  Single-crystalline membranes of functional materials enable the tuning of
properties via extreme strain states; however, conventional routes for
producing membranes require the use of sacrificial layers and chemical
etchants, which can both damage the membrane and limit the ability to make them
ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler
compound GdPtSb on graphene-terminated Al$_2$O$_3$ substrates. Despite the
presence of the graphene interlayer, the Heusler films have epitaxial registry
to the underlying sapphire, as revealed by x-ray diffraction, reflection high
energy electron diffraction, and transmission electron microscopy. The weak Van
der Waals interactions of graphene enable mechanical exfoliation to yield
free-standing GdPtSb membranes, which form ripples when transferred to a
flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic,
measurements on rippled membranes show a spontaneous magnetic moment at room
temperature, with a saturation magnetization of 5.2 bohr magneton per Gd.
First-principles calculations show that the coupling to homogeneous strain is
too small to induce ferromagnetism, suggesting a dominant role for strain
gradients. Our membranes provide a novel platform for tuning the magnetic
properties of intermetallic compounds via strain (piezomagnetixm and
magnetostriction) and strain gradients (flexomagnetism).
