Patent Description:
<FIG> shows a conventional magnetoresistive element <NUM> comprising a reference layer <NUM> having a reference magnetization <NUM>, comprises a tunnel barrier layer <NUM> and a sense layer <NUM> having a sense magnetization <NUM>. In <FIG>, the reference layer <NUM> comprises a synthetic antiferromagnetic (SAF) structure including a first reference sublayer <NUM> separated from a second reference sublayer <NUM> by a first non-magnetic spacer layer <NUM> such that the first reference sublayer <NUM> is antiferromagnetically coupled to the second reference sublayer <NUM>.

Sensor applications of the magnetoresistive element <NUM> require that the reference magnetization <NUM> is fixed such that it is not orientable by an external magnetic field to be measured. To that end, the reference magnetization <NUM> is pinned by a pinning layer <NUM>, such as an antiferromagnetic layer, by exchange coupling. In contrast, the sense magnetization <NUM> is free such that it can be aligned by the external magnetic field to be sensed.

Moreover, in order to obtain a good sensitivity to the external magnetic field to be measured, the sense magnetization <NUM> is saturated. However, the saturated sense magnetization <NUM> induces a local magnetic stray field, shown by numeral <NUM> in <FIG>, coupling with the reference layer <NUM> in a closed magnetic flux configuration. The magnitude of the local magnetic stray field <NUM> can reach values up to <NUM> Oe at the edges of the magnetoresistive element <NUM>.

The pinning layer <NUM> usually contains a certain amount of thermally unstable grains which can be switched upon application of the external magnetic field. The stray field <NUM> can locally disturb the exchange coupling of the pinning layer <NUM>, resulting in a hysteresis in the response of the magnetoresistive element <NUM> when the angle of the external magnetic field is varied. <FIG> reports simulated response of a 2D sensor comprising a magnetoresistive element <NUM> such as the one of <FIG>, when the external magnetic field is rotated clockwise (positive angles) and counterclockwise (negative angles). A hysteresis is visible between the clockwise and counterclockwise rotation.

A possible solution destined to minimize such hysteresis comprises enhancing the exchange coupling between the pinning layer <NUM> and the reference layer <NUM>. Alternatively, the reference layer <NUM> can comprises ferromagnetic materials having higher exchange stiffness, for example Co-rich alloys. However, there are only very limited alloys that have high exchange stiffness and that are compatible with the tunnel magnetoresistive technology. Another possible solution can include decreasing the thickness of the sense layer <NUM> such as to reduce the stray field <NUM>. However, this is detrimental to the signal-to-noise ratio of the magnetoresistive element <NUM>. Other solutions can include optimizing the growth of the pinning layer <NUM> and using a larger magnetoresistive element <NUM> such that the relative contribution of the edges of the magnetoresistive element <NUM> to reduce the response signal. The two latter solutions are not satisfactory.

<CIT> discloses a laminated structure composed of sandwiching a tunnel barrier layer between magnetic pinned layers each having multilayer structure and magnetic free layers each having multilayer structure. The magnetic pinned layer having multilayer structure, the tunnel barrier layer, and the magnetic free layer having multilayer structure are stacked in this order on a substrate.

<CIT> discloses a read sensor that includes an unbalanced SAF free layer structure. The unbalanced SAF free layer structure includes a first magnetic layer having a first magnetic moment value and a second magnetic layer having a second magnetic moment value that is different from the first magnetic moment value. A separation layer is included between the first magnetic layer and the second magnetic layer. The first magnetic layer and the second magnetic layer are antiferromagnetically coupled.

The present invention as defined in appended claim <NUM> concerns a magnetoresistive element comprising a tunnel barrier layer included between a reference layer having a reference magnetization and a sense layer having a sense magnetization. The sense layer comprises a SAF structure including a ferromagnetic first sense sublayer in contact with the tunnel barrier layer and separated from a ferromagnetic second sense sublayer by a first non-magnetic spacer layer such that the first sense sublayer is antiferromagnetically coupled to the second sense sublayer. The sense layer is configured such that a sense magnetic ratio defined as: <MAT> wherein MSFM1 and MSFM2 are the spontaneous magnetizations of, respectively, the first and second sense sublayers and tFM1 and tFM2 are the thicknesses of, respectively, the first and second sense sublayers. The sense magnetic ratio is between <NUM> and <NUM>. The second sense sublayer comprises a spontaneous magnetization that increases with increasing distance from the sense spacer layer.

