Magnetic memory and method for optimizing write current in a magnetic memory

Abstract

The invention provides methods and apparatus for for determining and providing optimum write bit line current and write word line current in an MRAM. A single reference potential is used to determine the values of the write line current and the bit line current. In determining the optimal values, asteroid curves representing bit line magnetic fields Hx generated by write bit line current IB and word line magnetic fields Hy generated by write word line current Iw for magnetization are considered, and an asteroid curve ACout is defined outside the asteroid curves of all memory cells taking manufacture variations and design margins into account. A write bit line current and a write word line current are selected such that the write current obtained by adding the write bit line current or currents and the write word line current, or the write power consumed by the bit line or lines and the write word line is minimized. Furthermore, in order to prevent multi-selection, the write bit line current and the write word line current are selected so that they generate a synthetic magnetic field on the curve between calculated points of the asteroid curve ACout.

Description

DETAILED DESCRIPTION OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for optimizing a write current in a magnetic memory device and to a magnetic memory device. More particularly, the present invention relates to methods for optimizing a write current in a magnetic random access memory (hereinafter referred to as MRAM) and to a magnetic memory device.

2. Background Art

Currently, an MRAM is receiving attention as a nonvolatile storage. The MRAM uses magnetic tunneling junction (hereinafter referred to as “MTJ”) device as its magnetic memory element.

FIG. 3 is a sectional view illustrating an exemplary structure of an MRAM memory cell. The memory cell shown in FIG. 3 has an MTJ device 12 and a transistor 50. The transistor 50 is formed on the main surface of a p-type semiconductor substrate 100 typically formed of silicon. On the main surface of the semiconductor substrate 100, n-type diffusion regions 101 and 102 are formed with a predetermined gap provided there between. A read word line RWL is formed between the n-type diffusion regions 101 and 102 on the semiconductor substrate 100. The read word line RWL corresponds to the gate of the transistor 50. Device isolation regions 103 and 104 are formed between the transistor 50 and other adjoining transistors (not shown).

The n-type diffusion region 101 is connected to a metal wire 107 through a contact hole 105. The metal wire 107 is connected to a ground potential node 130. The write word line WWL is formed above the metal wire 107 with an insulating film (not shown) between them. The n-type diffuision region 102 is connected to a metal wire 108 through a contact hole 106. The metal wire 108 is further connected to a metal wire 110 through a contact hole 109. The metal wire 110 is connected to a pad metal 112 through a contact hole 111. The pad metal 112 is a conductor for connecting the MTJ device 12 and the metal wire 110. The MTJ device 12 is formed on the pad metal 112. The MTJ device 12 includes a ferromagnetic free layer 120, an insulating layer 121 and a ferromagnetic pinned layer 122. The pinned layer 122 is designed to have a fixed magnetization direction so that the magnetization can not be reversed. The magnetization direction of the free layer 120 will be identical to or opposite from that of the pinned layer 122 according to data to be stored. A bit line BL is formed on the MTJ device 12.

The read operation of the MRAM memory cell described above will now be explained.

In a read operation, the read word line RWL is selected, and the transistor 50 turned ON. This causes the MTJ device 12 to be connected to a ground potential node Vss. At this time, a sense current passes through the bit line BL. The resistance of the MTJ device 12 is low when the direction of the magnetic field of the free layer 120 is the same as that of the pinned layer 122, while it is high when the direction of the magnetic field thereof is opposite from that of the pinned layer 122. Thus, data stored in a memory cell can be read by detecting the current through the MTJ device 12 or the voltage drop across the MTJ device 12.

The write operation of the MRAM will now be explained.

In a write operation, a write word line current I_w passes through a write word line WWL, and a write bit line current I_B passes through the bit line BL. The read word line RWL is not selected, so that the transistor 50 is OFF.

FIG. 4 illustrates the switching of the magnetization direction of the free layer 120. Referring to FIG. 4, the write bit line current I_B generates a bit line magnetic field in the direction of an easy magnetization axis of the free layer 120. The write word line current I_W generates a word line magnetic field in the direction of a hard magnetization axis of the free layer 120. The word line magnetic field lowers the intensity of the bit line magnetic field required for changing the magnetization direction.

FIG. 5 shows an asteroid curve illustrating a critical magnetic field for switching the magnetization direction. Referring to FIG. 5, the axis of abscissa indicates a bit line magnetic field H_x generated by the write bit line current I_B, while the axis of ordinate indicates a word line magnetic field H_y generated by the write word line current I_W. If a magnetic field H_x+H_y corresponding to the region inside the asteroid curve is generated, then the magnetization direction of the free layer 120 is not reversed, and the write operation is not performed. If a magnetic field H_x+H_y corresponding to the region outside the asteroid curve is generated, then the magnetization direction of the free layer 120 is determined by the magnetic field, and the write operation is performed.

One of the challenges to developing MRAMs is a large current required for generating the magnetic field in the write operation. For instance, the power consumed for reading performed every 10 ns in an MRAM is typically 5 mW. The same MRAM consumes 40 mW for writing under the same condition, spending far more power than in the read operation. In this case, a power source voltage is 2.5 V. Hence, the averaged value of the write current (write bit line current I_B +write word line current I_W) in the write operation is 16 mA. In actual operations, the write current flows for 2.5 ns during a write operation, so that the actual write current is 64 mA. In the write operation, therefore, much power is consumed and noise is generated, due to a write current with a large peak, leading to a possibility of a circuit malfunction.

FIGS. 6 and 7 are functional block diagrams illustrating the constructions related to the write operation of a memory cell array in the MRAM. In the twin cell type MRAM shown in FIG. 6, bit lines BLT and BLC are connected to each write circuit WC. The bit lines BLT and BLC are interconnected outside the memory cell array. The twin cells are disposed at the intersections of the bit lines BLT, BLC and the write word lines WWL. In a one transistor and one MTJ device type MRAM shown in FIG. 7, a bit line BL is connected between a write circuit WC1 and a write circuit WC2 disposed opposite from the write circuit WC1. The memory cells are disposed, corresponding to the intersections of the write word lines WWL and the bit lines BL. The MRAM shown in FIGS. 6 and 7 includes n bit lines per word. In response to a write address signal, one write word line and n bit lines are selected, and data is written to the memory cells located at the intersections of the write word line and the n bit lines. Thus, in the write operation, the write bit line current I_B passes through all the selected n bit lines as well as the write word line current I_w passing through the selected write word line. This means that the consumed power increases as the number of bit lines per word increases, and the probability of occurrence of noise increases accordingly. The write current should be preferably smaller to suppress power consumption and noise; however, an excessively small write current prevents a write operation from being accomplished.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided a method for optimizing write current in a magnetic memory having a plurality of bit lines, a plurality of word lines crossing the bit lines, and a plurality of memory cells disposed at intersections of the bit lines and the word lines, each of the memory cells including a free layer with reversible magnetization and a pinned layer with fixed magnetization, the method including a step for determining a distance r_B from the bit lines to the free layers, a distance r_W from the word lines to the free layers, and a number n of the bit lines through which write bit line current I_B passes in a write operation, and a step for determining the write bit line current I_B and the write word line current I_W so as to minimize the write current I_T by using expression (1) representing an asteroid curve expressed by a bit line magnetic field H_x generated by the bit line current I_B, a word line magnetic field H_y generated by the write word line current I_W passing through the word line in the write operation, and a predetermined constant H_K, expression (2) representing write current I_T obtained by adding the write bit line current I_B and the write word line current I_W, expression (3) representing the bit line magnetic field H_x generated by the bit line current I_B by using a predefined coefficient a, and expression (4) representing the word line magnetic field H_y generated by the word line current I_W by using the predefined coefficient a:[Expression 3]$\begin{array}{cc}{H}_x^{\frac{2}{3}}+H_y^{\frac{2}{3}}=H_k^{\frac{2}{3}}& \left(1\right)\\ I_T=n{I}_B+I_W& \left(2\right)\\ H_x=a\frac{I_B}{r_B}& \left(3\right)\\ H_y=a\frac{I_W}{r_W}& \left(4\right)\end{array}$

