Unexpectedly Wide Reversible Vortex Region Inβ-Pyrochlorerbos2o6: Bulk Magnetization Measurements

We study the extent of the reversible region in the vortex phase diagram of recently available RbOs2O6 single crystals [Rogacki , Phys. Rev. B 77, 134514 (2008)] by means of bulk magnetization measurements. We found that the irreversible magnetic response sets in at a field H-irr(T)similar to 0.3H(c2)(T) for 0.5 less than or similar to T/T-c less than or similar to 0.8 yielding a reversible vortex region that is wide in comparison with other low-T-c materials. The relevance of thermal fluctuations is limited since we estimate a Ginzburg number G(i)=5x10(-7). However, the relevance of quenched disorder is low since the critical-current density ratio at low fields and temperatures is of the order of that found in high T-c's. We therefore conclude that an intrinsically low bulk pinning magnitude favors the existence of an unexpectedly wide reversible vortex region in RbOs2O6.