Magnetic device to reduce reversal current in current-driven magnetic reversal and magnetic memory using same

1. A magnetic device comprising:

a first ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked, at least one layer of the magnetic layers having magnetization substantially fixed to a first direction, and two or more layers of the magnetic layers being ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane;

a second ferromagnetic layer having magnetization substantially fixed to a second direction;

a third ferromagnetic layer provided between the first ferromagnetic layer and the second ferromagnetic layer, the third ferromagnetic layer having a variable direction of magnetization;

a first intermediate layer provided between the first ferromagnetic layer and the third ferromagnetic layer;

a second intermediate layer provided between the second ferromagnetic layer and the third ferromagnetic layer; and

a couple of electrodes configured to provide write current between the first and second ferromagnetic layers to cause spin-polarized electrons to act on the third ferromagnetic layer so that the direction of magnetization of the third ferromagnetic layer is determined depending on a direction of the current,

the ferromagnetic coupling having a strength such that a parallel magnetic alignment of the magnetic layers is maintained when the write current is passed.

2. The magnetic device according to claim 1 , wherein the strength J of the ferromagnetic coupling satisfies a formula

J >( hP/ 2 e α)×( Iw/A )− tπMs 2

where Iw/A is a current density of the write current, t is a thickness of one of the magnetic layers, Ms is the magnetization of the one magnetic layer, h is the Planck's constant, P is a spin asymmetry of current, e is an electric charge, and α is a Gilbert damping constant.

3. The magnetic device according to claim 1 , wherein the nonmagnetic layers are formed from any one selected from the group consisting of copper (Cu), silver (Ag), gold (Au), ruthenium (Ru), iridium (Ir), rhodium (Rh) and an alloy containing at least one of them, and has a thickness of less than 2 nanometers.

4. The magnetic device according to claim 1 , wherein one of the first and the second intermediate layers is made of an insulator or a semiconductor, and other of the first and the second intermediate layers is made of a conductor.

5. The magnetic device according to claim 1 , wherein the third ferromagnetic layer includes alternately stacked magnetic and nonmagnetic layers, the magnetic layers being ferromagnetically coupled via the nonmagnetic layers.

6. A magnetic device comprising:

a first ferromagnetic layer having magnetization substantially fixed to a first direction;

a second ferromagnetic layer having magnetization substantially fixed to a second direction;

a third ferromagnetic layer provided between the first ferromagnetic layer and the second ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked and the magnetic layers are ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane, the third ferromagnetic layer having a variable direction of magnetization;

a first intermediate layer provided between the first ferromagnetic layer and the third ferromagnetic layer;

a second intermediate layer provided between the second ferromagnetic layer and the third ferromagnetic layer; and

a couple of electrodes configured to provide a current between the first and second ferromagnetic layers to cause spin-polarized electrons to act on the third ferromagnetic layer so that the direction of magnetization of the third ferromagnetic layer is determined depending on a direction of the current,

the ferromagnetic coupling having a strength such that magnetizations of all of the magnetic layers are reversed keeping parallel magnetic direction therebetween when the write current is passed.

7. The magnetic device according to claim 6 , wherein the strength J of the ferromagnetic coupling in the third ferromagnetic layer satisfies a formula

J >( hP/ 2 e α)×( Iw/A )− tπMs 2

where Iw/A is a current density of the write current, t is a thickness of one of the magnetic layers, Ms is the magnetization of the one magnetic layer, h is the Planck's constant, P is a spin asymmetry of current, e is an electric charge, and α is a Gilbert damping constant.

8. The magnetic device according to claim 6 , wherein the nonmagnetic layers are formed from any one selected from the group consisting of copper (Cu), silver (Ag), gold (Au), ruthenium (Ru), iridium (Ir), rhodium (Rh) and an alloy containing at least one of them, and has a thickness of less than 2 nanometers.

9. The magnetic device according to claim 6 , wherein one of the first and the second intermediate layers is made of an insulator or a semiconductor, and other of the first and the second intermediate layers is made of a conductor.