According to the method for optimizing write current, the write bit line current I_B and the write word line current I_W are determined so as to generate the magnetic field on the asteroid curve given by expression (1) and to minimize the write current I_T that is given by expression (2). Hence, the magnetization direction of the free layers can be securely determined, that is, the magnetization direction can be switched, if necessary, by a minimum write current I_T. Furthermore, since the write current I_T is minimized, occurrence of noise attributable to a change in the write current I_T can be suppressed.

According to another aspect of the present invention, there is provided a method for optimizing write current in a magnetic memory comprising a plurality of bit lines, a plurality of word lines crossing the bit lines, and a plurality of memory cells disposed at intersections of the bit lines and the word lines, each of the memory cells having a free layer with reversible magnetization and a pinned layer with fixed magnetization, the method including a step for determining a distance r_B from the bit lines to the free layers, a distance r_W from the word lines to the free layers, a number n of the bit lines through which write bit line current I_B passes in a write operation, a parasitic resistance R_B of the bit line, and a parasitic resistance R_W of the word line, and a step for determining the write bit line current I_B and the write word line current I_W so as to minimize write power P_d by using expression (5) representing an asteroid curve expressed by a bit line magnetic field H_x generated by the bit line current I_B, a word line magnetic field H_y generated by the word line current I_W and a predetermined constant H_K, expression (6) representing write power P_d consumed by the word lines and the bit lines by using the write bit line current I_B and the write word line current I_W, expression (7) representing the bit line magnetic field H_x generated by the bit line current I_B by using a predetermined coefficient a, and expression (8) representing the word line magnetic field H_y generated by the word line current I_W by using the coefficient a.[Expression 4]$\begin{array}{cc}{H}_x^{\frac{2}{3}}+H_y^{\frac{2}{3}}=H_k^{\frac{2}{3}}& \left(5\right)\\ P_d=n{I}_B^2{R}_B+I_W^2{R}_W& \left(6\right)\\ H_x=a\frac{I_B}{r_B}& \left(7\right)\\ H_y=a\frac{I_W}{r_W}& \left(8\right)\end{array}$

According to the method for optimizing write current, the write bit line current I_B and the write word line current I_W are determined so as to generate the magnetic field on the asteroid curve given by expression (5) and to minimize the write power P_d that is given by expression (6). Hence, the magnetization direction of the free layers can be securely determined or the magnetization direction can be switched if necessary, by a minimum write power P_d. Furthermore, since the write power P_d is minimized, excessive heat generation caused by write power can be restrained.

Preferably, the constant H_K is given by expression (9) using a predetermined design margin m₁ when the minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current I_B passes when H_y=0 or the minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the word line through which the write word line current I_W passes when H_x=0 is denoted by H_U.H_K=H_U+m₁  (9)

In this case, the asteroid curve given by expression (1) or (5) will be positioned outside a maximum asteroid curve among the asteroid curves that vary from a memory cell to another. Hence, the write bit line current I_B and the write word line current I_W are determined on the maximum asteroid curve, thereby making it possible to securely switch the magnetization direction of the free layers to be switched in any one of selected memory cells.

Preferably, the bit line magnetic field H_x and the word line magnetic field H_y are given by expressions (10) and (11) using predetermined design margins m₂ and m₃, respectively, when the maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when H_y=0 or the maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when H_x=0 is denoted by H_L, where m₂=m₃ or m₂≠m₃.|H_x|≦H_1x=H_L−m₂  (10)|H_y|≦H_2Y=H_L−m₃  (11)

In this case, the bit line magnetic field H_x and the word line magnetic field H_y are restricted by expressions (10) and (11), making it possible to prevent “multi-selection.”

According to yet another aspect of the present invention, there is provided a magnetic memory having a plurality of bit lines, a plurality of word lines crossing the bit lines, a plurality of magnetic memory elements disposed corresponding to intersections of the bit lines and the word lines, a reference potential generating means for generating a predetermined reference potential, a write bit line current controlling means for controlling write bit line current passing through the bit lines in a write operation on the basis of a reference potential generated by the reference potential generating means, and a write word line current controlling means for controlling write word line current passing through the word lines in the write operation on the basis of a reference potential generated by the reference potential generating means.

In this magnetic memory, a common reference potential is supplied to the write bit line current controlling means and the write word line current controlling means. The write bit line current controlling means controls a write bit line current on the basis of the reference potential, while the write word line current controlling means controls a write word line current on the basis of the same reference potential. Therefore, the write bit line current and the write word line current change in synchronization, so that their ratio can be always maintained to be constant.

Preferably, the write bit line current controlling means includes a first transistor having a gate for receiving a reference potential, and passing a write bit line current. The write word line current controlling means includes a second transistor having a gate for receiving a reference potential, and passing a write word line current.

Further preferably, the ratio of the channel width/channel length of the first transistor to the channel width/channel length of the second transistor is set to be substantially equal to the ratio of the write bit line current to the write word line current.

In this case, optimum write bit line current and write word line current can be set by appropriately setting the channel widths and channel lengths of the transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the characteristics of magnetic fields that can switch the magnetization of the free layer of an MTJ device used in MRAMs and the setting of the magnetic fields for designing the MRAM to explain the methods for optimizing the write current in the MRAM according to an embodiment of the present invention;

FIG. 2 is a functional block diagram showing a structure of the MRAM according to an embodiment of the present invention;

FIG. 3 is a sectional view showing an example of a construction of a memory cell of the MRAM (one transistor and one MTJ cell design);

FIG. 4 illustrates the relationship of the easy magnetization axis and the hard magnetization axis to the free layer, and the switching of magnetization direction, of the free layer of the MTJ device shown in FIG. 3;

FIG. 5 illustrates the characteristics of the magnetic fields that can switch the magnetization direction of the free layer of the MTJ device shown in FIG. 3;

FIG. 6 is a functional block diagram showing the configuration of a twin-cell type MRAM; and

FIG. 7 is a functional block diagram showing a configuration of an MRAM using one transistor and one MTJ device type cells.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment according to the present invention will now be explained in detail with reference to the accompanying drawings. In the drawings, like or corresponding components are assigned like reference numerals to avoid repeating the same description.

