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

Chromium nitride (CrN) films were epitaxially grown on $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ and MgO (001) substrates by pulsed laser deposition at $973\mathrm{K}$ under nitrogen radical irradiation, and the structural change of the films was investigated at around the Néel temperature of CrN $(\sim 270\mathrm{K})$ 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 $260\mathrm{K}$. On the other hand, on $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ 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 $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ 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 $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ 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.

Figure 1

Top view of CrN (001) plane in the Néel state. The distortion was exaggerated for clarity. Arrows denote the relative spin directions on Cr atoms. Solid and open circles represent Cr atoms with $z=0$ and $z=1∕2$, respectively.

Figure 2

Conventional $\omega \text{−}2\theta$ scan x-ray diffraction patterns of CrN epitaxial films.

Figure 3

Temperature-controlled x-ray diffraction patterns of CrN films. (a) $\mathrm{Cr}\mathrm{N}\left(001\right)∕\mathrm{Mg}\mathrm{O}\left(001\right)$ (film A); (b) $\mathrm{Cr}\mathrm{N}\left(111\right)∕\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ (film B).

Figure 4

Temperature dependences of $d$ spacing of CrN films.

Figure 5

Illustration of interatomic distances and bond angles calculated from reported bulk structure before and after the antiferromagnetic transition. Shaded circles: Cr; open circles: N.

Figure 6

Temperature dependence of resistivity of CrN films. Circles: $\mathrm{Cr}\mathrm{N}\left(001\right)∕\mathrm{Mg}\mathrm{O}\left(001\right)$ (film A), Squares: $\mathrm{Cr}\mathrm{N}\left(111\right)∕\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3\left(0001\right)$ (film B).