Magnetic domain observations of freestanding single crystal patterned Ni ₂MnGa films

Ni ₂MnGa thin films have been grown pseudomorphically on a 6-monolayer thick Sc _0.3Er _0.7As interlayer on GaAs(001) by molecular-beam epitaxy. They have a tetragonal structure (a=b=5.65Å and c=6.18Å) which is different from any of the known bulk phases. Magnetic measurements reveal Ni ₂MnGa to have an in-plane easy axis and a Curie temperature around 350 K. The magnetic properties of these films are given and compared to the corresponding measurements in bulk material. In contrast to bulk material, single crystal films have been predicted to exhibit exact austenite-martensite interfaces (without fine twinning of the martensite). Films have been patterned along the predicted interfaces using the conventional photolithography and reactive ion etching. The patterns are then released from the substrate by backside photolithography and selective wet chemical etching, to yield freestanding films. The martensitic transformation of the freestanding films has been observed slightly above the room temperature. Magnetic domain observations (by MFM) on the martensitic films are presented.