MAGNETORESISTIVE MEMORY DEVICES INCLUDING DUAL FREE LAYERS AND METHODS FOR MAKING AND OPERATING THE SAME

1 . A magnetoresistive memory cell, comprising:

a magnetic polarizer layer having a hard magnetization along a hard magnetization direction;

a first magnetic tunnel junction located on a first side of the magnetic polarizer layer and comprising a first reference layer having a first side facing the magnetic polarizer layer, a first free layer facing a second side of the first reference layer, and a first tunnel barrier layer located between the first free layer and the first reference layer; and

a second magnetic tunnel junction located on a second side of the magnetic polarizer layer and comprising a second reference layer having a second side facing the magnetic polarizer layer, a second free layer facing a first side of the second reference layer, and a second tunnel barrier layer located between the second free layer and the second reference layer,

wherein the magnetoresistive memory cell is configured to be programmed into three or four different memory states.

2 . The magnetoresistive memory cell of claim 1 , wherein:

the first reference layer faces the first side of the magnetic polarizer layer, is antiferromagnetically coupled to the magnetic polarizer layer, and has a first fixed magnetization direction that is antiparallel to the hard magnetization direction; and

the second reference layer faces the second side of the magnetic polarizer layer, antiferromagnetically coupled to the magnetic polarizer layer, and has a second fixed magnetization direction that is antiparallel to the hard magnetization direction.

3 . The magnetoresistive memory cell of claim 2 , wherein:

the first magnetic tunnel junction has a first parallel state resistance when a magnetization direction of the first free layer is parallel to the first fixed magnetization direction;

the second magnetic tunnel junction has a second parallel state resistance when a magnetization direction of the second free layer is parallel to the second fixed magnetization direction;

the second parallel state resistance is different from the first parallel state resistance;

the first magnetic tunnel junction has a first antiparallel state resistance when the magnetization direction of the first free layer is antiparallel to the first fixed magnetization direction;

the second magnetic tunnel junction has a second antiparallel state resistance when the magnetization direction of the second free layer is antiparallel to the second fixed magnetization direction; and

a sum of the first antiparallel state resistance and the second parallel state resistance is different from a sum of the first parallel state resistance and the second antiparallel state resistance.

4 . A magnetoresistive memory device comprising the magnetoresistive memory cell of claim 3 and a programming circuit configured to program magnetization directions of the first free layer and the second free layer into each of the magnetic configurations comprising:

a first magnetic configuration in which the first magnetic tunnel junction is in the first parallel state and the second magnetic tunnel junction is in the second parallel state;

a second magnetic configuration in which the first magnetic tunnel junction is in the first antiparallel state and the second magnetic tunnel junction is in the second parallel state;

a third magnetic configuration in which the first magnetic tunnel junction is in the first parallel state and the second magnetic tunnel junction is in the second antiparallel state; and

a fourth magnetic configuration in which the first magnetic tunnel junction is in the first antiparallel state and the second magnetic tunnel junction is in the second antiparallel state.

5 . The magnetoresistive memory device of claim 4 , wherein the first magnetic configuration, the second magnetic configuration, the third magnetic configuration, and the fourth magnetic configuration provide four different values for a sum of a first tunneling magnetoresistance of the first magnetic tunnel junction and a second tunneling magnetoresistance of the second magnetic tunnel junction.

6 . The magnetoresistive memory device of claim 4 , wherein the programing circuit is configured to program the magnetization directions of the first free layer and the second free layer into the first magnetic configuration by:

applying a relatively large first negative electron current to the magnetoresistive memory cell that flows in a first direction from the first reference layer to the first free layer, to switch the first magnetic tunnel junction into the first parallel state; and

applying a relatively small first positive electron current to the magnetoresistive memory cell that flows in a second direction from the second reference layer to the second free layer, which is opposite to the first direction, to switch the second magnetic tunnel junction into the second parallel state without disturbing the magnetization direction of the first free layer which requires a relatively large second positive electron current which is greater than the first positive electron current to switch into the first antiparallel state.

7 . The magnetoresistive memory device of claim 6 , wherein the programing circuit is configured to program the magnetization directions of the first free layer and the second free layer into the second magnetic configuration by applying the relatively large second positive electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction to the first antiparallel state.

8 . The magnetoresistive memory device of claim 7 , wherein the programing circuit is configured to program the magnetization directions of the first free layer and the second free layer into the third magnetic configuration by applying the relatively large first negative electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction to the first parallel state and to switch the second magnetic tunnel junction into the second antiparallel state.

9 . The magnetoresistive memory device of claim 8 , wherein the programing circuit is configured to program the magnetization directions of the first free layer and the second free layer into the fourth magnetic configuration by:

applying the relatively large second positive electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction into the first antiparallel state and to switch the second magnetic tunnel junction into the second parallel state; and

applying a relatively small second negative current which is smaller than the first negative current to the magnetoresistive memory cell to switch the second magnetic tunnel junction into the second antiparallel state, without disturbing the magnetization direction of the first free layer which requires the relatively large first negative current which is greater than the second negative current to switch into the first parallel state.

