Magnetic sensor

A magnetic sensor of the present invention has an elongate element portion having a magnetoresistive effect and a pair of elongate soft magnetic bodies that are arranged along the element portion on both sides of the element portion with regard to a short axis thereof. Each soft magnetic body includes a central portion that is adjacent to the element portion from one end to another end of the element portion with regard to a long axis direction thereof and a pair of end portions that protrude from the central portion in the long axis direction. A width of at least one of the end portions gradually decreases in a direction away from the central portion in at least a part of the end portions in the long axis direction thereof.

BACKGROUND OF THE INVENTION

Field of the Invention

The present application is based on, and claims priority from, JP Application No. 2017-250477, filed on Dec. 27, 2017, the disclosure of which is hereby incorporated by reference herein in its entirety.

The present invention relates to a magnetic sensor.

Description of the Related Art

As a sensor for detecting the position of a moving object, a magnetic sensor that has an element having a magnetoresistive effect is known (see JPH11-87804). A magnetic sensor moves relative to a magnet and thereby detects a change in an external magnetic field that is generated by the magnet, and calculates the moving distance of the moving object based on the change in the external magnetic field that is detected.

The magnetic sensor that is disclosed in JPH11-87804 has a giant magnetoresistive thin film that exhibits a magnetoresistive effect and soft magnetic thin films, as shown inFIG. 1thereof. In the magnetic sensor, the giant magnetoresistive thin film is elongate, and the soft magnetic thin films are provided on both sides of the giant magnetoresistive thin film with regard to the short axis direction thereof. The giant magnetoresistive thin film is rectangular, as viewed in the thickness direction thereof. In this magnetic sensor, the giant magnetoresistive thin film, which has poor sensitivity to a magnetic field, is combined with soft magnetic thin films in order to enhance the sensitivity to a magnetic field.

SUMMARY OF THE INVENTION

As disclosed in JPH11-87804, the sensitivity of a magnetic sensor is enhanced by providing soft magnetic thin films on both sides of the giant magnetoresistive thin film. In this arrangement, the sensitivity of a magnetic sensor is further enhanced as the soft magnetic thin films (soft magnetic bodies) become wider.

However, output noise in the long axis direction of the giant magnetoresistive thin film, which is perpendicular to the direction of a magnetically sensitive axis in a magnetic field (in this case, the direction of a short axis of the giant magnetoresistive thin film), increases as the soft magnetic thin films (soft magnetic bodies) become wider.

The present invention aims at providing a magnetic sensor that is capable of reducing output noise in the long axis direction in a magnetic field while enhancing the sensitivity.

A magnetic sensor of the present invention comprises: an elongate element portion having a magnetoresistive effect; and a pair of elongate soft magnetic bodies that are arranged along the element portion on both sides of the element portion with regard to a short axis thereof. Each soft magnetic body includes a central portion that is adjacent to the element portion from one end to another end of the element portion with regard to a long axis direction thereof and a pair of end portions that protrude from the central portion in the long axis direction. A width of at least one of the end portions gradually decreases in a direction away from the central portion in at least a part of the end portions in the long axis direction thereof.

According to the invention, it is possible to provide a magnetic sensor that is capable of reducing output noise in the long axis direction in a magnetic field while enhancing the sensitivity.

The above and other objects, features and advantages of the present invention will become apparent from the following descriptions with reference to the accompanying drawings which illustrate examples of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Explanation will be given about an embodiment, as well as modifications of the embodiment.

Magnetic sensor10of the embodiment is, for example, a sensor for detecting the position of a moving object (not shown) having a magnet, that is, a position sensor. Magnetic sensor10of the embodiment is configured to move relative to the above-mentioned magnet and thereby to detect a change in an external magnetic field that is generated by the magnet, and to calculate the moving distance of the moving object based on the change that is detected. Magnetic sensor10of the embodiment has a magnetically sensitive axis, which is the X axis inFIGS. 1A and 10, and detects a change in a magnetic field in the X axis direction that is generated by the moving object. In the following descriptions, the X axis inFIGS. 1A and 10, i.e., the axis parallel to the short axes of element portion20and soft magnetic body30, is referred to as a first axis, and the Y axis inFIGS. 1A and 10, i.e., the axis parallel to the long axes of element portion20and soft magnetic body30, is referred to as a second axis.

