75 As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements have been carried out to investigate the magnetic and electronic properties of the filled skutterudite metallic compound SrFe4As12. The temperature dependence of Knight shift K determined by the NQR spectrum under a small magnetic field (≤ 0.5 T) shows the similar temperature dependence of the magnetic susceptibility χ which exhibits a broad maximum at T * ∼ 50 K. The nuclear spinlattice relaxation rate divided by temperature, 1/T1T , increases with decreasing temperature and exhibits a broad maximum at T ∼ 70 K, similar to the case of χ. The temperature dependence of K and 1/T1T is reasonably explained by a simple model where we assume a concave-shaped band structure near the Fermi energy. Based on a Korringa ratio analysis using the T1 and K data, ferromagnetic spin fluctuations are found to exist in SrFe4As12. These results indicate that SrFe4As12 can be characterized to be a metal with ferromagnetic correlations and also the peculiar band structure responsible for the suppression of 1/T1T and K at low temperatures.PACS numbers:

We have succeeded in synthesizing samples of new filled skutterudite compounds SrT4As12 (T=Fe, Ru, and Os) using a high-pressure synthesis technique. These compounds have lattice constants of 8.351, 8.521, and 8.561 Å, respectively. The physical properties of the compounds are reported for the first time. The temperature dependences of the electrical resistivity for SrT4As12 (T=Fe, Ru, and Os) exhibit metallic behaviors. The electrical resistivity and magnetic measurements indicates that SrOs4As12 is a new superconductor with a transition temperature of 4.8 K. A broad maximum of magnetic susceptibility at around 50 K and a large electronic specific heat coefficient of 58 mJ mol-1 K-2 suggest that SrFe4As12 is a nearly ferromagnetic metal with spin fluctuations of Fe 3d electrons.

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Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe4As12 [Ding et al. Phys. Rev. B 98, 155149 (2018)], we have carried out 75 As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs4As12 with a superconducting transition temperature of Tc ∼ 4.8 K. Knight shift K determined by the NQR spectrum under a small magnetic field (≤ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/T1T , is nearly independent of temperature above ∼ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe4As12, we found that the values of |K| and 1/T1T in SrOs4As12 are smaller than those in SrFe4As12, indicating no obvious ferromagnetic spin correlations in SrOs4As12. From the temperature dependence of 1/T1 in the superconducting state, an s-wave superconductivity is realized.PACS numbers:

Filled skutterudite compound EuFe4As12 shows the highest magnetic ordering temperature of TC = 154 K among Eu-based skutterudite compounds, but its magnetic ground state has not been determined yet. Here, we performed 153 Eu nuclear magnetic resonance (NMR) and 75 As nuclear quadrupole resonance (NQR) measurements on EuFe4As12 to reveal its magnetic ground state as well as the physical properties from a microscopic point of view. From the temperature and magnetic field dependence of 153 Eu NMR spectrum in the magnetically ordered state, we found that the Eu ions are in Eu 2+ state with a nearly 7 µB corresponding to S = 7/2 spins. Combined with the magnetization measurements which show the reduced saturation moments of 4.5 µB/f.u., we determined the ground magnetic structure in EuFe4As12 to be ferrimagnetic where the Eu 2+ 4f and the Fe 3d ordered moments are ferromagnetically aligned in each sublattice but the moments between the sublattices are antiferromagnetically aligned. We also found the local distortion at the Eu site from the cubic symmetry in the magnetically ordered state. The relationship between the rattling motion of Eu atoms and the local symmetry of the Eu ions is discussed. From the 75 As NQR nuclear spin-lattice relaxation time measurements as well as 153 Eu NMR measurements, we found that the 4f electrons of the Eu ions are well described by the local moment picture in both the magnetic and paramagnetic metallic states.