^27Al-NMR study on a square-kagome lattice antiferromagnet

NMR study has been performed on an S=1/2 antiferromagnet KCu ₆ AlBiO ₄ (SO ₄) 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 Al-NMR spectra and the longitudinal relaxation time T₁ 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/T₁ obeyed the Arrhenius law with the thermal activation energy = 2K at low temperatures, suggesting that a small gap is formed in the spin excitation spectrum.