Observation of an inverse Doppler shift from left-handed dipolar spin waves

We report the experimental observation of an inverse Doppler shift from the motion of an ordinary solid object. The experiment used left-handed, or backward, spin waves in a magnetic thin film. As a pick-up antenna was moved toward the spin wave source, the measured frequency decreased. In contrast, an increase would be expected in everyday experience as the observer approaches the source. The backward spin wave wavelength in the experiment was 1.83 mm at 3 GHz, resulting in a Doppler shift of 546 Hz sec∕m, or about 50 times larger than would be observed on an ordinary electromagnetic wave at the same frequency. The measured shifts and dispersion relation agree well with the established theory.