Physical properties of Al doped Ba hexagonal ferrite thin films

We developed the thin film microwave magnetic material, M-type barium hexagonal ferrite (BaM) doped with Al, for signal processing devices operating above 40 GHz with little to no applied magnetic field. Al was chosen as the dopant material because it significantly increases the already strong anisotropy field of BaM. A series of thin film BaAlxFe12-xO19 samples, x ranging from 0 to 2 in 0.25 steps, were deposited on Pt templates using a metal-organic decomposition growth technique. The resulting films are polycrystalline and highly textured, with the hexagonal c-axis directed out of plane. These films are also self-biasing; easy axis hysteresis loops have a high squareness ratio, s, in the 0.83-0.92 range. As expected, the anisotropy field increases with x, ranging from 1.34 to 2.19 × 106 A/m (16.9-27.5 kOe) for x = 0-2, while the saturation magnetization Ms decreases with x, ranging from 0.334 to 0.175 × 106 A/m (4πMs = 4.2-2.2 kG) for x = 0-2. These values were measured at room temperature, but the temperature dependence of these quantities was also measured below room temperature, down to 30 K. The measured ferromagnetic resonance linewidths, on the order of 12-30 × 103 A/m (140–370 Oe) for compositions below x = 1, indicate device-quality films. Above a certain threshold, the linewidth increases linearly with frequency at a rate of 0.2-0.64 × 103 (A/m)/GHz (2.5-8 Oe/GHz) for x = 0–1, respectively. The behavior of the linewidth is correlated with the structural properties of the films measured using x-ray diffraction and atomic force microscopy. The results of magnetic force microscopy, Curie point measurements, spectral ellipsometry (index of refraction), and magneto-optical measurements are also included and discussed.