1. Preparation First, the description will be given of a precondition for optimizing a bit line write current and a word line write current according to the embodiment.

FIG. 1 shows asteroid curves in the embodiment according to the present invention. Referring to FIG. 1, the axis of abscissa indicates the bit line magnetic field H_x generated by a write bit line current, while the axis of ordinate indicates a word line magnetic field H_y generated by a write word line current.

Although the asteroid curve will vary, depending upon the locations of memory cells in an MRAM and variations in manufacturing conditions, it will generally remain within the hatched region shown in FIG. 1. A maximum asteroid curve AC_max constituting the outer edge of the hatched region is given by expression (12) shown below, while a minimum asteroid curve AC_min constituting the inner edge of the hatched region is given by expression (13) shown below.[Expression 5]$\begin{array}{cc}{H}_x^{\frac{2}{3}}+H_y^{\frac{2}{3}}=H_U^{\frac{2}{3}}& \left(12\right)\\ H_x^{\frac{2}{3}}+H_y^{\frac{2}{3}}=H_L^{\frac{2}{3}}& \left(13\right)\end{array}$

H_U in expression (12) denotes the minimum bit line magnetic field that makes it possible to reverse the magnetization of a free layer 120 (refer to FIG. 3) in any one of memory cells associated with selected bit lines BL in the MRAM when H_y=0 or the minimum word line magnetic field that makes it possible to reverse the magnetization of the free layer 120 in any one of memory cells associated with a selected write word line WWL in the MRAM when H_x=0. H_L in expression (13) denotes the maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layer 120 (see FIG. 3) in any one of memory cells associated with selected bit lines BL in the MRAM when H_y=0 or the maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layer 120 in any one of memory cells associated with a selected write word line WWL in the MRAM when H_x=0.

2. Setting design conditions An asteroid curve AC_out is defined with a predetermined design margin m₁ allowed between itself and a maximum asteroid curve AC_max outside the hatched region. In this embodiment, the outermost asteroid curve (hereinafter referred to as “the outer asteroid curve”) AC_out will be used. The outer asteroid curve AC_out is given by expression (14) shown below.[Expression 6]$\begin{array}{cc}{H}_x^{\frac{2}{3}}+H_y^{\frac{2}{3}}=H_k^{\frac{2}{3}}& \left(14\right)\end{array}$

where H_k denotes a predetermined constant and is given by expression (15) shown below when the predetermined design margin m₁ is used:H_k=H_U+m₁  (15)

It is necessary to restrict the bit line magnetic field H_x and the word line magnetic field H_y by expressions (16) and (17), respectively, shown below.|H_x|≦H_1x  (16)|H_y|≦H_2Y  (17)

where H_1x and H_2Y are defined by expressions (18) and (19) shown below by using predetermined design margins m₂ and m₃, respectively.H_1x=H_L−m₂  (18)H_2Y=H_L−m₃  (19)

If a bit line magnetic field H_x that leads to H_x>H_L is generated, then the magnetization direction of the free layer 120 in some memory cells will be changed merely by the bit line magnetic field H_x regardless of the presence of the word line magnetic field H_y. In this case, therefore, the data of memory cells that have not been selected by a write word line WWL will be also rewritten in addition to that of the memory cells selected by the write word line WWL. This is referred to as multi-selection. To prevent the multi-selection, the bit line magnetic field H_x and the word line magnetic field H_y are restricted as shown by expressions (16) and (17), respectively, taking the design margins m₂ and m₃ into account.

Referring back to FIG. 1, a point H₁ on the outer asteroid curve AC_out has its H_x component of H_1x and its H_Y component of H_1Y. Similarly, a point H₂ has its H_x component of H_2x and its H_Y component of H_2Y. Thus, a combination of an optimum write bit line current I_B and an optimum write word line current I_W are selected from among the combinations of the write bit line current I_B and the write word line current I_W for generating synthetic magnetic fields of the bit line magnetic field H_x and the word line magnetic field H_y lying on the curve between the point H₁ and the point H₂ on the outer asteroid curve AC_out.

The bit line magnetic field H_x generated around a bit line BL when the write bit line current I_B is passed through the bit line BL is given by expression (20) shown below.[Expression 7]$\begin{array}{cc}{H}_x=a\frac{I_B}{r_B}& \left(20\right)\end{array}$

where denotes a predefined coefficient, and r_B denotes the distance from the center of the cross-section of the bit line BL to the center of the cross-section of the free layer 120.

Similarly, the word line magnetic field H_Y generated around a write word line WWL when the write word line current I_W is passed through the write word line WWL is given by expression (21) shown below.[Expression 8]$\begin{array}{cc}{H}_y=a\frac{I_W}{r_W}& \left(21\right)\end{array}$

where r_W denotes the distance from the center of the cross-section of the write word line WWL to the center of the cross-section of the free layer 120.

To optimize the write bit line current I_B and the write word line current I_W, expressions (14), (16), (17), (20) and (21) are used to minimize the write current obtained by adding the write word line current I_W passing through a single selected write word line WWL and the write bit line currents I_B passing through a plurality of bit lines BL crossing the selected write word line WWL, or to minimize the power consumed by the write word line current I_W and the write bit line currents I_B.

The outer asteroid curve AC_out is symmetrical with respect to the H_X axis and the H_Y axis; therefore, the minimum write current is calculated using the first quadrant thereof.

A constant k_r is defined by expression (22) shown below:k_r≡r_W/r_B  (22)

where k_r, r_W, r_B≧0

Expression (23) is derived from expression (14) and expressions (20) through (22).[Expression 9]$\begin{array}{cc}{I}_B^{\frac{2}{3}}+{\left(\frac{I_W}{k_r}\right)}^{\frac{2}{3}}={\left(\frac{r_B}{a}\right)}^{\frac{2}{3}}{H}_k^{\frac{2}{3}}=I_{\mathrm{B0}}^{\frac{2}{3}}={\left(\frac{I_{\mathrm{W0}}}{k_r}\right)}^{\frac{2}{3}}& \left(23\right)\end{array}$

I_B0 denotes the write bit line current when I_W=0 and is defined by expression (24) shown below:I_B0=(r_B/a)H_k  (24)

I_W0 denotes the write word line current when I_B=0 and is defined by expression (25) shown below:I_W0=(r_W/a)H_k  (25)

Therefore, expression (26) shown below applies to the relationship between I_B0 and I_W0:I_WO=k_rI_B0  (26)

Meanwhile, from expressions (16) and (17), the bit line magnetic field H_x and the word line magnetic field H_y are subjected to the restrictions given by expressions (27) and (28) shown below:H_2x≦H_x≦H_1x  (27)H_1y≦H_y≦H_2y  (2 8)