10. A magnetic device comprising:

a first ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked, at least one layer of the magnetic layers having magnetization substantially fixed to a first direction, and two or more layers of the magnetic layers being ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane;

a second ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked, at least one layer of the magnetic layers having magnetization substantially fixed to a second direction, and two or more layers of the magnetic layers being ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane;

a third ferromagnetic layer provided between the first ferromagnetic layer and the second ferromagnetic layer, the third ferromagnetic layer having a variable direction of magnetization;

a first intermediate layer provided between the first ferromagnetic layer and the third ferromagnetic layer;

a second intermediate layer provided between the second ferromagnetic layer and the third ferromagnetic layer; and

a couple of electrodes configured to provide write current between the first and second ferromagnetic layers to cause spin-polarized electrons to act on the third ferromagnetic layer so that the direction of magnetization of the third ferromagnetic layer is determined depending on a direction of the current,

the ferromagnetic coupling having a strength such that magnetization of at least one of the magnetic layers is not reversed when the write current is passed.

11. The magnetic device according to claim 10 , wherein the strength J of the ferromagnetic coupling satisfies a formula

J >( hP/ 2 e α)×( Iw/A )− tπMs 2

where Iw/A is a current density of the write current, t is a thickness of one of the magnetic layers, Ms is the magnetization of the one magnetic layer, h is the Planck's constant, P is a spin asymmetry of current, e is an electric charge, and α is a Gilbert damping constant.

12. The magnetic device according to claim 10 , wherein the nonmagnetic layers are formed from any one selected from the group consisting of copper (Cu), silver (Ag), gold (Au), ruthenium (Ru), iridium (Ir), rhodium (Rh) and an alloy containing at least one of them, and has a thickness of less than 2 nanometers.

13. The magnetic device according to claim 10 , wherein one of the first and the second intermediate layers is made of an insulator or a semiconductor, and other of the first and the second intermediate layers is made of a conductor.

14. A magnetic memory comprising a memory cell in which a plurality of magnetic devices are provided in a matrix configuration with an insulator being interposed between the magnetic devices,

each of the magnetic devices having:

a first ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked, at least one layer of the magnetic layers having magnetization substantially fixed to a first direction, and two or more layers of the magnetic layers being ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane;

a second ferromagnetic layer having magnetization substantially fixed to a second direction;

a third ferromagnetic layer provided between the first ferromagnetic layer and the second ferromagnetic layer, the third ferromagnetic layer having a variable direction of magnetization;

a first intermediate layer provided between the first ferromagnetic layer and the third ferromagnetic layer;

a second intermediate layer provided between the second ferromagnetic layer and the third ferromagnetic layer; and

a couple of electrodes configured to provide write current between the first and second ferromagnetic layers to cause spin-polarized electrons to act on the third ferromagnetic layer so that the direction of magnetization of the third ferromagnetic layer is determined depending on a direction of the current,

the ferromagnetic coupling having a strength such that a parallel magnetic alignment of the magnetic layers is maintained when the write current is passed.

15. The magnetic memory according to claim 14 , wherein each of the magnetic devices on the memory cell is able to be accessed via a probe.

16. The magnetic memory according to claim 14 , wherein

a word line and a bit line are connected to each of the magnetic devices on the memory cell, and

information is able to be recorded to or read out from a particular one of the magnetic devices by selecting the word line and the bit line.

17. A magnetic memory comprising a memory cell in which a plurality of magnetic devices are provided in a matrix configuration with an insulator being interposed between the magnetic devices,

each of the magnetic devices having:

a first ferromagnetic layer having magnetization substantially fixed to a first direction;

a second ferromagnetic layer having magnetization substantially fixed to a second direction;

a third ferromagnetic layer provided between the first ferromagnetic layer and the second ferromagnetic layer in which magnetic layers and one or more nonmagnetic layers are alternately stacked and the magnetic layers are ferromagnetically coupled via the nonmagnetic layers while having easy axes of magnetization parallel to a film plane, the third ferromagnetic layer having a variable direction of magnetization;

a first intermediate layer provided between the first ferromagnetic layer and the third ferromagnetic layer;

a second intermediate layer provided between the second ferromagnetic layer and the third ferromagnetic layer; and

a couple of electrodes configured to provide a current between the first and second ferromagnetic layers to cause spin-polarized electrons to act on the third ferromagnetic layer so that the direction of magnetization of the third ferromagnetic layer is determined depending on a direction of the current,

the ferromagnetic coupling having a strength such that magnetizations of all of the magnetic layers are reversed keeping parallel magnetic direction therebetween when the write current is passed.

18. The magnetic memory according to claim 17 , wherein each of the magnetic devices on the memory cell is able to be accessed via a probe.

19. The magnetic memory according to claim 17 , wherein

a word line and a bit line are connected to each of the magnetic devices on the memory cell, and

information is able to be recorded to or read out from a particular one of the magnetic devices by selecting the word line and the bit line.