Magnetic sensor10of the embodiment has magnetoresistive element portion100that is constructed by element portion20and a pair of soft magnetic bodies30, as shown inFIGS. 1A and 10. As shown inFIG. 1B, magnetic sensor10of the embodiment has sensor portion200, in which magnetoresistive element portions100are bridge-connected to each other, and integrated circuit300having input terminal310that is electrically connected to sensor portion200, ground terminal320and external output terminals330,340etc.

Element portion20is, for example, elongate and includes a material that has a magnetoresistive effect, later described. A pair of soft magnetic bodies30is, for example, elongate and is arranged on both sides of element portion20with regard to the short axis thereof. Element portions20and soft magnetic bodies30are arranged alternately in the short axis direction of element portions20such that soft magnetic bodies30are arranged on both sides of element portions20, as shown inFIG. 1A. Element portions20are connected to each other at one end and the other end thereof with regard to the long axis direction thereof by means of electrodes40, forming a meander shape together with electrodes40. Element portions20and soft magnetic bodies30are arranged above a substrate (not shown).

Element portion20of the embodiment is, for example, elongate, as viewed in the thickness direction thereof, which is perpendicular both the long axis direction and to the short axis direction of element portion20.

Element portion20of the embodiment has, for example, a typical spin-valve type film configuration, as shown inFIG. 1D. Specifically, element portion20includes free layer151whose magnetization direction is changed depending on an external magnetic field, pinned layer153whose magnetization direction is pinned relative to the external magnetic field, spacer layer152that is positioned between and that is in contact both with free layer151and with pinned layer153, antiferromagnetic layer154that is adjacent to pinned layer153on the back side thereof, as viewed from spacer layer152. Free layer151, spacer layer152, pinned layer153and antiferromagnetic layer154are stacked above the above-mentioned substrate. Antiferromagnetic layer154fixes the magnetization direction of pinned layer153by the exchange coupling with pinned layer153. Pinned layer153may also have a synthetic structure in which two ferromagnetic layers sandwich a nonmagnetic intermediate layer. Spacer layer152is a tunneling barrier layer that is formed of a nonmagnetic insulator, such as Al2O3. Accordingly, element portion20of the embodiment functions as a tunneling magnetoresistive element (a TMR element). In other words, element portion20of the embodiment has a tunneling magnetoresistive effect. A TMR element is advantageous in that it has a larger MR ratio and a larger output voltage from the bridge circuit, for example, than a GMR element.

As shown inFIG. 1C, each soft magnetic body30of the embodiment is, for example, in line symmetry with regard to imaginary straight line L1(the dot-dash line) that passes through the center thereof with regard to the short axis direction and that extends in the long axis direction and also is in line symmetry with regard to imaginary straight line L2(the two-dot chain line) that passes through the center thereof with regard to the long axis direction and that extends in the short axis direction. In other words, each soft magnetic body30(including the shapes of end portions34) is in line symmetry both with regard to the long axis thereof and with regard to the short axis thereof, as viewed in a direction perpendicular both the long axis direction thereof and to the short axis direction thereof.

The pair of soft magnetic body30, which is arranged on both sides of element portion20, as mentioned above, has the function of enhancing the sensitivity of magnetic sensor10or the function as a yoke. Each soft magnetic body30of the embodiment is formed, for example, of NiFe, CoFe, CoFeSiB, CoZrNb or the like. As shown inFIG. 1C, soft magnetic bodies30are arranged on both sides of element portion20with regard to the short axis direction thereof and protrude outwards of both tip ends of element portion20with regard to the long axis direction thereof. In the following descriptions, a portion of soft magnetic body30that is adjacent to element portion20along element portion20from one end to the other end of element portion20with regard to the long axis direction thereof is referred to as central portion32, and a pair of portions of soft magnetic body30that protrude from central portion32in the long axis direction of element portion20is referred to as end portions34(seeFIG. 1C).

