WO2019093964A9

WO2019093964A9 - Magnetoresistance sensor with ac biasing and rectification detection

Info

Publication number: WO2019093964A9
Application number: PCT/SG2018/000007
Authority: WO
Inventors: Yihong Wu; Yanjun Xu; Yumeng Yang
Original assignee: National University Of Singapore
Priority date: 2017-11-13
Filing date: 2018-11-13
Publication date: 2019-07-18
Also published as: WO2019093964A1

Abstract

In one embodiment, a spin-orbit torque (SOT)-biased magnetic sensor is provided that utilizes both anisotropic magnetoresistance (AMR) and spin Hall magneto resistance (SMR) effects. The AMR/SMR sensor includes a sensing element made from a material that itself can produce a transverse bias by generating a SOT effective field in response to a sensing/biasing current. The sensing/biasing current may be a direct current (DC) or, preferably, an alternating current (AC). The use of a SOT effective field for biasing significantly simplifies the sensor structure. The use of AC for the sensing/biasing current significantly reduces noise, hysteresis and substantially eliminates DC offset of an output signal of the magnetic sensor.

Description

MAGNETORESISTANCE SENSOR WITH AC BIASING AND RECTIFICATION DETECTION

BACKGROUND

Technical Field

The present disclosure relates generally to magnetic sensors and more specifically to sensing elements for use in magnetic sensors.

Background Information

A wider variety of types of magnetic sensors have been developed for performing linear, angular position, rotation and other sensing tasks for consumer, industrial and scientific applications. Existing magnetic sensors may be classified into a number of types including fluxgate sensors, fiber-optic sensors, Hall-effect sensors, superconducting quantum interference device (SQUID) sensors, optical pumping sensors,

magnetoresistance (MR) sensors, giant magneto-impcdancc (GMI) sensors, among other types. MR sensors, in particular, have proven attractive for low to moderate field detection due to their high sensitivity, compactness, and ease of integration with other- types of devices.

MR sensors may be further classifies into a variety of sub-types including anisotropic magnetoresistance (AMR) sensors, giant magnetoresistance (GMR) sensors, spin-valve (SV) sensors, magnetic tunnel junction (MTJ) sensors, and planar Hall Effect (PHE) sensors, among other sub-types. In general, these MR sensors operate based on the detection of changes in longitudinal resistance when the MR sensor is subject to an external magnetic field, with the exception of PHE sensors that operate by detecting transverse resistance change in response to an external magnetic field. AMR sensors, in comparison to other sub-types such as GMR, SV and MTJ sensors, have proven more robust to electrostatic discharge and are considered by many to be easier to manufacture and use. In addition, AMR sensors also feature lower noise as compared to GMR and MTJ sensors. AMR sensors utilize the AMR effect that has its origin in spin-orbit coupling (SOC). The SGC is an effect that results in anisotropic scattering of electrons when they travel through magnetic materials. Materials exhibiting a normal AMR effect show a maximum resistivity when.thc current is parallel to the magnetization directio

, and a minimum resistivity when the current is perpendicular to the magnetization direction . At intermediate angles between the current direction and the magnetization direction, the resistivity of an AMR material is given by

where q is die angle between the current direction end the magnetization direction.

When the AMR effect is used in magnetic sensing, the magnetization direction is typically set at 45° with respect to the current direction at zero-field to maximize the sensitivity. This is apparent from the fact that the first derivative of p is maximum when p = 45°. When used in this configuration, an AMR sensor will respond linearly to an external field when the magnitude of the field is small.

To set the magnetization diivcticm to 45°, a. transverse bias is typically formed. There are many different ways to form a transverse bias. Two popular ways involve a soft adjacent layer (SAL) and a barber-pole structure, referred to respectively as SAL bias and barber-pole bias. In the case of SAL bias, a laminated structure is formed in which a .soft ferromagnetic layer (the SAL) is separated from a sensing element (in the form of a sensing layer) by a thin insulating spacer. The resistivity of the SAL is typically much larger than that of the sensing element, so that, when a charge current passes the tri-layer structure, a large portion of it will (low through the sensing element. The sensing current generates an Oersted field and thereby pushing the magnetization of the SAL away .from the direction of the easy axis of the sensing element. This tilt of magnetization causes magnetic charges at the edges of the SAL that in turn generate stray field and biases the magnetization of the sensing element into a desirable direction. The exact angle between the easy axis of the sensing element and the current direction (typically 45°) may be set easily through optimizing the thickness and material parameters of both the SAL and sensing element. Any deviation from the desirable angle can be adjusted via controlling the sensing current. Therefore, using SAL bias offers several advantages, such as providing an adjustable bias field, a relatively uniform bias field distribution, and a reduced demagnetizing field. However, it also has drawbacks, such as exhibiting a current shunting effect and is difficult to Implement in large size sensors, the latter being important for noise reduction.

