Magnetic memory device

• US 20070258281A1
• Filed: 11/30/2006
• Published: 11/08/2007
• Est. Priority Date: 05/04/2006
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Takemura, Riichiro, Takahashi, Hiromasa, Ito, Kenchi, Kawahara, Takayuki
• Granted Patent: US 7,443,718 B2
• Time in Patent Office: Days
• US Class Current: 365/158
• CPC Class Codes: G11C 11/15 using multiple magnetic layers G11C11/155 takes precedence

Abstract

A magnetic memory device comprises a magnetic tunnel junction (MTJ) having a ferromagnetic free layer, and exhibits a first, relatively high resistance state, and a second, relatively low resistance state. To write to the magnetic memory device a current I_MTJ is driven through the MTJ. For a first duration, the current is equal to a DC threshold current, being the DC current required to switch the multilayer structure between the first state and the second state. This induces a C-like domain structure in the free layer. For a second duration, the current I_MTJ is larger than the DC threshold current. This causes the MTJ to switch states. The current requited to cause switching is less than that required using a uniform current pulse.

Claims

1. A method of writing to a magnetic memory device comprising first and second leads, and a magnetoresistive multilayer structure arranged between said leads, said multilayer structure exhibiting a first, relatively high resistance state and a second, relatively low resistance state, the multilayer structure being switchable from the first state to the second state in response to a pulse of given duration and magnitude, the pulse magnitude being a current threshold magnitude which is a minimum current magnitude needed to switch the multilayer structure and which is dependent upon the duration of the pulse, the method comprising:

passing a current through the multilayer structure of first magnitude less than said threshold current magnitude, and

increasing the current through the multilayer structure thereby passing a current of second, higher magnitude which is less than said threshold current magnitude.

2. A method according to claim 1, wherein passing the current through the multilayer structure comprises:

passing the current through the multilayer structure of first magnitude less than said threshold current magnitude for a duration less than the given duration.

3. A method according to claim 1, wherein passing the current through the multilayer structure comprises:

passing the current through the multilayer structure of first magnitude between 0.1 and 0.5 of said current threshold magnitude.

4. A method according to claim 1, wherein passing the current through the multilayer structure of first magnitude comprises:

passing the current through the multilayer structure for between 0.1 and 0.4 of the given duration.

5. A method according to claim 4, wherein the given duration is less than 10 ns.

6. A method according to claim 1, wherein passing the current through the multilayer structure of first magnitude comprises:

passing the current through the multilayer structure at a fixed level for the duration.

7. A method according to claim 1, wherein passing the current through the multilayer structure comprises:

passing the current through the multilayer structure of first magnitude less than or equal to a DC threshold current, being the minimum DC current needed to switch the multilayer structure.

8. A method according to claim 1, wherein passing the current through the multilayer structure comprises:

passing a current through the multilayer structure of first magnitude less than said threshold current magnitude for a duration between 1 ns and 2 ns.

9. A method according to claim 1, wherein increasing the current comprises:

increasing the current to a second magnitude between 0.3 and 0.5 of said threshold magnitude.

10. A method according to claim 1, wherein increasing the current comprises:

increasing the current to a predetermined level and holding the current at that level for a period less than the given duration.

11. A method according to claim 1, wherein increasing the current comprises:

increasing the current to a predetermined level and holding the current at that level for a period between 0.1 and 0.4 of the given duration.

12. A method according to claim 11, wherein the given duration is less than 10 ns.

13. A method according to claim 1, wherein increasing the current comprises:

increasing the current to a predetermined level an holding the current at a fixed level.

14. A method according to claim 1, further comprising:

ramping current from a base current level to a predetermined current level.

15. A method according to claim 1, further comprising:

ramping current from a zero current level to a predetermined current level.

16. A method according to claim 1, further comprising:

ramping current up from the predetermined current level to the level of the current of a second, higher magnitude.

17. A method according to claim 1, further comprising:

ramping current down from the level of the current of a second, higher magnitude to a lower level.

18. A method according to claim 14, further comprising:

ramping current down from the level of the current of a second, higher magnitude to the base level.

19. A method according to claim 1, wherein the current threshold magnitude I needed to switch the multilayer structure is:

I=I_c0(1+C·

t_P<sup>−

1</sup>)

where I_c0 is a DC threshold current being the minimum DC current needed to switch the multilayer structure, C is a constant, and t_P is the duration of the pulse.

20. A memory comprising:

a magnetic memory device comprising;

first and second leads;

a magnetoresistive multilayer structure arranged between said leads, said multilayer structure exhibiting a first, relatively high resistance state and a second, relatively low resistance state, the multilayer structure being switchable from the first state to the second state in response to a pulse of given duration and magnitude, the pulse magnitude being a current threshold magnitude which is a minimum current magnitude needed to switch the multilayer structure and which is dependent upon the duration of the pulse; and

circuitry for controlling the magnetic memory device configured to perform a method according to claim 1.