Controlling the structural transition at the Néel point of CrN epitaxial thin films using epitaxial growth

Chromium nitride CrN films were epitaxially grown on -Al2O3 0001 and MgO 001 substrates by pulsed laser deposition at 973 K under nitrogen radical irradiation, and the structural change of the films was investigated at around the Néel temperature of CrN 270 K by temperature-controlled x-ray diffraction experiments. Bulk cubic CrN is known to show monoclinic distortion below the Néel temperature. The CrN film grown on MgO 001 with the CrN 001 plane parallel to the substrate surface, exhibited a clear structural change at around 260 K. On the other hand, on -Al2O3 0001 substrates, the CrN phase grew with its 111 planes parallel to the substrate surface, and showed no structural change at the Néel temperature. The different orientation of the epitaxial films can explain the different behavior of the films: The structural transition of bulk-CrN causes large variations in the interatomic distances and bond angles on the 111 plane, but varies little on the 001 plane. In the case of thin films, the -Al2O3 0001 substrate surface could prevent the 111 -oriented film from distorting its structure by fixing atom positions on the CrN 111 interfaces of the film. In accordance with the structural behavior of the films, the 111 -oriented CrN film on -Al2O3 0001 showed no anomaly in its metallic conductivity around the Néel temperature. On the other hand, the 001 -oriented CrN on MgO showed a steep increase in electrical conductivity, accompanied by a lattice distortion below the Néel point. These results highlight an example that epitaxy could be used to control the existence of structural transitions, further accompanied by an antiferromagnetic ordering, which is closely related to the electronic properties of materials.