Ultrahigh Sensitivity Ferromagnetic Resonance Measurement Based on Microwave Interferometer

An ultrahigh sensitivity ferromagnetic resonance (FMR) measurement technique based on a microwave interferometer was developed. A comparison between conventional and interferometer-based FMR showed a signal-to-noise ratio (SNR) improvement of as much as 42 dB. This largely improved SNR allowed clear resolution of the FMR signal of the uniform mode on a 100 nm diameter, 5 nm thick CoFeB single nanodot, and even higher order modes on a 400 nm diameter CoFeB single nanodot. A system noise measurement confirmed that the minimum noise level of this technique is set by Johnson noise at the input of the low noise amplifier, which is more than one order of magnitude smaller than the FMR signal obtained from the 100 nm CoFeB single nanodot.