NiMn, IrMn, and NiO exchange coupled CoFe multilayers for microwave applications

We compare the static and microwave properties of AF/CoFe/AF multilayers; with NiMn, IrMn, and NiO as AF-layers, respectively. These exchange-coupled multilayers combine very high saturation magnetization (4 pi M(s)) with soft rotational behaviors. NiMn has the best exchange coupling and exhibits very high fFMR (5-10 GHz) and good thermal stability up to 150 degrees C. However, its resistivity is rather low (175 mu Omega center dot cm), and requires very large thickness to be efficient leading to low filling ratio. IrMn exhibits an intermediate exchange coupling leading to fFMR in the range of 3-6 GHz. It is a good compromise regarding resisitivity (275 mu Omega center dot cm), and the reduced thickness for mu(r) 100 at such frequencies. The high resistive NiO exhibits permeability spectra with moderate fFMR around 3-5 GHz but allows the possibility to stop the eddy currents by lamination. However, NiO and IrMn both exhibit linear decrease of their FMR versus temperature. This work shows that NiMn, IrMn, and NiO multilayers provide suitable magnetic properties for microwave applications, and exhibit complementary properties on selected bandwidths which allow to address the 3-10 GHz range.