Prototype spin-dependent tunneling isolators integrated with integrated circuit electronics

Low power, fast speed, small size, wide temperature range, and common-mode-noise-rejection capability are some of the attributes of magnetoresistive devices for galvanic isolation over conventional devices such as optical and capacitive devices. We have fabricated prototype galvanic isolators using spin-dependent tunneling materials. The tunnel junctions have been deposited by rf diode sputtering and the ${\mathrm{Al}}_2{\mathrm{O}}_3$ barriers are formed by depositing a thin layer of Al, then oxidizing it with oxygen contained plasma. The junctions are then patterned using photolithography techniques to define the pinned and free layers separately. A series of tunnel junctions are connected in a Wheatstone bridge form and are fabricated directly on top of the integrated circuit (IC) electronics that are used to process the signals from the bridge. In its core magnetics design, this digital device simply employs a one-bit memory cell capable of operating at 5 V. The functions of the devices are tested using a function generator, an IC driver chip, a probe station, two dc power supplies, and a high-speed network analyzer. The power consumption for the isolator is <1 mW when operating at 2 MHz, a factor of 10 lower than the giant magnetoresistance product of similar functions. The operating speed of this device reaches 60 MHz, limited by the design of the electronics.