If the write bit line current at H₁ in FIG. 1 is denoted as I_B1 and the write word line current at H₁ is denoted as I_W1, while the write word line current at H₂ in FIG. 1 is denoted as I_W2 and the write bit line current at H₂ is denoted as I_B2, then the currents I_B1, I_W1, I_W2 and I_B2 will be defined by expressions (29) through (32), respectively, as shown below:[Expression 10]$\begin{array}{cc}{I}_{\mathrm{B1}}\equiv \frac{r_B}{a}{H}_{1x}& \left(29\right)\\ I_{\mathrm{W1}}\equiv {k_r\left(I_{\mathrm{B0}}^{\frac{2}{3}}-I_{\mathrm{B1}}^{\frac{2}{3}}\right)}^{\frac{2}{3}}=\frac{r_W}{a}{\left(H_k^{\frac{2}{3}}-H_{1x}^{\frac{2}{3}}\right)}^{\frac{2}{3}}=\frac{r_W}{a}{H}_{1y}& \left(30\right)\\ I_{\mathrm{W2}}\equiv \frac{r_W}{a}{H}_{2y}& \left(31\right)\\ I_{\mathrm{B2}}\equiv {\left\{I_{\mathrm{B0}}^{\frac{2}{3}}-{\left(\frac{I_{\mathrm{W2}}}{k_r}\right)}^{\frac{2}{3}}\right\}}^{\frac{2}{3}}=\frac{r_B}{a}{\left(H_k^{\frac{2}{3}}-H_{2y}^{\frac{2}{3}}\right)}^{\frac{2}{3}}=\frac{r_B}{a}{H}_{2x}& \left(32\right)\end{array}$

As is obvious from FIG. 1, relations expressed as 0<I_B2<I_B1<I_B0 and 0<I_W1<I_W2<I_W0 hold.

From expressions (27) through (32), the write bit line current I_B and the write word line current I_W are required to satisfy expressions (33) and (34) shown below:I_B2≦I_B≦I_B1  (33)I_W1≦I_W≦I_W2  (34)

3. Method for Optimization by Minimizing Write Current

The description will now be given of a method for optimizing the write bit line current I_B and the write word line current I_W by minimizing the write current obtained by adding the write bit line currents I_B and the write word line current I_W under the precondition described above.

The write current I_T passing in the write operation is given by expression (35) shown below:I_T=nI_B+I_W  (35)

Accordingly, a combination of the write bit line current I_B and the write word line current I_W that minimizes the write current I_T is selected from among the combinations of the write bit line current I_B and the write word line current I_W that satisfy both expressions (23) and (35). The selected combination indicates optimum write bit line current I_B and the write word line current I_W. Expression (36) shown below is derived from expressions (23) and (35):[Expression 11]$\begin{array}{cc}{I}_T=n{I}_B+{k_r\left(I_{\mathrm{B0}}^{\frac{2}{3}}-I_B^{\frac{2}{3}}\right)}^{\frac{2}{3}}=\frac{n}{k_r}{\left(I_{\mathrm{W0}}^{\frac{2}{3}}-I_W^{\frac{2}{3}}\right)}^{\frac{3}{2}}+I_W& \left(36\right)\end{array}$

Based on expression (36), d²I_T/dI²_B>0 in a region defined by 0<I_B<I_B0; therefore, the write current I_T is a convex function of the bit line current I_B in the above region. Accordingly, the write current I_T takes a local minimum value when dI_T/dI_B=0. Hereinafter, the write bit line current I_B that gives the local minimum value of I_T will be denoted by I_BTmin.

3.1. Case Where I_B2<I_BTmin<I_B1

If I_B2<I_BTmin<I_B1, then I_BTmin is given by expression (37) shown below.[Expression 12]$\begin{array}{cc}{I}_{\mathrm{BT}\min }=\frac{k_r^3}{{\left(n^2+k_r^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{B0}}=\frac{r_W^3}{{\left(n^2{r}_B^2+r_W^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{B0}}=\frac{r_B{r}_W^3}{a{\left(n^2{r}_B^2+r_W^2\right)}^{\frac{3}{2}}}{H}_k& \left(37\right)\end{array}$

From expression (23), the write word line current I_WTmin that gives the local minimum value in this case is given by expression (38) shown below.[Expression 13]$\begin{array}{cc}\begin{array}{c}{I}_{\mathrm{WT}\min }=\frac{n^3{k}_r}{{\left(n^2+k_r^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{B0}}=\frac{n^3{r}_B^2{r}_W}{{\left(n^2{r}_B^2+r_W^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{B0}}\\ =\frac{n^3}{{\left(n^2+k_r^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{W0}}=\frac{n^3{r}_B^3}{{\left(n^2{r}_B^2+r_W^2\right)}^{\frac{3}{2}}}{I}_{\mathrm{W0}}\\ =\frac{n^3{r}_B^3+r_W}{a{\left(n^2{r}_B^2+r_W^2\right)}^{\frac{3}{2}}}{H}_k\end{array}& \left(38\right)\end{array}$

Based on expressions (35), (37) and (38), the minimum write current I_Tmin in this case is given by expression (39) shown below.[Expression 14]$\begin{array}{cc}\begin{array}{c}{I}_{T\min }=\frac{n{k}_r}{\sqrt{n^2+k_r^2}}{I}_{\mathrm{B0}}=\frac{n{r}_W}{\sqrt{n^2{r}_B^2+r_W^2}}{I}_{\mathrm{B0}}\\ =\frac{n}{\sqrt{n^2+k_r^2}}{I}_{\mathrm{W0}}=\frac{n{r}_B}{\sqrt{n^2{r}_B^2+r_W^2}}{I}_{\mathrm{W0}}\\ =\frac{n{r}_B{r}_W}{a\sqrt{n^2{r}_B^2+r_W^2}}{H}_k\end{array}& \left(39\right)\end{array}$

In this case, I_BTmin is determined as the optimum write bit line current, while I_WTmin is determined as the optimum write word line current.

3.2. Case Where I_BTmin>I_B1

If I_BTmin>I_B1, then the write current I_T takes a minimum value in the region wherein the write bit line current I_B is larger than I_B1. Based on expression (33), the write bit line current I_B must be I_B1 or less; therefore, the write bit line current I_B that minimizes the write current I_T is I_B1. Hence, the minimum write current I_Tmin in this case is given by expression (40) shown below.

[Expression 15]I_Tmin=nI_B1+I_W1=1/a(nr_BH_1x+r_WH_1y)  (40)

In this case I_B1 is determined as the optimum write bit line current, while I_W1 is determined as the optimum write word line current.

3.3. Case Where I_BTmin<I_B2

If I_BTmin<I_B2, then the write current I_T takes a minimum value in the region wherein the write bit line current I_B is smaller than I_B2. Based on expression (33), the write bit line current I_B must be I_B2 or more. Therefore, the write bit line current I_B for minimizing the write current I_T in this case is I_B2. Hence, the minimum write current I_Tmin in this case is given by expression (41) shown below:

[Expression 16]I_Tmin=nI_B2+I_W2=1/a(n r_BH_2x+r_WH_2y)  (41)

In this case, I_B2 is determined as the optimum write bit line current, while I_W2 is determined as the optimum write word line current.

