JPH07307501A - Magnetic field sensor - Google Patents

Abstract

PURPOSE: To make the hysteresis curve not substantially irregular and reduce the hysteresis. CONSTITUTION: The sensor has a magnetoresistance element having a substrate having two ferroelectric or layers 2, 4 separated by a nonmagnetic intermediate layer 3; the layers 2, 4 showing a single axis in-plane anisotropy the directions of which are mutually perpendicular, resulting in a more uniform variation of the magnetizing direction of a sensor caused by an external magnetic field and corresponding magnetizing curve shows a less hysteresis. When the sensor is used as a magnetic head, an improved S/N ratio is obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention comprises a magnetoresistive element having a substrate exhibiting uniaxial in-plane anisotropy and having two ferromagnetic layers separated by a nonmagnetic intermediate layer. The present invention relates to a magnetic field sensor. The present invention also relates to a magnetic head including such a magnetic field sensor.

[0002]

Magnetic field sensors are used in particular with compasses to measure position and rotational speed. Another important application relates to magnetic heads. Such magnetic field sensors are used to read information magnetically stored on a read medium such as a magnetic tape.

The magnetoresistive element having the above-described structure is especially used in the magazine "Japan Society of Applied Physics" published in 1991.
ed Physics) 69 (8), p. 4774. More specifically, this publication includes a silicon substrate coated with a Ta nuclear layer, which is intermediate between the first ferromagnetic layer of NiFe (layer thickness 6.2 nm) and Cu (layer thickness 2.2 nm). Layer and NiFe (layer thickness 4.0n
m) and a second ferromagnetic layer of FeMn (layer thickness 7.0 nm) are sequentially formed to describe a magnetic field sensor having a double structure. The known magnetic field sensor also includes a cap layer that minimizes oxidation.

The first ferromagnetic layer of NiFe has a uniaxial in-plane anisotropy and a relatively small coercive force, so that this layer has a magnetization curve centered at zero field strength. The magnetic properties of the second ferromagnetic layer are ferromagnetic NiFe and antiferromagnetic.
It is largely determined by the exchange coupling with FeMn. For this reason, the magnetization curve of the second so-called bias ferromagnetic layer deviates toward a magnetic field strength having a finite value. As a result, the magnetization curves of both ferromagnetic layers do not match. The second ferromagnetic layer also exhibits uniaxial in-plane anisotropy controlled by antiferromagnetic FeMn. Anisotropy directions of both ferromagnetic layers become parallel,
The magnetizations of these layers are therefore parallel or antiparallel depending on the presence and direction of the externally applied magnetic field.

[0005]

In the known layer structure, the resistance as a function of the applied external magnetic field (H) (ΔR /
The change in R) can be measured by a relatively simple method. FIG. 1 of the publication mentioned above shows the magnetoresistance curve thus measured. FIG. 1 shows two hysteresis curves with significantly different shapes. The curve corresponding to the reversal of the magnetization of the "biased" ferromagnetic layer shows a relatively high hysteresis on the order of 100 Oe (= 8 kA / m). Due to the exchange coupling with the FeMn layer mentioned above, this hysteresis is about 400 Oe (= 32 k
A / m) magnetic field strength is deviated. The other curves centered on an external magnetic field of 0 Oe are dominated by changes in the magnetization of the "unbiased" NiFe layer. The change in magnetization of the "biasless" NiFe layer and the resulting change in resistance are very important in using this type of magnetic field sensor in a magnetic head. Upon using the known magnetic field sensor, a change in resistance of up to 4.1% was confirmed at room temperature.

According to another experiment conducted by the applicant,
Actually, it is shown that the hysteresis of other magnetoresistive curves centered on an external magnetic field of 0 Oe is about 10 Oe (= 0.8 kA / m). It was further confirmed that this curve had a fairly irregular shape. When using this known magnetic field sensor, such irregularities result in relatively high noise levels. This adversely affects the S / N ratio.

The object of the present invention is to alleviate the problems mentioned above. The invention is particularly aimed at providing a magnetic field sensor which has a hysteresis curve with few irregularities and whose hysteresis is considerably smaller than that of known magnetic field sensors.

[0008]

These and other objects of the invention are achieved by a magnetic field sensor of the type described above, which according to the invention is such that the anisotropy direction of one ferromagnetic layer is It is characterized by being orthogonal to the anisotropic direction of the other ferromagnetic layer.

The invention is based on the recognition that the magnetization reversal of a known magnetic field sensor is caused by the process of the domain wall because the anisotropy directions of both ferromagnetic layers are parallel. Such inversion causes a more or less stepwise change in magnetization and a stepwise change in resistance dominated thereby. This phenomenon causes a noticeable noise level in the magnetic field sensor. Such noise is generally called "Barkhausen noise".

