# Patent

## Title
Magnetic random access memory cell with a dual junction for ternary content addressable memory applications

## Abstract

An MRAM cell including a first tunnel barrier layer between a soft ferromagnetic layer having a free magnetization and a first hard ferromagnetic layer having a first storage magnetization; a second tunnel barrier layer between the soft ferromagnetic layer and a second hard ferromagnetic layer having a second storage magnetization; the first storage magnetization being freely orientable at a first high predetermined temperature threshold and the second storage magnetization being freely orientable at a second predetermined high temperature threshold; the first high predetermined temperature threshold being higher than the second predetermined high temperature threshold. The MRAM cell can be used as a ternary content addressable memory (TCAM) and store up to three distinct state levels. The MRAM cell has a reduced size and can be made at low cost.

## Background
FIELD
The present invention concerns a magnetic random access memory (MRAM) cell with a dual magnetic tunnel junction to be used as a ternary content addressable memory.
BACKGROUND
TCAM (ternary content addressable memory) is an important class of memory device widely used for internet networks. These elements work on matching input data with stored address data. One feature of such elements is that they require storing 3 distinct states 1, 0 and “don't care”. Normal implementations of such a device require a very large number of transistors to enable such functions and this leads to extremely large die sizes.
A typical implementation of a static random access memory (SRAM) TCAM cell consists of a ternary storage containing two SRAM cells which combines ten to twelve transistors. It also has comparison logic, which is basically a XNOR gate using four additional pass transistors. Hence very large cells size of fourteen to sixteen transistors, hence a costly device. Conventional TCAM cells are often provided as a two standard SRAM cells with four or more transistors designed to implement the exclusive-OR (EOR) function.
Unlike a RAM chip, which has simple storage cells, each individual memory bit in a fully parallel TCAM has its own associated comparison circuit to detect a match between the stored data bit and the input data bit. TCAM chips are thus considerably smaller in storage capacity than regular memory chips. Additionally, match outputs from each cell in the data word can be combined to yield a complete data word match signal. The associated additional circuitry further increases the physical size of the TCAM chip. Furthermore, CAM and TCAM as it is done today (using SRAM elements) is intrinsically volatile, meaning that the data are lost when the power is turned off. As a result, every comparison circuit needs being active on every clock cycle, resulting in large power dissipation. With a large price tag, high power and intrinsic volatility, TCAM is only used in specialized applications where searching speed cannot be accomplished using a less costly method.
Emerging memory technology and high-speed lookup-intensive applications are demanding ternary content addressable memories with large word sizes, which suffer from lower search speeds due to large cell capacitance.
SUMMARY
The present disclosure concerns a magnetic random access memory (MRAM) cell comprising a soft ferromagnetic layer having a magnetization that can be freely aligned; a first hard ferromagnetic layer having a first storage magnetization; a first tunnel barrier layer comprised between the soft ferromagnetic layer and the a first hard ferromagnetic layer; a second hard ferromagnetic layer having a second storage magnetization; and a second tunnel barrier layer comprised between the soft ferromagnetic layer and the second hard ferromagnetic layer; wherein the first storage magnetization can be freely oriented at a first high predetermined temperature threshold and the second storage magnetization can be freely oriented at a second predetermined high temperature threshold; the first high predetermined temperature threshold being higher than the second predetermined high temperature threshold.
In an embodiment, the magnetic element can further comprise a first antiferromagnetic layer pinning the first storage magnetization below the first high predetermined temperature threshold, and a second antiferromagnetic layer pinning the second storage magnetization below the second predetermined high temperature threshold.
In another embodiment, the first hard ferromagnetic layer can have a first junction resistance-area product and the second hard ferromagnetic layer can have a second junction resistance-area product that is substantially equal to the first junction resistance-area product.
The present disclosure also pertains to a method for writing to the MRAM cell comprising:
heating the magnetic element to a temperature above the first predetermined high temperature threshold;
applying a write magnetic field in a first direction such as to align the first storage magnetization and the second storage magnetization in accordance with the write magnetic field.
In an embodiment, said write magnetic field can be applied in a first direction for storing a first data or in a second direction for storing a second data.
In another embodiment, the method can further comprise cooling the magnetic element (2) below the second predetermined high temperature threshold.
In yet another embodiment, the method can further comprise:
cooling the magnetic element to an intermediate temperature comprised below the first predetermined high temperature threshold and above the second predetermined high temperature threshold;
applying the write magnetic field in a second direction opposed to the first direction, such as to align the second storage magnetization in the second direction in accordance with the write magnetic field for storing a third data; and cooling the magnetic element below the second predetermined high temperature threshold.
Also disclosed is a method for reading the MRAM cell, comprising:
measuring an initial resistance value of the magnetic element with the stored data;
providing a first search data to the sense layer and determining the matching between the first search data and the stored data; and providing a second search data to the sense layer and determining the matching between the second search data and the stored data.
The MRAM cell disclosed herein can be used as a ternary content addressable memory. The MRAM cell can store three distinct state levels “1”, “0” and “X” (don't care) and be used as a matching device thus allowing an implementation as a TCAM cell with a drastically reduced cell size and cost.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which:
FIG. 1 shows a view of a magnetic random access memory (MRAM) cell comprising a first storage layer, a second storage layer and a sense layer according to an embodiment; and
FIGS. 2a to c illustrate the orientation of the magnetization of the first and second storage layer, and of the sense layer.



DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS
In an embodiment illustrated in FIG. 1, a magnetic random access memory (MRAM) cell 1 comprises a magnetic element 2 that is formed from a dual magnetic tunnel junction comprising a first tunnel barrier layer 22 having a first junction resistance-area product RA1 and a first hard ferromagnetic layer, or first storage layer 23, having a first storage magnetization 230. The magnetic element 2 further comprises a second tunnel barrier layer 24 having a second junction resistance-area product RA2 and a second hard ferromagnetic layer, or second storage layer 25 having a second storage magnetization 250. A soft ferromagnetic layer, or sense layer 21, having a sense magnetization 210 that can be freely aligned is comprised between the first and second tunnel barrier layer 22, 24.
In the example of FIG. 1, the magnetic element 2 further comprises a first antiferromagnetic layer 20 and a second antiferromagnetic layer 26. The first antiferromagnetic layer 20 is adapted to exchange couple the first storage layer 23 such that the first storage magnetization 230 can be freely oriented at a first high temperature threshold Tw1, and pinned below this temperature. The second antiferromagnetic layer 26 is adapted to exchange couple the second storage layer 25 such that the second storage magnetization 250 can be freely oriented at a second high temperature threshold Tw2, and pinned below this temperature. In an embodiment, the first predetermined high temperature threshold Tw1 is greater than the second predetermined high temperature threshold Tw2.
In an illustrative example, the first storage layer 23 can be made from a NiFe/CoFeB-based alloy and the first antiferromagnetic layer 20 can be made from an IrMn-based alloy. The second storage layer 25 can be made from a CoFeB/NiFe-based alloy and the second antiferromagnetic layer 26 can be made from a FeMn-based alloy. The sense layer 21 is preferably made from a CoFeB-based alloy.
In an embodiment, the first junction resistance-area product RA1 is substantially equal to the second junction resistance-area product RA2. A first tunnel magneto resistance TMR1 of the first tunnel barrier layer 22, first storage layer 23 and the sense layer 21 will then be substantially the same as a second tunnel magneto resistance TMR2 of the second tunnel barrier layer 24, second storage layer 25 and the sense layer 21. The first and second tunnel barrier layer 22, 24 are preferably made from MgO with where both the first and second junction resistance-area products RA1, RA2 are substantially equal to 20 ohm □m2.
The MRAM cell 1 can further comprises a current line (not represented) in electrical communication with one end of the magnetic element 2 and a selection transistor (also not represented) in electrical communication with the other end of the magnetic element 2.
In an embodiment, the MRAM cell 1 is written using a thermally assisted write operation. More particularly, providing a first write data to the MRAM cell 1 comprises the steps of:
heating the magnetic element 2 to/at a temperature above the first predetermined high temperature threshold Tw1, such as to free the first and second storage magnetizations 230, 250;
applying a write magnetic field in a first direction such as to align both the first storage magnetization 230 and the second storage magnetization 250 in that first direction in accordance with the write magnetic field; and
cooling the magnetic element 2 to a temperature that is below the second predetermined high temperature threshold Tw2 such that the first and second storage magnetization 230, 250 are pinned in the written state by the first and second antiferromagnetic layer 20, 26, respectively.
After cooling and in the absence of the write magnetic field, the sense layer 21 is in the equilibrium state and its sense magnetization 210 is oriented antiparallel to the first and second storage magnetization 230, 250. A first stored data “0” thus corresponds to the magnetic element 2 having a first state level. The orientation of the first storage magnetization 230, the second storage magnetization 250 and of the sense magnetization 210 are illustrated in FIGS. 2a to c. In FIGS. 2a to c the first and second antiferromagnetic layers 20, 26 are not represented. More particularly, FIG. 2a represents the orientation of storage magnetizations 230, 250 and of the sense magnetization 210 for the first stored data “0”.
In another embodiment, a second write data can be provided to the magnetic element 1 by performing the heating step described above. The write magnetic field is then applied a second direction that is opposed to the first direction, such as to align both the first storage magnetization 230 and the second storage magnetization 250 in the second direction. After cooling to a temperature below the second predetermined high temperature threshold Tw2, and in the absence of the write magnetic field (equilibrium), the sense magnetization 210 is oriented antiparallel to the first and second storage magnetization 230, 250. The second stored data “1” thus corresponds to the magnetic element 2 having a second state level (see FIG. 2b).
In another embodiment, providing a third write data is performed by:
heating the magnetic element 2 to/at a temperature above the first predetermined high temperature threshold Tw1, such as to free the first and second storage magnetizations 230, 250;
applying the write magnetic field in the first direction such as to align both the first storage magnetization 230 and the second storage magnetization 250 in the first direction in accordance with the write magnetic field;
cooling the magnetic element 2 to/at an intermediate temperature that is comprised below the first predetermined high temperature threshold Tw1 and above the second predetermined high temperature threshold Tw2 such as to pin the first storage magnetization 230 by the first antiferromagnetic layer 20 such that the second storage magnetization 250 can be freely oriented;
