Critical behavior of nanocrystalline gadolinium: Evidence for a new universality class

We report on how nanocrystal size affects the critical behavior of the rare-earth metal Gd near the ferromagnetic-to-paramagnetic phase transition. The asymptotic critical behavior of the coarse-grained polycrystalline sample (with an average crystallite size of $L \cong \unit[100]{\mu m}$) is that of a (pure) \textsl{uniaxial dipolar} ferromagnet, as is the case with single-crystal Gd, albeit the width of the asymptotic critical region (ACR) is reduced. As the grain size approaches $\sim \unit[30]{nm}$, the ACR is so narrow that it could not be accessed in the present experiments. Inaccessibly narrow ACR for $L \sim \unit[30]{nm}$ and the continuous increase in the width of ACR as $L$ decreases from $\unit[16]{nm}$ to $\unit[9.5]{nm}$ basically reflects a crossover to the \textsl{random uniaxial dipolar} fixed point caused by the quenched random-exchange disorder prevalent at the internal interfaces (grain boundaries).