Self-referenced magnetic random access memory (MRAM) and method for writing to the MRAM cell with increased reliability and reduced power consumption

MRAM cell including a magnetic tunnel junction including a sense layer, a storage layer, a tunnel barrier layer and an antiferromagnetic layer exchange-coupling the storage layer such that the storage magnetization can be pinned when the antiferromagnetic layer is below a critical temperature and freely varied when the antiferromagnetic layer is heated at or above the critical temperature. The sense layer is arranged such that the sense magnetization can be switched from a first stable direction to another stable direction opposed to the first direction. The switched sense magnetization generates a sense stray field being large enough for switching the storage magnetization according to the switched sense magnetization, when the magnetic tunnel junction is heated at the writing temperature. The disclosure also relates to a method for writing to the MRAM cell with increased reliability and reduced power consumption.

FIELD

The present disclosure concerns a self-referenced magnetic random access memory (MRAM) cell comprising a sense layer, a storage layer, and a tunnel barrier layer, and a method for writing to the MRAM cell with increased reliability and reduced power consumption by using a stray field induced by the sense layer.

DESCRIPTION OF RELATED ART

A MRAM cell using the so-called self-referenced reading operation typically comprise (seeFIG. 1) a magnetic tunnel junction2formed of a magnetic storage layer23having a first storage magnetization234which direction can be changed from a first stable direction to a second stable direction, a thin insulating layer22, and a sense layer21having a sense magnetization210with a reversible direction. The self-referenced MRAM cell allows for performing the write and read operation with low power consumption and an increased speed. The self-referenced MRAM can further be read by using a dynamic reading operation having improved robustness against variability from one MRAM cell to another. InFIG. 1, the storage layer23is represented as a synthetic storage layer including a first storage layer231and a second storage layer233being separated by a spacer layer232.

Switching of the storage magnetization during a write operation of the self-referenced MRAM cell can be performed by using magnetostatic interactions between the storage and the sense layer. Such switching is often called dipolar-induced switching and allows for using a switching field having lower magnitude than switching field used in non-self-referenced MRAM cells.FIG. 2shows the MRAM cell during the self-referenced writing operation. A magnetic field42is applied such as to switch the sense magnetization210. The writing operation further involves heating the magnetic tunnel junction2at or above the critical temperature of an antiferromagnetic layer (layer24inFIG. 1) at which the storage magnetization234can be freely oriented. Heating is performed while the switching field42is being applied such that the storage magnetization can be oriented in accordance with the switched sense magnetization210and the switching field42. The magnetic tunnel junction is then cooled below the critical temperature such as to pin the storage magnetization in its written state.FIG. 3shows the MRAM cell1after the write operation with the sense magnetization210being switched with the magnetic field42and the storage magnetization234being also switched in its written direction by the magnetic field42and the switched sense magnetization210.

During such write operation however, the storage magnetization may not be fully recovered, i.e., aligned along an easy axis of the storage layer, once the magnetic tunnel junction has been cooled below the critical temperature. Indeed, the storage magnetization may remain orientable due to restoration of the exchange coupling of the antiferromagnetic layer during cooling, while the storage layer is in a non-saturated state, leading to a magnetically frustrated configuration. This can yield reduced reproducibility of the writing operation.

EP2276034 discloses MRAM cell and a method for writing the MRAM cell comprising switching a magnetization direction of said storage layer to write data to said storage layer.

EP2575135 discloses a method for writing and reading an MRAM cell wherein a net local magnetic stray field couples the storage layer with the sense layer.

US2009027948 concerns an MRAM cell including a first magnetic layer arrangement having a magnetization which corresponds to a predefined ground state magnetization, a non-magnetic spacer layer coupled to the first layer arrangement, a second magnetic layer arrangement disposed on the opposite side of the non-magnetic spacer layer with regard to the first magnetic layer arrangement, the second magnetic layer arrangement having a magnetization fixation temperature that is lower than the magnetization fixation temperature of the first magnetic layer arrangement, and at least a portion of the second magnetic layer arrangement having a closed magnetic flux structure in its demagnetized state.

U.S. Pat. No. 5,966,323 discloses a low switching field magnetoresistive tunnelling junction memory cell including a first exchange coupled structure and an exchange interaction layer so as to pin the magnetic vectors of the pair of layers anti-parallel, a second exchange coupled structure having a pair of magnetoresistive layers and an exchange interaction layer so as to pin the magnetic vectors of the pair of layers anti-parallel. Each of the first and second exchange coupled structures, and hence the memory cell has no net magnetic moment.

EP2109111 discloses a method for writing an MRAM cell comprising a current line wherein the current line has a first function for passing a first portion of current for heating the junction, and a second function for passing a second portion of current in order to switch the magnetization of the first magnetic layer.

