Dynamic properties of charged walls in ion implanted garnets

Abstract

The dynamic behavior of the charged wall in the ion implanted layer of contiguous disk bubble devices is determined for the first time without the disturbing influence of a bubble domain being coupled to the charged wall. The charged wall was stabilized next to a nonimplanted region by an applied in-plane field Hx and was oscillated along ŷ by an rf field hy sin ωt. Oscillations in Faraday magneto-optic contrast signals proportional to the wall oscillations were detected with a high-speed photomultiplier (PMT) when a focused laser beam was obliquely incident on the region of the charged wall. The drive frequency ω was provided by an rf tracking generator which was phase locked to a spectrum analyzer set to detect the fundamental amplitude response of the PMT signals. Our measurements were made on the doped yttrium iron garnet (YIG) layers Gd, Tm, Ga:YIG and Eu, Tm, Ga: YIG typically used for driving 5- and 1-μm bubbles, respectively. The charged wall amplitude rolloff with frequency interpreted in terms of an overdamped linear oscillator reveals a relaxation frequency near 10 MHz when Hx = 30 Oe, for example, in contrast to ∼0.5 MHz when the bubble is attached. The linear mobility of the charged wall μcw = 6600 cm/sec Oe determined in Gd, Tm, Ga: YIG from the relaxation frequency contrasts with an effective mobility μeff = 250 cm/sec Oe when the bubble is attached. These results confirm our earlier assumption [B. E. Argyle, et al., IEEE Trans. MAG-14, 593 (1978)] that the damping which affects the upper limiting frequency of operation in this contiguous disc material comes mainly from the bubble domain and almost none (i.e., ≲5%) from the charged wall. In addition, no velocity saturation effects are observed in the isolated charged wall at least up to 12.8 MHz. The maximum velocity 28 000 cm/sec inferred from the maximum harmonic amplitude at this frequency is at least 25 times the saturation velocity of an isolated bubble, e.g., 1100 cm/sec, in the nonimplanted film. A simple theoretical model taking into account restoring torques on the wall magnetization due to in-plane anisotropy and wall demagnetization predicts velocity saturation will occur at yet several times larger velocity than the maximum response we can observe in the present experiment.