Unexpectedly wide reversible vortex region in $\beta$-pyrochlore RbOs${}_2$O${}_6$

Abstract

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 $H_{\mathrm{irr}}(T)\sim 0.3H_{c2}(T)$ for $0.5\lesssim T/T_{\mathrm{c}}\lesssim 0.8$, yielding a reversible vortex region that is uncommonly wide in comparison with typical low-$T_{\mathrm{c}}$ materials. The relevance of thermal fluctuations is limited since we estimate a Ginzburg number $G_{\mathrm{i}}=5\times 10^{-7}$. 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${}_{\mathrm{c}}$'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.