SYSTEM AND METHOD FOR SUPPRESSING LOW FREQUENCY MAGNETIC NOISE IN MAGNETO-RESISTIVE SENSORS

Abstract

A system for suppressing low frequency magnetic noise from magnetoresistive sensors, the system including at least one magneto-resistive sensor including a free magnetic layer having a variable magnetisation, and a system for modifying magnetisation of the free magnetic layer, wherein the system for modifying magnetisation of the free layer is adapted to drive dynamics of the magnetisation of the free magnetic layer.

Claims

1. A system for suppressing low frequency magnetic noise from magnetoresistive sensors, said system comprising: at least one magnetoresistive sensor comprising a free magnetic layer having a variable magnetisation; means for modifying the magnetisation of the free magnetic layer, wherein said means for modifying magnetisation of the free magnetic layer are adapted to drive dynamics of the magnetisation of the free magnetic layer to prevent it from being trapped and reduce low frequency magnetic noise.

2. The system for suppressing low frequency magnetic noise according to claim 1, wherein the magnetisation of the free magnetic layer has a spatially inhomogeneous configuration.

3. The system for suppressing low frequency magnetic noise according to claim 2, wherein the magnetisation of the free magnetic layer is in a vortex configuration.

4. The system for suppressing low frequency magnetic noise according to claim 3, wherein the means for modifying magnetisation of the free magnetic layer are adapted to drive dynamics of the magnetisation of the free layer including moving the vortex.

5. The system for suppressing low frequency magnetic noise according to claim 2, wherein the free magnetic layer includes a stack of free magnetic layers, each free magnetic layer of the stack including a spatially inhomogeneous magnetisation.

6. The system for suppressing low frequency magnetic noise according to claim 5, wherein the free magnetic layers are in direct contact two by two.

7. The system for suppressing low-frequency magnetic noise according to claim 1, wherein the means for modifying magnetisation of the free magnetic layer comprise means for injecting a direct electric current into the magnetoresistive sensor.

8. The system for suppressing low-frequency magnetic noise according to claim 1, wherein the free magnetic layer magnetisation modifying means comprises means for injecting an alternating electric current into the magnetoresistive sensor.

9. The system for suppressing low-frequency magnetic noise according to claim 7, wherein a current density injected into the magnetoresistive sensor is greater than a predetermined critical density, the predetermined critical density being greater than or equal to 6.2.Math.10.sup.10 A/m.sup.2.

10. The system for suppressing low-frequency magnetic noise according to claim 1, wherein the means for modifying magnetisation of the free magnetic layer comprises means for generating an oscillating magnetic field.

11. The system for suppressing low-frequency magnetic noise according to claim 10, wherein the oscillating magnetic field applied is greater than a predetermined critical field.

12. The system for suppressing low-frequency magnetic noise according to claim 1, further comprising means adapted to measure a resistance of the magnetoresistive sensor.

13. A method for suppressing low frequency magnetic noise associated with the measurement of an external magnetic field by a measurement device comprising a magneto-resistive sensor, said magneto-resistive sensor comprising a free magnetic layer having a variable magnetisation, said method comprising: a. placing the free magnetic layer of the magneto-resistive sensor into a predetermined magnetisation state; b. driving, using means for modifying magnetisation of the free magnetic layer, dynamics of the magnetisation of the free magnetic layer to prevent it from being trapped and reduce low frequency magnetic noise.

14. The method according to claim 13, wherein the magnetisation of the free magnetic layer is in a vortex configuration and the driving of the dynamics of the magnetisation of the free magnetic layer includes a step of moving the vortex.

15. The method according to claim 13, further comprising measuring the resistance of the magnetoresistive sensor.

16. The method according to claim 15, wherein the producing of the dynamics of the magnetisation of the free magnetic layer and the measuring of the resistance of the magneto-resistive sensor are performed simultaneously.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0062] Further characteristics and advantages of the invention will become clearer from the description given below, by way of indicating and in no way limiting purposes, with reference to the appended figures, among which:

[0063] FIG. 1A shows one embodiment of the system according to the invention.