SUMMARY

The present disclosure concerns a self-referenced MRAM cell comprising a magnetic tunnel junction including a sense layer having a sense magnetization; a storage layer having a storage magnetization; a tunnel barrier layer comprised between the sense and the storage layers; and an antiferromagnetic layer exchange-coupling the storage layer such that the storage magnetization can be pinned when the antiferromagnetic layer is below a critical temperature and freely varied when the antiferromagnetic layer is heated at or above the critical temperature; wherein said sense layer is arranged such that the sense magnetization can be switched from a first stable direction to another stable direction opposed to the first direction; the switched sense magnetization generating a sense stray field being large enough for switching the storage magnetization according to the switched sense magnetization, when the magnetic tunnel junction is heated at the writing temperature.

The sense magnetization can be larger than the net storage magnetization. The sense layer can have a coercive field being higher than a net storage magnetic stray field induced by the storage magnetization. The sense layer can have a magnetic anisotropy. The magnetic anisotropy can comprise at least one of an elliptical shape or a hard magnetic material.

The storage magnetization can comprise a first storage layer, a spacer layer and a second storage layer; the storage magnetization comprising a first storage magnetization of the first storage layer and a second storage magnetization of the second storage layer, the spacer layer magnetically coupling the first storage magnetization antiparallel with the second storage magnetization. The antiferromagnetic layer can exchange-couple the first storage layer. The sense layer can have a thickness being larger than the difference between the thickness of the first storage layer and the thickness of the second storage layer multiplied by the ratio of the storage magnetization to the sense magnetization.

The present disclosure further concerns a method for writing to the MRAM cell comprising:

switching the sense magnetization from a first direction to a second direction opposed to the first direction, the switched sense magnetization generating a local sense stray field; and

passing a heating current pulse in the magnetic tunnel junction for heating the magnetic tunnel junction at or above the critical temperature such as to switch the storage magnetization in accordance with the sense stray field;

wherein the method further comprises the step of turning off the write magnetic field; and wherein applying a heating current pulse is performed after turning off the write magnetic field.

The self-referenced MRAM cell using the method for write disclosed herein allows for better reliability during the writing operation compared to writing a conventional self-referenced MRAM cell due to the writing sequence allowing the storage magnetization to relax in a more stable magnetic configuration.

The self-referenced MRAM cell in combination with the writing method allows for reducing power consumption since the write magnetic field is applied only for switching the sense magnetization and does not need to overcome the intrinsic switching field of the storage layer during switching of the first storage magnetization. Moreover, since the write magnetic field can be small, the same magnetic field can be used for both the write magnetic field and the first and second read magnetic fields.

In the present description, the expression “magnetization” is used indifferently to describe a magnetic moment of the magnetic layer and the magnetization induced by the magnetic moment.

DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS

FIG. 4shows a MRAM cell1arrangement according to an embodiment. The MRAM cell1comprises a magnetic tunnel junction2comprising a sense layer21having a first magnetization210; a storage layer23; a tunnel barrier layer22comprised between the sense and the storage layers21,23; and an antiferromagnetic layer24exchange-coupled with the storage layer23. In the example ofFIG. 4, the storage layer23is a synthetic storage layer including a first storage layer231having a first storage magnetization234, a second storage layer233having a second storage magnetization235, the first and second ferromagnetic layers231,233being separated by a spacer layer232. The first and second storage layers231,233can be made of a material such as, for example, cobalt iron (CoFe), cobalt iron boron (CoFeB), nickel iron (NiFe), Cobalt (Co), etc. The thickness of the first and second storage layer231,233can be comprised, for example, between 1 nm and 10 nm.

The dimensions (e.g., thickness) of the spacer layer232may be selected to cause the first and second storage layers231and233to be magnetically coupled, via RKKY interaction, so that the first storage magnetization234is oriented anti-parallel with the second magnetization235. The thickness may depend on the material that the spacer layer232is formed from. For example, the spacer layer232can be made from a non-magnetic material selected from the group comprising, for example, ruthenium (Ru), rhenium (Re), rhodium (Rh), tellurium (Te), yttrium (Y), chromium (Cr), iridium (Ir), silver (Ag), copper (Cu), etc. In an embodiment, the thickness may be between about 0.2 nm and 3 nm. However, other thicknesses may be suitable to couple the two storage layers231and233.

The exchange-coupling between the antiferromagnetic layer24and the synthetic storage layer23is such that the first storage magnetization234is pinned below a critical temperature of the antiferromagnetic layer24and can be freely oriented at and above the critical temperature. The antiferromagnetic layer24can be made from a manganese-based alloy, such as IrMn, PtMn or FeMn, or any other suitable materials. The sense layer21is not exchange biased.

