We report the results of As-nuclear quadrupole resonance (NQR) in LaFe 4 As 12 which shows itinerant ferromagnetic order at 5.2 K. We used high quality sample grown by high pressure synthesis. Appreciably broadened 75 As-NQR spectrum over the frequency range from 53.5 to 57.5 MHz observed below T C is the first microscopic evidence for a ferromagnetic order in LaFe 4 As 12 . In the paramagnetic state, Nowak et al. [Phys. Rev. B 79 (2009) 214411] reported that the nature of the low-energy spin excitations is explained by the SCR (self consistent renormalization) theory for weakly itinerant ferromagnetic metals from the results of NQR and NMR (nuclear magnetic resonance) measurements in the temperature range of 20−300 K. However, in the present As-NQR study, nuclear spin lattice relexation rate 1/T 1 is anomalously enhanced below 100 K, suggesting the extra low-energy spin excitations other than the conventional SCR mechanism.KEYWORDS: NQR, skutterudite, LaFe 4 As 12 Filled skutterudite compounds RT 4 X 12 (R=rare earth or alkali ion; T =Fe, Ru, Os; X=P, As,Sb) have attracted much attention because of their rich variety of physical phenomena, such as metal-insulator transitions observed in PrRu 4 P 12 and PrFe 4 P 12 , 2, 3) field-independent heavy electron behavior in SmOs 4 Sb 12 , 4) heavy electron superconductivity in PrOs 4 Sb 12 , 5) and so on. These varieties of intriguing ground states are believed to arise from the hybridization between conduction and f -electrons originated in the unique caged crystal structure where a R ion is surrounded by twelve X atoms.Such a unique caged structure also induces anomalous physical phenomena at low temperatures. The "guest" R ion enclosed in an over-sized cage can vibrate within an anharmonic potential, the so called "rattling".6) Theoretically, the interaction between the "rattling" and conduction electrons are regarded as multi-channel Kondo effect and gives rise to the heavy fermion behavior. 7) Therefore, the study of "rattling" state in RT 4 X 12 is an another issue to clarify the varieties of the intriguing ground states. Many studies have been done for other caged compounds, e.g., clathrate compound, 8) β-pyrochlore oxide 9) and so on. Among filled skutterudite compounds, La-based filled skutterudites LaFe 4 X 12 (X=P, As, Sb) without f ion have been attracted much attention because of their anomalous magnetic properties.10) In LaFe 4 X 12 , it is reported that the electronic state is a correlated itinerant electron system arising from Fe 3d electrons. In the paramagnetic state, when X is varied from P to As, and Sb, the electronic specific heat coefficient (γ s ) increases from 57, 78 and 200 mJ/(K 2 mol), respectively. These features are consistent with the theoretical prediction that the electronic density of states at Fermi level, N(E F ), monotonically increases with increasing of the lattice constant. [10][11][12] However, their ground states are different with one another; for X=P, it is reported that the electronic state changes from * E-mail address: ...
