Non-stoichiometry and antiferromagnetic phase transition of NaCl-type CrN thin films prepared by reactive sputtering

Non-stoichiometric CrN1+x (0.0 less than or equal to x less than or equal to 0.2) thin films with a NaCl-type structure have been prepared by reactive sputtering of chromium metal target in Ar/N-2 mixed gas. Nearly stoichiometric CrN thin films are obtained at the N-2 mixing ratio of 20% and the total pressure of 0.67 Pa. Magnetic susceptibility, electrical resistivity and X-ray diffraction measurements indicate that these films show an antiferromagnetic first order phase transition at around 260 K. With the increase in N-2 partial pressure, metastable over-stoichiometric CrN1+x thin films are obtained. The compositions of samples prepared in pure N-2 gas are almost CrN1.2 and the lattice constant is 2% greater than that of the nearly-stoichiometric bulk CrN. Over-stoichiometric CrN1+x thin films do not show the first order antiferromagnetic phase transition. Their chi-T curves show a broad maximum at around 90 K. The electrical resistivity at room temperature decreases with increasing the nitrogen composition and the temperature coefficient of resistivity is always negative. The composition dependence of magnetic and electrical properties correlates to the variation in the density of state near the Fermi level detected by XPS.