In the case of barber pole bias, conductive ships are placed on top of the active sensing dement and are aligned in 45º with respect to the easy axis of the sensing element. In this way, the current flow direction in the active layer will be aligned 45° with respect to the magnetization direction. A primary drawback of barber pole bias is that only a small portion of the sensing element is active. Further, processes for forming this land of structure ate generally complex, thereby increasing the overall cost of the AMR sensor.

In actual AMR sensors, in addition to transverse bias, the sensor also typically needs a longitudinal bias to stabilize the domain structure in order to reduce Barkhausen noise caused by domain wall motion. So far, the most widely studied longitudinal bias scheme is the contiguous (or abutted) junction scheme. This scheme uses pennaiienl magnets positioned to either side of, and abutting, the sensing element to control bias. There are a number of factors involved in forming a proper bias in this scheme. Key among them is selection of a proper material with an appropriate thickness, and the control of the junction shape between the permanent magnet and the sensing element of the AMR sensor.

One significant drawback of a contiguous (or abutted) junction scheme is that the bias field usually is not uniform. It is normally stronger at the two edges and weaker at the center. If the center portion is properly biased, then it is unavoidable that the edge regions will be over-biased, leading to the formation of so-called dead regions. These inactive regions will generally degrade the sensitivity of the AMR sensor. The influence of the dead regions becomes more prominent when sensor width becomes smaller.

An alternative scheme that can suppress the effect of die inactive region involves a lead overlaid structure. In this scheme, contact electrodes are extended over the abutted junction and thus form a direct electrical contact with the inactive region of the AMR sensor. However, comparative studies of magnetic noise in sensors with a contiguous junction and lead overlay showed that magnetic noise is twice as large as Johnson noise for the lead overlay design, while it is comparable with Johnson noise for the contiguous junction design. The higher magnetic noise is attributed to a weaker longitudinal bias field with the lead overlaid design. Although the uniformity of bias may be improved by other bias techniques such as exchange bias from an antiferromagnet, tliis generally leads to a degradation of sensor sensitivity.

The aforementioned patterned permanent magnet bias of the contiguous (or abutted) junction scheme is in principle only suitable for small sensors. For a large- AMR sensor, one typically relies on a shape anisotropy field for longitudinal biasing. In this scheme, however, the magnetic domain of the sensing element can be disturbed easily by a large external field. Once new domains are nucleated, magnetic noise will increase and thus degrade the signal-to-noise ratio. One frequent measure to counter this adverse effect is to perform a SET/RESET operation on the sensing element. In a SET/RESET operation, an external alternating magnetic field switches the magnetization alternatively in two opposite directions along the easy axis. Conventionally, one or more conducting coils are required to generate the alternating field, which very often requires a very large current. The need for conducting coils typically results fo increased manufacturing cost, increased bulkiness of the sensor, and increased power consumption.

In general, all the above-discussed traditional biasing schemes significantly increase the number of process steps necessary in the manufacture of an AMR sensor. Moreover, the mostly commonly used biasing scheme, patterned permanent magnet bias, often results in non-uniform bias field in the sensor area.

In addition to shortcomings of traditional biasing schemes, AMR sensors face other challenges. In a typical AMR sensor, electrodes are formed . at ends of the sensing element for supplying a driving current and detecting voltage change in response to an external field. In order to remove the zero- field voltage (or DC offset) and extract the useful signal induced by external field only, the sensor is often configured into a Wheatstone bridge with four elements. When the four elements are identical, in principle, the DC offset can be completely removed. However, in practice, it is impossible to make the four elements identical and therefore, some DC offset always exists. Additional measures are typically required to remove this DC offset. Accordingly, there is a need for new designs for magnetic sensors that address the shortcomings of traditional biasing schemes and the issues of removing the DC offset. It. would be desirable if such a new magnetic sensor could make conventional magnetic bias unnecessary and completely remove the DC offset without the need for any additional post-detection signal processing, while being simple to manufacture.

SUMMARY

A spin-orbit torque (SOT)-b:iascd magnetic sensor is provided that utilizes both anisotropic magneioresislatice (AMR) and spin Hall magnctaresistance (SMR) effects. The AMR/SMR sensor includes a sensing element made from a material that itself can produce a transverse bias by generating a SOT effective field in response to a sensingfoiasing current. The sensing/biasing current may be a direct current (DC) or, preferably, on alternating current (AC). The use of a SOT effective field for biasing significantly simplifies the sensor structure. The use of AC for the sensing/biasing current significantly reduces noise, hysteresis and substantially eliminates DC offset of an output signal of the magnetic sensor.