However, when the anisotropy direction of the non-biased NiFe layer is perpendicular to the anisotropy direction of the biased NiFe layer, the change in the magnetization direction of the non-biased NiFe layer is controlled by the external magnetic field, and the Instead, it occurs according to a coherent rotating mechanism, resulting in a fairly low noise level. Therefore, the magnetoresistive curve has a more regular shape and lower hysteresis. To obtain a useful sensor, the magnetic field direction of the applied external magnetic field is unbiased NiFe.
It must be orthogonal to the anisotropic direction of the layer. “Orthogonal” should be understood to mean that the anisotropic directions of both ferromagnetic layers intersect each other at an angle of 90 ° ± 20 °.

It is emphasized that the inventive concept of orthogonal anisotropy does not apply only to magnetic field sensors comprising biased and non-biased ferromagnetic layers.
The concept of the invention is as described in the publications mentioned above,
It can also be used in magnetic field sensors with several ferromagnetic layers with different coercive forces. Furthermore, it is emphasized that the magnetic field sensor of the present invention may comprise more than one ferromagnetic layer. A magnetic field sensor having a multilayer structure based on two or more ferromagnetic layers has the advantage of a relatively high detection sensitivity.

In principle, any ferromagnetic composition can be used as the material for the ferromagnetic layer. Especially preferred is C
o, Ni, Fe or alloys thereof. A magnetic field sensor comprising at least one layer of biased NiFe is preferred for preferred magnetization properties. Here NiFe is 20 ± 2% Fe and 80 ± 2%
It means an alloy with Ni. This material is soft magnetic and has a very low coercive force and a very low reluctance. Furthermore, with this material, uniaxial in-plane anisotropy can be realized in a simple manner in any direction by forming the NiFe layer under a magnetic field having a relatively weak strength. The anisotropy thus generated is parallel to the applied magnetic field direction.

If the first ferromagnetic layer is composed of unbiased NiFe, doped or undoped Co, Fe and / or Ni can be used for the second ferromagnetic layer. In the first type of magnetic field sensor, the composition of the second layer is selected so that the second layer has a coercive force different from the coercive force of the first layer. In this case, a satisfactory result is obtained when the second ferromagnetic layer is composed of an alloy containing Co and Pt, which has an influence.

However, even better results are obtained with a second type of magnetic field sensor in which the second layer is composed of a biased ferromagnetic layer, preferably biased NiFe. Such a biased NiFe layer is transformed into a layer of FeMn (50% each)
It can be obtained by forming a layer of NiFe. Preferably the thickness of the ferromagnetic layer is not more than 10 nm and 1.5n
Be no thinner than m. When the layer thickness exceeds 10 nm, the magnetoresistive effect of the magnetic field sensor becomes very small. When the layer thickness is less than 1.5 nm, the magnetic permeability of the non-biased layer is very small.

The intermediate layer needs to be made of a non-magnetic material. This intermediate layer acts to magnetically decouple both ferromagnetic layers. Materials that can be particularly preferably used for the intermediate layer are metals and metal alloys. For this, Au, A
It has been found that noble metals such as g, Pt and Cu can be used particularly suitably. Best results have been obtained with Cu or Cu alloy interlayers. The thickness of the intermediate layer is preferably not more than 5 nm and not less than 1.5 nm. 1.5 nm
The thinner intermediate layer results in poor decoupling between both ferromagnetic layers. When the thickness of the intermediate layer exceeds 5 nm, the “decoupling effect” hardly increases.

In the case of the second type of magnetic field sensor, the improvement was also achieved by providing a very thin Co layer of, for example, 0.2 to 0.4 nm between the ferromagnetic layer and the intermediate layer. As described in the publications mentioned above, such an arrangement considerably increases the magnetoresistive effect.

Depending on the shape of the magnetic field sensor, the ferromagnetic layers can be cross-anisotropically formed in various ways. In the case of a magnetic field sensor having ferromagnetic layers with various coercive forces, cross anisotropy can be formed as follows. For example, a first ferromagnetic layer of unbiased NiFe is formed on a flat surface under the influence of a magnetic field parallel to the plane of the substrate. After forming the intermediate layer,
A second ferromagnetic layer having a stronger coercive force is formed. Then, the second ferromagnetic layer is magnetized in a relatively strong magnetic field for a short time, and this magnetic field is made to be orthogonal to the anisotropic direction of the first ferromagnetic layer and to the plane of the substrate. Make them parallel.

For a type of magnetic field sensor having a first ferromagnetic layer of unbiased NiFe and a second ferromagnetic layer of biased NiFe, cross anisotropy can be formed as follows.
According to the first method, a magnetic field can be applied during the formation of both ferromagnetic layers, which magnetic field must be parallel to the surface of the substrate on which the ferromagnetic layers are formed. During the formation of the second ferromagnetic layer, the direction of this magnetic field is rotated about 90 ° around the normal to the surface of the substrate. According to the second method, it is also possible to instead form the ferromagnetic layers that lie in the plane of the magnetic field acting in the same direction when forming both ferromagnetic layers. In this case, the anisotropic directions of both ferromagnetic layers are parallel to each other. afterwards,
It is necessary to heat the magnetic field sensor for a short time and cool it in a magnetic field that is orthogonal to the anisotropic direction of the first ferromagnetic layer and is parallel to the substrate surface. As a result, the anisotropic direction of the first ferromagnetic layer is 90 ° rotated in the direction of the applied magnetic field. This technique is known under the name "magnetic cooling".
The second method can be used even when the order of the biased and non-biased ferromagnetic layers on the substrate is reversed.

