Bias-adjusted giant magnetoresistive (GMR) devices for magnetic random access memory (MRAM) applications
A bias-adjusted giant magnetoresistive (GMR) device includes a ferromagnetic reference layer, which has a magnetization that remains relatively fixed when a range of magnetic fields is applied, and a ferromagnetic switching layer, which has a magnetization that can be changed by applying a relatively small magnetic field. In MRAM applications, the switching layer stores data in the form of the particular orientation of its magnetization relative to the magnetization of the reference layer. At least one of the reference and switching layers is split into at least two ferromagnetic layers separated by one or more layers of a nonmagnetic conductor, such that the hysteresis curve of resistance versus applied magnetic field is substantially symmetric about zero applied magnetic field.
1. A magnetoresistive device having a resistance that can be changed by applying a magnetic field, said magnetoresistive device comprising:
a reference layer, said reference layer having a first magnetization direction that remains substantially fixed when said magnetic field is applied;
a switching layer, said switching layer having a second magnetization direction that changes orientation relative to said first magnetization direction when said magnetic field is applied; and
a first nonmagnetic conductor layer between said reference layer and said switching layer,
wherein at least one of said reference and switching layers includes at least a first ferromagnetic layer, a second ferromagnetic layer, and a second nonmagnetic conductor layer between said first and second ferromagnetic layers, wherein magnetic coupling between said first and second ferromagnetic layers is predominantly magnetostatic,
wherein said magnetoresistive device has a first state, in which said magnetoresistive device has a first zero-field resistance, and a second state, in which said magnetoresistive device has a second zero-field resistance,
wherein said magnetoresistive device can be switched from said first state to said second state by applying said magnetic field in a first direction, provided that said magnetic field exceeds a first threshold magnitude, and said magnetoresistive device can be switched from said second state to said first state by applying said magnetic field in a second direction opposite to said first direction, provided that said magnetic field exceeds a second threshold magnitude, and
wherein said first and second threshold magnitudes are substantially equal.
2. The magnetoresistive device of claim 1 , wherein said first nonmagnetic conductor layer comprises copper.
3. The magnetoresistive device of claim 1 , wherein said second nonmagnetic conductor layer comprises copper.
4. The magnetoresistive device of claim 1 , further comprising:
an antiferromagentic layer magnetically coupled to said reference layer.
5. The magnetoresistive device of claim 2 , wherein said magnetoresistive device is a spin valve.
6. The magnetoresistive device of claim 1 , wherein said reference layer has a higher switching field than said switching layer.
7. The magnetoresistive device of claim 6 , wherein said magnetoresistive device is a pseudo spin valve.
8. The magnetoresistive device of claim 7 , wherein said reference layer is thicker than said switching layer.
9. The magnetoresistive device of claim 1 , wherein said first and second magnetization directions are substantially parallel in said first state and said first and second magnetization directions are substantially antiparallel in said second state.