In one embodiment, a SOT- biased magnetic sensor includes a sensing element having an easy axis and a hard axis, and including a stack of planar films of which at least one film is a magnetic film with in-plane magnetic anisotropy, A pair of electrodes are formed at the ends of the sensing element along the easy axis. The sensing/biasing current may be either a DC or an AC current. When the DC or AC current passes through the sensing element in the easy axis direction, an SOT effective field is generated in the hard axis direction, which will bias the magnetization away from the easy axis direction. The angle between ihe magnetization and current direction can be adjusted by Ihe magnitude of the current, lii the case of a DC current, the bias angle may be fixed, i.e., at 45°. When an external field is present in rhe hard axis direction, the sensor will respond to the external field to induce a change in the voltage across the two electrodes, which is detected as the output signal. On the other hand, when an AC current is applied, the magnetization oscillates around the easy axis with the maximum bias angle depending on the amplitude of the AC current. In this cases, when an external field is applied, the sensor will generate both first and second harmonic signals with respect to the bias current frequency. The second harmonic signal is proportional to the external field and thus gives Ihe output signal. The output signal can be detected using either the lock-in technique or rectification detection. In the latter case, one simply detects the DC component of the second harmonic signal.

In a second embodiment, a SOT -biased Wheatstone bridge magnetic sensor includes four sensing elements arranged in a Wheatstone bridge, each having an easy axis and a hard axis and. including a stack of planar films of which at least one film is a magnetic fi lm with in-plane magnetic anisotropy. Four electrodes are formed at ends of ones of the four sensing elements along the easy axis. A current source is configured to apply an AC sensing/biasing between a first pair of the four electrodes, the AC sensing/biasing current operates as both a sensing current having a response to external magnetic fields in accordance to the change in the sensing element’s resistance and a biasing current that generates a SOT effective field providing a transverse bias to the sensing element. A measurement device is configured to measure the response to external magnetic fields based on voltage change between a second pair of the four electrodes.

The change in voltage can be detected using either a lock-in technique or rectification detection (i.e., detecting the DC component of the second harmonic signal).

It should be understood that a Variety of additional features and alternative embodiments based on current-induced perpendicular switching in single magnetic layer, maybe implemented other than those discussed in this Summary. This Summary is intended simply as a brief introduction to the reader, arid does not indicate or imply that the examples mentioned herein cover all aspects of the disclosure, or are necessary or essential aspects of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description below refers to the accompanying drawings of example embodiments, of which:

Fig. 1a is a schematic diagram of an example SOT-biased.magnetic sensor utilizing both AMR and SMR effects; Fig. 1b is a schematic diagram of an example cross section of the sensing element of Fig. la, showing an example where the stack of planer films includes a HM/FM bilayer;

Fig. 1c is a schematic diagram illustrating formation of a SOT effective field in the example HM/FM bilayer of Fig. 1 b;

Fig.2 is a schematic diagram of an example SOT-biased Wheatstone bridge magnetic sensor;

Fig. 3 is a schematic diagram illustrating an example SET/RESET operation of a conventional AMR sensor driven by a DC current;

Fig.4 is a scanning electron micrograph and schematic diagram of four example sensing elements with a - 800 mm, which are connected to form a Wheatstone bridge;

Fig. 5a is a response cuive of an example SOT-biased SMR/AMR Wheatstone bridge in which the scnsing/biasing current is DC;

Fig. 5b is a response curve of the same example SOT-biased SMR/AMR Wheatstone bridge sensor as Fig. 5a in which the scnsing/biasing current is AC;

Fig. 5c is a.response curve of the same example SOT-biased SMR/AMR Wheatstone bridge sensor of Fig. 5a, in which the sensing/biasing current is AC with an adjustable DC offset;

Fig. 6a is a graph of angle dependent magnetoresistance for an example single NiFe (1.8)/Pt (2) sensing element with the dimension of 800mm x 200 mm at 300K;

Fig. 6b is a graph of temperature dependence of AMR and SMR ratio for the same example sensing element as Fig. 6a;

Fig. 6c is a graph df magnetoresistance response of one of the sensing elements of an example Wheatstone bridge sensor;

Fig.6d is a graph of the output signal of the example Wheatstone full bridge sensor; Fig. 7 is a graph comparing the detectivity for an example NiFe (1.8 nm/Ft (2 nm) Wheatstone bridge SOT-biased SMR/AMR sensor with a dimension of 800 mm x

200 mm for both DC and AC biasing with commercial AMR sensors; and

Fig. 8 is a table that compares the detectivity, power, sensitivity and field range of two example SOT-biased SMR/AMR sensors fabricated with a = 200 mm and b = 50 mm, respectively, to commercial sensors,

