  In the intermediate mixed state (IMS) of type-II/1 superconductors, vortex
lattice (VL) and Meissner state domains coexist due to a partially attractive
vortex interaction. Using a neutron-based multiscale approach combined with
magnetization measurements, we study the continuous decomposition of a
homogeneous VL into increasingly dense domains in the IMS in bulk niobium
samples of varying purity. We find a universal temperature dependence of the
vortex spacing, closely related to the London penetration depth and independent
of the external magnetic field. The rearrangement of vortices occurs even in
the presence of a flux freezing transition, i.e. pronounced pinning, indicating
a breakdown of pinning at the onset of the vortex attraction.
