Nonvolatile Programmable Spin-Logic Gates Show Potential in Reconfigurable Computing
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RESEARCH/RESEARCHERS Nonvolatile Programmable SpinLogic Gates Show Potential in Reconfigurable Computing Researchers at Siemens AG and the University of Bielefeld, Germany, have made programmable spin-logic gates based on spin-dependent tunneling (STD) elements. The SDT elements form the spin-logic gates, allowing programmable logic operations. For reconfigurable computing using highly magnetoresistive thin-film magnetic structures, it is essential that a programming operation be performed rapidly and with no practical limitation of programming cycles. In spin logic, the programming information (i.e., the logic inputs and outputs) is nonvolatile and power consumption is low. The researchers chose a tunneling system because it exhibits good readout-voltage levels for submicrometer elements and compatibility to nonvolatile memory. The major challenges of such a tunneling system are that it needs a homogeneous device-fabrication process and homogeneous tunneling-current distribution or a tunneling barrier. The researchers showed that such requirements could be met down to an area of 0.6 µm2 per SDT element. Using a seven-mask lithography process, R. Richter and colleagues fabricated an IrMn/CoFe/Ru/CoFe/Al2O3/ NiFe tunneling structure, in which the artificial antiferrimagnet hard subsystem, CoFe (2.0 nm)/Ru (0.9 nm)/CoFe (3.0 nm), was exchange-biased to the antiferromagnet IrMn. As reported in the February 18 issue of Applied Physics Letters, field-programmable gate arrays (FPGAs) were constructed from three-input fieldprogrammable spin-logic gates (in order to compare with semiconductor-based FPGAs). Each gate consisted of six SDT elements. A current in a switch line generated a magnetic field to switch the soft magnetic NiFe layer and, hence, the corresponding SDT element between its highand low-resistance states. A clock line ran on top and perpendicular to the switch line. These two lines were electrically isolated from each other and from the SDT elements. The device was typically operated at a tunneling current of 5 µA with a corresponding voltage of ~300 mV across each SDT element. The voltage across a switch line led to a current and, hence, to a magnetic field at the location of the selected SDT element. In the six-element configuration, three elements were chosen as reference bits, and the other three served as logic input bits. The voltages defining the inputs Boolean 0 and Boolean 1 were chosen such that the corresponding magnetic 166
fields were sufficient to switch the soft magnetic layer of any selected SDT element. Thus, programming was as fast as performing a logic operation. Additionally, the researchers reported no limitation of programming cycles. The researchers showed the feasibility of a hybrid field-programmable spinlogic gate based on SDT elements in steady-state and clocked operation. They achieved working hybrid spin-logic gates with higher integration density and the possibility of more than three inputs. The programming information and the logic inputs/output were nonvolatile. The researc
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