening increased with lowering temperatures and leveled off below 3 K, where FWHM reached the value as large as 0.1 T, implying that the ground state is magnetic one, consistent with previous reports. On the other hand, the longitudinal nuclear spin relaxation rate \(1/T_1\) $1/T_1$ obeyed the Arrhenius law with the thermal activation energy \(\Delta =\) $\mathrm{\Delta }=$ 2K at low temperatures, suggesting that a small gap is formed in the spin excitation spectrum.