Phase reciprocity of spin-wave excitation by a microstrip antenna

Using space-, time-, and phase-resolved Brillouin light-scattering spectroscopy we investigate the difference in phase of the two counterpropagating spin waves excited by the same microwave microstrip transducer. These studies are performed both for backward volume magnetostatic waves and magnetostatic surface waves in an in-plane magnetized yttrium iron garnet film. The experiments show that for the backward volume magnetostatic spin waves (which are reciprocal and excited symmetrically in amplitude) there is a phase difference of $\pi$ associated with the excitation process and thus the phase symmetry is distorted. On the contrary, for the magnetostatic surface spin waves (which are nonreciprocal and unsymmetrical in amplitude) the phase symmetry is preserved (there is no phase difference between the two waves associated with the excitation). Theoretical analysis confirms this effect.

Figure 1

(a) Experimental setup for BVMSW geometry. Spin waves were excited using a microstrip antenna. (b) Space resolved BLS measurement of the intensity of two counterpropagating spin waves in the BVMSW geometry approximately 40 ns after their excitation. (c) Experimental setup for MSSW geometry. (d) Space resolved BLS measurement of two counterpropagating spin waves in the MSSW geometry approximately 40 ns after their excitation.

Figure 2

(Color online) Phase accumulation for BVMSW geometry. The measured excitation phase is $\left(1.1\pm 0.4\right)\pi$. The applied bias magnetic field is 1835 Oe. Spin-wave carrier frequency and wave vector are 7.132 GHz and $\left(-198\pm 1\right){\text{cm}}^{-1}$, respectively. The inset shows a micrograph of the used structure.

Figure 3

(Color online) Phase accumulation for MSSW geometry. The measured excitation phase is $\left(-0.16\pm 0.17\right)\pi$. The applied bias magnetic field is 1825 Oe. Spin-wave carrier frequency and wave vector are 7.132 GHz and $\left(75\pm 1\right){\text{cm}}^{-1}$, respectively. The inset shows a micrograph of the used structure.

Figure 4

Summary of the performed measurements. The expected behavior is clearly visible. The different excitation phase values for the lowest absolute values of the wave vectors can be explained by the distortion due to the propagation close to the ferromagnetic resonance frequency.