Magnetoresistive logic cell and method of use
View Patent ↗A magnetoresistive logic cell (MRLC) is described that includes two MTJs in series that share a common free layer (CFL). The relative magnetization orientations of the CFL and the switchable reference layer (SRL) in MTJ-1 dominate the overall resistance of the MRLC without regard to the fixed magnetization orientation of the nonswitchable reference layer in MTJ-2. High and low resistance states of the MRLC occurs based on the relative magnetization orientations of SRL and CFL. This behavior allows the MRLC to be used as a logical comparator. The CFL is switched by STT effect by application of selected relatively short voltage pulses that do not switch the SRL. A voltage-induced switching principle can be used with MRLC embodiments of the present invention to switch the SRL to parallel or anti-parallel with respect to the magnetization CFL in both perpendicular and in-plane anisotropy embodiments.
1. A method of using magnetoresistive logic cell (MRLC) having first and second magnetic tunnel junctions (MTJs) in series that include a nonswitchable reference layer with a fixed magnetization direction, a switchable reference layer with a switchable magnetization direction and a free layer common to first and second MTJs switchable magnetization direction and wherein the first MTJ has a substantially higher resistance-area product and effective capacitance than the second MTJ, the method comprising:
writing a first data bit value in the switchable reference layer by first setting a first or second magnetization direction of the free layer in relation to fixed magnetization direction nonswitchable reference layer by applying a first voltage pulse to the MRLC, the first voltage pulse having a first set of characteristics including a first duration selected to switch the magnetization direction of the free layer in relation to the fixed magnetization direction of the nonswitchable reference layer with a positive voltage pulse switching the magnetization direction of the free layer in the first magnetization direction and a negative voltage pulse switching the magnetization direction of the free layer in the second magnetization direction and then applying a second voltage pulse to the MRLC to set the magnetization direction of the switchable reference layer, the second voltage pulse having a second set of characteristics including a second duration longer than the first duration selected to switch the magnetization direction of the switchable reference layer in relation to the magnetization direction of the free layer with a voltage pulse of a first amplitude switching the magnetization direction of the switchable reference layer to parallel to the magnetization direction of the free layer and a voltage pulse of a second amplitude switching the magnetization direction of the switchable reference layer to antiparallel the magnetization direction of the free layer;
writing a second data bit value in the free layer by setting a first or second magnetization direction of the free layer in relation to fixed magnetization direction nonswitchable reference layer by applying a third voltage pulse to the MRLC, the third voltage pulse having the first set of characteristics including a first duration and first shape with a positive voltage pulse switching the magnetization direction of the free layer in the first magnetization direction and a negative voltage pulse switching the magnetization direction of the free layer in the second magnetization direction; and
reading a high or low resistance of the MRLC as a result of a logical operation using first and second data bit values, where the high resistance indicates that the magnetization direction of the free layer and the switchable reference layer are antiparallel and the low resistance indicates that the magnetization direction of the free layer and the switchable reference layer are parallel.
2. The method of claim 1 wherein the second duration is greater than 1 ns.
3. The method of claim 1 wherein the second duration is greater than 10 ns.
4. The method of claim 1 wherein the first voltage pulse having the first set of characteristics switches the free layer by spin transfer torque (STT) effect.
5. The method of claim 1 wherein the second voltage pulse having the second set of characteristics switches the magnetization direction of the switchable reference layer in relation to the magnetization direction of the free layer by voltage-induced switching.
6. The method of claim 1 wherein the second voltage pulse switches the reference layer by changing a magnetic anisotropy in the first reference layer.
7. The method of claim 1 wherein the free layer and the reference layer have perpendicular anisotropy and the second voltage pulse is a negative voltage.
8. The method of claim 1 wherein the free layer and the reference layer have in-plane anisotropy and the second voltage pulse is a positive voltage.