The tunnel barrier layer22is a thin layer, typically in the nanometer range and can be formed, for example, from any suitable insulating material, such as alumina or magnesium oxide.

In an embodiment represented inFIG. 5, a dipolar-induced method for writing to the MRAM cell1comprises switching the sense magnetization210from a first direction to a second switched direction, opposed to the first direction. InFIG. 4, the first direction if the sense magnetization210is represented towards the left side of the page and the switched second direction is shown inFIG. 5towards the right side of the page. Switching the sense magnetization210can comprise applying a write magnetic field42having a suitable magnitude. The second direction of the switched sense magnetization210is oriented in accordance to the direction of the write magnetic field42. The write magnetic field42is applied prior to heating the magnetic tunnel junction2at a read temperature, for example at room temperature. Thus, during applying the write magnetic field42, the magnetic tunnel junction2is at a temperature being below the critical temperature and the storage magnetization remains pinned by the antiferromagnetic layer24. The switched sense magnetization210generates a local sense stray field shown by the arrow indicated by numeral60inFIG. 5. The sense stray field60induces in turn a magnetic coupling between the sense magnetization210and the first and second storage magnetizations234,235in a closed magnetic flux configuration.

The write magnetic field42can be applied by passing a write current41in an upper current line3in electrical communication with one end of the magnetic tunnel junction2, disposed on top of the magnetic tunnel junction2, in example ofFIG. 2. Alternatively, the write current41can be passed in a field line (not represented) located above the upper current line3or at the other end of the magnetic tunnel junction2.

Once the sense magnetization210has been switched, the write magnetic field42is turned off. After turning off the write magnetic field42, a heating current pulse31is passed in the magnetic tunnel junction2such as to heat the magnetic tunnel junction2at a writing temperature corresponding to the critical temperature or being above the critical temperature of the antiferromagnetic layer24, and thus, free the first storage magnetization234(seeFIG. 6). The heating current pulse31can be passed in the magnetic tunnel junction2via the upper current line3and a lower current line4electrically connected to the other end of the magnetic tunnel junction2.

Once the magnetic tunnel junction2has reached the writing temperature, one of the first and second storage magnetization234,235is switched due to the presence of the sense stray field60. In fact, the sense stray field60switches the larger of the first storage magnetization234and the second storage magnetization235. In the example ofFIG. 6, the first storage magnetization234is larger than the second storage magnetization235and is switched by the sense stray field60, in accordance with the direction of the sense stray field60. Due to the anti-parallel coupling between first and second storage layers231,233, the second storage magnetizations235will also switch in order to remain antiparallel to the first storage magnetizations234(seeFIG. 6). In the example ofFIG. 6, the second storage magnetization235is switched parallel to the sense magnetization210resulting in a low resistance R1of the MRAM cell1. The storage magnetization234,235is thus switched in the absence of the write magnetic field42, i.e., one of the first and second storage magnetizations234,235, depending on their relative magnitude, is switched by the sense stray field60and the other storage magnetization235,234is switched antiparallel in the opposite direction due to the anti-parallel coupling induced by the spacer layer232.

After switching of the first and second storage magnetizations234,235, the heating current pulse31can be turned off such as to cool the magnetic tunnel junction2, for example at the read temperature that is below the critical temperature, such as to pin the first storage magnetization234in the written state.

FIG. 8reports chronograms of the write magnetic field42and of the heating current pulse31according to the method for writing to the MRAM cell1as disclosed herein. In particular, the abscissa represents a time scale and the ordinate the magnitude for the write magnetic field42and a magnitude of the heating current pulse31.FIG. 8shows the onset of the heating current pulse31located after the end of the write magnetic field pulse42. Also reported inFIG. 8are chronograms of the sense magnetization210and of the first storage magnetization234where the ordinate represents the direction of the magnetizations210,234. The chronograms show the sense magnetization210and the first storage magnetization234being switched sequentially upon application of the write magnetic field42and of the heating current pulse31, respectively. In the example ofFIG. 8, the first direction of the sense magnetization210and of the first storage magnetization234is opposed to their switched orientation, as illustrated inFIG. 4andFIG. 5.FIG. 7reports chronograms of the write magnetic field42and of the heating current pulse31in the case of a conventional method for writing a conventional self-referenced MRAM cell (such as the one shown inFIGS. 1 to 3). As shown inFIG. 7, the heating current pulse31is located while the write magnetic field42is applied. Switching of the first storage magnetization234and of the sense magnetization210occur simultaneously under the action of the write magnetic field42.