The present invention further concerns a 2D magnetic sensor comprising a plurality of the magnetoresistive element as described above.

The ratio of the magnetic moment results in a non-null magnetic moment of the sense layer and a net stray field on the level of pinned layer will that is significantly suppressed.

The magnetoresistive element disclosed herein has a reduced hysteresis response when measuring an external magnetic field varying angularly. The magnetoresistive element has improved sensitivity, signal to noise ratio and has better sensor lifetime.

The present invention further concerns a 2D magnetic sensor comprising a plurality of the magnetoresistive element.

Exemplar embodiments of the invention are disclosed in the description and illustrated by the drawings. It is noted that the present invention as defined in appended claim <NUM> encompasses embodiments according to <FIG>.

With reference to <FIG>, a magnetoresistive element <NUM> is shown comprising a reference layer <NUM> having a reference magnetization <NUM> and a sense layer <NUM> having a sense magnetization <NUM>. A tunnel barrier layer <NUM> is included between the reference layer <NUM> and the sense layer <NUM>. The reference layer <NUM> comprises a reference SAF structure including a first reference sublayer <NUM>, a second reference sublayer <NUM> in contact with the tunnel barrier layer <NUM> and a non-magnetic reference spacer layer <NUM> between the first and second reference sublayers <NUM>, <NUM> such that the first reference sublayer <NUM> is antiferromagnetically coupled to the second reference sublayer <NUM>. The sense layer <NUM> comprises a sense SAF structure including a first sense sublayer <NUM> in contact with the tunnel barrier layer <NUM> and separated from a second sense sublayer <NUM> by a non-magnetic first sense spacer layer <NUM> such that the first sense sublayer <NUM> is antiferromagnetically coupled to the second sense sublayer <NUM>.

Preferably, the sense magnetization <NUM> is saturated.

In an embodiment, the magnetic moment of the first sense sublayer <NUM> is smaller than the magnetic moment of the second sense sublayer <NUM>. More particularly, a sense magnetic ratio ΔM, i.e., the ratio of the magnetic moment of the first sense sublayer <NUM> to the magnetic moment of the second sense sublayer <NUM>, can be defined by Equation (<NUM>): <MAT> where MSFM1 corresponds to the spontaneous magnetization of the first sense sublayer <NUM>, tFM1 corresponds to the thickness of the first sense sublayer <NUM>, MSFM2 corresponds to the spontaneous magnetization of the second sense sublayer <NUM> and tFM2 corresponds to the thickness of the second sense sublayer <NUM>.

In a preferred embodiment, the sense magnetic ratio ΔM is between <NUM> and <NUM>.

<FIG> reports the ratio of the SAF stray field HAFM to the FM stray field HFM as a function of the sense magnetic ratio ΔM. Here, the SAF stray field HAFM corresponds to the net stray field <NUM> generated by the sense SAF structure <NUM>, i.e., the stray field resulting from the different sense sublayers <NUM>, <NUM>. The FM stray field HFM corresponds to the stray field generated by the sense layer <NUM> comprising a single ferromagnetic layer or several ferromagnetically coupled ferromagnetic layers. The SAF stray field HAFM was calculated for the first sense sublayer <NUM> having a thickness of <NUM> and for the second sense sublayer <NUM> having a thickness between <NUM> and <NUM>. The FM stray field HFM was calculated for a ferromagnetic layer having a thickness of between <NUM> and <NUM>. <FIG> shows that a sense magnetic ratio ΔM of <NUM> yields a ratio of the SAF stray field HAFM to the FM stray field HFM of <NUM>%.

<FIG> reports the ratio of the SAF stray field HAFM to the FM stray field HFM as a function of the thickness of the second sense sublayer <NUM>. The first sense sublayer <NUM> has a thickness of <NUM>. When the ratio of the SAF stray field HAFM to the FM stray field HFM is null, and when the sense magnetic ratio ΔM is null, the sense layer <NUM> loses its capability to sense the external magnetic field. This corresponds to the second sense sublayer <NUM> having a thickness of <NUM> in the example of <FIG>.

