We report low-temperature calorimetric, magnetic, and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh(2)Si(2) ( T(N)=70 mK) as a function of magnetic field B. While for fields exceeding the critical value B(c0) at which T(N)-->0 the low-temperature resistivity shows an AT2 dependence, a 1/(B-B(c0)) divergence of A(B) upon reducing B to B(c0) suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.

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AC and DC magnetic susceptibilities have been measured on frustrated pyrochlore ferromagnets Ho2Sn2O7 and Ho2Ti2O7 at temperatures down to 100 mK. In the AC magnetic susceptibility, a dramatic drop of χ' (in-phase component) and a single maximum of χ'' (quadrature component) are observed at around 1 K. The frequency dependence of these peak positions indicates the presence of a slow magnetic relaxation at low temperature, whose relaxation time obeys the Arrhenius formula with the energy barrier Eb = 19.6 K and 27.5 K for Ho2Sn2O7 and Ho2Ti2O7, respectively. Strong irreversibility is also observed in the temperature dependence of the DC magnetization of Ho2Sn2O7 at 1 kOe below Tp~ 0.75 K; the zero-field-cooling (ZFC) curve indicates a very sharp peak at Tp, whereas the field-cooling curve has no anomaly at Tp and increases monotonically with decreasing T. In the DC magnetization process of the ZFC state at 0.46 K, a steep increase of the moment occurs above 2 kOe, and the irreversibility disappears at around ~15 kOe where the moment ferromagnetically saturates. These results indicate a clustering of magnetic moment whose size is of the order of a few tetrahedra, consistent with a recently proposed `spin ice' model.

We report the magnetic properties of pyrochlore rare earth oxides R 2 Sn 2 O 7 (R=Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm and Yb). The magnetic susceptibility has been measured at temperatures down to 0.1 K. Among them, some compounds show the magnetic anomaly below 1 K. Nd 2 Sn 2 O 7 and Gd 2 Sn 2 O 7 show an antiferromagnetic ordering at 0.9 K and 1.0 K, respectively. Tb 2 Sn 2 O 7 shows a ferromagnetic ordering at 0.87 K. Although both Er 3þ and Yb 3þ are a Kramers ion, no magnetic anomaly is observed at least down to 0.15 K. We found that Dy 2 Sn 2 O 7 and Ho 2 Sn 2 O 7 show 'spin ice' behavior. In addition, it is suggested that Pr 2 Sn 2 O 7 is a new spin ice compound.

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