  We examine the role of intrinsic chain susceptibility anisotropy in magnetic
field directed self-assembly of a block copolymer using \textit{in situ} X-ray
scattering. Alignment of a lamellar mesophase is observed on cooling across the
disorder-order transition with the resulting orientational order inversely
proportional to the cooling rate. We discuss the origin of the susceptibility
anisotropy, $\Delta\chi$, that drives alignment, and calculate its magnitude
using coarse-grained molecular dynamics to sample conformations of
surface-tethered chains, finding $\Delta\chi\approx 2\times10^{-8}$. From
field-dependent scattering data we estimate grains of $\approx1.2$ $\mu$m are
present during alignment. These results demonstrate that intrinsic anisotropy
is sufficient to support strong field-induced mesophase alignment and suggest a
versatile strategy for field control of orientational order in block
copolymers.