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

An Example SOT-Biased SMR/AMR Sensor

Fig. la is a schematic diagram of an example SOT-biased magnetic sensor utilizing both AMR and SMR effects. In this example, the AMR/SMR sensor includes a single sensing element 110 Formed as a stack of planar films with in-plane magnetic anisotropy, which is able to generate a SOT effective field when a charge current flows through it When viewed from above, the stack may have an elongated (e.g., elliptical shape). The long axis of the shape is aligned with an easy axis (i.e. an energetically favorable direction of magnetization) of the sensing clement 110 and a short axis of the shape is aligned with a hard axis (i.e. an energetically unfavorable direction of magnetization) of the sensing element 110, such that a stable domain can be achieved using shape anisotropy.

Two electrodes are coupled to the sensing element 110, being formed at the ends of the long axis (easy axis). The electrodes 120, 130 are used to both supply a sensing/biasing current with a current source 140 and to measure an output signal with a measurement device 150. The sensing/biasing current operates as a sensing current whose response to external magnetic fields is measured to produce an output signal, and -a biasing current that is used to generate SOT effective fields. The SOT effective fields include two types. A field- like effective field is generated in the plane of the sensing element 110, and perpendicular to the direction the current is applied. A damping-like effective field is generated that is out-of-plane. The field-like effective field acts as a transverse bias field to set a working point for the sensor and to ensure it responds linearly to an external transverse in-plane field (i.e. a field in the plane of ihe sensing element 110 that is perpendicular to the current direction ) with maximized sensitivity. It also functions as a longitudinal bias field to suppress domain wall nudeation and propagation. Accordingly, structures used in conventional magnetic sensors to provide Iran# vers bias control, such as a SAL of barber pole, or to provide longitudinal bias control, such as a patterned permanent magnet, can be avoided.

The sensing/biasing current may be a direct current (DC). When driven by DC, tlic current may be controlled such that the SOT effective field generated is able to bias the magnetization of the sensing element 1 10 to be 45° away from the direction the current is applied. Under this bias condition, the sensing element 110 will exhibit maximum sensitivity with a linear response to a transverse in-plane field, When an external field is applied, voltage change across the sensing element 110, between the electrodes 120, 130, is proportional to the Strength of the external field and therefore may be measured as the output signal.

Preferably, however, the sensing/hiasing current is an alternating current (AC). When driven by AC, a time-varying AC biasing field is generated that is directly proportional to the current. The AC biasing field deflects the magnetization of the sensing element 110 and makes it oscillate around the easy axis. When an external field is applied, with its direction perpendicular to the easy axis, the time average (i.e. DC component) of the voltage change across the sensing dement 110, between the electrodes 120, 130, is proportional to the strength of the external field and therefore may be measured as the output signal. The second harmonic of the voltage change is also proportional lo the strength of the external field and, alternatively, may be measured as the output signal.

DC offsets will be substantially eliminated automatically in AC biasing during the averaging process, or rectification readout. Other residual offsets due to the external environment can be reduced to nearly zero by adding a small DC offset to the AC sensing/biasing current. In addition to these benefits, AC biasing may also significantly reduce noise and hysteresis of the sensor by suppressing domain movement. It may function similar to that of a SET/RESET operation on a conventional AMR sensor, while avoiding the costly, bulky, power-consuming coil traditionally used to generate the SET/RESET field. A SET/RESET -like field may be generated automatically by the sensing current through SOT, enabling the AMR/SMR. sensor to be made extremely compact and power efficient.

Fig. lb is a schematic diagram of an example cross-section of the sensing element 110 of Fig. la, stowing an example where the stack of planar films includes a heavy meal (HM)/ ferromagnet (FM) bilaycr 160. A current passing through the HM/FM bilaycr 160 will generate an SOT effective field that is in the plane of the bilayer, which as discussed above functions as an effective transvers bias field and longitudinal. Mas field. The HM layer of the HM/FM bilayer 160 may be constructed from a variety of different materials, including platinum (Pt), palladium (Pd), tantalum (Ta), tungsten (W), lead (Pb), niobium (Nb), among others. The FM layer of the HM/FM. bilayer 160 may be constructed from a variety of different materials, including cobalt (Co), iron (Fe), nickel (Ni), cobalt iron boron (CoFeB), gadolinium (Gd), yttrium iron garnet (YIG), ferrites, and alloys of Co, Fe, Ni, CoFeB or Gd, among others.