As shown inFIG. 1C, width W of central portion32is, for example, larger than width W0of element portion20, which makes magnetic sensor10of the embodiment more sensitive than a magnetic sensor in which width W of central portion32is equal to or less than width W0of element portion20. Central portion32, which is a portion of soft magnetic body30other than both end portions34with regard to the long axis direction thereof, is rectangular, as viewed in the thickness direction (seeFIG. 1C).

Length L of each end portion34. i.e., the protruding length from the boundary with central portion32is, for example, equal to or less than width W of central portion32.

For example, there is a relationship that is expressed by formula (1) between length L of each end portion34and width W of central portion32
W/2=<L(formula 1)

In the present embodiment, the width of each end portion34gradually decreases as the distance from central portion32in the long axis direction of element portion20increases and becomes zero at the tip end, as shown inFIG. 1C. Specifically, the edge of each end portion34is expressed by the curve that is stipulated by formula (2) in the first quadrant of a Cartesian coordinate and by a curve that is in line symmetry with the curve of formula (2) with regard to the Y axis (imaginary straight line L1) in the second quadrant of the Cartesian coordinate, as viewed in a direction that is perpendicular both to the long axis direction and to the short axis direction of element portion20, wherein in the Cartesian coordinate, the coordinates of the middle point of imaginary boundary line L3(seeFIGS. 1C and 2A) between end portion34and central portion32are (0,0), imaginary boundary line L3is the X axis, and imaginary straight line L1that passes through the coordinates of the middle point and extends in the long axis direction is the Y axis.
(X,Y)=(W/2×cosθ,L×sin (θ+P×sin (2θ))  (formula 2)
whereW : the width of central portion32L : the length of end portion34P : strain angle, for example, −20°≤P≤+30°θ: angle in the Cartesian coordinate system (seeFIG. 2A),

The angle θ is between the imaginary boundary line L3and a line extending between the middle point (0,0) and (X,Y) as shown inFIG. 2A.

In the present embodiment, the edge of each end portion34is expressed by substituting the following numerical values into formula (2) and is shaped into a first shape, as shown in the first quadrant ofFIG. 2A.W=2000 (nm)L=1000 (nm)P=0 (°)

In the embodiment, the chamfering rate (%) of end portions34is, for example, 5(%) or more, as viewed in a direction that is perpendicular both to the X axis direction and to the Y axis direction. In the following description, a direction that is perpendicular both to the X axis direction and to the Y axis direction is referred to as a Z axis. The Z axis corresponds to a direction that is perpendicular both to the long axis direction and the short axis direction of element portion20and each soft magnetic body30.

The chamfering rate (%) is defined as follows. As viewed in the Z axis direction inFIG. 1E, suppose that sides of central portion32along the long axis direction are sides32Y and that an imaginary line that passes through the tip end of end portions34, i.e., the tip end of soft magnetic body30with regard to the long axis direction thereof, that is perpendicular to sides32Y and that extends in the X axis direction between extensions of both sides32Y is imaginary line30X. Further, suppose that a part of extensions of both sides32Y that connect central portion32to respective ends of imaginary line30X are imaginary lines32Y. Further, suppose that the area of portion CH that is defined by both imaginary lines32YA, imaginary line30X and the edge of end portion34, i.e., a value obtained by subtracting the area of end portion34from the area of the minimum rectangular region that envelops end portion34is chamfering area S1, and a product of length L of end portions34and width W of central portion32, i.e., the area of the minimum rectangular region that envelops end portion34is area S2of end portion34before chamfered, as viewed in the Z axis direction. Then, chamfering rate R (%) is expressed as formula (3)
R=S1/S2×100(%)  (formula 3)

Next, the effect of the present embodiment will be explained by comparing various shapes of the present embodiment, i.e., the first to fourth shapes (seeFIGS. 2A to 2D), to a comparative example (seeFIG. 3). The comparison is conducted based on the measurement of output of each magnetic sensor versus time duration for applying a magnetic field of a predetermined strength to each magnetic sensor in the second axis direction, as shown in the graph ofFIG. 4. In the descriptions of the comparative example, when the same elements are used in the comparative example as in the present embodiment, the names and reference numerals in the present embodiment will be used.

