High speed low power magnetic devices based on current induced spin-momentum transfer
A high speed, low power method to control and switch the magnetization direction and/or helicity of a magnetic region in a magnetic device for memory cells using spin polarized electrical current. The magnetic device comprises a reference magnetic layer with a fixed magnetic helicity and/or magnetization direction and a free magnetic layer with a changeable magnetic helicity and/or magnetization direction. The fixed magnetic layer and the free magnetic layer are preferably separated by a non-magnetic layer. The fixed and free magnetic layers may have magnetization directions at a substantially non-zero angle relative to the layer normal. A current can be applied to the device to induce a torque that alters the magnetic state of the device so that it can act as a magnetic memory for writing information. The resistance, which depends on the magnetic state of the device, is measured to read out the information stored in the device.
1. A magnetic device comprising:
a pinned magnetic layer with a first magnetization vector with a first magnetization direction that is fixed;
a free magnetic layer with a second magnetization vector with a second magnetization direction that is changeable and has four stable magnetic states;
a first non-magnetic layer spatially separating the free magnetic layer and the pinned magnetic layer;
a read-out magnetic layer with a third magnetization vector with a third magnetization direction that is fixed; and
a second non-magnetic layer that spatially separates the free magnetic layer and the read-out magnetic layer, wherein application of current pulses traversing each of the layers of the magnetic device switches the magnetization vector of the free magnetic layer between the four stable magnetic states.
2. The magnetic device of claim 1 , wherein the second magnetization direction represents two bits of information.
3. The magnetic device of claim 1 , wherein the read-out magnetic layer is aligned such that each of the four stable magnetic states of the free magnetic layer has a different resistance.
4. The magnetic device of claim 3 , wherein an in-plane component of the read-out magnetic layer magnetization vector is not parallel to or offset by 45 degrees from any of the four stable magnetic states of the free magnetic layer.
5. The magnetic device of claim 1 , wherein the second non-magnetic layer is configured to minimize mutual magnetic interaction between the free magnetic layer and the read-out magnetic layer.
6. The magnetic device of claim 1 , wherein the first magnetization direction is perpendicular to a plane of the free magnetic layer, and wherein the second changeable magnetization direction is perpendicular to an axis extending longitudinally through the magnetic device.
7. The magnetic device of claim 1 , wherein the four stable magnetic states of the second magnetization vector are separated by 90 degrees.
8. The magnetic device of claim 1 , wherein the magnetic device is pillar-shaped, and wherein the pinned magnetic layer, the first non-magnetic layer, the free magnetic layer, the second non-magnetic layer, and the read-out magnetic layer are less than approximately 200 nm laterally and each layer is approximately 1 nm to 50 nm thick.
9. The magnetic device of claim 1 , wherein the pinned magnetic layer, the free magnetic layer, and the read-out magnetic layer are comprised of Co, Ni, Fe, an alloy of Co and Ni, an alloy of Co and Fe, an alloy of Ni and Fe, an alloy of Co, Ni, Fe, or permalloy Ni1-xFex.
10. The magnetic device of claim 1 , wherein the pinned magnetic layer, the free magnetic layer, and the read-out magnetic layer are comprised of a non-magnetic metal and an alloy, wherein the alloy comprises an alloy of Co and Ni, an alloy of Co and Fe, an alloy of Ni and Fe, or an alloy of Co, Ni and Fe, such that the non-magnetic metal and the alloy are ferromagnetically ordered at room temperature.
11. The magnetic device of claim 10 , wherein the non-magnetic metal comprises Cu, Pd or Pt.
12. The magnetic device of claim 1 , wherein the pinned magnetic layer, the free magnetic layer, and the read-out magnetic layer are comprised of NiMnSb and a conducting magnetic oxide.
13. The magnetic device of claim 12 , wherein the conducting magnetic oxide comprises CrO2 or Fe3O4.
14. The magnetic device of claim 1 , wherein the non-magnetic layers comprise Cu, Cr, Au, Ag or Al.
15. A method of magnetic switching, the method comprising:
applying an electric current comprising a first current pulse followed by a second current pulse wherein the second current pulse is out of phase with the first current pulse to a magnetic device comprising a first magnetic layer with a first magnetization vector and a second magnetic layer with a second magnetization vector having four stable magnetic states over a sub-nanosecond period of time; and
stopping application of the electric current when the second magnetization vector changes from one of the four stable magnetic states to another of the four stable magnetic states.
16. The method of claim 15 , wherein the magnetization vector represents two bits based upon the rotation of the magnetization vector.
17. The method of claim 15 , further comprising transferring spin-momentum of the pinned magnetic layer to the free magnetic layer while the electric current is applied.
18. The method of claim 15 , wherein the magnetic device further comprises a read-out magnetic layer having a third magnetization vector.
19. The method of claim 15 , further comprising reading a state of the second magnetization vector based upon the third magnetization vector of the read-out magnetic layer of the magnetic device.
20. The method of claim 19 , further comprising:
measuring a resistance through at least the read-out magnetic layer and the free magnetic layer; and
determining the state of the second magnetization vector based upon the resistance.
21. The method of claim 15 , wherein the current is applied through the pinned magnetic layer, the free magnetic layer, and the read-out magnetic layer.