Thus, according to this embodiment, if n, r_B and r_W are given, and H_k, H_1x and H_2y are determined, then the optimum write bit line current I_B and optimum word line current I_W for minimizing the write current IT can be determined.

3.4. Example of Calculation

Table 1 shows an example in which the write bit line current and the write word line current are optimized for minimizing write current when n and k_r(=r_W/r_B) take different predetermined values.

TABLE 1
		IBTmin/	IWTmin/	IWTmin/	ITmin/	ITmin/	ITmin/
n	kr	IB0	IB0	IW0	IB0	IW0	IW0
1	1.0	0.354	0.354	0.354	0.707	0.707	—
4	5.0	0.476	1.22	0.244	3.12	0.625	—
8	5.0	0.149	3.05	0.610	4.24	0.848	—
4	10.0	0.800	0.512	0.0512	3.71	0.371	0.385	(*1)
8	10.0	0.476	2.44	0.244	6.25	0.625	—
16	10.0	0.149	6.10	0.610	8.48	0.848	—
32	10.0	0.0265	8.70	0.870	9.54	0.954	1.05	(*2)
8	15.0	0.687	1.56	0.104	7.06	0.471	0.471	(*1)
16	15.0	0.320	5.82	0.388	10.9	0.730	—
32	15.0	0.0765	11.1	0.742	13.6	0.905	0.916	(*2)
8	20.0	0.800	1.02	0.0512	7.43	0.371	0.385	(*1)
16	20.0	0.476	4.88	0.244	12.5	0.625	—
32	20.0	0.149	12.2	0.610	17.0	0.848	—
64	20.0	0.0265	17.4	0.870	19.1	0.954	1.05	(*2)

If r_B takes a fixed value, then I_B0 also takes a fixed value. Thus, I_BTmin, I_WTmin and I_Tmin can be calculated on the basis of comparison with I_B0. Similarly, if r_W takes a fixed value, the I_W0 also takes a fixed value. Thus, I_WTmin and I_Tmin can be calculated on the basis of comparison with I_W0.

Referring to Table 1, I_BTmin/I_B0 is given by expression (37). I_WTmin/I_B0 and I_WTmin/I_W0 are given by expression (38). I_Tmin/I_B0 and I_Tmin/I_W0 are given by expression (39). I_Tmin/I_W0 in the rightmost column is given by expression (40) or (41).

In the rightmost column of Table 1, it is assumed that H_1x=H_2y=0.65×H_k(I_B1=0.65×I_B0, I_B2=0.125×I_B0). In the column, (*1) indicates the value of I_Tmin/I_W0 when I_B=I_B1, because I_BTmin>I_B1 for these rows. Furthermore, (*2) indicates the value of I_Tmin/I_W0 when I_B=I_B2, because I_BTmin<I_B2 for the rows.

4. Optimizing Method by Minimizing Write Power

As another alternative method, the write bit line current I_B and the write word line current I_W may be optimized by minimizing power consumption in write operations (hereinafter referred to as gwrite power h) in place of the above write current I_T.

Write power P_d due to parasitic resistance of the write word line WWL L is given by expression (42) shown below:

[Expression 17]P_d=nI_B²R_B+I_W²R_W  (42)

where R_B denotes a parasitic resistance of the bit line BL, and R_W denotes a parasitic resistance of the write word line WWL.

For the convenience of calculation, k_R is defined as k_R=R_W/R_B, and R_W and P_d of expression (42) is normalized by R_B. The normalized write power P is defined by expression (43) shown below:[Expression 18]$\begin{array}{cc}P\equiv \frac{P_d}{R_B}=n{I}_B^2+k_R{I}_W^2& \left(43\right)\end{array}$

Deleting I_W from expression (43) by using expression (23) leads to expression (44) shown below;[Expression 19]$\begin{array}{cc}\begin{array}{c}P=n{I}_B^2+k_r^2{k_R\left(I_{\mathrm{B0}}^{\frac{2}{3}}-I_B^{\frac{2}{3}}\right)}^3\\ =\left(n-k_r^2{k}_R\right)I_B^2+3k_r^2{k}_R{I}_{\mathrm{B0}}^{\frac{2}{3}}{I}_B^{\frac{4}{3}}-3k_r^2{k}_R{I}_{\mathrm{B0}}^{\frac{4}{3}}{I}_B^{\frac{2}{3}}+\\ k_r^2{k}_R{I}_{\mathrm{B0}}^2\end{array}& \left(44\right)\end{array}$

The value of I_B that gives the local extreme values of P can be obtained by solving dP/dI_B=0. The value of I_B is given by expression (45) when n−k_r²k_R¹0:[Expression 20]$\begin{array}{cc}{I}_B={\left\{\frac{-k_r\left(k_r{k}_R\pm \sqrt{n{k}_R}\right)}{n-k_r^2{k}_R}\right\}}^{\frac{3}{2}}{I}_{\mathrm{B0}}& \left(45\right)\end{array}$

When n−k_r²k_R¹0, P may be regarded as a cubic function of I_B^2/3.I_B^2/3 is a monotone increasing function of I_B in the region of interest. In the vicinity of local extreme values, therefore, it may be said that the behavior of P as the function of I_B is similar to the behavior of P as the function of I_B^2/3.

In the vicinity of the values of I_B given by expression (45), P behaves as a cubic function of I_B^2/3 as shown in expression (44). If n−k_r²k_R<0, then k_rk_R−(nk_R)^1/2>0; therefore, the value I_BPmin that is the smaller value of I_B given by expression (45) is given by expression (46) shown below. This I_BPmin value is a candidate of the bit line current for minimizing the normalized write power P and eventually the write power P_d.[Expression 21]$\begin{array}{cc}\begin{array}{c}{I}_{\mathrm{BP}\min }={\left(\frac{k_r\sqrt{k_R}}{\sqrt{n}+k_r\sqrt{k_R}}\right)}^{\frac{3}{2}}{I}_{\mathrm{B0}}\\ ={\left(\frac{r_W\sqrt{R_W}}{r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}}\right)}^{\frac{3}{2}}{I}_{\mathrm{B0}}\\ ={\left(\frac{r_W\sqrt{R_W}}{r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}}\right)}^{\frac{3}{2}}\frac{r_B}{a}{H}_k\end{array}& \left(46\right)\end{array}$

If n−k_r²k_R>0, then a value that is the larger value of I_B given by expression (45) provides a candidate of the write bit line current for minimizing the write power P_d and is also given by expression (46).

