Low temperature magnetic properties of frustrated pyrochlore ferromagnets Ho2Sn2O7 and Ho2Ti2O7

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 chi' (in-phase component) and a single maximum of chi " (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 E-b = 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 T-p similar to 0.75 K; the zero-field-cooling (ZFC) curve indicates a very sharp peak at T-p, whereas the field-cooling curve has no anomaly at T-p 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 similar to 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.