Giant magnetoresistive random-access memories based on current-in-plane devices

A giant magnetoresistive random-access memory (GMRAM) is a nonvolatile memory consisting of magnetic memory devices integrated with standard semiconductor electronics. In GMRAMs, magnetic multilayer devices, such as "pseudo-spin-valve" (PSV) and "spin-valve" (SV) devices, are made of layers of nanometer-thick materials, and are used to store information and to allow the data to be read. PSV and SV devices are giant magnetoresistive (GMR) current-in-plane (CIP) devices that are alternative memory devices to current-perpendicular-to-plane devices such as magnetic tunnel junction devices. In this paper, GMR CIP magnetic device operation and characteristics are described that have supported the demonstration of GMRAMs that have passed nonvolatility and complex write and read memory pattern tests at write and read cycle times down to approximately 50 ns. The GMRAM technology implementation described here is based on a 1R0T architecture in which a single GMR PSV device defines the bit cell and provides write and read selectivity so no transistors are needed per bit cell. A PSV bit is stored by setting the direction of magnetization in the storage layer. A PSV bit is read by determining the orientation of the sense layer with respect to the storage layer using the GMR effect that such magnetic multilayers produce when current flows in the magnetic device. The GMR effect is a magnetoresistive variable-resistance spin effect in which the resistance depends on the relative magnetization between storage and sense layers in the magnetic multilayer. In a magnetic multilayer that exhibits the GMR effect, resistance is maximized when the relative magnetization between the storage and sense layers is antiparallel; resistance is minimized when the relative magnetization between the storage and sense layers is parallel. The GMR effect produces a change in resistance that, when excited with a read current, induces a signal that distinguishes between a binary "1" and "0". Write and read characteristics of CIP PSV devices are described in terms of write switching, read switching, resistive, and magnetoresistive properties of individual PSV devices and statistical ensembles of PSV devices fabricated on bulk Si and CMOS underlayers. Based on experimental work and modeling, magnetization reversal has been inferred to be rotational, including irreversible rotations that correspond to switching, reversible rotations that anticipate switching, and reversible rotations that complete reversal from switching to saturation in the opposite direction. Magnetoresistive and magnetic switching properties along with nonvolatility, nondestructive readout, and potentially unlimited cyclability make PSV and SV devices potential options as nonvolatile memory elements for GMRAMs.