[0064] FIG. 1B shows one embodiment of the system according to the invention.

[0065] FIG. 1C shows one embodiment of the system according to the invention.

[0066] FIG. 2 shows one exemplary embodiment of a stack of layers used for making a GMR or TMR type MR sensor.

[0067] FIG. 3 shows the noise response of the system according to the invention.

[0068] FIG. 4 shows the resistance response of the system according to the invention.

[0069] FIG. 5 shows the noise reduction as a function of the strength of the oscillating magnetic field applied in the embodiment represented in FIG. 1C.

[0070] FIG. 6 shows the noise reduction as a function of the frequency of the oscillating magnetic field applied in the case of the embodiment represented in FIG. 10.

[0071] FIG. 7 shows the reduction of magnetic noise as a function of the frequency of the alternating electric current injected in the case of the embodiment represented in FIG. 1B.

[0072] FIG. 8 illustrates the steps of the method for suppressing low frequency magnetic noise according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0073] FIG. 1A shows a first embodiment 111 of the low frequency magnetic noise suppression system according to the invention. The system 111 comprises an MR sensor 101 and means 102 for modifying magnetisation of the free magnetic layer of the MR sensor 101. According to this embodiment, the means 102 comprise means for injecting a direct electric current I.sub.DC into the MR sensor 101. The low frequency magnetic noise suppression system 111 may further comprise means 103 for measuring the resistance of the MR sensor 101.

[0074] Advantageously, in the embodiment 111, the MR sensor 101 is supplied with a direct current I.sub.DC which makes it possible to drive dynamics of the magnetisation in the free magnetic layer of the sensor 101 and thus to reduce or even suppress the noise of magnetic origin. The measurement of this current I.sub.DC or of the voltage across the sensor 101 gives access to the variation of the resistance of the sensor 101 and thus to the magnetic signal to be detected.

[0075] According to one embodiment, the dynamics of the magnetisation of the free layer of the system 111 can be sustained. This mode of operation is also referred to as the self-oscillating regime. Alternatively, the system may operate in a damped or subcritical regime. To achieve the sustained oscillation regime, it is necessary to inject a current density greater than a critical density. The critical current density required to reach the self-oscillation regime is determined by measuring the radio frequency power emitted by the sensor when a DC current is injected.

[0076] FIG. 1B illustrates a second embodiment 112. The low frequency magnetic noise suppression system 112 comprises an MR sensor 101 and means 104 for modifying magnetisation of the free layer of the MR sensor 101. The means 104 are adapted to inject an alternating electric current I.sub.AC into the MR sensor 101. The low frequency magnetic noise suppression system 112 may further comprise means 103 for measuring the resistance of the MR sensor 101.

[0077] Advantageously, in the embodiment 112, the MR sensor 101 is supplied with an alternating electric current I.sub.AC which makes it possible to drive dynamics in the free layer of the sensor 101 and thus to reduce or even suppress the noise of magnetic origin. The measurement of this current I.sub.AC or of the voltage across the sensor 101 makes it possible to measure variation of the resistance of the sensor 101 and thus the intensity of the magnetic field to be detected.

[0078] According to one embodiment, the dynamics of the magnetisation of the free layer of the system 112 can be sustained. This mode of operation is also referred to as the self-oscillating regime. Alternatively, the system may operate in a damped or subcritical regime. To achieve the sustained oscillation regime, it is necessary to inject a current density greater than a critical density.

[0079] FIG. 1C illustrates a third embodiment 113. The low frequency magnetic noise suppression system 113 comprises an MR sensor 101 and means 106 for modifying magnetisation of the free layer of the MR sensor 101. The means 106 are adapted to apply an oscillating magnetic field in proximity to the MR sensor 101. The low frequency magnetic noise suppression system 112 may further comprise means 105 for injecting a DC or AC current into the MR sensor 101.

