Setup

One issue with the measurement principle of directly observing the reflected microwave power, is that generally, there is a large background from scattered radiation absorbed by the diode microwave probe that is unrelated to FMR. Although this measurement is still possible, and has been preformed this way before [4], often the observed dip of signal at resonance is only about 1% of the actual signal. To improve the signal to noise ratio, the applied field, H (∼ 1000 Oe), is modulated a small amount, δH (∼ 0.5 Oe) ≪ H, as a function of time, so that a lock-in amplifier can be used in tandem with the microwave probe to determine the change in microwaves detected arising from the change in absorption of the magnetic sample only. This causes the observed spectrum to be the magnetic field derivative of the absorption spectrum rather than the direct absorption spectrum, as the lock-in amplifier observes the RMS change in absorption, dχ, when the field is varied slightly by dB, and thus measures dχ/dB as opposed to χ. With some additional analysis, the resonance field, line-width, and Gilbert damping can still be derived from the absorption field derivative (see Data Collection and Analysis).

With this, Fig. 5 schematically depicts the experimental setup. A YIG device is placed on a coplanar stripline which injects microwave radiation (of frequency 1-30GHz) into it. The device is placed face side down on the stripline so as to maximize inductive coupling with the field to the coplanar stripline [4]. The YIG and stripline are between electromagnets which produce a constant steady applied magnetic field from a power supply. Between these electromagnets are smaller modulating coils, which are connected to a function generator and are used to provide a time variance in the magnetic field. Next to the YIG device sits a Hall Probe, which is used to measure the applied magnetic field from the electromagnets. Additionally, a microwave diode sits next to the YIG device as a probe to the absorption of microwaves by the YIG sample. The output voltage from this diode is sent to a lock-in amplifier, which uses the modulation from the waveform generator as a reference to filter out microwave absorption from the microwave diode that wasn’t related to magnetic interactions. Finally, all data collection and measurement controlling devices, i.e. the Hall Probe, lock-in amplifier, waveform generator, magnet power supply, and microwave generator, are connected to a computer so that the experiment can be controlled automatically using LabView software. After the first round of measurements and analysis was completed, an amplifier was inserted between the microwave generator and the YIG device. This allows the microwave power to be increased so that the nonlinear damping phenomenon can be observed.