A Magnetic Random-Access Memory (MagRAM) is an array of bistable magnetic memory elements with semiconductor amplifier and addressing circuitry. MagRAMs are in the early stages of development, which has been motivated by a need for nonvolatile memories with high densities and unlimited cyclability -- a combination of properties that has not been achieved in nonvolatile electronic RAMs. In principle, the magnetic memory elements in MagRAMs can be made free of fatigue and thus capable of unlimited cyclability. Magnetic memory elements provide signals of reasonable magnitude that can be amplified by semiconductor electronic circuits, and offer the additional advantage of radiation hardness.

Figure 1. A MagRAM is an array of bistable magnetic memory elements within a matrix of semiconductor electronic circuitry that provides amplification, latching, and addressing.

In a MagRAM, data is stored in the magnetic states of the magnetic memory elements, which are hysteretic. The data is read from these elements by using the magnetoresistive effect to sense their magnetization states. Figure 1 is a simplified schematic diagram of a 16-bit MagRAM. A designated bit element is addressed, for reading or writing, by the application of appropriate currents to the word-line (row) conductors and sensing-line (column) conductors that intersect at that element. The current in the word-line conductor generates the magnetic field to write a bit in the designated element. A bit (0 or 1) is written in an element by applying a sensing current IS, together with a writing word current -IW for a 0 or +IW for 1. Nondestructive readout of the bit is effected by applying IS with (a) a word current -IR followed by (b) a word current +IR (IR<IW). During readout, the analog sensing amplifier and latch act together to convert the change in voltage on the sensing line to a bit. The currents IW,IS, and IR are chosen according to the hysteretic and magnetoresistive properties and the need to prevent spurious writing in inactive cells crossed by active word-line conductors.

Figure 2. This Prototype 16-Bit MagRAM and Display Unit contains discrete integrated-circuit chips that would be combined into a single chip in an advanced production version.

A low-density 16-bit prototype MagRAM based on the concept is illustrated in Figure 2. This assembly is made from discrete subsystems in the sense that the functional blocks indicated in Figure 1 are implemented by means of interconnecting separate integrated-circuit chips. Subsequent development efforts are expected to lead to the integration of all magnetic and electronic MagRAM components onto a single chip that would feature high memory density and low power consumption.

This work was done by Romney Katti and Brent Blaes of Caltech for NASA's Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

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This article first appeared in the February, 1999 issue of NASA Tech Briefs Magazine.

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