applying the write magnetic field in the second direction such as to align the second storage magnetization 250 in the second direction in accordance with the write magnetic field; and
cooling the magnetic element 2 to a temperature below the second predetermined high temperature threshold Tw2 such that the first and second storage magnetization 230, 250 are pinned in the written state by the first and second antiferromagnetic layer 20, 26, respectively.
In this latter configuration, the sense magnetization 210 can be oriented either in the first or second direction, i.e., parallel or antiparallel to the first and second storage magnetization 230, 250. The third stored data “X” thus corresponds to the magnetic element 2 having a third intermediate state level (see FIG. 2c).
Heating the magnetic element 2 can be performed by passing a heating current (not shown) in the magnetic element 2, via the current line. Cooling the magnetic element 2 to the intermediate temperature threshold can thus be performed by reducing the heating current intensity, and cooling the magnetic element 2 to the low temperature threshold can be achieved by blocking the heating current. Applying the write magnetic field can be performed by passing a field current (not shown) in the current line.
According to an embodiment, a read operation of the written MRAM cell 1 comprises:
measuring an initial resistance value Ro of the magnetic element 2 with the stored data;
providing a first search data “0” to the sense layer 21 and determining the matching between the first search data and the stored data; and providing a second search data “1” to the sense layer 21 and determining the matching between the second search data and the stored data.
Measuring the initial resistance Ro is performed by passing a read current in the magnetic element 2 in the absence of external magnetic field (zero field). Providing the first and second search data “0”, “1” comprises applying the read magnetic field in a first and second direction, respectively, such as to orient the sense magnetization 210 in accordance with the read magnetic field direction. Determining the matching between the first and second search data and the stored data comprises measuring a first read resistance R1 and a second read resistance R2 by passing the read current in the magnetic element 2 when the magnetic field is applied in the first and second direction, respectively.
The read operation disclosed herein is a self-referenced based read operation in the sense that the resistance of the magnetic element 2 is measured for the first and second search data “0”, “1” (first and second read resistance) and the use of the reference cell is not required. Such self-referenced based read operation has also been disclosed in European patent application EP2276034 by the present applicant. Moreover, the read operation disclosed herein comprises determining the matching between the first and second search data and the stored data, instead of simply reading a stored value “0” or “1” as in a conventional read operation.
In the case of the first stored data “0” the initial resistance Ro has a high value (Rmax1+Rmax2) that is determined by a first high resistance (Rmax1) due to the antiparallel orientation of the sense magnetization 210 with the first storage magnetization 230 and a second high resistance (Rmax2) due to the antiparallel orientation of the sense magnetization 210 with the second storage magnetization 250 (see Table I). Providing the first search data “0” orients the sense magnetization 210 antiparallel to the first and second storage magnetization 230, 250, and the measured value of the first read resistance R1 is high (Rmax1+Rmax2). Providing the second search data “1” orients the sense magnetization 210 parallel to the first and second storage magnetization 230, 250, and the measured value of the second read resistance R2 is low (Rmin1+Rmin2), where Rmin1 is a first low resistance due to the parallel orientation of the sense magnetization 210 with the first storage magnetization 230, and Rmin2 is a second low resistance due to the parallel orientation of the sense magnetization 210 with the second storage magnetization 230. Here, the difference between R2 and R1 corresponds to □R=(Rmax1+Rmax2) (Rmin1+Rmin2).
In the case of the second stored data “1” the initial resistance Ro has a high value (Rmax1+Rmax2). Providing the first search data “0” orients the sense magnetization 210 parallel to the first and second storage magnetization 230, 250 and the measured value of the first read resistance R1 is low (Rmin1+Rmin2). Providing the second search data “1” orients the sense magnetization 210 antiparallel to the first and second storage magnetization 230, 250 and the measured value of the second read resistance R2 is high (Rmax1+Rmax2).
In the case of the third stored data “X” the initial resistance Ro has an intermediate value (Rmin1+Rmax2). Providing the first search data “0” orients the sense magnetization 210 parallel to the first storage magnetization 230 and antiparallel to the and second storage magnetization 250. Providing the second search data “1” orients the sense magnetization 210 antiparallel to the first storage magnetization 230 and parallel to the second storage magnetization 250. The measured value for both the first and second read resistance R1, R2 corresponds to an intermediate value (Rmin1+Rmax2). The same value of the first and second read resistance R1, R2 for the two search data “0” and “1” corresponds to a “don't care” state level of the MRAM cell 1, since the output (read resistance) is insensitive to the input state (search data). Table 1 reports the different resistance values measured for the initial resistance Ro and first and second read resistance R1, R2.











