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Journal of Applied Physics
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Dynamic properties of charged walls in ion implanted garnets

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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 y 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.

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Journal of Applied Physics

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