Method and apparatus for reading a magnetoresistive memory

• US 6,134,138 A
• Filed: 07/30/1999
• Issued: 10/17/2000
• Est. Priority Date: 07/30/1999
• Status: Expired due to Term

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Lu, Yong, Zhu, Theodore
• Primary Examiner(s): PHAN, TRONG Q
• Application Number:
  US09/365,308
• Time in Patent Office: 445 Days
• Field of Search: 365/171, 365/173, 365/158
• US Class Current: 365/158
• CPC Class Codes: G11C 11/16 using elements in which the storage effect is based on magnetic spin effect
• CPC Class Codes: G11C 11/1675 Writing or programming circuits or methods

Abstract

A method and apparatus for reading a magnetoresistive memory is disclosed wherein the wordline current is turned off during selected sensing operations. This substantially eliminates the noise that is typically injected by the wordline current into the bit structures during the sensing operations, which increases the signal-to-noise ratio on the sense lines. This, in turn, significantly increases the speed of the sensing operations and thus the read access time of the memory. Substantial power savings are also realized.

Claims

1. A method for reading the state of a magnetoresistive bit structure having a soft layer and a hard layer, the method comprising the steps of:

applying a magnetic field to the magnetoresistive bit structure in a first direction to set the state of the soft layer in a first direction;

substantially removing the magnetic field;

sensing a first sensed resistance of the magnetoresistive bit structure;

applying a magnetic field to the magnetoresistive bit structure in an opposite direction to the first direction to set the state of the soft layer in a second opposite direction;

substantially removing the magnetic field; and

sensing if the resistance of the bit structure has increased or decreased relative to the first sensed resistance.

2. A method according to claim 1, wherein said sensing steps include the step of applying a sense current to the bit structure, and detecting a resulting voltage drop.

3. A method according to claim 1, wherein the bit structure is included in a sense line that includes at least one other magnetoresistive bit structure connected in series to form a sense line.

4. A method for sensing the state of a magnetoresistive bit structure of a magnetoresistive memory, the magnetoresistive bit structure being connected in series with one or more other magnetoresistive bit structures to form a sense line, each of the magnetoresistive bit structures having a soft layer and a hard layer with a common easy axis, the method comprising the steps of:

applying a magnetic field to the magnetoresistive bit structure in a first direction parallel to the easy axis to set the state of the soft layer in a first direction;

removing the magnetic field;

sensing a first sensed resistance of the sense line;

applying a magnetic field to the magnetoresistive bit structure in an opposite direction to the first direction to set the state of the soft layer in a second opposite direction;

removing the magnetic field; and

sensing a second sensed resistance of the sense line.

5. A method according to claim 4, further comprising the step of determining if the first sensed resistance is larger or smaller than the second sensed resistance.

6. A method according to claim 4, wherein said applying steps include the step of providing a wordline current through a wordline that extends adjacent the magnetoresistive bit structure.

7. A method according to claim 6, wherein said current providing step provides current in a first direction to apply the magnetic field in the first direction, and provides current in the opposite direction to apply the magnetic field in the second opposite direction.

8. A method according to claim 6, wherein said first applying step includes the step of providing a wordline current through a first wordline that extends adjacent a first side of the magnetoresistive bit structure, and said second applying step includes the step of providing a wordline current through a second wordline that extends adjacent a second opposite side of the magnetoresistive bit structure.

9. A method for reading the state of a magnetoresistive bit structure having a soft layer and a hard layer, the soft layer having an initial state, the method comprising the steps of:

sensing a first sensed resistance of the magnetoresistive bit structure with the soft layer in the initial state;

setting the soft layer of the magnetoresistive bit structure to an opposite state;

sensing if the resistance of the bit structure has increased or decreased relative to the first sensed resistance;

setting the soft layer back to the initial state.

10. A method according to claim 9 wherein said first setting step includes the steps of:

applying a magnetic field to the magnetoresistive bit structure in a first direction to set the state of the soft layer to the opposite state; and

substantially removing the magnetic field.

11. A method according to claim 10 wherein said second setting step includes the steps of:

applying a magnetic field to the magnetoresistive bit structure in a second opposite direction to set the state of the soft layer back to the initial state; and

substantially removing the magnetic field.