A sense magnetic ratio ΔM between <NUM> and <NUM> provides a good sensitivity of the magnetoresistive element <NUM> to the external magnetic field. Moreover, it reduces the net stray field <NUM> on the reference layer <NUM> such that the response of the magnetoresistive element <NUM> to an angularly varying external magnetic field shows substantially no hysteresis. <FIG> shows the variation of a signal measured by the magnetoresistive element <NUM> (such as a resistance value) as a function of the angle of the external magnetic field being measured. Here, the net stray field <NUM> generated by the sense layer <NUM> has a magnitude of <NUM> Oe. Almost no hysteresis is observed.

The magnetoresistive element <NUM> described herein can have lower magnetic noise and higher tunnel magnetoresistance (TMR) by using thick magnetic layers in the sensing layer <NUM>. The reduction of the net stray field <NUM> generated by the sense layer <NUM> and acting on the pinned reference layer <NUM> can further have enhanced stability to high temperature, improved life-time stability and improved overall performance.

As shown in <FIG> and <FIG>, the ratio of the magnetic moments in the first and second sense sublayers <NUM>, <NUM> can be varied in order to optimize the net stray field <NUM> by selecting the thickness of the first and second sense sublayers <NUM>, <NUM>, by selecting the composition of the first and second sense sublayers <NUM>, <NUM> and/or by other parameters of the sense layer <NUM>.

In one aspect, the first and second sense sublayers <NUM>, <NUM> can comprise a ferromagnetic material such as a ferromagnetic alloy based on any one of Fe, Co, Ni, for example CoFe or NiFe. At least one of the first and second sense sublayers <NUM>, <NUM> can further comprise a non-magnetic element such as B, Ta, Ru or W or a combination of these elements. More particularly, the first sense sublayer <NUM> comprises nonmagnetic elements in order to dilute the ferromagnetic material constituting the first sense sublayer <NUM> and decrease its spontaneous magnetization <NUM>.

In another aspect, the second sense sublayer <NUM> has a greater thickness than the first sense sublayer <NUM>.

In one aspect, the first sense spacer layer <NUM> can comprise a non-magnetic material such as, but not limited to, Ru, W, Mo or Ir or a combination of these elements.

With reference to <FIG>, the magnetoresistive element <NUM> is shown according to another embodiment, wherein the first sense sublayer <NUM> is antiferromagnetically coupled to the second sense sublayer <NUM>, and wherein the second sense sublayer <NUM> comprises a gradient of the sense spontaneous magnetization <NUM>. More particularly the sense spontaneous magnetization <NUM> in the second sense sublayer <NUM> increases with increasing distance from the first sense spacer layer <NUM>.

The net spontaneous magnetization <NUM> of the second sense sublayer <NUM> can be adjusted to compensate the spontaneous magnetization <NUM> of the first sense sublayer <NUM> such as to adjust the sense magnetic ratio ΔM, for example between <NUM> and <NUM>. Here, MSFM2 corresponds to the net spontaneous magnetization of the second sense sublayer <NUM>.

Since the magnitude of the stray field <NUM> decreases as the cube of the distance, the larger sense magnetization <NUM> in the portion of the second sense sublayer <NUM> farthest from the reference layer <NUM> does not contribute significantly to the net stray field <NUM> at the level of the reference layer <NUM>. On the other hand, the larger sense magnetization <NUM> allows for increasing the TMR of the magnetoresistive element <NUM>.

The second sense sublayer <NUM> has a sense spontaneous magnetization <NUM> that is smaller, for example at least two times, than the sense spontaneous magnetization <NUM> of the first sense sublayer <NUM> and, thus, generate a smaller net stray field <NUM> on the reference layer <NUM>. On the level of the reference layer <NUM>, the net stray field <NUM> produced by the second sense sublayer <NUM> is smaller than the one produced by the first sense sublayer <NUM>.

