Fluctuation Dissipation Theorem

In our experiment, we have measured spin noise spectrum of a Dysprosium Titanate sample at equilibrium, from 1.2K to 4K using a DC-SQUID. From the Fluctuation Dissipation (FD) theorem, we know that statistical fluctuations in a physical variable of a system are related to the linear response to a small force applied to the system. In this chapter our experimental results of flux noise coming from a sample of Dy2Ti2O7 are discussed in the context of previous boundary-free AC susceptibility measurements of Dysprosium Titanate.