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We are interested in using ferromagnetic materials to engineer more energy-efficient transistors and logic gates. Transistors today are limited by the energy dissipated per switching operation, and this heat dissipation can potentially be greatly reduced by using a collective effect, such as the collective switching of magnetic moments. In our research we are designing and fabricating a promising instantiation of magnetic logic, using the switching of a magnetic domain wall in a soft ferromagnet. The state of the logic gate is read out using a magnetic tunnel junction. This gate is nonvolatile and can have a fanout greater than one; additionally, each device is a universal NAND gate. Figure 1 shows a cartoon of the device, for a soft ferromagnet with magnetic moments parallel to the plane of the wire.
The logic gate uses current-induced domain wall motion in a soft ferromagnetic wire such as NiFe or CoFeB to write the state of the device. We ensure that there is only one 180° transverse domain wall by depositing an antiferromagnet such as IrMn on each end of the wire, creating an exchange bias that fixes the net magnetic moment of the ends. The output current of the device will depend on the tunnel magnetoresistance (TMR) of the tunnel junction, using an insulating tunnel barrier such as MgO. TMR values from 300% to 600% have been observed at room temperature [1] , allowing a possible fanout up to 6.
The device is fabricated using electron-beam lithography and UHV sputter deposition. We are using micromagnetic simulations to understand the scaling of the device with size, and we are implementing the device in circuit designs.
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- Figure 1: (a) Soft ferromagnet, (b) Antiferromagnet, (c) Tunnel barrier, and (d) Synthetic antiferromagnet magnetic tunnel junction.
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- Figure 2: Device fabrication. The soft layer is sputtered NiFe, with the ends exchange biased by IrMn. A U-shape is used to easily set the magnetization of the ends with one step. The tunnel junction will be placed between the center contacts.
- S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K. Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, and H. Ohno. “Tunnel magnetoresistance of 604% at 300 K by suppression of Ta diffusion in CoFeB/ MgO/ CoFeB pseudo-spin-valves annealed at high temperature.” Applied Physics Letters, vol. 93, p. 082508, 2008. [↩]