Ferromagnetic spin correlations in the filled skutterudite SrFe₄As₁₂ revealed by ⁷⁵As NQR-NMR studies

Recently new filled-skutterudite compounds SrT₄As₁₂ (T = Fe, Ru, Os) have been synthesized, which provides a new opportunity of systematic studies of the effects of different d electron of 3d, 4d and 5d in the systems. In this study, as the first step for the systematic studies, we have carried out nuclear quadrupole resonance (NQR) and nuclear magnetic resonance (NMR) measurements on the filled skutterudite SrFe₄As₁₂ in order to investigate the magnetic properties from a microscopic point of view. Using the NQR spectra under small magnetic field, we have succeeded in determining Knight shift K. The temperature dependences of both K and the nuclear spin-lattice relaxation rate divided by temperature, 1/T₁T, have been reasonably explained by the simple model in which a concave-shaped band structure near the Fermi energy is assumed. The Korringa ratio parameter is found to be much smaller than unity, indicating the existence of ferromagnetic fluctuations. These NMR/NQR results evidence that SrFe₄As₁₂ is a metal with ferromagnetic spin correlations.