We report on the synthesis and measurements of the magnetic susceptibility, electrical resistivity, and specific heat of ROs 2 Al 10 (R = Pr, Nd, Sm, and Gd). All compounds are isostructural with a Kondo semiconductor CeOs 2 Al 10 showing an unusual phase transition at T 0 = 28.6 K. For R = Pr, no magnetic transitions were observed down to 0.4 K, whereas compounds with R = Nd, Sm, and Gd undergo antiferromagnetic transitions at T N = 2.2 K, 12.5 K, and 18 K, respectively. Moreover, successive magnetic transitions occur for R = Sm and Gd. The resistivity of the compounds with R = Sm and Gd increases on cooling below T N due to the superzone-gap formation. The fact that T N = 18 K for R = Gd is lower than T 0 for R = Ce indicates that the phase transition in CeOs 2 Al 10 is not originated from the RKKY interaction alone but from the anisotropic hybridization of the 4 f state with conduction bands.KEYWORDS: ROs 2 Al 10 , antiferromagnetic transition, successive magnetic transition, superzone gap Cerium-based intermetallic compounds CeT 2 Al 10 (T = Ru and Os), which crystallize in the orthorhombic YbFe 2 Al 10 -type structure, 1, 2) have been studied extensively because of the occurrence of anomalous phase transition accompanying the formation of an energy gap.3-6) The transition temperature of T 0 = 27 K for T = Ru and 28.6 K for T = Os are exceptionally high for the antiferromagnetic (AFM) transition temperature in the Ce intermetallics. The formation of energy gap is manifested below T 0 by the upturn in the electrical resistivity and thermally activated behaviors in the magnetic susceptibility, specific heat, and spin-lattice relaxation rate of 27 Al-NMR/NQR. 5,7,8) In order to explain these features, charge density wave transition and spin-Peierls one have been expected.9, 10) However, the combined studies of powder neutron diffraction and µ + SR study evidenced the existence of AFM order of Ce 4 f moments whose values are 0.34 µ B /Ce for T = Ru and less than 0.2 µ B /Ce for T = Os.11, 12) However, it is still puzzling why the value of T 0 is so high despite the reduced Ce moments by the strong c-f hybridization effect. Interestingly, the c-f hybridization results in the formation of a pseudogap, which develops at the temperatures above T 0 .13) The fact that T 0 disappears suddenly at the critical pressure P C = 4 GPa for T = Ru and 2.5 GPa for T = Os is unaccountable by the Doniach phase diagram which assumes the competition between the c-f hybridization and RKKY interaction. 5,14) In this paper, we have synthesized and studied the magnetic and transport properties of ROs 2 Al 10 (R = Pr, Nd, Sm, and Gd) in order to estimate the strength of RKKY interaction in CeOs 2 Al 10 .Polycrystalline samples of ROs 2 Al 10 were prepared by arcmelting the stoichiometric amounts of the constituent elements under an Ar atmosphere. The obtained buttons were annealed at 1000• C for 3 days. Powder x-ray diffraction patterns indicated that the compounds ROs 2 Al 10 are isostructural with CeOs 2 Al 10 . The obtained lattice p...

We have carried out muon spin relaxation (SR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru 1-x Fe x ) 2 Al 10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin gap formation as evident from the bulk measurements in the end members. Our zero-field SR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with T N =27, 26 and 21 K respectively. On the other hand the SR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively exhibit a temperature independent KuboToyabe term confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 2 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1.The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting gap in CeT 2 Al 10 (T=Ru and Os) compounds in terms of hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.