Parallel-Pumped Spin-Wave Instability Threshold Characteristics in Polycrystalline Yttrium Iron Garnet Excited by the Varied Rising-Edge Microwave Pulses

摘要

The parallel-pumped spin-wave instability (SWI) threshold power, $P_{mathrm {th}}$ , of polycrystalline yttrium iron garnet (YIG) excited by microwave pulses with the varied rising-edge time was investigated theoretically and experimentally. The SWI threshold model associated with the rising-edge time $t_{r}$ is developed in terms of the attributes of half-frequency spin waves in the applied static fields $H_{mathrm {ext}}$ . The results manifest a significant increase in microwave-excited SWI threshold power level when the rising-edge time $t_{r}$ is shortened from 600 to 100 ns at the X-band. In addition, the rising-edge microwave pulses with different time scale distinctly present different threshold responses under the left-wing field region of the butterfly curves. The SWI thresholds associated with the spin-wave initial amplitude grow with increasing $H_{mathrm {ext}}$ when $t_{r}$ is smaller than the spin-wave relaxation time ( $t_{r}~ boldsymbol {le }~200$ ns), while SWI thresholds are more sensitive to the $k$ -dependent spin-wave linewidth, which decreases with the increasing $H_{mathrm {ext}}$ as $t_{r}$ exceeds the spin-wave relaxation time ( $t_{r} ~ boldsymbol {ge }~400$ ns). The calculated responses of the SWI threshold are in close agreement with the measurements.