A dipolar coupling can occur between the storage layer23and the sense layer21. Such dipolar coupling is caused by a first local storage magnetic stray field55induced by the first storage magnetization234and a second local storage magnetic stray field56induced by the second storage magnetization235. The first and second storage magnetic stray fields55,56, are shown inFIG. 5coupling the first and second storage magnetizations234,235with the sense magnetization210of the sense layer21in a closed magnetic flux configuration. The magnitude of the dipolar coupling, or the net storage magnetic stray field57, corresponds to the sum of the first and second storage magnetic stray fields55,56.

Switching the storage magnetization234,235using the sense stray field60requires that the sense stray field60is larger than the net storage magnetic stray field57, when the magnetic tunnel junction2is at the writing temperature. This is achieved by the sense magnetization210being larger than the net magnetization of the storage layer23, the net magnetization of the storage layer corresponding to the sum of the first and second storage magnetization234,235. The larger sense magnetization210can be achieved by using high magnetization materials for the sense layer21, such as materials used in permanent magnets, or by a suitable thickness of the sense layer21, or by a combination of the two former conditions. The suitable thickness of the sense layer21can comprise a thickness being larger than the storage magnetization234,235multiplied by the difference between the thickness of the first storage layer231and the thickness of the second storage layer233, divided by the sense magnetization210.

The dipolar-induced writing method further requires that the sense magnetization210remains stable in the switched orientation after turning off the write magnetic field42. The stability of the switched sense magnetization210can be achieved by the sense layer21having a coercive field being higher than the net storage magnetic stray field57. Such high coercivity of the sense layer21can be achieved by the sense layer21having a magnetic anisotropy, and/or the sense layer21comprising a hard magnetic material. The magnetic anisotropy can comprise shape anisotropy, for example wherein the sense layer21has an elliptical shape, or magnetocrystalline anisotropy. The hard magnetic material can comprise one the materials used in permanent magnets.

The MRAM cell1is not limited to the configuration of the above embodiment as long as the MRAM cell1can be written using the writing method disclosed herein. For example, the storage layer23can comprise only the first storage layer231having the first storage magnetization234being switched by the sense stray field60induced by the sense magnetization210. Here, the net local magnetic stray field corresponds to the sole contribution of the first storage magnetic stray field55and the net storage magnetization corresponds to the first storage magnetization234.

According to an embodiment, a read operation of the MRAM cell1comprises a first read cycle including applying a first read magnetic field52adapted for aligning the sense magnetization210in a first direction, in accordance with the first orientation of the first read magnetic field52. The first read magnetic field52can be applied by passing a first read field current51having a first polarity in the upper current line3. The first direction of the sense magnetization210is then compared with the second storage magnetization235by passing a sense current32though the magnetic tunnel junction2. The voltage measured across the magnetic tunnel junction2yields a corresponding first resistance value R1of the magnetic tunnel junction2(corresponding to the high or low resistance RH, RL).

The read operation of the MRAM-based cell1can further comprise a second read cycle comprising applying a second read magnetic field54adapted for aligning the sense magnetization210in a second direction opposed to the first direction, in accordance with the second orientation of the second read magnetic field54. The second read magnetic field54can be applied by passing a second read field current53having a second polarity in the upper current line3. The second direction of the sense magnetization210is then compared with the second storage magnetization235by passing the sense current32though the magnetic tunnel junction2. Measuring a voltage across the magnetic tunnel junction2when the sense current32is passed through the magnetic tunnel junction2yields a corresponding second resistance value R2of the magnetic tunnel junction2. The data written in the MRAM cell1can then be determined by a difference between the first and second resistance value R1, R2.

In an embodiment, a magnetic memory device (not represented) can comprise a plurality of the MRAM cells1arranged in rows and columns. The magnetic memory device can further comprise one or a plurality of the upper current line3that connect the MRAM cells1along a row, and one or a plurality of the lower current line4coupled to the MRAM cells1along a column. The magnetic memory device can further comprise a device package, the plurality of the MRAM cells1being disposed within the device package.

An advantage of the MRAM cell1and of the method for writing the MRAM cell1includes increased reproducibility of the writing operation since the storage magnetization (i.e., the first and/or second storage magnetization234,235) will spontaneously relax to a lowest energy state without being constraint by an external field, such as the write magnetic field42, which could induce spin-flopping configurations in the storage magnetizations234,235.

The MRAM cell1in combination with the writing method allows for reducing power consumption since the write magnetic field42is applied only for switching the sense magnetization210and does not need to overcome the intrinsic switching field of the storage layer23. Since the write magnetic field42can be small the same magnetic field can be used for both the write magnetic field42and the first and second read magnetic fields52,53. Moreover, an improved reliability of the configuration of the storage magnetization234,235in the written state can be achieved, yielding an improved reliability of the resistance levels of the magnetic tunnel junction2.

REFERENCE NUMBERS AND SYMBOLS