Weak ferromagnetic ordering in the anomalous field-insensitive heavy-fermion state in SmOs4Sb12SmOs4Sb12

Zero field (ZF) and transverse field (TF) muon spin relaxation and rotation (μSRμSR) study has been carried out in filled-skutterudite SmOs4Sb12SmOs4Sb12 in order to investigate the magnetically robust heavy-fermion (HF) state and the weak ferromagnetic anomaly appearing below ∼2.5K. A large-amplitude oscillating signal appears in the ZF-μSRμSR spectra at low temperatures, confirming that the weak ferromagnetic anomaly is an intrinsic bulk property. Two components with the fraction ratio of ∼2:1∼2:1 exist both in the ZF-μSRμSR spectra of the ferromagnetically ordered state and the TF-μSRμSR FFT spectra in applied fields along the 〈001〉 direction. This observation can be explained consistently based on the most probable muon stopping site indicated from the value of the static nuclear dipolar width in Kubo–Toyabe function in the high-temperature ZF-μSRμSR spectra. Analysis reveals that the spontaneous magnetic moment MsMs lies along the 〈001〉 direction and the size of MsMs is largely suppressed, indicating that the weak ferromagnetic moment is carried by itinerant heavy quasiparticles.