[0080] Advantageously, the means 105 for injecting an AC or DC current allows measurement of the voltage across the sensor. The variation of the resistance of the sensor 101 thus makes it possible to measure the magnetic field to be detected. The low frequency magnetic noise suppression system 113 may further comprise means 103 for measuring the resistance of the MR sensor 101.

[0081] The means 106 for modifying magnetisation of the free layer of the sensor may comprise coils or a field line close to the sensor which are powered by an electrical current oscillating at radio frequencies or RF.

[0082] According to one embodiment, the dynamics of the free layer magnetisation of the system 113 may be sustained. This mode of operation is also referred to as the self-oscillating regime. Alternatively, the system may operate in a damped or subcritical regime. To achieve the sustained oscillation regime, it is necessary to apply an oscillating magnetic field greater than a critical oscillating field.

[0083] FIG. 2 shows a typical stack 200 of MR sensor consisting of a cover layer 201, a first ferromagnetic layer 202 having a free magnetisation, a non-magnetic layer 203, a second ferromagnetic layer 204 having a fixed magnetisation and a buffer layer 205. The two ferromagnetic layers 202 and 204 are thus separated by a non-magnetic spacer 203, being respectively metallic for a GMR and insulating for a TMR. The layers represented in FIG. 2 may each include a stack of layers, including different materials and thicknesses chosen to achieve the desired function.

[0084] The second ferromagnetic layer 204, referred to as the “reference” layer, has a magnetisation independent of the magnetic field to be detected. The first so-called “free” ferromagnetic layer 202 has a magnetisation that follows the magnetic field to be detected. The buffer layer allows re-growth on the substrate. The protective layer protects the sensor from oxidation especially and allows electrical contacts to be continued. The reference layer, the free layer, the protective layer and the buffer layer may be comprised of one or more layers.

[0085] According to one embodiment, the first free ferromagnetic layer 202 comprises a plurality of free magnetic layers. The free magnetic layers may be spaced apart two by two, for example by means of a non-magnetic layer. Said non-magnetic layer implements for example an indirect coupling between the two adjacent free magnetic layers. According to an alternative, the free magnetic layers may be in contact two by two, so as to improve the measurable magnetoresistance signal across the stack. Said free magnetic layers are in partial or total contact. When the free magnetic layers are in direct contact, the dynamics of the resulting magnetisation is also improved.

[0086] According to one embodiment, the MR sensor is a TMR sensor comprised of a Si/SiO2 type stack for the substrate/buffer layer/PtMn(15)/CoFe.sub.29 (2.5)/Ru (0.85)/CoFeB (1.6)/CoFe_30 (2.5)/MgO (1)/FeB (6)/MgO (1)/cover layer. The numbers in brackets here indicate the layer thicknesses in nm. This stack can be fabricated into a pillar of typically 300 nm diameter connected by metal contacts. The magnetisation of the reference layer PtMn (15)/CoFe.sub.29 (2.5)/Ru (0.86)/CoFeB (1.6)/CoFe.sub.30 (2.5) is locked in the plane of the thin layers and the magnetisation of the free FeB layer (6) is stabilised in a vortex state by virtue of its thickness and the size of the pad.

[0087] This vortex configuration is interesting because it allows a linear response of the sensor as a function of the field over a wide range of magnetic fields to be measured. The size of the pads and the MR stacking allow the field response and linearity range to be controlled. The 1/f and RTN noise in these sensors sharply increases over this linearity range and is of magnetic origin, which limits performance of the sensor over its operating range. Advantageously, the use of the system according to the invention makes it possible to reduce the noise associated with this type of sensor while keeping linearity over a wide range of magnetic fields to be measured.

