Low temperature magnetic properties of frustrated pyrochlore ferromagnets Ho2Sn2O7and Ho2Ti2O7

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Abstract
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.