4.1. Case Where I_B2£I_BPmin£I_B1

Because d²P/dI_B²>0 in the region of 0<I_B<I_B0, both P and P_d are convex functions in the region. Hence, if I_B2£I_BPmin£I_B1, then P_d takes a minimum value at I_BPmin given by expression (46). The minimum power consumption P_dmin is given by expression (47) shown below by substituting (46) for I_B in expression (44).[Expression 22]$\begin{array}{cc}\begin{array}{c}{P}_{d\min }=R_B{I}_{\mathrm{B0}}^2\frac{n{k}_r^2{k}_R}{{\left(\sqrt{n}+k_r\sqrt{k_R}\right)}^2}=\frac{n{r}_W^2{R}_B{R}_W}{{\left(r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}\right)}^2}{I}_{\mathrm{B0}}^2\\ =R_W{I}_{\mathrm{W0}}^2\frac{n}{{\left(\sqrt{n}+k_r\sqrt{k_R}\right)}^2}=\frac{n{r}_B^2{R}_B{R}_W}{{\left(r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}\right)}^2}{I}_{\mathrm{W0}}^2\\ =\frac{n{r}_B^2{r}_W^2{R}_B{R}_W}{{a^2\left(r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}\right)}^2}{H}_k^2\end{array}& \left(47\right)\end{array}$

The write word line current I_WPmin for the minimum power consumption P_dmin is given by expression (48) shown below from expressions (23) and (46):[Expression 23]$\begin{array}{cc}\begin{array}{c}{I}_{\mathrm{WP}\min }={k_r\left(\frac{\sqrt{n}}{\sqrt{n}+k_r\sqrt{k_R}}\right)}^{\frac{3}{2}}{I}_{\mathrm{B0}}=\frac{r_W}{r_B}{\left(\frac{r_B\sqrt{n{R}_B}}{r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}}\right)}^{\frac{3}{2}}{I}_{\mathrm{B0}}\\ ={\left(\frac{\sqrt{n}}{\sqrt{n}+k_r\sqrt{k_R}}\right)}^{\frac{3}{2}}{I}_{\mathrm{W0}}={\left(\frac{r_B\sqrt{n{R}_B}}{r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}}\right)}^{\frac{3}{2}}{I}_{\mathrm{W0}}\\ ={\left(\frac{r_B\sqrt{n{R}_B}}{r_B\sqrt{n{R}_B}+r_W\sqrt{R_W}}\right)}^{\frac{3}{2}}\frac{r_W}{a}{H}_k\end{array}& \left(48\right)\end{array}$

If n−k_r²k_R=0, then P in expression (44) reduces to a quadratic function of I_B^2/3. This quadratic function is also a convex function, so that P_d takes a minimum value P_dmin at I_BPmin and I_WPmin. P_dmin, I_BPmin and I_WPmin are given by expressions (49) through (51), respectively, shown below:

In this case, I_BPmin is determined as the optimum write bit line current, while I_WPmin is determined as the optimum write word line current.[Expression 24]$\begin{array}{cc}{P}_{d\min }=\frac{n}{4}{I}_{\mathrm{B0}}^2{R}_B=\frac{1}{4}{I}_{\mathrm{W0}}^2{R}_W& \left(49\right)\\ I_{\mathrm{BP}\min }=\frac{I_{\mathrm{B0}}}{2\sqrt{2}}& \left(50\right)\\ I_{\mathrm{WP}\min }=\frac{I_{\mathrm{W0}}}{2\sqrt{2}}& \left(51\right)\end{array}$

4.2. Case Where I_BPmin>I_b1

Independently of the value of n−k_r²k_R, P_d is a convex function of I_B in the region defined by 0<I_B<I_B0. If I_BTmin>I_B1, then the write power P_d takes a minimum local value P_dmin in the region wherein the write bit line current I_B is larger than I_B1. Based on expression (33), the write bit line current I_B must be I_B1 or less; hence, the write bit line current I_B for minimizing the write power P_d in this case is I_B1. Thus, the minimum write power P_dmin in this case is given by expression (52) shown below:

[Expression 25]P_dmin=nI_B1²R_B+I_W1²R_W  (52)

In this case, I_B1 is determined as the optimum write bit line current, while I_W1 is determined as the optimum write word line current.

4.3. Case Where I_BPmin<I_B2

If I_BPmin<I_B2, then the write power P_d takes a local minimum value P_dmin in the region wherein the write bit line current I_B is smaller than I_B2. Based on expression (33), the write bit line current I_B must be I_B2 or more. In this case, therefore, the write bit line current I_B for minimizing the write power P_d is I_B2. Hence, the minimum write power P_dmin in this case is given by expression (53) shown below:

[Expression 26]P_dmin=nI_B2²R_B+I_W2²R_W  (53)

In this case, I_B2 is determined as the optimum write bit line current, while I_W2 is determined as the optimum write word line current.

4.4. Example of Calculation

Table 2 shows an example in which the write bit line current and the write word line current are optimized for minimizing write power when n, k_R and k_r respectively take different predetermined values.

TABLE 2
n	kR	kr	IBPmin/IB0	IWPmin/IB0	IWPmin/IW0	Pdmin/(IB02RB)	Pdmin/(IW02RW)	Pdmin/(IW02RW)
1	1	1	0.354	0.354	0.354	0.250	0.250	—
128	1	5	0.170	2.89	0.578	12.0	0.481	—
256	1	5	0.116	3.33	0.665	14.5	0.580	0.582	(*2)
8	1	10	0.688	1.04	0.104	4.86	0.0486	0.0494	(*1)
16	1	10	0.604	1.53	0.153	8.16	0.0816	—
64	1	10	0.414	2.96	0.296	19.8	0.198	—
256	1	10	0.239	4.83	0.483	37.9	0.379	—
32	2	15	0.701	1.45	0.0966	19.9	0.0443	0.0456	(*1)
64	2	15	0.619	2.15	0.143	33.7	0.0750	—
256	2	15	0.430	4.23	0.282	83.2	0.185	—
64	1	20	0.604	3.05	0.153	32.7	0.0816	—
128	4	20	0.688	2.07	0.104	77.8	0.0486	0.0494	(*1)
256	4	20	0.604	3.05	0.153	131	0.0816	—

Referring to Table 2, I_BPmin/I_B0 is given by expression (46). I_WPmin/I_B0 and I_WPmin/I_W0 are given by expression (48). P_dmin/(I_B0²R_B) and P_dmin/(I_W0²R_W) are given by expression (47). P_dmin/(I_W0²R_W) in the rightmost column is given by expression (52) or (53).

In the rightmost column of Table 2, it is assumed that H_1x=H_2y=0.65×H_k(I_B1=0.65×I_B0, I_B2=0.125×I_B0), as in Table 1 above. In the column, (*1) indicates the value of P_dmin/(I_W0²R_W) when I_B=I_B1, because I_BTmin>I_B1 for these rows. Furthermore, (*2) indicates the value of P_dmin/(I_W0²R_W) when I_B=I_B2, because I_BTmin<I_B2 for the rows.

Thus, according to the embodiment, optimum write bit line current I_BTmin and write word line current I_WTmin can be determined on the basis of the asteroid curve. More specifically, to suppress the occurrence of noise or to minimize the load on a power circuit, optimum write bit line current I_BTmin and write word line current I_WTmin can be determined so as to minimize the write current I_T. To restrain heat generation, optimum write bit line current I_BPmin and write word line current I_WPmin can be determined so as to minimize the write power P_d.