In an alternative to a HM/FM bilaycr 160, the imsing element 110 may be constructed such that the stack of films includes other numbers and types of layers that are able to generate an SOT effective field. For example, the sensing element 1 10 may be a HM/antifeniomagnet (AFM) bilayer. Likewise, the sensing element 110 may be a HM/AFM multilayer such as a FM/AFM/HM trilayer, an AFM/HM/FM trilayer, an AFM/FM/HM trilayer or an HM/AFM/FM trilayer, where the AFM is constructed of ferromanganese (FeMn), iridium manganese (TfMn), nickel iron (NiFe), platinum manganese (PtMn), nickel manganese (NiMn), or platinum nickel manganese (PtNiMn), other alloys, manganese (Mn), chromium (Cr), nickel oxide (NiO), cobalt oxide (CoO), other oxides, or copper manganese arsenide (CuMnAs), among others. Tn addition, the sensing element 110 maybe a FM/ topological insulator (TI) bilayer, a magnetic TI, a dilute magnetic semiconductor a PM/transition metal dichalcogenide (TMD) heterostructure, a FM/Wcylc metal or semimetal bilaycr, among oilier possibilities.

Fig. 1 c is a schematic diagram illustrating formation of a SOT effective field in the example HM/FM bilaycr 160 of Fig. lb. As discussed above, the SOT effective field may serve as a transverse bias field. When an AC current

passes through a sensing element 1 10 comprising of a FM/HM bilayer 140, the voltage across the two ends of the sensing element can be given as follows:

The time-average of V, or DC component, is given by:

Here, I_o is the amplitude of applied AC current, w is the angular frequency, a is the ratio of SOT effective field over applied current, H_FL— al_osinwt, DR is magnetoresistance, R_o is resistance of the sensing element, H_k is the uniaxial anisotropy field, H_D is the shape anisotropy field, H_y is the applied field, and Q is the angle between magnetization (M) and easy axis direction. Q may be determined by the energy minimization method and is given by sinR =

it is clear from equation (2) that, although the sensing current is

an AC current, the output signal has a DC component that is proportional to the external magnetic field. This means that the sensor exhibits a linear response to an external field without the requirement that Q must be 45° at H_y = 0 . This is in sharp contrast to DC biasing in which Q must be set at 45° at H_y 0 in order to ensure a linear response of the sensor. It is also clear that an output signal can, alternatively, be obtained by detecting the second harmonic of the signal given by Eq. (1), for example, using a lock-in technique.

In addition to being used singly, the AMR/SMR sensor of Figs la-c can further be configured into a Wheatstone bridge with four nominally identical sensing elements. Fig. 2 is a schematic diagram of an example SOT-biased Wheatstone bridge magnetic sensor. As discussed above, each sensing element 1 10 may be a FM/HM bilayer or other structure with in-plane magnetic anisotropy that is able to generate a SOT when a current flows through it. Two of the four electrodes of the bridge may be employed to supply the sensing/biasing current and the other two electrodes may be employed to detect the voltage change caused by the external field. If DC sensing/biasing current is supplied to the Wheatstone bridge, it may produce voltage change of double amplitude as compared with that of a single element, without a large DC component due to sensor resistance. In practice, however, it is difficult to make the four sensing elements 110 truly identical, and therefore a DC biased Wheatstone bridge configuration still may have a sizable DC offset that may be undesirable for practical applications. Preferably, the sensing/biasing current is AC. With an AC sensing/biasing current an output signal of nearly zero offset may be produced.

When an AC current passes through two electrodes as shown in Fig. 2, the voltage across the other two electrodes is given as follows:

Flere, AR₀ is the offset resistance caused by non-identical sensing elements due to manufacturing processes, and the remaining parameters are the same as those used in equation (1) and (2). The time-average of V, or DC component, is given by:

Again, it is clear from equation (4) that, although the sensing current is an AC current, the output signal has a DC component that is proportional to the external magnetic field. This means that the sensor exhibits a linear response to external field without the requirement that Q must be set at 45° at H_y = 0. Compared to the case of a single sensing element, the Wheatstone bridge configuration leads to a sensor with much smaller AC noise, since the large AC signal due to the resistance of each sensing element has been cancelled out. It has also has a smaller DC offset and thermal drift.

The DC offset, if any, can be further reduced by adding a DC offset to

sensing/biasing current. Alternatively, lock-in detection may be employed to detect the second harmonic that is also proportional to the external field.

