1. Field of the Invention
The present invention relates to a readout circuit of a magnetic memory.
2. Related Background Art
An MRAM (Magnetic Random Access Memory) has a structure in which a TMR (Tunnel Magnetoresistance) element is located at each of intersections between bit lines and word lines routed in a lattice-shaped pattern. An ordinary TMR element is comprised of a three-layer structure of ferromagnetic layer/nonmagnetic insulating layer/ferromagnetic layer having a nonmagnetic layer between two ferromagnetic layers. The ferromagnetic layers are normally made of a magnetic element of a transition metal (Fe, Co, or Ni) or an alloy of magnetic elements of transition metals (CoFe, CoFeNi, NiFe, or the like) in the thickness of not more than 10 nm, and the nonmagnetic insulating layer is made of Al2O3, MgO, or the like.
One ferromagnetic layer (fixed layer) forming the TMR element is a layer in which an orientation of magnetization is fixed, and the other ferromagnetic layer (magnetosensitive layer or free layer) is a layer in which an orientation of magnetization rotates according to an external magnetic field. A popularly used structure for the fixed layer is an exchange coupling type in which one ferromagnetic layer is given an antiferromagnetic layer (FeMn, IrMn, PtMn, No, or the like).
Memory information “1” or “0” is defined according to a state of orientations of magnetizations in the two ferromagnets forming the TMR element, i.e., depending upon whether the directions of magnetizations are parallel or antiparallel. When the orientations of magnetizations in these two ferromagnets are antiparallel, the value of electric resistance in the thickness direction is larger than when the orientations of magnetizations are parallel.
Therefore, the information “1” or “0” is read out by applying an electric current in the thickness direction of the TMR element and measuring a resistance value or a voltage value of the TMR element by the MR (magnetoresistance) effect.
A conventionally proposed method for writing the information “1” or “0” is to rotate the orientation of magnetization in the magnetosensitive layer of the TMR element by action of a magnetic field established when an electric current is applied to wires located near the TMR element, and in recent years, a writing method by spin injection is also becoming popular.
Patent Document 1 (Published Japanese Translation of PCT Application No. 2002-533863) discloses the readout circuit of the magnetic memory having a plurality of magnetic memory cells arranged on intersecting points between bit lines and word lines, and describes that a magnetoresistance of a target cell is compared with a magnetoresistance of a reference cell to enable output of the result.
Patent Document 2 (Japanese Patent Application Laid-Open No. 2001-266567) describes the readout circuit of the magnetic memory which utilizes one switch (transistor and diode) and two tunnel junctions to realize a differential method and realize excellent S/N ratios.
Patent Document 3 (Japanese Patent Application Laid-Open No. 2001-325791) describes the readout circuit of the magnetic memory in which potentials given to the word lines are kept equal, so as to prevent inflow of an electric current from an unintended path.
In the above-described conventional technologies, the information “0” or “1” upon readout is determined by comparing a detected value from each magnetoresistive element with a reference value.