Strong Evidence that the Paramagnetic-Spin Glass Transition in a Magnetic Field is a Crossover

An understanding of the nature of the paramagnetic to spin glass phase transition has been lacking due to and interplay between time, temperature and magnetic-field, and the lack of a directly observable correlation length scale. In this paper we present TRM data that ties the characteristic time scales (associated with the growth of spin glass correlations) with the temperature, as the spin glass transition temperature is approached from below T<0.97 Tg. As a function of temperature in a fixed magnetic field, we have found a crossover timescale t_co. For tco spin glass correlations are observed to grow whereas for t>tco, correlations can grow to the length scale associated with the t_co timescale, but no larger. We have analyzed the structure of the crossover line for magnetic fields ranging from 9.6 G to 96 G. The magnetic field suppresses the growth of correlations. The data strongly suggests that in a magnetic field the spin glass phase transition is a dynamic crossover, while in zero field there is likely a phase transition. The Droplet Model (Phys. Rev. B 38, 373) qualitatively predicts this behavior.