The Low Temperature Magnetic Relaxation of Dy₂Ti₂O₇

Abstract

The recent formulation of the monopole picture of spin ice[1, 2, 3], has lead to several studies of the signatures of monopole excitations[2, 4], as well as studies of the motion of magnetic monopoles[5, 6, 7]. In this thesis low temperature dynamics of the dipolar spin ice material Dy₂Ti₂O₇ are examined through SQUID measurements of the dc magnetic relaxation. The results are compared to recent ac susceptibility measurements[8] and new Monte Carlo simulations. It is found that instead of the relaxation being a single exponential decay with a temperature-activated relaxation time, which is what is expected from the dipolar spin ice model[9, 10, 11], the relaxation is a stretched exponential that develops into a long-time tail. The relaxation has a temperature-activated relaxation time, τ(T) = τo exp(∆E/kT), that has an energy barrier, ∆E/k, of ∼ 9K, as opposed to the ∼ 5 K energy barrier predicted by the dipolar spin ice model. By comparison to dynamic Monte Carlo simulations the stretched exponential behaviour is attributed to surface effects of the sample and the long-time tail is explained by a small amount (0.3 %) of stuffed Dy spins in the material. The 9 K energy barrier is explained by assuming that the basic spin flip process is not fully temperature independent and instead has an energy barrier of 4 K. This study should bring to light the importance of taking material defects and surface effects into account as one would in an electric conductor, whose material defects lead to resistance. Even in the case of the spin ice materials, which are usually assumed to be extremely “clean”, defects can play an important role in the dynamics and this study is the first instance where resistance meets “magneticity”.