Two-bit magnetoresistive random-access memory cell

1. A two-bit spin orbit torque (SOT) magnetoresistive random-access memory (MRAM) cell, the MRAM cell comprising:

a first heavy metal layer;

a first magnetic tunnel junction (MTJ) coupled to the first heavy metal layer, wherein the first MTJ has a first cross-sectional area;

a second MTJ connected in series with the first MTJ, wherein the second MTJ has a second cross-sectional area that is different than the first cross-sectional area, wherein the second MTJ shares a reference layer with the first MTJ; and

a second heavy metal layer coupled to the second MTJ, wherein:

the first heavy metal layer is connected to a first word line and to a bit line through a first transistor;

the second heavy metal layer is connected to a second word line and to the bit line through a second transistor;

the first heavy metal layer is connected to a third word line and to a source line through a third transistor; and

the second heavy metal layer is connected directly to the source line.

2. The MRAM cell of claim 1 , wherein the first MTJ comprises:

a first free layer in direct contact with the first heavy metal layer;

the reference layer; and

a first tunnel barrier disposed between the first free layer and the reference layer.

3. The MRAM cell of claim 2 , wherein the second MTJ comprises:

a second free layer in direct contact with the second heavy metal layer;

the reference layer; and

a second tunnel barrier disposed between the second free layer and the reference layer.

4. The MRAM cell of claim 2 , wherein the first tunnel barrier is a layer of MgO.

5. The MRAM cell of claim 1 , wherein the first cross-sectional area is smaller than the second cross-sectional area.

6. The MRAM cell of claim 5 , wherein the first MTJ and the second MTJ have different cross-sectional areas, and wherein the shared reference layer has the same cross-sectional area as the second MTJ.

7. The MRAM cell of claim 1 , wherein the first heavy metal layer and the second heavy metal layer comprise one or more layers of a metal that exhibits spin orbit torque properties.

8. The MRAM cell of claim 1 , wherein the first heavy metal layer and the second heavy metal layer comprise one or more layers of metals selected from the group consisting of platinum, tungsten, tantalum, and manganese.

9. The MRAM cell of claim 1 , wherein the first MTJ is stacked on top of the second MTJ.

10. The MRAM cell of claim 1 , wherein the MRAM cell is part of a memory array that comprising a plurality of two-bit MRAM cells.

11. The MRAM cell of claim 1 , wherein:

each of the first and second heavy metal layers comprises one or more metals selected from the group consisting of Pt, Ta, Mn, and W; and

the shared reference layer comprises a ferromagnetic metal.

12. A method of forming a two-bit spin orbit torque (SOT) MRAM cell, the method comprising:

forming a second heavy metal layer;

forming a magnetic tunnel junction (MTJ) stack that comprises:

a second MTJ above the second heavy metal layer, the second MTJ having a second cross-sectional area; and

a first MTJ in series with the second MTJ, the first MTJ having a first cross-sectional area, wherein the first cross-sectional area is different than the second cross-sectional area, and wherein the first MTJ shares a reference layer with the second MTJ; and

forming a first heavy metal layer above the first MTJ, wherein:

the first heavy metal layer is connected to a first word line and to a bit line through a first transistor;

the second heavy metal layer is connected to a second word line and to the bit line through a second transistor;

the first heavy metal layer is connected to a third word line and to a source line through a third transistor; and

the second heavy metal layer is connected directly to the source line.

13. The method of claim 12 , wherein forming the MTJ stack comprises:

forming an initial material stack, wherein the initial material stack comprises:

a second free layer;

a second tunnel barrier disposed on top of the second free layer;

the reference layer disposed on top of the second tunnel barrier;

a first tunnel barrier disposed on top of the reference layer; and

a first free layer disposed on top of the first tunnel barrier;

etching portions of the first heavy metal layer, the first free layer, and the first tunnel barrier to a first width;

encapsulating the etched first heavy metal layer, the first free layer, and the first tunnel barrier using a dielectric material; and

etching the reference layer, the second tunnel barrier, and the second free layer using the encapsulated layers as a hardmask such that a cross-sectional area of the second tunnel barrier and the second free layer is larger than a cross-sectional area of the first tunnel barrier and the first free layer.

14. The method of claim 12 , wherein the first cross-sectional area is smaller than the second cross-sectional area.

15. The method of claim 12 , wherein the first heavy metal layer and the second heavy metal layer comprise one or more layers of a metal that exhibits spin orbit torque properties.

16. The method of claim 12 , wherein the first heavy metal layer and the second heavy metal layer comprise one or more layers of metals selected from the group consisting of platinum, tungsten, tantalum, and manganese.

17. A method of programming a two-bit MRAM cell, the method comprising:

receiving a write command to program a first bit in a two-bit MRAM cell that comprises a first magnetic tunnel junction (MTJ) in series with a second MTJ, wherein:

an area of the first MTJ is different than an area of the second MTJ;

the first MTJ is coupled to a first heavy metal layer;

the first heavy metal layer is connected to a bit line using a first transistor;

the first heavy metal layer is connected to a source line using a second transistor;

the second MTJ is connected to a second heavy metal layer;

the second heavy metal layer is connected to the bit line using a third transistor; and

the second heavy metal layer is directly connected to the source line;

determining that the first bit is to be stored in a particular MTJ, wherein the particular MTJ is one of the first MTJ or the second MTJ; and

selectively activating one or more of the first, second, and third transistors to cause current to flow through a heavy metal layer that is associated with the particular MTJ.

18. The method of claim 17 , wherein the particular MTJ is the first MTJ, and wherein selectively activating one or more of the first, second, and third transistors comprises:

activating the first and third transistors to cause current to flow through the first heavy metal layer,

wherein the second transistor is not activated.

19. The method of claim 17 , wherein the particular MTJ is the second MTJ, and wherein selectively activating one or more of the first, second, and third transistors comprises:

activating the second transistor to cause current to flow through the second heavy metal layer,

wherein the first and third transistors remain not activated.

20. A system for programming a two-bit MRAM cell, wherein the system comprises:

an MRAM cell comprising a first magnetic tunnel junction (MTJ) in series with a second MTJ, wherein:

an area of the first MTJ is different than an area of the second MTJ;

the first MTJ is coupled to a first heavy metal layer;

the first heavy metal layer is connected to a bit line using a first transistor;

the first heavy metal layer is connected to a source line using a second transistor;

the second MTJ is connected to a second heavy metal layer;

the second heavy metal layer is connected to the bit line using a third transistor; and

the second heavy metal layer is directly connected to the source line; and

a processor configured to perform a method comprising:

receiving a write command to program a first bit in the MRAM cell;

determining that the first bit is to be stored in a particular MTJ, wherein the particular MTJ is one of the first MTJ or the second MTJ; and

selectively activating one or more of the first, second, and third transistors to cause current to flow through a heavy metal layer that is associated with the particular MTJ.

21. The system of claim 20 , wherein the particular MTJ is the first MTJ, and wherein selectively activating one or more of the first, second, and third transistors comprises:

activating the first and third transistors to cause current to flow through the first heavy metal layer,

wherein the second transistor is not activated.

22. The system of claim 20 , wherein the particular MTJ is the second MTJ, and wherein selectively activating one or more of the first, second, and third transistors comprises:

activating the second transistor to cause current to flow through the second heavy metal layer,

wherein the first and third transistors remain not activated.