[0088] FIG. 3 shows the noise response measured on TMR pillars of 350 nm diameter and comprising the stack of layers described in paragraphs [0057-0059]. FIG. 3 shows the rise in magnetic noise in the sensor operating regime, that is around zero field and over the linearity range of the sensor, with this magnetic noise being suppressed at high magnetic field. The graph in FIG. 3 shows the Hooge parameter as a function of the external magnetic field applied in the plane of the thin layers and in the direction parallel to the direction of the magnetisation of the reference layer, that is along the sensitivity axis of the sensor. The Hooge parameter determines the 1/f noise amplitude and is extracted from the noise spectral density measurement. The circles indicate the transition from the state of the sensor with the free layer magnetisation parallel to the reference layer magnetisation P to the state of the sensor with the two anti-parallel magnetisations AP. The squares illustrate the reverse transition from the AP to the P configuration. The chirality and polarisation of the vortex are indicated by the letters P and C respectively. Noise reduction has been verified for all 4 states of the vortex: +P, −P, +C and −C.

[0089] FIG. 4 illustrates the resistance response under the same conditions as those set in FIG. 3. The sensor response is linear over the range of −30 Oe to 80 Oe (where 1 Oersted is equal to 1000/(4π) A□m.sup.−1 in international system units).

[0090] FIG. 5 illustrates the reduction in low frequency magnetic noise by virtue of using the system according to the invention in the configuration of FIG. 1C.

[0091] The graph in FIG. 5 represents the Hooge parameter as a function of the strength of the applied RF magnetic field to drive a vortex dynamics of the free layer magnetisation according to the invention. The dots connected by a solid line represent the Hooge parameter measured in the configuration 113 of FIG. 1C. The dashed line represents the magnetic noise measured without an applied magnetic field, that is when the system according to the invention is not used. FIG. 5 thus shows that the system according to the invention is effective in reducing low frequency magnetic noise.

[0092] FIG. 6 illustrates the Hooge parameter as a function of the frequency of the oscillating magnetic field applied to drive a vortex dynamics of the free layer magnetisation according to the invention. The dots connected by a solid line represent the Hooge parameter measured in the configuration 113 of FIG. 1C. The dashed line represents the magnetic noise measured without an applied magnetic field, that is when the system according to the invention is not used. As in the case of FIG. 5, FIG. 6 thus shows that the system according to the invention is effective in reducing low frequency magnetic noise.

[0093] In the cases of FIGS. 5 and 6, the MR sensor is in the self-oscillating regime with an applied DC current of 8 mA and a perpendicular magnetic field of 4 kOe (where 1 Oersted is equal to 1000/(4π) A□m.sup.−1 in international system units).

[0094] According to one embodiment, an AC line positioned above the free layer is used to apply an oscillating magnetic field parallel to the plane of the disc with an RF current injected into the line 106 of FIG. 1C. It is possible to further reduce the noise, by moving into the self-oscillation regime, allowing the noise value in the parallel state of the sensor magnetisations (the lowest in general) to be approached while keeping the advantage of the linearity of vortex-based TMR sensors.

[0095] FIG. 7 illustrates the Hooge parameter as a function of the frequency of the AC electric current injected into the MR sensor according to the configuration 112 illustrated in FIG. 1B. The measurement was made for an RF current in the sensor having power −25 dBm, corresponding to about 3 μW. As in FIGS. 5 and 6, the dots connected by a solid line correspond to the measured Hooge parameter and show the noise reduction compared to the dashed line, corresponding to the case when the system according to the invention is not used.

[0096] Measurements illustrated in FIGS. 5, 6 and 7 indicate that the system according to the invention allows the reduction of magnetic noise by a factor 3 in the configurations considered here. A gain of a factor 3 is thus directly obtained on the signal to noise ratio. It should be noted that in order to obtain this gain of a factor 3 on the signal to noise ratio without using additional AC excitation, it would be necessary to increase averaging of the measurement by a factor 10.

[0097] Advantageously, the measuring means 103 is configured to measure the variation in sensor resistance by measuring a low frequency component of a signal from the sensor. The signal from the sensor is, for example, an electric voltage or an electric current. The low frequency component of the signal advantageously has a maximum frequency of less than 1 MHz, for example less than 50 kHz.

