RF Magnetic Properties of FeCoB/Al₂O₃/FeCoB Structure With Varied Al₂O₃ Thickness

Physical vapor deposition prepared FeCoB(100 nm)/Al₂O₃/FeCoB(100 nm) sandwich structures with different Al₂O₃thickness (2 to 15 nm) were studied. The magnetization curves showed that the in-plane coercive field and saturation field along the easy axis decreased with increase in Al₂O₃thickness. The saturation magnetization field 4πMₛ$for all samples is above 1.6 T. The coercive field remained below 0.5 Oe for Al$₂O₃$thickness greater than 6 nm, approximately when the antiferromagnetic coupling switched to ferromagnetic coupling. The existence of optical modes in ferromagnetic resonance (FMR) curves confirmed antiferromagnetic coupling in samples with Al₂O₃thickness below 6 nm. At 8.5 GHz, the sandwich structure with a 3 nm Al₂O₃layer displayed a narrow linewidth of only 29 Oe, indicating lower magnetic loss in the sandwich structure compared to the single layer FeCoB with a thickness of 200 nm that exhibited a linewidth of 50 Oe. Also, all sandwich structures demonstrated real relative permeability of above 600 up to 1.5 GHz. The FeCoB(100 nm)/Al₂O₃(3 nm)/FeCoB(100 nm) structure with low coercive field, high magnetization, and low FMR linewidth presents great potential in RF magnetic application.