Evidence of impurity and boundary effects on magnetic monopole dynamics in spin ice

Abstract

In the highly degenerate spin-ice ground state, flipped spins give rise to magnetic charges, or monopoles, which form a measurable current in a magnetic field. The low-temperature relaxation dynamics of spin-ice materials now reveal that defects can impede monopole flow—creating a magnetic analogue of electrical resistance. Electrical resistance is a crucial and well-understood property of systems ranging from computer microchips to nerve impulse propagation in the human body. Here we study the motion of magnetic charges in spin ice and find that extra spins inserted in Dy2Ti2O7 trap magnetic monopole excitations and provide the first example of how defects in a spin-ice material obstruct the flow of monopoles—a magnetic version of residual resistance. We measure the time-dependent magnetic relaxation in Dy2Ti2O7 and show that it decays with a stretched exponential followed by a very slow long-time tail. In a Monte Carlo simulation governed by Metropolis dynamics we show that surface effects and a very low level of stuffed spins (0.30%)—magnetic Dy ions substituted for non-magnetic Ti ions—cause these signatures in the relaxation. In addition, we find evidence that the rapidly diverging experimental timescale is due to a temperature-dependent attempt rate proportional to the monopole density.