In one aspect, the second sense sublayer <NUM> comprises a gradient of nonmagnetic impurities. More particularly, the second sense sublayer <NUM> comprises nonmagnetic impurities in a concentration that decreases with increasing distance from the first sense spacer layer <NUM>. The increasing content of nonmagnetic impurities dilutes the ferromagnetic material of the second sense sublayer <NUM> towards the first sense spacer layer <NUM>.

<FIG> shows a detail of the second sense sublayer <NUM> according to another aspect. Here, the second sense sublayer <NUM> comprises a plurality of ferromagnetic sense bi-layers 232bl, wherein each sense bi-layer 232bl includes a low spontaneous sense layer <NUM> and a high spontaneous sense layer <NUM>. The high spontaneous sense layer <NUM> has a sense spontaneous magnetization <NUM> higher than the one of the low spontaneous sense layer <NUM>. The thicknesses of the low spontaneous sense layer <NUM> relative to the thicknesses of the high spontaneous sense layer <NUM> decreases with increasing distance from the first sense spacer layer <NUM>.

Any one of, alone or in combination, the thickness of the second sense sublayer <NUM>, the gradient of the sense spontaneous magnetization <NUM> or the arrangement of the sense bi-layers 232bl, can be adjusted in order to obtain the ratio of the magnetic moment of the first sense sublayer <NUM> to the magnetic moment of the second sense sublayer <NUM> between <NUM> and <NUM>, and to decrease the net stray field <NUM> applied on the reference layer <NUM>.

The configuration of the magnetoresistive element <NUM> shown in <FIG> allows for obtaining a low stray field <NUM> and have a sense magnetic ratio ΔM between <NUM> and <NUM> using only a single sense spacer layer <NUM>. In the case the second sense sublayer <NUM> comprises a plurality of ferromagnetic sense bi-layers 232bl, the term MSFM2 corresponds to the net spontaneous magnetization of the plurality of ferromagnetic sense bi-layers 232bl and tFM2 corresponds to the thickness of the second sense sublayer <NUM> comprising the plurality of ferromagnetic sense bi-layers 232bl. <FIG> illustrates the magnetoresistive element <NUM> according to yet another embodiment, wherein the second sense sublayer <NUM> comprises a proximal second sense sublayer 232a and a distal second sense sublayer 232b. The distal second sense sublayer 232b has a sense spontaneous magnetization <NUM> that is at least two times higher than the sense spontaneous magnetization <NUM> of the proximal second sense sublayer 232a. The proximal second sense sublayer 232a is ferromagnetically coupled to the distal second sense sublayer 232b.

Here, the sense magnetic ratio ΔM does not depend on a specific arrangement of the first and second sense layers <NUM>, 232a, 232b but rather on the net magnetic moment of these layers. More particularly, the term MSFM2 tFM2 in Equation (<NUM>) corresponds to MSFM2a tFM2a + MSFM2b tFM2b, where MSFM2a and tFM2a respectively correspond to the spontaneous magnetization and thickness of the proximal second sense sublayer 232a and where MSFM2b and tFM2b, respectively correspond to the spontaneous magnetization and thickness of the distal second sense sublayer 232b.

In one aspect, the lower sense spontaneous magnetization <NUM> of the proximal second sense sublayer 232a relative to the distal second sense sublayer 232b can be obtained by including nonmagnetic impurities in the ferromagnetic proximal second sense sublayer 232a such as to dilute the spontaneous magnetization of the ferromagnetic material. Alternatively or in combination, the relative lower sense spontaneous magnetization <NUM> of the proximal second sense sublayer 232a can be obtained by the distal second sense sublayer 232b having a greater thickness that the thickness of the proximal second sense sublayer 232a.

The sense spontaneous magnetization <NUM> of the proximal second sense sublayer 232a can be adjusted to compensate the stray field generated by the first sense sublayer <NUM> and decrease the net stray field <NUM> at the reference layer <NUM>. Since the magnitude of the stray field <NUM> decreases as the cube of the distance, the stray field generated by the thicker distal second sense sublayer 232b has a negligible contribution in the net stray field <NUM> at the reference layer <NUM>. The larger sense magnetization <NUM> of the distal second sense sublayer 232b allows for increasing the TMR of the magnetoresistive element <NUM>.