10 . The magnetoresistive memory cell of claim 1 , wherein the magnetoresistive memory cell further comprises:

a selector element;

a first antiferromagnetic coupling layer located between the magnetic polarizer layer and the first reference layer and providing the antiferromagnetic coupling therebetween; and

a second antiferromagnetic coupling layer located between the magnetic polarizer layer and the second reference layer and providing the antiferromagnetic coupling therebetween.

11 . The magnetoresistive memory cell of claim 1 , wherein:

the first free layer has a first area in a horizontal plane; and

the second free layer has a second area less than the first area in the horizontal plane. wherein the second lateral dimension is less than the first lateral dimension.

12 . The magnetoresistive memory cell of claim 11 , wherein the magnetoresistive memory cell further comprises a tubular dielectric spacer laterally surrounding and contacting the second magnetic tunnel junction and not contacting the first magnetic tunnel junction.

13 . The magnetoresistive memory cell of claim 1 , wherein:

the first free layer has a first thickness; and

the second free layer has a second thickness different than the first thickness.

14 . The magnetoresistive memory cell of claim 1 , wherein:

the first free layer has a magnetic damping coefficient; and

the second free layer has a second magnetic damping coefficient different than the second magnetic damping coefficient.

15 . The magnetoresistive memory cell of claim 1 , wherein the magnetic polarizer layer comprises a hard magnet layer.

16 . The magnetoresistive memory cell of claim 1 , wherein:

the first reference layer underlies the magnetic polarizer layer;

the first free layer underlies the first reference layer;

the second reference layer overlies the magnetic polarizer layer; and the second free layer overlies the second reference layer.

17 . A method of operating the magnetoresistive memory cell of claim 1 , comprising programming the magnetoresistive memory cell into the three or the four different memory states.

18 . The method of claim 17 , wherein the programming the magnetoresistive memory cell into three or four different memory states comprises the four memory states selected from

a first memory state in which the first magnetic tunnel junction is in a first parallel state and the second magnetic tunnel junction is in a second parallel state;

a second memory state in which the first magnetic tunnel junction is in a first antiparallel state and the second magnetic tunnel junction is in the second parallel state;

a third memory state in which the first magnetic tunnel junction is in the first parallel state and the second magnetic tunnel junction is in a second antiparallel state; or

a fourth memory state in which the first magnetic tunnel junction is in the first antiparallel state and the second magnetic tunnel junction is in the second antiparallel state.

19 . A method of operating a magnetoresistive memory device of claim 4 , comprising:

programing the magnetization directions of the first free layer and the second free layer into the first magnetic configuration by:

applying a relatively large first negative electron current to the magnetoresistive memory cell that flows in a first direction from the first reference layer to the first free layer, to switch the first magnetic tunnel junction into the first parallel state; and

applying a relatively small first positive electron current to the magnetoresistive memory cell that flows in a second direction from the second reference layer to the second free layer, which is opposite to the first direction, to switch the second magnetic tunnel junction into the second parallel state without disturbing the magnetization direction of the first free layer which requires a relatively large second positive electron current which is greater than the first positive electron current to switch into the first antiparallel state;

programing the magnetization directions of the first free layer and the second free layer into the second magnetic configuration by applying the relatively large second positive electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction to the first antiparallel state;

programing the magnetization directions of the first free layer and the second free layer into the third magnetic configuration by applying the relatively large first negative electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction to the first parallel state and to switch the second magnetic tunnel junction into the second antiparallel state; and

programing the magnetization directions of the first free layer and the second free layer into the fourth magnetic configuration by:

applying the relatively large second positive electron current to the magnetoresistive memory cell to switch the first magnetic tunnel junction into the first antiparallel state and to switch the second magnetic tunnel junction into the second parallel state; and

applying a relatively small second negative current which is smaller than the first negative current to the magnetoresistive memory cell to switch the second magnetic tunnel junction into the second antiparallel state, without disturbing the magnetization direction of the first free layer which requires the relatively large first negative current which is greater than the second negative current to switch into the first parallel state.

20 . A method of forming a magnetoresistive memory device, comprising:

forming a layer stack comprising, in order from bottom to top, a first continuous free layer, a first continuous tunnel barrier layer, a first continuous reference layer, a continuous magnetic polarizer layer, a second continuous reference layer, a second continuous tunnel barrier layer, and a second continuous free layer;

patterning the second continuous free layer, the second continuous tunnel barrier layer, and the second continuous reference layer into first pillar structures comprising second magnetic tunnel junctions;

forming tubular dielectric spacers around the first pillar structures; and

patterning the first continuous free layer, the first continuous tunnel barrier layer, the first continuous reference layer, the continuous magnetic polarizer layer using the first pillar structures and the tubular dielectric spacers to form first magnetic tunnel junctions having a smaller horizontal area than the second tunnel junctions.