The method of measurement will be explained with reference toFIG. 4

In the measurement, a magnetic field of a predetermined strength is applied to magnetic sensors10of the present embodiment in various shapes (the first to fourth shapes) and magnetic sensor10A of the comparative example in the second axis direction. The outputs of magnetic sensors10,10A that are measured when the magnetic field is applied are recorded, for example, every 0.1 second. Then, the output spectra of magnetic sensors10,10A are compared to each other. The spectrum of magnetic sensor10of the present embodiment of the first shape and the spectrum of magnetic sensor10A of the comparative example are shown in the graph ofFIG. 4. InFIG. 4, output spectra are normalized such that half the difference between the maximum values and the minimum values of the outputs of respective magnetic sensors10,10A are 1.

It is preferable that a magnetic sensor having a magnetically sensitive axis in the X axis direction ideally outputs zero when a magnetic field is applied in the second axis direction. Actually, however, when a magnetic field is applied to the magnetic sensor in the second axis direction, each soft magnetic body30is considered to be magnetized in the second axis direction and additionally magnetized in the first axis direction in an unstable manner. Due to the magnetization that is directed in the first axis direction, unstable magnetic field component Bx that is directed in the first axis direction is applied to element portion20. Thus, the magnetic sensor generates an output that is affected by the application of magnetic field component Bx. In other words, for each magnetic sensor10,10A, each spectrum inFIG. 4corresponds to output noise that is caused by the magnetic field applied in the second axis direction.

Accordingly, it is preferable that the output spectrum of each magnetic sensor10,10A be as flat as possible with regard to the time duration. It is further preferable that the output be as low as possible, that is, as close to zero as possible. In the following descriptions, the output noise that is caused by the magnetic field applied in the second axis direction is simply referred to as “output noise”.

The configuration of magnetic sensor10A of the comparative example will be explained in detail with reference toFIG. 3. Both tip ends of end portions34A of magnetic sensor10A of the comparative example with regard to the long axis direction thereof are planes that extend in the short axis direction thereof. The width of each end portion34A of the comparative example is the same as the width W of central portion32A. Accordingly, unlike the present embodiment (seeFIG. 1C), the width of each end portion34A of the comparative example does not gradually decrease as the distance in the long axis direction from central portion32A increases. Magnetic sensor10A of the comparative example has the same configuration as magnetic sensor10of the present embodiment (seeFIG. 1C) except for the above.

Referring to the graph ofFIG. 4, the output of the comparative example is sharply changed several seconds after the measurement started (in the graph ofFIG. 4, three to four seconds after the measurement started). In other words, a jump occurs in the output spectrum. The “spectrum jump” is indicated by the reference sign ND inFIG. 4. Consideration about the output will be given after the explanation about the measurements of the present embodiment.

In the present descriptions, output noise whose output, which is normalized in the aforementioned manner, is changed at a rate of 0.001 or more per 0.1 (second), i.e., output noise whose output, which is normalized in the aforementioned manner, is changed at a rate of 0.1(%) or more per 0.1 (second) is defined to be the “spectrum jump ND” mentioned above.

Next, explanation will be given about the output spectra of the present embodiments, i.e., the first to fourth shapes (seeFIGS. 2A to 2D).

An output spectrum of magnetic sensor10having end portions34of the first shape (seeFIGS. 1C, 2A) was observed, as shown inFIG. 4. Spectrum jump ND in the output spectrum, which was observed in the comparative example, was not observed, as shown in the graph ofFIG. 4.

