Anisotropic heat transport via monopoles in the spin-ice compound Dy_2Ti_2O_7

摘要

We report a study of the thermal conductivity κ of the spin-ice material Dy_2Ti_2O_7. From the anisotropic magnetic-field dependence of κ and by additional measurements on the phononic reference compounds Y_2Ti_2O_7 and (Dy_(0.5)Y_(0.5))_2Ti_2O_7, we are able to separate the phononic and the magnetic contributions to the total heat transport, i.e., κ_(ph) and κ_(mag), respectively, which both depend on the magnetic field. The field dependence of κ_(ph) arises from the noncollinear magnetic moments of the Dy ions, which tend to align along the field direction. For κ_(mag), we observe a highly anisotropic magnetic-field dependence, which correlates with the corresponding magnetization data reflecting the different magnetic-field induced spin-ice ground states. The magnitude of κ_(mag) increases with the degree of the ground-state degeneracy. This anisotropic field dependence as well as various hysteresis effects suggest that κ_(mag) is essentially determined by the mobility of the magnetic monopole excitations in spin ice.