[0098] It is known to the person skilled in the art to measure the value of an external magnetic field by separating a high frequency component from a signal from a sensor and determining the frequency shift of the high frequency component when the external field is applied or not. The high frequency component of said signal has a frequency greater than 1 MHz, for example in the order of a few gigahertz. This method does not measure the variation in resistance of the sensor, but only the variation in frequency of the high frequency component. This method, known to the person skilled in the art, implements, for example, a polarisation tee to separate the high frequency component from the low frequency component.

[0099] The measuring means 103 according to the invention, on the other hand, makes it possible to measure directly the variation in the resistance of the sensor. It also has the advantage of not resorting to a polarisation tee and is therefore simpler. A low-pass filter can be used to eliminate the high frequency component of the signal from the sensor.

[0100] FIG. 8 illustrates the method PRO for suppressing low frequency magnetic noise associated with the measurement of an external magnetic field by a measurement device comprising a magneto-resistive sensor.

[0101] The method PRO comprises a first step PL comprising placing the free magnetic layer of the magneto-resistive sensor into a predetermined state of magnetisation.

[0102] Advantageously, this step makes it possible to determine the magnetisation state of the free layer of the MR sensor. Placing the magnetisation of the free layer into a well-determined state is essential to be able to effectively drive its dynamics during the implementation of the method according to the invention.

[0103] According to one embodiment, the state of magnetisation of the free layer is a magnetisation in a vortex configuration.

[0104] The method PRO further comprises a step DY of driving dynamics of the magnetisation of the free magnetic layer.

[0105] Advantageously, this step reduces low frequency magnetic noise by preventing the magnetisation of the free magnetic layer from being trapped in defects in the layer.

[0106] When the magnetisation of the free magnetic layer is in a vortex configuration, the dynamics of the free layer may include moving the core of the vortex in the plane of the layer, preventing it from being trapped and reducing low frequency magnetic noise.

[0107] According to one embodiment, the method PRO according to the invention further comprises a step RES of measuring the resistance of the MR sensor. Advantageously, this step makes it possible to measure the external magnetic field.

[0108] According to one embodiment, the step DY of driving dynamics of the free magnetic layer and the step RE of measuring the resistance of the MR sensor are performed simultaneously. In other words, the dynamics of the free layer magnetisation is driven only during the measurement of the external magnetic field.

[0109] Advantageously, this makes it possible to limit power consumption during the implementation of the method according to the invention, since the dynamics of the magnetisation of the free layer is driven only during the operation of measuring the external magnetic field.

[0110] Advantageously, when the AC dynamics of the vortex core is produced by a current injected into the sensor, the same current can be used to read the magnetic response of the sensor without an additional power source.

[0111] The PRO method according to the invention may further comprise a step of determining conditions necessary to drive dynamics of the free layer magnetisation.

[0112] In other words, the PRO method according to the invention may comprise a step of determining properties of the free layer magnetisation modifying means in order to achieve the desired dynamics of the free layer magnetisation.

[0113] For example, the current and/or magnetic field and/or frequency and/or amplitude conditions necessary to drive free layer magnetisation dynamics may be previously measured on the MR sensor using a spectrum analyser.

[0114] Typically, for configurations 111 and 112, current densities of a few 10{circumflex over ( )}7 A/cm.sup.2 (that is in the order of one mA for the sensor sizes considered) are required and induce gyrotropic mode frequencies ranging from 100 MHz to 600-700 MHz typically for standard magnetic materials. Note that for coupled vortex systems, it is possible to substantially increase this frequency range. For the configuration 113, an AC field in the order of 100 MHz should be applied.

[0115] According to one embodiment, the critical current density of the DC current is greater than or equal to 6.2□10.sup.10 A/m.sup.2. Thus, the flow of a 6 mA current over a stack as described with reference to FIG. 2, and having for example a diameter of 350 nm, allows a self-oscillation regime of the magnetisation to be reached and thus an improved noise reduction. The frequency range of the magnetisation dynamics corresponding to this critical current density may be the radio frequency range, for example around 300 MHz, for example between 200 MHz and 400 MHz. In the aforementioned example, the frequency of the magnetisation dynamics may be 240 MHz.