British University Announces First Submicron Magnetic Logic Device

Physicists at the University of Durham (Durham, UK) have fabricated a magnetic NOT gate that can operate at room temperature. It is the first wholly magnetic logic device to be formed on a microchip, and offers a key to what could become a completely new micro- and nano-magnetic chip technology.

The Durham NOT gate consists of a track of a naturally ferromagnetic alloy shaped like an inverted "Y." The magnetism of this ferromagnetic alloy tends to run parallel to its track length and points in one of two opposite directions. A single ferromagnetic track can contain different regions, each magnetized in one of these two directions. Where these opposite magnetizations meet, a transition region or "domain wall" is formed in which the magnetization direction is gradually rotated across the thickness of the domain wall. Domain walls can be moved along magnetic tracks by applying an external magnetic field. These two different magnetization directions can be used to code for binary logic states of 0 and 1 for storing a binary digit.

The device was switched between the two states using a suitable magnetic field whose vector rotated with time in the sample plane. A domain wall should pass around a corner in a magnetic track if the field and corner have the same chirality in the direction concerned. A domain wall is thus pushed around the curved input arm of the Y-shaped section. The magnetization of the inverted Y shape's central stem is reversed before the domain wall leaves the output arm of the Y-shaped track. The magnetization of the output arm is now opposite to that of the input arm and the central stem magnetization has been reversed, so the state of the logic circuit has been switched. When the chirality of the magnetic field was reversed, the domain wall was shown to propagate in the opposite direction around a bend, as expected.

Extremely thin Permalloy strips (80% nickel, 20% iron) ~200 nm wide and 5 nm thick were used in this work. The workers calculated that the domain walls should be ~100 nm wide. Structures incorporating nickel-iron alloy junctions were fabricated by ion beam milling (using 30 keV Ga+ ions) of thermally evaporated Permalloy films on silicon substrates. The wire corners had a radius of 1 µm.

The magnetization was analyzed using a magneto-optical Kerr effect (MOKE) magnetometer, with averaging of the data from successive magnetic field cycles for several minutes.

The group connected up to 11 NOT gates directly together in a circular closed loop operated as a shift register. The NOT gate input was provided by feeding back the output magnetization from the four-sided loop, avoiding the need for an external domain wall input. The gates operated without failure as the switching position and domain wall passed around the circuit up to 100,000×. Several domain walls can circulate in a multi-gate circuit.

The researchers plan other devices using the same technology, including an AND gate to compare two inputs and perform full-scale calculations. They expect to develop a fully functioning logic system within about a year. The frequency of operation of circuits demonstrated was only 27 Hz, because an iron-cored electromagnet was used to apply the rotating field. On a high-frequency strip line, the operating speed would ultimately be limited by the domain wall propagation time through a single gate. An operating frequency of >200 MHz has been calculated for devices with a 1 µm radius, with this frequency increasing as the gate size is reduced. The switching energy is estimated to be 35 eV per transition, far less than required for present low-power processes.

Some advantages of using magnetic microdevices are the non-volatility of the information they hold during power failure, the possibility of high-density integration of the individual gates, and the expected low power consumption. The high carrier density in metals and the absence of multilayer heterostructures that need precise alignment should greatly facilitate scaling to nanometer dimensions. These devices should be very resistant to high doses of ionizing radiation, such as those that occur in space work. Fabrication costs should be very low, because only a single metal layer is needed.

Conventional electronics are based fundamentally on electronic charge movement. There is rapidly growing interest in technologies based essentially on electron spin, for which the term "spintronics" is being used. Its possible use in computers for high-density data storage and fast switching is of great interest. Many problems need to be solved before commercialization, such as a reliable technique for feeding domain walls into a circuit, methods of generating the required rotating magnetic switching field from wires fixed into a chip, and the development of interfaces with electronic circuits.

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