TABLE 1







stored data “0”
stored data “1”
stored data “X”



















Ro (zero field)
Rmax1 + Rmax2
Rmax1 + Rmax2
Rmin1 + Rmax2


R1 (search data “0”)
Rmax1 + Rmax2
Rmin1 + Rmin2
Rmin1 + Rmax2


R2 (search data “1”)
Rmin1 + Rmin2
Rmax1 + Rmax2
Rmin1 + Rmax2









The MRAM cell 1 disclosed herein can thus be used as a ternary content addressable memory. The MRAM cell 1 can store three distinct state levels “1”, “0” and “X” (don't care) and be used as a matching device thus allowing an implementation as a TCAM cell with a drastically reduced cell size and cost. Table 2 reports the matching between the first and second search data and the stored data. More particularly, determining the matching further comprises comparing the measured first and second read resistance (R1, R2) to the initial resistance value (Ro). A match corresponds to no change between the initial resistance value Ro and the value of the first and second read resistance R1, R2.










TABLE 2






stored data “0”
stored data “1”
stored data “X”







search data “X”
No change 
No change (match)
No change


(zero field)
(match)

(match)


search data “0”
No change 
R2 decreases by ΔR
No change



(match)

(match)


search data “1”
R2 decreases
No change (match)
No change



by ΔR

(match)









An additional advantage of the MRAM cell 1 is that, during the write operation, both the first and second tunnel barrier layer 22, 24 contribute in heating the magnetic element 2 at the first and second predetermined high temperature threshold Tw1, Tw2. Consequently, the power required when passing the heating current for heating the magnetic element 2 can be reduced by a factor of about √2 as compared to the power required in a conventional magnetic tunnel junction comprising a single tunnel barrier layer. This results in an enhanced endurance of the MRAM cell 1 to voltage cycling during the write operation.
In addition, the MRAM cell 1 disclosed herein allows for comparing a “don't-care” search data. This can be handled by a second read at zero field. Clearly this leads to no resistance change and thus results in a match irrespective or the written state (search data “X”).
REFERENCE NUMBERS AND SYMBOLS


