Cornell University researchers have demonstrated a new way to write information to magnetic material that could lead to new computer memory chips that will have a very high storage capacity and will be non-volatile, meaning they would not require a constant electric current flowing to maintain stored information.
Dan Ralph, Cornell assistant professor of physics, says the effect was demonstrated in devices between 10 nanometers and 100 nanometers across. If the effect can be commercially harnessed with 10-nanometer devices, he says, it will make possible single chips capable of storing terabits (trillions of bits) of information. A nanometer is one-billionth of a meter, or about three times the diameter of an atom.
The researchers have shown that a small electric current passed through a "sandwich" of two layers of magnetic material separated by a copper conductor can controllably switch the orientation of the "magnetic moment" -- that is, flip the north and south poles -- of a small magnetized area. The experiment, which confirms theoretical predictions, is reported in the Aug. 6, 1999, issue of Science magazine.
Using the facilities of the Cornell Nanofabrication Facility, the researchers created two thin layers of cobalt separated by a copper spacer. The upper cobalt layer is less than 10 nanometers thick, the copper layer is 4 nanometers thick, and the lower cobalt layer is about 100 nanometers thick. A tiny copper electrode touches the top of the upper layer.
By passing an electrical current perpendicularly through the layers, the researchers were able to reverse the orientation of the magnetic moments in a region of the thinner cobalt layer, while leaving the thicker layer unchanged. The result was that the north and south poles in the two layers pointed in non-parallel directions. Reversing the current flipped the thin layer back so that the moments in the two layers were again parallel.
The orientation of the magnetization in the two layers can be read by simply passing a much weaker electric current through the two layers, because the resistance is much higher when the two moments are non-parallel. This means the effect could be used to create a computer memory chip in which each tiny magnetized area represents a "one" when the moments are parallel or a "zero" when they are opposed.
The effect works, the researchers said in their paper, because electrons passing through a magnetic material are "spin polarized," meaning that they are spinning in the same direction, with their axes lined up. When the electrons previously polarized by passing through the thick magnetic layer collide with the thin one, they apply a twisting force that can cause the magnetic moment in that layer to switch orientation. Electrons that pass through the thin layer first are reflected back off the thick layer, with reversed polarization, to have the same effect.
Such an effect was predicted by John Slonczewski, a theoretical physicist at the IBM research laboratory in Yorktown Heights, N.Y. Previously the only means by which the magnetic poles could be reoriented in magnetic devices was through the application of an external magnetic field.
The simple switching effect only occurs in films four nanometers thick or less, Ralph says. With thicker films, spin-polarized currents induce more complicated motions of the magnetic moments, yet to be studied in detail. The initial experiment was conducted in an apparatus cooled to 4.2 degrees above absolute zero by liquid helium. In later work, reported in a paper, "Current-Driven Magnetization Reversal and Spin Wave Excitations in Co/Cu/Co Pillars," submitted to Physical Review Letters the researchers demonstrated the same effect at room temperatures.
The Science paper is titled "Current-Induced Switching of Domains in Magnetic Multilayer Devices." The authors are Ralph, graduate student Edward Myers, former graduate student Richard Louie (now at Pacific Lutheran University, in Tacoma, Wash.), post-doctoral research associate Jordan Katine and Robert Buhrman, the J. E. Sweet Professor of Engineering.