An output spectrum of magnetic sensor10having end portions34of the second shape (seeFIG. 2B) was observed (not shown). The output spectrum was similar to the output spectrum of the first shape. In other words, spectrum jump ND in the output spectrum, which was observed in the comparative example, was not observed in the second shape. Each parameter in formula (2) for magnetic sensor10having end portions34of the second shape is set to be as follows.W=2000 (nm)L=2000 (nm)P=0 (°)

An output spectrum of magnetic sensor10having end portions34of the third shape (seeFIG. 2C) was observed (not shown). The output spectrum was similar to the output spectra of the first and second shapes. In other words, spectrum jump ND in the output spectrum, which was observed in the comparative example, was not observed in the third shape. Each parameter in formula (2) for magnetic sensor10having end portions34of the third shape is set to be as follows.W=2000 (nm)L=2000 (nm)P=−20 (°)

An output spectrum of magnetic sensor10having end portion34of the fourth shape (seeFIG. 2D) was observed (not shown). The output spectrum was similar to the output spectrum of the first shape. In other words, spectrum jump ND in the output spectrum, which was observed in the comparative example, was not observed in the fourth shape. The output spectrum of the fourth shape was less flat than the output spectra of the second and third shapes. Each parameter in formula (2) for magnetic sensor10having end portions34of the fourth shape is set to be as follows.W=2000 (nm)L=2000 (nm)P=20 (°)

Next, consideration will be given based on the comparison of the output spectra between the embodiments in various shapes, i.e., the first to fourth shapes, and the comparative example.

Spectrum jump ND was not observed in the output spectra of the embodiments in various shapes, i.e., the first to fourth shapes, unlike the comparative example. Further, the output spectra of the embodiments in various shapes, i.e., the first to fourth shapes are flatter than the output spectrum of the comparative example. The inventor thinks that the reason is as follows.

In the comparative example, the width of end portion34A of soft magnetic body30A (seeFIG. 3) does not gradually decrease from central portion32toward the tip end as the distance from central portion32in the long axis direction increases (the width of the tip end is not zero) and the tip end is a flat plain that extends in the short axis direction of element portion20. Thus, unstable magnetic field component Bx that is directed from outward to inward in the short axis direction is considered to occur in end portions34A, as shown in the enlarged view of end portion34A inFIG. 3.

To the contrary, in the embodiments in various shapes, unlike the comparative example, end portions34(seeFIGS. 2A to 2D) of soft magnetic body30A gradually narrow from central portion32toward the tip ends as the distance from central portion32in the long axis direction increases (the width of the tip end is zero). Thus, in end portions34of the embodiments in various shapes, unstable magnetic field component Bx (seeFIG. 3) that is directed from outward to inward in the short axis direction of end portion34A is less apt to occur, as compared to end portions34A of the comparative example, and even if it occurs, it is smaller than that of the comparative example. As a result, spectrum jump ND that was observed in the comparative example was not observed in the output spectra of the embodiments, as shown inFIG. 4.

Accordingly, magnetic sensor10of the present embodiment is capable of reducing output noise or limiting spectrum jump ND while enhancing sensitivity. This effect is especially advantageous in the present embodiment, in which the width of soft magnetic body30is larger than width W0of element portion20, because the output noise tends to increase. Furthermore, this effect is especially advantageous in the present embodiment, because S/N ratio can be improved when element portion20has a tunneling magnetoresistive effect.

In addition, as mentioned above, the second shape (FIG. 2B) and the third shape (FIG. 2C) show flatter output spectra or small changes in output than the fourth shape (FIG. 2D). The inventor thinks that this is because the second and third shapes have a larger rate of change in the width of end portion34than the fourth shape, although the second to fourth shapes have the same length L of end portion34.

A specific embodiment of the invention has been described. However, the present invention is not limited to the embodiment mentioned above. For example, the following modifications are included in the scope of the present invention.