12. A magnetic storage device for storing at least one bit of information, the magnetic storage device having at least one magnetoresistive bit structure having a soft layer and a hard layer, comprising:

applying means for applying a magnetic field to the magnetoresistive bit structure in a first direction and a second direction to set the state of the soft layer;

sensing means for sensing a sensed resistance of a sense line that includes the magnetoresistive bit structure; and

control means for controlling said applying means and said sensing means, wherein said control means first enables said applying means to apply a magnetic field to the magnetoresistive bit structure in the first direction to set the state of the soft layer in a first state, said control means then disabling said applying means, which substantially removes the magnetic field, said control means then initiating said sensing means to sense a first sensed resistance of a sense line that includes the magnetoresistive bit structure, said control means then disabling said sensing means, and initiating said applying means to applying a magnetic field to the magnetoresistive bit structure in an opposite direction to the first direction to set the state of the soft layer in a second opposite state;

said control means then disabling said applying means, which substantially removes the magnetic field, said control means then enabling said sensing means to sense if the resistance of the sense line has increased or decreased relative to the first sensed resistance.

13. A magnetic storage device according to claim 12, wherein said applying means comprises a wordline.

14. A magnetic storage device according to claim 13, wherein said applying means comprises a wordline driver for driving a wordline current in a first direction and a second direction through the wordline.

15. A magnetic storage device according to claim 12, wherein said applying means comprises a first wordline located adjacent a first side of said magnetoresistive bit structure and a second wordline located adjacent a second opposite side of said magnetoresistive bit structure.

16. A magnetic storage device according to claim 15, wherein said applying means further comprises one or more wordline drivers for selectively driving a wordline current to either the first wordline or the second wordline.

17. A magnetic storage device according to claim 12, wherein said sensing means comprises a current source.

18. A magnetic storage device according to claim 12, wherein said sensing means comprises a voltage source.

19. In a memory having a number of magnetoresistive bit structures each having a soft layer and a hard layer and arranged in a number of rows, wherein selected magnetoresistive bit structures in each row are connected together to form a corresponding sense line, the memory further having a number of wordlines, wherein each wordline extends adjacent to selected magnetoresistive bit structures, the memory also including one or more wordline drivers for providing a wordline current to each wordline, wherein each of said wordline drivers is capable of providing a wordline current in a first direction and a second opposite direction, the memory further including one or more sensing blocks for sensing a resistance of the sense lines, the improvement comprising:

a controller for controlling selected wordline drivers and selected sensing blocks, said controller enabling a selected wordline driver to provide a wordline current in a first state to set the soft layer of a selected magnetoresistive bit structure to a first state, said controller then disabling said selected wordline driver, which substantially removes the wordline current from the selected wordline, said controller then initiating said sensing block to sense a first sensed resistance of the sense line that includes the selected magnetoresistive bit structure, said controller then disabling said sensing block, and initiating said selected wordline driver to provide a wordline current in the second opposite direction to set the soft layer of the selected magnetoresistive bit structure to a second opposite state, said controller then disabling said selected wordline driver, which substantially removes the wordline current from the selected wordline, said controller then enabling said sensing block to sense if the resistance of the sense line that includes the selected magnetoresistive bit structure has increased or decreased relative to the first sensed resistance.

20. In a memory having a number of magnetoresistive bit structures each having a soft layer and a hard layer and arranged in a number of rows, wherein selected magnetoresistive bit structures in each row are connected together to form a corresponding sense line, the memory further having a number of wordlines, wherein selected pairs of wordlines extends above and below a selected magnetoresistive bit structure, the memory also including one or more wordline drivers for providing a wordline current to each wordline, wherein each of said wordline drivers is capable of alternatively providing a wordline current to the wordlines of a selected pair of wordlines, the memory further including one or more sensing blocks for sensing a resistance of the sense lines, the improvement comprising:

a controller for controlling selected wordline drivers and selected sensing blocks, said controller enabling a selected wordline driver to provide a wordline current in a first one of a selected pair of wordlines to set the soft layer of a selected magnetoresistive bit structure in a first state, said controller then disabling said selected wordline driver, which substantially removes the wordline current from the selected wordline pair, said controller then initiating said sensing block to sense a first sensed resistance of the sense line that includes the selected magnetoresistive bit structure, said controller then disabling said sensing block, and initiating said selected wordline driver to provide a wordline current in the other one of the selected pair of wordlines to set the soft layer of the selected magnetoresistive bit structure in a second opposite state, said controller then disabling said selected wordline driver, which substantially removes the wordline current from the selected wordline pair, said controller then enabling said sensing block to sense if the resistance of the sense line that includes the selected magnetoresistive bit structure has increased or decreased relative to the first sensed resistance.