Nonlocal effects in magnetization of high-$\kappa$ superconductors

The reversible magnetization $M(T,H)\mathrm{}$ in high quality single crystals of $\mathrm{Lu}({\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{B}}_2\mathrm{C}$ with $x=0\mathrm{}-0.09$ is measured in a broad $T,H$ domain and interpreted within London theory corrected for nonlocality of the current – vector-potential relation. Profound deviations in the data from the standard London result, $M\propto \ln {(H}_{c2\mathrm{}}/H)$ in intermediate fields $H_{c1\mathrm{}}\ll H\ll H_{c2\mathrm{}},$ are seen in clean samples at low $T$’s.Unlike strongly anisotropic high-$T_c$ compounds, for nearly isotropic borocarbides this behavior cannot be attributed to fluctuations of weakly interacting pancake vortices. We show that the nonlocal London model describes qualitatively and consistently the whole set of $M(T,H)\mathrm{}$ data and, in particular, its temperature and the mean-free path dependence. The scaling field $H_0,$ which arises in ${M=M(H/H}_0)$ due to nonlocality, is found to be nearly proportional to the the field $H_2$ at which the vortex lattice undergoes the symmetry change (the “square-to-hex” transition).