We study the extent of the reversible region in the vortex phase diagram of recently available RbOs$_2$O$_6$ single crystals \cite{ Rogacki08a} by means of bulk magnetization measurements. We found that the irreversible magnetic response sets in at a field $H_{ \rm irr}(T) \sim 0.3 H_{ \rm c2}(T)$ for $0.5 \lesssim T/T_{ \rm c} \lesssim 0.8$, yielding a reversible vortex region that is uncommonly wide in comparison with typical low-$T_{ \rm c}$ materials. The relevance of thermal fluctuations is limited since we estimate a Ginzburg number $G_{ \rm i} = 5 \times 10^{ -7}$. However, the relevance of quenched disorder is unconventionally low since the critical-current density ratio at low fields and temperatures is of the order of that found in high-T$_{ \rm c}$'s. We therefore conclude that an intrinsically low bulk pinning magnitude favors the existence of an unexpectedly wide reversible vortex region that seems to be generic to $\beta$-pyrochlores.
Categories: cond-mat.supr-con