5. Write Current Control Circuit

FIG. 2 is a functional block diagram showing the structure of an MRAM according to an embodiment of the present invention. Referring to FIG. 2, MRAM1 includes a memory cell array 2, a row decoder 3, a column decoder 4 and a write current control circuit 5.

A row decoder 3 receives a row address signal input from an outside source and selects a single write or read word line from a plurality of write or read word lines. A column decoder 4 receives a column address signal input from an outside source and selects one or more bit lines from a plurality of bit lines. The write current control circuit 4 controls the write word line current supplied to the word line selected by the row decoder 3 and also controls the write bit line current or currents supplied to the bit line or bit lines selected by the column decoder 4.

The write current control circuit 4 includes a reference potential generating circuit 51, a write bit line current control circuit 52 and a write word line current control circuit 53. The reference potential generating circuit 51 includes a P-channel MOS transistor 54 and a constant-current source 55. The P-channel MOS transistor 54 and the constant-current source 55 are connected in series between a power source potential (VDD) node 56 and a ground potential node 57, the P-channel MOS transistor 54 being diode-connected. The reference potential generating circuit 51 generates a reference potential Vref and supplies the reference potential Vref to both the write bit line current control circuit 52 and the write word line current control circuit 53.

The write bit line current control circuit 52 has a plurality of P-channel MOS transistors Tr1 through Trn (n being a natural number). The sources of the P-channel MOS transistors Tr1 through Trn are connected to the power source potential node 56, and the drains thereof are connected to bit line current supply source lines BLCS1 through BLCSn, respectively. The reference potential Vref is commonly supplied from the reference potential generating circuit 51 to the gates of the P-channel MOS transistors Tr1 through Trn.

The write word line current control circuit 53 has a P-channel MOS transistor 531. The source of the P-channel MOS transistor 531 is connected to the power source potential node 56, and the drain thereof is connected to a write word line current supply source line WLCS. The reference potential Vref is supplied from the reference potential generating circuit 51 to the gate of the P-channel MOS transistor 531.

Thus, the write bit line current control circuit 52 controls the write bit line currents according to the reference potential Vref, while the write word line current control circuit 53 controls the write word line current according to the same reference potential Vref. Thus, as the reference potential Vref increases, the write bit line current and the write word line current both decrease, whereas the write bit line current and the write word line current both increase as the reference potential Vref decreases. This means that the write bit line current and the write word line current change in the same direction and in a mutually interlocked manner.

Here, the channel width/channel length (W/L) of the P-channel MOS transistors Tr1 through Trn in the write bit line current control circuit 52 and that of the P-channel MOS transistor 531 in the write word line current control circuit 53 are determined as described below.

The optimum write bit line current I_B and the optimum write word line current I_W that pass in the write operations are determined according to the write current optimizing method described before. Hence, the bit line write current control circuit 52 must supply the optimum write bit line current I_B to each bit line and also supply the optimum write word line current I_W to the selected word line. The same reference potential Vref is supplied to the write bit line current control circuit 52 and the write word line current control circuit 53. Thus, the W/L values are set such that those of the P-channel MOS transistors Tr1 through Trn are different from that of the P-channel MOS transistor 531, thereby supplying optimum write bit line current I_B and the write word line current I_W.

More specifically, the W/L ratio of the P-channel MOS transistors Tr1 through Trn to the P-channel MOS transistor 531 is set to be substantially equal to the ratio of the optimum write bit line current I_B to the optimum write word line current I_W. In Table 1, for example, if n=8 and k_r=10.0, then I_BTmin/I_B0=0.476 and I_WTmin/I_B0=2.44. Hence, the W/L of the P-channel MOS transistors Tr1 through Trn and the W/L of the P-channel MOS transistor 531 are set such that the aforesaid W/L ratio is 0.476/2.44.

As described above, the write current control circuit 5 according to the embodiment controls the write bit line current and the write word line current on the basis of the same reference potential Vref, permitting the ratio thereof to remain constant.

The reference potential Vref can be adjusted by adjusting the current value of the constant-current source 55. This makes it possible to set the absolute values of the write bit line current and the write word line current at appropriate values.

While an embodiment of the present invention has been described, the aforesaid embodiment is merely an example for embodying the invention. It is to be understood, therefore, that the invention is not limited to the disclosed embodiment. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention.

Claims

1. A magnetic memory comprising: a plurality of bit lines; a plurality of word lines crossing the bit lines; a plurality of magnetic memory elements disposed at intersections of the bit lines and the word lines, each of the memory cells comprising a free layer with reversible magnetization and a pinned layer with fixed magnetization; a reference potential generating circuit configured to generate a predetermined reference potential; a write bit line current controlling circuit configured to control write bit line current passing through the bit lines in a write operation on the basis of a reference potential generated by the reference potential generating means; and a write word line current controlling circuit configured to control write word line current passing through the word lines in the write operation on the basis of a reference potential generated by the reference potential generating circuit.

2. The magnetic memory of claim 1, wherein the write bit line current controlling circuit includes a first transistor having a gate for receiving the reference potential to pass the write bit line current, and the write word line current controlling circuit includes a second transistor having a gate for receiving the reference potential to pass the write word line current.

3. The magnetic memory of claim 2, wherein a ratio of a channel width/channel length of the first transistor to a channel width/channel length of the second transistor is set to be substantially equal to a ratio of the write bit line current to the write word line current.

4. The magnetic memory of claim 1, wherein the write bit line current is optimized.

5. The magnetic memory of claim 1, wherein the write bit line current is optimized by: determining a distance rB from the bit lines to the free layers, a distance rW from the word lines to the free layers, and a number n of the bit lines through which write bit line current IB passes in a write operation; and determining the write bit line current IB and the write word line current IW so as to minimize the write current IT by using expression (1) representing an asteroid curve expressed by a bit line magnetic field Hx generated by the bit line current IB, a word line magnetic field Hy generated by the write word line current IW passing through the word lines in the write operation, and a predetermined constant HK, expression (2) representing write current IT obtained by adding the write bit line current IB and the write word line current IW, expression (3) representing the bit line magnetic field Hx generated by the bit line current IB by using a predetermined coefficient a, and expression (4) representing the word line magnetic field Hy generated by the word line current IW by using the predetermined coefficient a: Hx23+Hy23=Hk23(1)IT=n⁢ ⁢IB+IW(2)Hx=a⁢IBrB(3)Hy=a⁢IWrW.(4)

6. The magnetic memory of claim 5, wherein the constant HK is given by expression (5) using a predetermined design margin m1 when a minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current IB passes when Hy=0 or a minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with a word line through which the write word line current IW passes when Hx=0 is denoted as HU: HK=HU+m1  (5).

7. The magnetic memory of claim 6, wherein the constant HK is given by expression (5) using a predetermined design margin m1 when a minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current IB passes when Hy=0 or a minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with a word line through which the write word line current IW passes when Hx=0 is denoted as HU: HK=HU+m1  (5).

