Spin-transfer based MRAM with reduced critical current density
View Patent ↗A magnetic random access memory device include a spin torque MRAM cell ( 100 ) having a reduced switching current (I c ) wherein standard materials may be used for a free layer ( 108 ). A fixed magnetic element ( 112 ) polarizes electrons passing therethrough, and the free magnetic element ( 108 ) having a first plane anisotropy comprises a first magnetization ( 130 ) whose direction is varied by the spin torque of the polarized electrons. An insulator ( 110 ) is positioned between the fixed magnetic element ( 112 ) and the free magnetic element ( 108 ), and a keeper layer ( 104 ) positioned contiguous to the free magnetic element ( 108 ) and having a second plane anisotropy orthogonal to the first plane anisotropy, reduces the first plane anisotropy and hence reduces the spin torque switching current (I c ). The keeper layer ( 104 ) may include alternating synthetic antiferromagnetic layers ( 132, 134 ) of magnetization approximately equal in magnitude and opposite in direction.
1. A magnetic random access memory device comprising:
a fixed magnetic element;
a free magnetic element having a first magnetization and a first anisotropy;
an insulator positioned between the fixed magnetic element and the free magnetic element; and
a keeper layer positioned contiguous to the free magnetic element and having a second anisotropy orthogonal to the first anisotropy.
2. The magnetic random access memory device of claim 1 wherein the keeper layer comprises a plurality of magnetic layers separated by nonmagnetic spacer layers.
3. The magnetic random access memory device of claim 2 wherein the magnetic layers comprise alternating layers of one of a) cobalt and platinum, and b) cobalt and palladium and c) cobalt and nickel.
4. The magnetic random access memory device of claim 2 wherein the magnetization of each magnetic layer is approximately equal in magnitude and opposite in direction to the magnetization of the adjacent magnetic layer.
5. The magnetic random access memory device of claim 2 wherein each spacer layer produces antiferromagnetic exchange coupling between the magnetic layers.
6. The magnetic random access memory device of claim 1 wherein the keeper layer is magnetostatically coupled, and exchange decoupled, from the free layer.
7. The magnetic random access memory device according to claim 1 further comprising a spin torque switching current that is reduced by the keeper layer.
8. The magnetic random access memory device according to claim 1 further comprising a spacer layer positioned between the keeper layer and the free magnetic element.
9. The magnetic random access memory device of claim 8 wherein the spacer layer comprises an electrically conducting non-magnetic material.
10. A magnetic random access memory device comprising:
a fixed magnetic element that polarizes electrons passing therethrough;
a free magnetic element having a first magnetization whose direction is varied by the spin torque of the polarized electrons, and having a first anisotropy;
an insulator positioned between the fixed magnetic element and the free magnetic element; and
a keeper layer positioned contiguous to the free magnetic element that reduces the first anisotropy, the keeper layer having a second anisotropy orthogonal to the first anisotropy, wherein a spin torque current is reduced by the reduction of the first plane anisotropy.
11. The magnetic random access memory device of claim 10 wherein the keeper layer comprises a plurality of magnetic layers separated by nonmagnetic spacer layers.
12. The magnetic random access memory device of claim 11 wherein the magnetic layers comprise alternating layers of one of a) cobalt and platinum, and b) cobalt and palladium and c) cobalt and nickel.
13. The magnetic random access memory device of claim 11 wherein the magnetization of each magnetic layer is approximately equal in magnitude and opposite in direction to the magnetization of the adjacent magnetic layer.
14. The magnetic random access memory device of claim 11 wherein each spacer layer produces antiferromagnetic exchange coupling between the magnetic layers.
15. The magnetic random access memory device of claim 10 wherein the keeper layer is magnetostatically coupled, and exchange decoupled, from the free layer.
16. The magnetic random access memory device according to claim 10 further comprising a spacer layer positioned between the keeper layer and the free magnetic element.
17. The magnetic random access memory device of claim 11 wherein the spacer layer comprises an electrically conducting non-magnetic material.