Fig. 3 is a schematic diagram illustrating an example SET/RESET operation of a conventional AMR sensor driven by a DC current. In a conventional AMR sensor, a large external field can cause the formation of multiple domains in the sensing element, which may induce Barkhausen noise. One well-established technique to suppress the domain wall nucleation and motion-induced noise is to saturate the magnetization alternatively in opposite directions along the easy-axis, which is referred to as a SET/RESET operation. A SET/RESET operation requires an additional coil or pair of coils 310 to generate an AC excitation field. In order to obtain a sufficiently large magnetic field, current as high as several amperes is typically required. This unavoidably increases the power consumption and complexity in designing and manufacturing the sensor. The

SET/RESET operation is known to bo able to reduce the noise by a factor of 2-3. In contrast, an AC driven SOT-biased AMR/SMR sensor according to the above teachings may provide a solution without the need for any additional coils. Instead of saturating the field alternatively in the easy axis direction, magnetization oscillates around a fixed direction along the easy axis. This effectively suppresses domain wall nucleation and motion, thereby reducing noise.

Experimental Results

The operation and performance of above described AC dri ven SOT-biased AMR/SMR sensor may b e better understood by considering results of experimental testing in one example lest, an AC driven SQT-biased Wheatstone bridge sensor with ellipsoidal shaped NiFe (1.8)/Pt (2) bi layers was fabricated. The long to short axis ratio was a/b 4, with a = 800 mm. The distance (L) between the two electrical contacts far each sensing element was kept a/3, Fig. 4 is a scanning electron micrograph (SEM) and schematic diagram of tour example sensing elements with a 800 mm, which are connected to form a Wheatstone bridge. When a current source is connected to the top and bottom terminals as depicted in Fig. 4, the magnetization of the sensing elements, 1 and 4, are rotated to the direction opposite to that of the sensing elements, 2 and 3, with respect to the easy axis, leading to a linear response to the external field that is detected as a voltage signal from the other two terminals of the bridge.

Fig. 5a is a response curve of an example SOT-biased SMR/AMR Wheatstone bridge in which the sensing/biasing current is DC In Fig. 5a. the DC biasing current densities is j_DC = 5.6 x 105 A/cm². Due to non-identicalness of the four sensing elements, there is a sizable DC offset in the readout signal. However, for the same sensor, the DC offset is reduced significantly when it is biased using an AC current. Fig. 5b is a response curve of the same example SOT-biased SMR/AMR

Wheatstone bridge sensor as Fig. 5a in which the sensing/biasing current is AC, In Fig.

5b, the frequency is 5000 Hz and a current density of J_RMS = 5.6 x 105 A/cm . The DC offset can be completely removed by adding a small DC offset to the driving AC current.

Fig, 5c is a response curve of the same example SOT-biased SMR/AMR

Wheatstone bridge sensor of Fig 5a, in which the sensing/biasing current is AC with an adjustable DC offset. The sensor performance is independent of AC currant frequency as long as the magnetization can respond to the SOT effective field generated by the current.

As discussed above, the sensing elements may be constructed as FM/I-IM bilayers, among other possibilities. 1ft ultrathin FM/HM bilayers, SMR is even larger than the AMR. Including both the AMR and SMR, the longitudinal resistance (R_xx) of an example NiFe/Pt bilayer can be expressed as:

Here, R_y is the resistance with the magnetization in y-direetion (i.e., transverse direction}, DR_AMR represents the size of resistance change induced by AMR,

represents the size of resistance change induced by SMR, and f is the angle between in-plane magnetization and current. It is apparent that both AMR and SMR exhibit the same angle dependence. To have a more quantitative understanding, angle dependent MR measurement was performed on a single NiFe (1.8)/Pt (2) sensing element with the dimension of 800 mm x 200 mm to determine the respective AMR and SMR contributions. In this measurement, the ma,gnetization of the sample was aligned by a constant field of 30 kOe, and R_xx was recorded at 300 K while rotating the field in zx and zy planes.

Fig. 6a is a graph of angle dependent magnetoresistance for an example SingleNiFe (1.S)/Pt (2) sensing element with the dimension of 800mi^h x 200 pm at 300K. As can be seer: from Fig. 6a, both AMR (

dependence) and SMR (

dependence) effects are clearly observed. The SMR ratio of 2.3x10^-3 is around twice that of the AMR ratio of 1 x 10^-3, suggesting that around 2/3 of the MR signal comes from the SMR, and 1/3 is from AMR at room temperature. Fig, 6b is a graph of temperature dependence of AMR and SMR ratio for the same example sensing element as Fig. 6a, it maybe observed from the figure that the SMR is much less sensitive to temperature than the AMR; AMR drops about 66% from 250 K to 400 K. whereas the SMR remains almost constant. The sensor output is mainly from the SMR effect, and therefore an AMR/SMR sensor will generally be more thermally stable that a conventional AMR sensor.