For example, in the embodiment, the width of end portion34of soft magnetic body30gradually decreases as the distance in the long axis direction from central portion32increases and becomes zero at the tip end (seeFIG. 1C). However, as shown inFIG. 5Aillustrating magnetic sensor10B of a first modification, the width of end portion34does not have to become zero at the tip end as long as the width of end portion34of soft magnetic body30gradually decreases as the distance in the long axis direction from central portion32increases. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

In the embodiment, the width of each end portion34of soft magnetic body30gradually decreases as the distance in the long axis direction from central portion32increases (seeFIG. 1C). However, as shown inFIG. 5Billustrating magnetic sensor10C of a second modification, the width of at least one of end portions34of soft magnetic body30may gradually decrease as the distance in the long axis direction from central portion32increases. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

In the embodiment, each end portion34of soft magnetic body30is in line symmetry with regard to imaginary straight line L1(seeFIG. 1C). However, as shown inFIG. 5Cillustrating magnetic sensor10D of a third modification, each end portion34of soft magnetic body30does not have to be in line symmetry with regard to imaginary straight line L1as long as the width of each end portion34of soft magnetic body30gradually decreases as the distance in the long axis direction from central portion32increases. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

In the embodiment, end portions34of soft magnetic body30are in line symmetry with each other with regard to imaginary line L2(seeFIG. 1C). However, as shown inFIG. 5Dillustrating magnetic sensor10E of a fourth modification, end portions34of soft magnetic body30do not have to be in line symmetry with each other with regard to imaginary line L2as long as the width of each end portion34of soft magnetic body30gradually decreases as the distance in the long axis direction from central portion32increases. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

As shown inFIG. 5Eillustrating magnetic sensor10F of a fifth modification, the rate of decrease (change) in the long axis direction of the width of each end portion34may vary as long as the width of each end portion34gradually decreases as the distance in the long axis direction from central portion32increases. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

In the embodiment and the first modification etc., the widths of end portions34of soft magnetic body30gradually decrease as the distance in the long axis direction from central portion32(or the boundary between end portion34and central portion32) increases (seeFIGS. 1C and 5A). However, as shown inFIG. 5Fillustrating magnetic sensor10G of a sixth modification, the widths of end portions34of soft magnetic body30do not have to gradually decrease from the boundary with central portion32to the tip ends as the distance in the long axis direction from central portion32increases, as long as the width of end portion34of soft magnetic body30gradually decreases in a direction away from central portion32in at least a part of end portion34in the long axis direction thereof. This modification is also capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example.

Chamfering rate R of end portion34is, for example, 5(%) or more in the embodiment, and the embodiment and any modification that meets this requirement is capable of reducing output noise while enhancing sensitivity, as compared to the above-mentioned comparative example. However, taking into account the consideration mentioned above, any modification having chamfering rate R of less than 5(%) (not shown) shows an effect that is similar to the effect of the embodiment as long as the widths of end portions34of soft magnetic body30gradually decrease from the side of central portion32to the tip ends as the distance in the long axis direction from central portion32increases.

In the embodiment, the spacer layer that constitutes element portion20is a tunneling barrier layer, and element portion20is a TMR element. However, the spacer layer that constitutes element portion20may be a nonmagnetic conductive layer that is formed of a nonmagnetic metal, such as Cu, in order to form element portion20as a giant magnetoresistive element (GMR element). Element portion20may also be an anisotropic magnetoresistive element (AMR element). These modifications are also capable of reducing output noise while enhancing sensitivity, as compared to the comparative example.

In the embodiment, the width of central portion32of soft magnetic body30is larger than width W0of element portion20(seeFIG. 1C). However, the width of central portion32of soft magnetic body30may be equal to or smaller than width W0of element portion20(not shown). Taking into account the fact that output noise occurs as a trade-off for enhancing the sensitivity of magnetic sensor10by providing soft magnetic body30, the modification also has the effect of the above-mentioned embodiment.

An embodiment in which one from among the embodiment and the first to sixth modifications is combined with an element of other embodiment/modifications is also included in the scope of the present invention. For example, a modification of magnetic sensor10B according to the first modification (seeFIG. 5A), in which one of soft magnetic bodies30is replaced with one of soft magnetic bodies30according to the third modification, is included in the scope of the present invention.

Magnetic sensor10of the embodiment has been described by taking a position sensor as an example. However, magnetic sensor10of the embodiment may be a sensor other than a position sensor as long as magnetic sensor10detects a magnetic field that is applied in the first axis direction. For example, magnetic sensor10may be a sensor, such as an angle sensor, an encoder or the like.

Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims.