Magnetism modulation in Cu-doped AlN via coupling between AlN thin film and ferroelectric substrate

Cu-doped AlN (Cu:AlN) thin films were deposited on non-ferroelectric substrate (Si (1 1 1)) and ferroelectric substrates (LiTaO₃ and LiNbO₃) by direct current reactive magnetron sputtering. It was discovered that all the deposited Cu:AlN thin films showed room temperature ferromagnetism and the average magnetic moment (AMM) per Cu atom of the film increased with the decrease of the angle (θ) between AlN film plane and the c-axis of ferroelectric substrate. A giant AMM as high as 2.23 μ_B/Cu was obtained when Cu:AlN thin film was deposited on LiTaO₃(110) with θ of 0 degree, while a small AMM of about 0.11 μ_B/Cu was observed when Cu:AlN thin film was deposited on LiNbO₃ (0 0 6) with θ of 90°. It was discovered that the AMM of the film increased with decreasing of θ. The ferromagnetism modulation was ascribed to the coupling between AlN piezoelectric film and ferroelectric substrate, resulting from the change of host lattice structure and spontaneous polarization of AlN thin film. The in-plane anisotropy of AMM and the annealing relaxation results confirmed the anomalous coupling.