Magnetotransport properties of microstructured AlCu₂Mn Heusler alloy thin films in the amorphous and crystalline phase

We have studied the resistance, magnetoresistance and Hall effect of AlCu₂Mn Heusler alloy thin films prepared by flash evaporation on substrates cooled at ⁴He liquid temperature. The as-prepared samples were amorphous and were annealed stepwise to induce the transformation to the crystalline phase. The amorphous phase is metastable up to above room temperature and the transition to the crystalline phase was observed by means of resistance measurements. Using transmission electron microscopy, we have determined the structure factor S(K) and the pair correlation function g(r), both results indicate that amorphous AlCu₂Mn is an electronic stabilized phase. The X-ray diffraction of the crystallized film shows peaks corresponding to the well ordered L2₁ phase. The resistance shows a negative temperature coefficient in both phases. The magnetoresistance (MR) is negative in both phases, yet larger in the crystalline state compared to the amorphous one. The magnetic properties were studied further by anomalous Hall effect measurements, which were present in both phases. In the amorphous state, the anomalous Hall effect disappears at temperatures below 175 K and is present up to above room temperature in the case of crystalline AlCu₂Mn.