  We investigate the spin-glass transition in the strongly frustrated
well-known compound $Fe_2TiO_5$. A remarkable feature of this transition,
widely discussed in the literature, is its anisotropic properties: the
transition manifests itself in the magnetic susceptibly only along one axis,
despite $Fe^{3+}$ $d^5$ spins having no orbital component. We demonstrate,
using neutron scattering, that below the transition temperature $T_g = 55 K$,
$Fe_2TiO_5$ develops nanoscale surfboard shaped antiferromagnetic regions in
which the $Fe^{3+}$ spins are aligned perpendicular to the axis which exhibits
freezing. We show that the glass transition may result from the freezing of
transverse fluctuations of the magnetization of these regions and we develop a
mean-field replica theory of such a transition, revealing a type of magnetic
van der Waals effect.
