Current‐Access Magnetic Bubble Circuits

Experimental and theoretical results from our work on current‐access technology show promise for high‐density, ∼10⁷ bits/cm², and high‐frequency, f > 1 MHz, bubble devices. We have operated current‐access devices where the bubble‐driving fields derive from two patterned conducting sheets instead of orthogonal field coils. Margins for generation, propagation, and transfer were studied on 8‐μm periods at 1 MHz. These 8‐μm period structures typically required 1.5 mA/μm per conducting sheet and dissipated 14 μW/bit. Single conducting‐sheet, current‐access circuitry also propagates bubbles but offers less design flexibility. We present design criteria, magnetic field equations, and design curves. Implementation of these devices required new magnetic materials with quality factor Q comparable to available garnets, yet higher mobility and lower dynamic coercivity. Of the three systems, (YLuSmCa)₃(FeGe)₅O₁₂, (LaLuSmCa)₃(FeGe)₅O₁₂, and (LaLuSm)₃(FeGa)₅O₁₂, the last appears best suited; some room temperature characteristics of the composition La_0.6Lu_2.1Sm_0.3Ga_0.9Fe_4.1O₁₂ are 4πM_s = 470G, μ = 750 cmS⁻¹‐Oe⁻¹, d = 1.6 μm, and ΔH_c = 2 Oe across a bubble for the threshold of motion. Necessary improvements in processing were made with a radio‐frequency, chlorine‐containing plasma etch which produced metal patterns identical to those of the etch mask. We anticipate that current‐access devices, when compared to conventional field‐access devices, will achieve higher data rates, lower power consumption per bit, and greater storage densities with existing processing technologies.