Magnetic random access memory

1. A magnetic random access memory comprising:

a first magneto resistive element; and

a first current source circuit which gives a first electric bias to the first magneto resistive element when data in the first magneto resistive element is read,

wherein a value of the first electric bias changes depending on temperature without depending on a power supply potential.

2. The magnetic random access memory according to claim 1 , further comprising:

a search circuit which searches for an optimum value of the first electric bias at a specific temperature, wherein the optimum value is a value at which a signal difference of signals respectively obtained from two magneto resistive elements in different states is maximized.

3. The magnetic random access memory according to claim 2 , wherein

the search circuit is constituted of a potential generating circuit which outputs first and second clamp potentials having different values, and a signal difference comparing circuit which compares the signal difference when the first clamp potential is used with the signal difference when the second clamp potential is used, and the values of the first and second clamp potentials are decided on the basis of control signals.

4. The magnetic random access memory according to claim 1 , wherein

the value of the first electric bias is controlled so that a potential difference produced between both ends of the first magneto resistive element will be half of an MR ratio when “0” bias is applied to the first magneto resistive element.

5. The magnetic random access memory according to claim 4 , wherein

the potential difference produced between both ends of the first magneto resistive element changes depending on temperature without depending on the power supply potential.

6. The magnetic random access memory according to claim 1 , further comprising: a BGR circuit which gives a reference current to the first current source circuit, wherein a value of the reference current changes depending on temperature without depending on the power supply potential.

7. The magnetic random access memory according to claim 6 , wherein

temperature dependency of the reference current is decided on the basis of the control signal.

8. The magnetic random access memory according to claim 6 , wherein

temperature dependency of the reference current is different among a plurality of temperature ranges separated by at least one temperature point.

9. The magnetic random access memory according to claim 6 , wherein

temperature dependency of the reference current is such that the value of the reference current decreases with a rise in temperature.

10. The magnetic random access memory according to claim 6 , wherein

temperature dependency of the reference current is such that the value of the reference current increases with the rise in temperature.

11. The magnetic random access memory according to claim 1 , further comprising:

a second magneto resistive element which generates a reference potential; and

a second current source circuit which gives a second electric bias to the second magneto resistive element when data in the first magneto resistive element is read,

wherein a value of the second electric bias changes depending on temperature without depending on the power supply potential.

12. The magnetic random access memory according to claim 11 , wherein

the value of the second electric bias is controlled so that a potential difference produced between both ends of the second magneto resistive element will be half of the MR ratio when “0” bias is applied to the second magneto resistive element.

13. The magnetic random access memory according to claim 12 , wherein

the potential difference produced between both ends of the second magneto resistive element changes depending on temperature without depending on the power supply potential.

14. The magnetic random access memory according to claim 1 , further comprising:

means for limiting the value of the first electric bias to a constant value on the basis of a clamp potential; and

a clamp potential generating circuit which generates the clamp potential,

wherein a value of the clamp potential changes depending on temperature without depending on the power supply potential.

15. The magnetic random access memory according to claim 11 , further comprising:

means for limiting the value of the second electric bias to a constant value on the basis of a clamp potential; and

a clamp potential generating circuit which generates the clamp potential,

wherein a value of the clamp potential changes depending on temperature without depending on the power supply potential.

16. The magnetic random access memory according to claim 1 , further comprising:

a monitor circuit which monitors the first electric bias when temperature dependency of the first electric bias is evaluated.

17. The magnetic random access memory according to claim 1 , further comprising:

a setting circuit which receives the control signal to decide temperature dependency of the first electric bias when temperature dependency of the first electric bias is evaluated.

18. The magnetic random access memory according to claim 17 , wherein

the setting circuit stores tuning information to decide temperature dependency of the first electric bias after the evaluation, and decides temperature dependency of the first electric bias on the basis of the tuning information during an ordinary operation.

19. The magnetic random access memory according to claim 1 , further comprising:

a sense amplifier which, when data in the first magneto resistive element is read, judges a value of the data on the basis of the reference potential; and

a reference potential generating circuit which generates the reference potential, wherein a value of the reference potential changes depending on temperature without depending on the power supply potential.

20. The magnetic random access memory according to claim 6 , wherein

temperature dependency of the reference current is decided on the basis of a control signal which decides a resistance ratio among a plurality of resistive elements constituting the BGR circuit.

21. A memory system comprising:

the magnetic random access memory according to claim 1 ; and

a controller controlling an operation of the magnetic random access memory,

wherein the controller has a search circuit which searches for the optimum value of the electric bias at a specific temperature, and the optimum value is a value at which a signal difference of signals respectively obtained from two magneto resistive elements in different states is maximized.