The low sensitivity of AMR/SMR sensor to thermal drifi is also manifested in the negligible drift of forward and backward MR curves measured at typical bias condition. Fig. 6c is a graph of magnetoresistance response of one of the sensing elements of an example Wheatstone bridge sensor. The sensing element is a NiFe (1.8 am)/Pt (2 am) bilayer at a biasing current density of 3.67 x 105 A/cm², which is required to achieve the maximum sensitivity. The external field was swept in a Ml loop, from -1 Oe to +1 Oe and then back to - 1 Oe. It is clear that the backward magnetoresistance response curve is almost Overlapping with the forward one, suggesting that, during the full loop sweeping measurement, thermal drift due to Joule heating was negligible. The same full loop field sweeping measurement has also been performed on· a Wheatstone full bridge sensor. Fig. 6d is a graph of the output signal of an example Wheatstone Mi bridge sensor. Again, the sensing elements are NiFe(1.8 nm)/Pt(2 am) bilayers and the external field was swept in a foil loop, from -1 Oe to +1 Oe and then back to -1 Oe. The overlapping of forward and backward output signal versus external field further demonstrates that thermal drift is small.

In order to estimate the detectivity of the magnetic sensor, standard noise characterization was performed in a magnetically shielded cylinder made of seven layers of m-metals, The magnetic sensor was powered by a battery and its bridge output voltage was amplified by a low-noise amplifier. A dynamic signal analyzer was used to acquire the noise power spectrum. The equivalent field noise, or detectivity, was obtained by dividing the noise voltage spectral density over the sensor sensitivity. Fig. 7 is a graph comparing the detectivity for an example NiFe (1.8 nm)/Pt (2 nm) Wheatstone bridge SOT-biased SMR/AMR sensor with a dimension of 800 mm x 200 mm for both DC and AC biasing with commercial AMR sensors. The commercial AMR sensors include a Honeywell Aerospace HMC 1001 sensor and a Honeywell Aerospace HMC 1021 sensor. The results show that AC -biasing can reduce the noise level by a factor of two as compared to DC biasing . In addition, it can be seen that the detectivity of Wheatstone bridge SOT-biased SMR/AMR sensor is comparable to that of commercial sensors even though the design is much simpler. It should be remembered that results shown are just for purposes of example. Sensors with much lower noise may be obtained by optimizing the device structure and manufacturing processes.

Similar measurements were performed on another sensor with a = 200 mm and b = 50 mm. Fig. 8 is a table that compares the detectivity, power, sensitivity and field range of two example SOT-biased SMR/AMR sensors fabricated with a - 200 mm and h = 50 mm, respectively, to commercial sensors. A.s can be seen, the field range of a SOT-biased SMR/AMR sensor can be adjusted by the size and aspect ratio of the sensor. Compared to commercial sensors, the power consumption of the SOT-biased SMR/AMR sensors is one order of magnitude smaller.

Concluding Comments and Alternatives

It should be appreciated that details included m the various example embodiments presented above are merely provided for purposes of illustration, and are not intended to limit the scope, applicability, or configuration of the invention. In general, it should be understood that the various components in the example embodiments presented above may be made from differing materials, implemented in different combinations or otherwise formed or used different ly without departing from the intended scope of the invention. What is claimed is:

Claims

1. A spin-orbit torque (SOT)-biased magnetic sensor comprising;

a sensing element having an easy axis and a hard axis, the sensing element including a stack of planar film s of which at least one film is a magnetic film with in-plane magnetic anisotropy;

a pair of electrodes each formed at an end of the sensing element along the easy axis; and

a current source configured to apply an alternating current (AC) sensing/biasing current between the electrodes, the AC sensing/biasing current to operate as a sensing current having a response to external magnetic fields in accordance to the change in the sensing element’s resistance and a biasing current that generates a SOT effective field providing a transverse bias to the sensing element.

2. The SOT-biased magn etic sensor of claim 1, wherein the magnetic response is produced at least in part by spin Hail magneto resistance (SMR),

3. The SOT -biased magnetic sensor of claim 1 , further comprising;

a measurement device configured to measure response to external magnetic fields as a direct current (DC) component of voltage change between the electrodes.

4. T he SOT-biased magnetic sensor of claim 1, further comprising:

a measurement device configured to measure response to external magnetic fields as a second harmonic of voltage change between the electrodes.

5. The SOT-biased magnetic sensor of claim 1 , wherein the sensing element is one of tour sensing dements arranged in a Wheatstone bridge, each having an easy axis and a hard axis and including a stack of planar films with in-plane magnetic anisotropy, and the pair of electrodes are ones of four electrodes of the Wheatstone bridge, and the current source is confi gured to apply the AC sensing/biasing current to a first pair of the four electrodes. and a measurement device is configured to measure response to external magnetic fields based on voltage change between a second pair of foe four electrodes.

6. The SOT-biased magnetic sensor of claim 5. wherein the measurement devise is configured to measure response to external magnetic fields as a direct current (DC) component of the voltage change between the second pair of electrodes.