9. The method of claim 1 wherein the logical operation is a XOR or XNOR.
10. The method of claim 1 wherein the second voltage pulse has a slower rise time than the first voltage pulse.
11. A method of operating a magnetoresistive logic cell (MRLC) having first and second magnetic tunnel junctions (MTJs) in series that include a nonswitchable reference layer with a fixed magnetization direction, a switchable reference layer with a switchable magnetization direction and a free layer common to first and second MTJs switchable magnetization direction and wherein the first MTJ has a substantially higher resistance-area product and effective capacitance than the second MTJ, the method comprising:
setting the magnetization direction of the switchable reference layer by applying a sequence of first and second pulses to the MRLC, the first pulse setting the magnetization direction of the free layer in relation to the fixed magnetization direction of the nonswitchable reference layer and the second pulse setting the magnetization direction of the switchable reference layer in relation to the magnetization direction of the free layer;
setting magnetization direction of the free layer by applying a third pulse to the MRLC; and
reading a high or low resistance of the MRLC, where the high resistance indicates that the magnetization direction of the free layer and the switchable reference layer are antiparallel and the low resistance indicates that the magnetization direction of the free layer and the switchable reference layer are parallel;
wherein the first and third pulses have a first set of characteristics including a first duration and a first shape selected to switch the magnetization direction of the free layer in relation to the fixed magnetization direction of the nonswitchable reference layer with a positive voltage pulse switching the magnetization direction of the free layer in the first magnetization direction and a negative voltage pulse switching the magnetization direction of the free layer in the second magnetization direction, the second voltage pulse set the magnetization direction of the switchable reference layer, and
the second voltage pulse having a second set of characteristics including a second duration longer than the first duration and a second shape selected to switch the magnetization direction of the switchable reference layer in relation to the magnetization direction of the free layer with a voltage pulse of a first amplitude switching the magnetization direction of the switchable reference layer to parallel to the magnetization direction of the free layer and a voltage pulse of a second amplitude switching the magnetization direction of the switchable reference layer to antiparallel the magnetization direction of the free layer.
12. The method of claim 11 wherein the magnetization directions of the free layer and the switchable reference layer correspond to first and second data bit values and reading the high or low resistance of the MRLC gives a result of a logical XOR or XNOR operation on first and second data bit values.
13. The method of claim 11 wherein the second duration is greater than 1ns.
14. The method of claim 11 wherein the second duration is greater than 10 ns.
15. The method of claim 11 wherein the first voltage pulse having the first set of characteristics switches the free layer by spin transfer torque (STT) effect.
16. The method of claim 11 wherein the second voltage pulse having the second set of characteristics switches the magnetization direction of the switchable reference layer in relation to the magnetization direction of the free layer by voltage-induced switching.
17. The method of claim 11 wherein the second voltage pulse switches the reference layer by changing a magnetic anisotropy in the first reference layer.
18. The method of claim 11 wherein the free layer and the reference layer have perpendicular anisotropy and the second voltage pulse is a negative voltage.
19. The method of claim 11 wherein the free layer and the reference layer have in-plane anisotropy and the second voltage pulse is a positive voltage.
20. The method of claim 11 wherein the second voltage pulse has a slower rise time than the first voltage pulse.
21. A method of using a magnetoresistive logic cell (MRLC) formed in sequence of nonswitchable reference layer (NSRL), first junction layer, free layer, second junction layer, switchable reference layer (SRL), first junction layer having a substantially lower resistance-area product than second junction layer, the method comprising:
writing first data bit to the SRL by:
setting the magnetization direction of the free layer to be parallel or anti-parallel to the magnetization direction of the NSRL by applying a first voltage pulse having a first duration; and
then setting magnetization direction of the SRLto be parallel or anti-parallel to the magnetization direction of the free layer by applying a second voltage pulse having a second duration longer than the first duration;
writing second data bit to the free layer by:
setting the magnetization direction of the free layer to be parallel or anti-parallel to the magnetization direction of the NSRL by applying a third voltage pulse, having the first duration; and
reading resistance of the MRLC as a result of a logic operation using first and second data bit values.