Magnetic random access memory devices including multi-bit cells
View Patent ↗A magnetic random access memory (“MRAM”) cell includes: (1) a first magnetic layer having a first magnetization direction and a magnetic anisotropy axis; (2) a second magnetic layer having a second magnetization direction; and (3) a spacer layer disposed between the first magnetic layer and the second magnetic layer. The MRAM cell also includes a field line magnetically coupled to the MRAM cell and configured to induce a write magnetic field along a magnetic field axis, and the magnetic anisotropy axis is tilted relative to the magnetic field axis. During a write operation, the first magnetization direction is switchable between m directions to store data corresponding to one of m logic states, with m>2, at least one of the m directions is aligned relative to the magnetic anisotropy axis, and at least another one of the m directions is aligned relative to the magnetic field axis.
1. A memory device comprising:
at least one magnetic random access memory (MRAM) cell including
a first magnetic layer having a first magnetization direction and a magnetic anisotropy axis;
a second magnetic layer having a second magnetization direction; and
a spacer layer disposed between the first magnetic layer and the second magnetic layer; and
a field line magnetically coupled to the MRAM cell and configured to induce a write magnetic field along a magnetic field axis,
wherein the magnetic anisotropy axis is tilted relative to the magnetic field axis, and
wherein, during a write operation, the first magnetization direction is switchable between m directions to store data corresponding to one of m logic states, with m>2, at least one of the m directions is aligned relative to the magnetic anisotropy axis, and at least another one of the m directions is aligned relative to the magnetic field axis.
2. The memory device of claim 1 , wherein m=4.
3. The memory device of claim 2 , wherein the MRAM cell is a 2-bit cell.
4. The memory device of claim 1 , wherein the magnetic anisotropy axis is tilted at an angle θ relative to the magnetic field axis, with 0°<θ<90°.
5. The memory device of claim 4 , wherein 35°<θ<55°.
6. The memory device of claim 4 , wherein the field line extends along a field line axis, and the magnetic anisotropy axis is tilted at an angle (90°−θ) relative to the field line axis.
7. The memory device of claim 1 , wherein a cross-sectional shape of the first magnetic layer has an aspect ratio greater than 1.
8. The memory device of claim 7 , wherein the cross-sectional shape has a longest dimension axis, and the magnetic anisotropy axis corresponds to the longest dimension axis.
9. The memory device of claim 1 , wherein the MRAM cell further includes a pinning layer adjacent to the first magnetic layer, and the pinning layer is configured to stabilize the first magnetization direction relative to a threshold temperature.
10. The memory device of claim 9 , further comprising a bit line electrically coupled to the MRAM cell, and wherein, during the write operation, the bit line is configured to apply a heating current to heat the MRAM cell above the threshold temperature, such that the first magnetization direction is temporarily unpinned prior to cooling of the MRAM cell below the threshold temperature.
11. The memory device of claim 10 , wherein, prior to cooling of the MRAM cell below the threshold temperature, the field line is configured to maintain the write magnetic field, such that the first magnetization direction is aligned relative to the magnetic field axis.
12. The memory device of claim 10 , wherein, prior to cooling of the MRAM cell below the threshold temperature, the field line is configured to induce the write magnetic field, such that the first magnetization direction is aligned relative to the magnetic field axis, and the field line is then deactivated, such that the first magnetization direction is aligned relative to the magnetic anisotropy axis.
13. The memory device of claim 10 , wherein, during a read operation, the bit line is configured to apply a read current to determine a resistance of the MRAM cell, and the resistance is indicative of a degree of alignment between the first magnetization direction and the second magnetization direction.
14. The memory device of claim 9 , wherein the field line is electrically coupled to the MRAM cell, and wherein, during the write operation, the field line is configured to apply a write current to induce the write magnetic field and to heat the MRAM cell above the threshold temperature, such that the first magnetization direction is temporarily unpinned prior to cooling of the MRAM cell below the threshold temperature.
15. The memory device of claim 14 , wherein, prior to cooling of the MRAM cell below the threshold temperature, the field line is configured to maintain the write magnetic field, such that the first magnetization direction is aligned relative to the magnetic field axis.
16. The memory device of claim 14 , wherein, prior to cooling of the MRAM cell below the threshold temperature, the field line is configured to induce the write magnetic field, such that the first magnetization direction is aligned relative to the magnetic field axis, and the field line is then deactivated, such that the first magnetization direction is aligned relative to the magnetic anisotropy axis.
17. The memory device of claim 14 , wherein, during a read operation, the field line is configured to apply a read current to determine a resistance of the MRAM cell, and the resistance is indicative of a degree of alignment between the first magnetization direction and the second magnetization direction.
18. A method of operating a memory device, comprising:
providing a magnetic random access memory (MRAM) cell having a storage magnetization direction and a magnetic anisotropy axis, wherein the storage magnetization direction is pinned relative to a threshold temperature;
during a first write operation, heating the MRAM cell above the threshold temperature, such that the storage magnetization direction is temporarily unpinned prior to cooling of the MRAM cell below the threshold temperature; and
during the first write operation and while the storage magnetization direction remains unpinned,
inducing a first write magnetic field along a magnetic field axis, such that the storage magnetization direction is aligned relative to the magnetic field axis; and
deactivating the first write magnetic field, such that the storage magnetization direction is aligned relative to the magnetic anisotropy axis, wherein the magnetic anisotropy axis is tilted at an angle θ relative to the magnetic field axis, with 0°<θ<90°.
19. The method of claim 18 , wherein 35°<θ<55°.
20. The method of claim 18 , further comprising:
during a second write operation, heating the MRAM cell above the threshold temperature, such that the storage magnetization direction is temporarily unpinned prior to cooling of the MRAM cell below the threshold temperature; and
during the second write operation, inducing a second write magnetic field along the magnetic field axis, such that the storage magnetization direction is aligned relative to the magnetic field axis, wherein the second write magnetic field is maintained prior to cooling of the MRAM cell below the threshold temperature.