7. The SOT-biased magnetic sensor of claim 5, wherein the measurement devise is configured to measure response to external magnetic fields as a second harmonic of the vol tage change between the second pair of electrodes.

8. The SOT-biased magnetic sensor of claim 5, wherein foe AC sensing/biasing current includes an adjustable direct current (DC) offset, the measurement device is configured to measure response to external magnetic fields as a DC component of the voltage change between the second pair of electrodes, and the DC offset is selected to substan tially eliminate a DC offset of an output signal that measures response.

9. The SOT-biased magnetic sensor of claim 5. wherein the AC sensing/biasing current includes an adjustable direct current (DC) offset, the measurement device is configured to measure response to external magnetic fields as a second harmonic of the voltage change between the other two electrodes, and the DC offset is selected to substantially eliminate a DC offset of an output signal that measures response.

10. The SOT-biased magnetic sensor of claim 1 , wherein the stack of foe sensing element is selected from the group consisting of: a heavy meal {HM)/ferromagnet (FM) bilayer. an antiferromagnet (AFM) Mayer, a FM/AFM/HM Mayer, an AFM/HM/FM trilayer, art APM/FM/MM Mayer and an HM/AFM/FM Mayer.

11. The SOT-biased magnetic sensor of claim 10. wherein the stack of the sensing element includes at least a HM selected from the group consisting of platinum (Pt), palladium (Pd), tantalum (Ta), tungsten (W), lead (Pb), and niobium (Nb).

12. The SOT-biased magnetic sensor of claim 10, wherein the stack of the sensing element includes at least a FM selected From the group consisting of cobalt (Co), iron (Fe), nickel (Ni), cobalt iron boron (CoFeB), gadolinium (Gd), yttrium iron garnet (YIG), ferrites, and alloys of Co, Fe. Ni. CoFeB or CM.

13. The SOT-biased magnetic sensor of claim 10, wherein the stack of the sensing element includes at least an AFM selected from the group consisting of ferromanganese (FeMn), iridium manganese (IrMn), nickel iron (NiFe), platinum manganese (PtMn), nickel manganese (NiMiti), or platinum nickel manganese (PtNiMn), manganese (Mn), chromium (Cr), nickel oxide (NiO), cobait oxide (CoO) and copper manganese arsenide (CuMnAs).

14. The SOT-biased magnetic sensor of claim 1 , wherein the stack of the sensing element is selected from the group consisting of: a ferromagnet (FM) / topological insulator (Tl) bilayer, a dilute magnetic semiconductor a FM/transition metal dichaicogenide (TMD)heterostmeture, and a FM/Weyle metal or semimetal bilayer.

15. A spin-orbit torque (SOT)-biased Wheatstone bridge magnetic sensor, comprising: four sensing elements arranged in a Wheatstone bridge, each having an easy axis and a hard axis and including a stack of planar films of which at least one film is a magnetic film with in-plane magnetic anisotropy; four electrodes formed at ends of the ones of the four seasing elements along the easy axis thereof;

& current source configured to apply an alternating current (AC) sensing/biasing between a first pair of the four electrodes, the AC sensing/biasing current to operat e as a sensing current having a response to external magnetic fields in accordance to the change in the sensing element 's resistance and a biasing current that generates a SOT effective field providing a transverse bias to the sensing element; and

a measurement device configured to measure response to external magnetic fields based on voltage change between a second pair of the four electrodes.

16, The SOT -biased Wheatstone bridge magnetic sensor of claim 15. wherein the magnetic response is produced at least in part by spin Hall magneto resistance (SMR).

17. The SOT-biased Wheatstone bridge magnetic sensor of claim 15, wherein the measurement device is configured to measure response to external magnetic fields as a direct current (DC) component of the voltage change between the second pair of electrodes,

18. The SOT-biased Wheatstone bridge magnetic sensor of claim 15, wherein the measurement device is configured to measure response to external magnetic fields as a second harmonic of the voltage change between the second pair of electrodes.

19. The SOT-biased Wheatstone bridge magnetic sensor of claim 15, wherein the AC sensing/biasing current includes an adjustable direct current (DC) offset, the measurement device is configured to measure response to external magnetic fields as a DC component of the voltage change between the second pair of electrodes, and the DC offset is selected to substantially eliminate a DC offset of an output signal that measures response.

20. The SOT-biased magnetic sensor of claim 15, wherein the AC sensing/biasing current includes an adjustable direct current (DC) offset, the measurement device is configured to measure response to external magnetic fields as a second harmonic of the voltage change between the other two electrodes, and the DC offset is selected to substantially eliminate a DC offset of an output signal that measures response.