MRAM Overview
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How MRAM Works

MRAM (Magnetoresistive Random Access Memory) uses electron spin to store data. Memory cells are integrated on an integrated circuit chip, and the function of the resulting device is like a semiconductor static RAM (SRAM) chip, with potentially higher density and the added feature that the data are nonvolatile, that is data are retained with power off. Typical “classic”or “conventional” MRAM uses spin-dependent tunnel junction memory cells and magnetic row and column write lines as illustrated below:

   MRAM Overview (02:51)

The spin-dependent tunnel junction produces a large change in resistance depending on the predominant electron spin in a storage layer. The tunnel barrier (dark green in the figure above) is as thin as a few atomic layers--so thin that electrons can “tunnel” through the normally insulating material, causing a resistance change.

Row and column magnetic write lines allow data to be written to a selected cell in a two-dimensional array:

MRAM array

Data are written by small electrical currents in the write lines that create a magnetic fields, which flip electron spins in the spin-dependent tunnel junction storage layer, thus changing the junction’s resistance. Data is read by the tunneling current or resistance through the tunnel junction.

Next-generation MRAM could reduce cell size and power consumption. Potential next-generation designs include Spin-Momentum Transfer, Magneto-Thermal MRAM, and Vertical Transport MRAM. Spin-Momentum Transfer (also “Spin-Transfer,” “Spin Injection,” or “Spin Torque Transfer”) MRAM is based on changing the spin of storage electrons directly with an electrical current rather than an induced magnetic field. This method has the potential to significantly reduce MRAM write currents, especially with lithographic feature sizes less than 100 nanometers. M-T MRAM uses a combination of magnetic fields and ultra-fast heating from electrical current pulses to reduce the energy required to write data. Vertical Transport MRAM (VMRAM) is a high-density type of MRAM that employs current perpendicular to the plane to switch spintronic memory elements.

References and further reading

Jim Daughton, "Magnetic Spin Devices: 7 Years from Discover to Product. Where now? (.pdf)," 2004 Materials Research Society Fall Meeting Technical Program, Symposium X--Frontiers of Materials Research--Innovations to Impact, December 1, 2004, Boston, Mass.

Dr. Jim Daughton, "Spintronics Applications at NVE," First Annual Center for Nanoscale Systems Nanotechnology Symposium, May 14, 2004, Cornell University, Ithaca, N.Y.

James M. Daughton, "Advanced Magnetoresistive Random Access Memory (MRAM) (.pdf)," February 7, 2001.

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