Low temperature magnetic properties of frustrated pyrochloreferromagnets Ho2Sn2O7 and Ho2Ti2O7

Abstract

AC and DC magnetic susceptibilities have been measured on frustratedpyrochlore ferromagnets Ho2Sn2O7 and Ho2Ti2O7 attemperatures down to 100 mK.In the AC magnetic susceptibility, a dramatic drop of χ' (in-phasecomponent) and a singlemaximum of χ'' (quadrature component) are observed ataround 1 K. The frequency dependence of these peak positions indicates thepresence of a slowmagnetic relaxation at low temperature, whose relaxation time obeys theArrhenius formula with the energy barrier Eb = 19.6 K and27.5 K forHo2Sn2O7 andHo2Ti2O7, respectively. Strong irreversibility is also observed inthe temperature dependence of the DC magnetization of Ho2Sn2O7 at 1kOe belowTp~ 0.75 K; the zero-field-cooling (ZFC) curve indicates a verysharp peak at Tp, whereas the field-cooling curve has no anomaly atTp andincreases monotonically with decreasing T. In the DC magnetizationprocess of the ZFC state at 0.46 K, a steep increase of the moment occursabove 2 kOe, and theirreversibility disappears at around~15 kOe where the moment ferromagnetically saturates. These resultsindicate a clustering of magnetic moment whose size is of the order of a fewtetrahedra, consistentwith a recently proposed `spin ice' model.