Bloch line `crystallization' as intrinsic pinning mechanism in ferrimagnetic YIG films*

The present intense drive to develop current-switched magnetic storage media has lead to a renewed interest in ferrimagnetic garnet films which, for several decades, were the focus of devices exploiting manipulation of magnetic `bubbles'. In such uniaxial materials, the appearance of Bloch lines in structured domain walls strongly influences their \textit{dynamic} properties in an applied magnetic field. Here we show that the \textit{static} magnetic properties of garnet films can also be profoundly influenced due to \textit{crystallization} of Bloch lines into a square lattice along adjacent domain walls. This rigid lattice \textit{intrinsically} pins domain walls and suppresses the expected expansion/contraction of magnetic domains in an applied field. Even in the pinned regime, ultra-sensitive scanning Hall probe measurements reveal the \textit{nanoscale motion} of magnetic blocks in the walls comprising an integer number of Bloch-lines. Although the estimated displacements ($\sim $2-25 nm) are very much smaller than the domain period, we observe highly correlated motion across many domain walls, driven by the strongly interacting Bloch line lattice.