    1 MRAM cell
    2 magnetic element
    20 first antiferromagnetic layer
    21 sense layer
    22 first tunnel barrier layer
    23 first storage layer
    24 second tunnel barrier
    25 second storage layer
    26 second antiferromagnetic layer
    210 sense magnetization
    230 first storage magnetization
    250 second storage magnetization
    ΔR difference between R1 and R2 
    RA1 first junction resistance-area product
    RA2 second junction resistance-area product
    Rmax1 first high resistance value
    Rmax2 second high resistance value
    Rmin1 first low resistance value
    Rmin2 second low resistance value
    Ro initial resistance
    R1 first read resistance
    R2 second read resistance
    TMR1 first tunnel magneto resistance
    TMR2 second tunnel magneto resistance
    Tw1 first predetermined high temperature threshold
    Tw2 second predetermined high temperature threshold

## Claims

1. A method for writing up to three distinct write data to a MRAM cell including a magnetic element comprising a soft ferromagnetic layer having a magnetization that can be freely aligned; a first hard ferromagnetic layer having a first storage magnetization; a first tunnel barrier layer comprised between the soft ferromagnetic layer and the first hard ferromagnetic layer; a second hard ferromagnetic layer having a second storage magnetization; and a second tunnel barrier layer comprised between the soft ferromagnetic layer and the second hard ferromagnetic layer; the first storage magnetization being be freely orientable at a first high predetermined temperature threshold and the second storage magnetization being freely orientable at a second predetermined high temperature threshold; the first high predetermined temperature threshold being higher than the second predetermined high temperature threshold; the method comprising:
heating the magnetic element to a temperature above the first predetermined high temperature threshold; and
applying a write magnetic field such as to align the first storage magnetization and the second storage magnetization in accordance with the write magnetic field.

2. Method according to claim 1, wherein said write magnetic field is applied in a first direction for storing a first data or in a second direction for storing a second data.

3. Method according to claim 2, further comprising the step of cooling the magnetic element below the second predetermined high temperature threshold.

4. Method according to claim 1, wherein said write magnetic field is applied in a first direction; said method further comprising the steps of:
cooling the magnetic element to an intermediate temperature comprised below the first predetermined high temperature threshold and above the second predetermined high temperature threshold;
applying the write magnetic field in a second direction opposed to the first direction, such as to align the second storage magnetization in the second direction in accordance with the write magnetic field for storing a third data; and
cooling the magnetic element below the second predetermined high temperature threshold.

5. A method for writing up to three distinct write data to a MRAM cell including a magnetic element comprising a soft ferromagnetic layer having a magnetization that can be freely aligned; a first hard ferromagnetic layer having a first storage magnetization; a first tunnel barrier layer comprised between the soft ferromagnetic layer and the first hard ferromagnetic layer; a second hard ferromagnetic layer having a second storage magnetization; and a second tunnel barrier layer comprised between the soft ferromagnetic layer and the second hard ferromagnetic layer; the first storage magnetization being be freely orientable at a first high predetermined temperature threshold and the second storage magnetization being freely orientable at a second predetermined high temperature threshold; the first high predetermined temperature threshold being higher than the second predetermined high temperature threshold; the method comprising:
heating the magnetic element to a temperature above the first predetermined high temperature threshold;
applying a write magnetic field such as to align the first storage magnetization and the second storage magnetization in accordance with the write magnetic field; and
reading the MRAM cell including the steps of:
measuring an initial resistance value of the magnetic element with the stored data,
providing a first search data to the sense layer and determining the matching between the first search data and the stored data, and
providing a second search data to the sense layer and determining the matching between the second search data and the stored data.

6. The method according to claim 5, wherein the step of measuring the initial resistance comprises the step of passing a read current in the magnetic element in the absence of external magnetic field.

7. The method according to claim 5, wherein the step of providing the first search data comprises the steps of: applying the read magnetic field in a first direction and providing the second search data comprises applying the read magnetic field in a second direction; such as to orient the sense magnetization in the first and second direction, respectively.

8. The method according to claim 5, wherein the step of determining the matching between the first and second search data and the stored data comprises the steps of: measuring a first read resistance and measuring a second read resistance by passing the read current in the magnetic element with the magnetic field being applied in the first and second direction, respectively.

9. The method according to claim 5, wherein the step of determining the matching further comprises the step of comparing the measured first and second read resistance to the initial resistance value.