8. The magnetic memory of claim 7, wherein the bit line magnetic field Hx and the word line magnetic field Hy are given by expressions (6) and (7) using predetermined design margins m2 and m3, respectively, when a maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hy=0 or a maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hx=0 is denoted as HL where:

9. A method for providing an optimized write current in a magnetic memory, the method comprising: providing a plurality of bit lines; providing a plurality of word lines crossing the bit lines; providing a plurality of magnetic memory elements disposed at intersections of the bit lines and the word lines, each of the magnetic memory elements comprising a free layer with reversible magnetization and a pinned layer with fixed magnetization; providing a reference potential generating circuit configured to generate a predetermined reference potential; providing a write bit line current controlling circuit configured to control write bit line current passing through the bit lines in a write operation on the basis of a reference potential generated by the reference potential generating means; and providing a write word line current controlling circuit configured to control write word line current passing through the word lines in the write operation on the basis of a reference potential generated by the reference potential generating circuit.

10. The method of claim 9, wherein the write word line current is determined by: determining a distance rB from the bit lines to the free layers, a distance rW from the word lines to the free layers, and a number n of the bit lines through which write bit line current IB passes in a write operation; and determining the write bit line current IB and the write word line current IW so as to minimize the write current IT by using expression (1) representing an asteroid curve expressed by a bit line magnetic field Hx generated by the bit line current IB, a word line magnetic field Hy generated by the write word line current IW passing through the word lines in the write operation, and a predetermined constant HK, expression (2) representing write current IT obtained by adding the write bit line current IB and the write word line current IW, expression (3) representing the bit line magnetic field Hx generated by the bit line current IB by using a predetermined coefficient a, and expression (4) representing the word line magnetic field Hy generated by the word line current IW by using the predetermined coefficient a: Hx23+Hy23=Hk23(1)IT=n⁢ ⁢IB+IW(2)Hx=a⁢IBrB(3)Hy=a⁢IWrW.(4)

11. The method of claim 10, wherein the constant HK is given by expression (5) using a predetermined design margin m1 when a minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current IB passes when Hy=0 or a minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with a word line through which the write word line current IW passes when Hx=0 is denoted as HU where: HK=HU+m1  (5).

12. The method of claim 9, wherein the bit line magnetic field Hx and the word line magnetic field Hy are given by expressions (6) and (7) using predetermined design margins m2 and m3, respectively, when a maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hy=0 or a maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hx=0 is denoted as HL: |Hx|≦H1x=HL−m2  (6) |Hy|≦H2Y=HL−m3  (7).

13. A method for optimizing write current in a magnetic memory comprising a plurality of bit lines, a plurality of word lines crossing the bit lines, and a plurality of memory cells disposed at intersections of the bit lines and the word lines, each of the memory cells comprising a free layer with reversible magnetization and a pinned layer with fixed magnetization, the method comprising: determining a distance rB from the bit lines to the free layers, a distance rW from the word lines to the free layers, and a number n of the bit lines through which write bit line current IB passes in a write operation; and determining the write bit line current IB and the write word line current IW so as to minimize the write current IT by using expression (1) representing an asteroid curve expressed by a bit line magnetic field Hx generated by the bit line current IB, a word line magnetic field Hy generated by the write word line current IW passing through the word lines in the write operation, and a predetermined constant HK, expression (2) representing write current IT obtained by adding the write bit line current IB and the write word line current IW, expression (3) representing the bit line magnetic field Hx generated by the bit line current IB by using a predetermined coefficient a, and expression (4) representing the word line magnetic field Hy generated by the word line current IW by using the predetermined coefficient a: Hx23+Hy23=Hk23(1)IT=n⁢ ⁢IB+IW(2)Hx=a⁢IBrB(3)Hy=a⁢IWrW.(4)

14. The method of claim 13, wherein the constant HK is given by expression (5) using a predetermined design margin ml when a minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current IB passes when Hy=0 or a minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with a word line through which the write word line current IW passes when Hx=0 is denoted as HU: HK=HU+m1  (5).

15. The method of claim 13, wherein the bit line magnetic field Hx and the word line magnetic field Hy are given by expressions (6) and (7) using predetermined design margins m2 and m3, respectively, when a maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hy=0 or a maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hx=0 is denoted as HL: |Hx|≦H1x=HL−m2  (6)

16. A method of determining a write line current value comprising: Considering asteroid curves representing bit line magnetic fields Hx generated by write bit line current IB and word line magnetic fields Hy generated by write word line current IW for magnetization; defining an asteroid curve ACout outside the asteroid curves of all memory cells taking manufacture variations and design margins into account; and selecting a write bit line current and a write word line current such that the write current obtained by adding the write bit line current and the write word line current is minimized.

17. The method of claim 16, further comprising selecting the write bit line current and the write word line current such that they generate a synthetic magnetic field on the curve between calculated points of the asteroid curve Acout.

18. The method of claim 16, wherein the a magnetic memory comprises a plurality of bit lines, a plurality of word lines crossing the bit lines, and a plurality of memory cells disposed at intersections of the bit lines and the word lines, each of the memory cells comprising a free layer with reversible magnetization and a pinned layer with fixed magnetization, and wherein selecting a write bit line current and a write word line current further comprises: determining a distance rB from the bit lines to the free layers, a distance rW from the word lines to the free layers, and a number n of the bit lines through which write bit line current IB passes in a write operation; and determining the write bit line current IB and the write word line current IW so as to minimize the write current IT by using expression (1) representing an asteroid curve expressed by a bit line magnetic field Hx generated by the bit line current IB, a word line magnetic field Hy generated by the write word line current IW passing through the word lines in the write operation, and a predetermined constant HK, expression (2) representing write current IT obtained by adding the write bit line current IB and the write word line current IW, expression (3) representing the bit line magnetic field Hx generated by the bit line current IB by using a predetermined coefficient a, and expression (4) representing the word line magnetic field Hy generated by the word line current IW by using the predetermined coefficient a: Hx23+Hy23=Hk23(1)IT=n⁢ ⁢IB+IW(2)Hx=a⁢IBrB(3)Hy=a⁢IWrW.(4)

19. The method of claim 18, wherein the constant HK is given by expression (5) using a predetermined design margin m1 when a minimum bit line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with the bit line through which the write bit line current IB passes when Hy=0 or a minimum word line magnetic field that makes it possible to reverse the magnetization of the free layers in any one of the memory cells associated with a word line through which the write word line current IW passes when Hx=0 is denoted as HU: HK=HU+m1  (5).

20. The method of claim 18, wherein the bit line magnetic field Hx and the word line magnetic field Hy are given by expressions (6) and (7) using predetermined design margins m2 and m3, respectively, when a maximum bit line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hy=0 or a maximum word line magnetic field that makes it impossible to reverse the magnetization of the free layers in any one of the memory cells when Hx=0 is denoted as HL: |Hx|≦H1x=HL−m2  (6) |Hy|≦H2Y=HL−m3  (7).