<FIG> illustrates a variant of the magnetoresistive element <NUM> shown in <FIG>, wherein the second sense sublayer <NUM> comprises a proximal second sense sublayer 232a separated from a distal second sense sublayer 232b by a non-magnetic second sense spacer layer <NUM>. The second sense spacer layer <NUM> can comprise a non-magnetic material such as, but not limited to, Ru, W, Mo or Ir or a combination of these elements. In this configuration, the distal second sense sublayer 232b is antiferromagnetically coupled to the proximal second sense sublayer 232a.

Similarly to the configuration of the magnetoresistive element <NUM> shown in <FIG>, the thickness of the proximal second sense sublayer 232a can be adjusted to compensate the stray field generated by the first sense sublayer <NUM> and decrease the net stray field <NUM> at the reference layer <NUM>. The stray field generated by the thicker distal second sense sublayer 232b has a negligible contribution in the net stray field <NUM> at the reference layer <NUM>. The larger sense magnetization <NUM> of the distal second sense sublayer 232b allows for increasing the TMR of the magnetoresistive element <NUM>.

In a variant not illustrated, the sequence: "first sense layer <NUM> / first sense spacer layer <NUM> / second sense layer <NUM>" can be repeated a plurality of times forming a multi-layered structure. Such multi-layered structure can have a spontaneous magnetization that is lower than the spontaneous magnetization of the distal second sense layer 232b. The multi-layered structure can be strongly coupled to the distal second sense layer 232b.

The thickness of the first sense layer <NUM> and/or the second sense layer <NUM>, as well as the thickness of the proximal and distal second sense layers 232a, 232b in the magnetoresistive element <NUM> according to the configuration of <FIG> and <FIG>, can be adjusted in order to decrease the stray field <NUM>.

<FIG> shows a detail of the sense layer <NUM> according to an embodiment, wherein the sense layer <NUM> further comprises an intermediate ferromagnetic sense layer <NUM> on each side of the first sense spacer layer <NUM> and in contact with the first sense spacer layer <NUM>. The intermediate ferromagnetic sense layer <NUM> can be a nanolayer, for example have a thickness of about <NUM>. The intermediate ferromagnetic sense layer <NUM> can comprises any one of a Co or CoFe -based alloy. Preferably, the intermediate ferromagnetic sense layer <NUM> has a high spontaneous magnetization. For example, the intermediate ferromagnetic sense layer <NUM> can comprises a CoFe alloy containing <NUM> to <NUM>% wt Fe.

Improved sensing layer structure with two or more antiferromagnetically coupled sublayers. Proper choice of sense layer materials and sense layer thickness provides significant reduction of hysteresis in sensor angular response, improves sensitivity, signal to noise ratio and longevity of sensor lifetime.

Claim 1:
A magnetoresistive element (<NUM>) for a 2D magnetic sensor, the magnetoresistive element (<NUM>) comprising a tunnel barrier layer (<NUM>) included between a reference layer (<NUM>) having a reference magnetization (<NUM>) and a sense layer (<NUM>) having a sense magnetization (<NUM>);
wherein the sense layer (<NUM>) comprises a synthetic antiferromagnetic (SAF) structure including a ferromagnetic first sense sublayer (<NUM>) in contact with the tunnel barrier layer (<NUM>) and separated from a ferromagnetic second sense sublayer (<NUM>) by a first non-magnetic sense spacer layer (<NUM>) such that the first sense sublayer (<NUM>) is antiferromagnetically coupled to the second sense sublayer (<NUM>); and
wherein the sense layer (<NUM>) is configured such that a sense magnetic ratio (ΔM) defined as: <MAT>
wherein MSFM1 and MSFM2 are the spontaneous magnetizations of, respectively, the first and second sense sublayers (<NUM>, <NUM>) and tFM1 and tFM2 are the thicknesses of, respectively, the first and second sense sublayers (<NUM>, <NUM>); and
characterized in that the sense magnetic ratio (ΔM) is between <NUM> and <NUM>; and
in that the second sense sublayer (<NUM>) comprises a spontaneous magnetization (<NUM>) that increases with increasing distance from the first sense spacer layer (<NUM>).