[0019]

Embodiments of the magnetic field sensor of the present invention will be described in detail with reference to the drawings. For clarity, not all illustrated elements are shown to scale. FIG. 1 diagrammatically shows the structure of a known magnetic field sensor. Only the magnetoresistive material is shown for clarity and electrical connections are omitted. The illustrated material comprises a non-magnetic substrate 1 of eg Si, on which a first ferromagnetic layer 2 of eg bias-free NiFe is formed. For example, the Cu intermediate layer 3 is formed on the ferromagnetic layer 2. The second ferromagnetic layer 4 is deposited on the intermediate layer 3. The second ferromagnetic layer 4 can be formed of a ferromagnetic material such as a CoPt alloy having a coercive force different from that of NiFe. This example has the advantage of forming a Co or NiFe sublayer between the CoPt layer and the intermediate layer. However, the second ferromagnetic layer 4 can also be composed of non-biased NiFe. When the second ferromagnetic layer 4 is composed of non-biased NiFe, the second ferromagnetic layer 4 is composed of a NiFe sublayer and, for example, a FeMn sublayer, and the NiFe sublayer is formed between the intermediate layer and the FeMn layer. Deploy.

The ferromagnetic layers of the known magnetic field sensor are formed such that they exhibit a uniaxial in-plane anisotropy in a parallel orientation.
Depending on the manufacturing method, the magnetization directions of both ferromagnetic layers are parallel or antiparallel in the absence of an external magnetic field. When an external magnetic field H is applied parallel to the direction of magnetization of the magnetic field sensor, the magnetization of the first ferromagnetic layer 2 is correspondingly oriented (FIG. 1a). When the direction of the external magnetic field is reversed, the first
The magnetization direction of the ferromagnetic layer 2 is also reversed (Fig. 1b). Due to the stronger coercive force of the second ferromagnetic layer 4, ie the biasing effect on the second ferromagnetic layer 4, the magnetization direction of the second ferromagnetic layer 4 is reversed only when a fairly strong external magnetic field is applied.

FIG. 2 shows a part of the magnetic field sensor of the present invention. The combination of layers is the same as the combination of magnetic field sensors shown in FIG. Both ferromagnetic layers exhibit uniaxial in-plane anisotropy. The anisotropic direction of the first ferromagnetic layer 2 extends so as to cross the anisotropic direction of the second ferromagnetic layer 4.

When there is no external magnetic field (H = 0), the magnetization of the first ferromagnetic layer 2 extends transversely to the magnetization of the second ferromagnetic layer 4. This magnetization direction is determined by the anisotropy of these ferromagnetic layers which are oriented at an angle of approximately 90 ° to each other. This state is shown in FIG. 2a. When the magnetic field H is applied parallel to the magnetization direction of the second ferromagnetic layer 4, the first
The magnetization direction of the ferromagnetic layer 2 rotates in the direction of the applied magnetic field. The magnetization direction of the first ferromagnetic layer 2 is parallel to the magnetization direction of the second ferromagnetic layer 4 depending on the direction of the applied magnetic field (FIG. 2c).
Or facing non-parallel (Fig. 2d).

The rotation mechanism of the magnetization of the first ferromagnetic layer 2 of the magnetic field sensor of the present invention is different from the rotation mechanism of the magnetization of the first ferromagnetic layer 2 of the known magnetic field sensor. In the magnetic field sensor of FIG. 1, inversion occurs due to the deviation of the domain wall. This is caused by the intermittent change of the magnetization direction. In the magnetic field sensor of the invention of FIG. 2, the magnetization rotation is generated by a (coherent) rotation action. This process occurs due to a more uniform change in magnetization direction.

FIG. 3a shows the magnetoresistance curve measured by the conventional magnetic field sensor, and FIG. 3b shows the magnetoresistance curve measured by the magnetic field sensor of the present invention. Both magnetic field sensors were manufactured as follows. Ta (3nm), NiFe (8nm), Cu on a silicon substrate by DC magnetron sputtering
Layers of (2.5 nm), NiFe (6 nm), FeMn (8 nm) and Ta (2 nm) were sequentially formed. Apply these layers to a basic pressure of 5.10 to 7 Torr and
It was formed by Ar sputtering pressure of 7.10 to 3 Torr. First
During the sputtering of the second NiFe layer with the NiFe and FeMn layers, a magnetic field with an intensity of about 150 Oe (= 12 kA / m) was applied in the plane of these layers. In the known manufacture of magnetic field sensors, the direction of the magnetic field was the same during sputtering of these layers. In this way, a magnetic field sensor was obtained in which the plane anisotropy directions of both ferromagnetic layers were parallel to each other. In the manufacture of the magnetic field sensor of the present invention, a known magnetic field sensor is used in a magnetic field for a short time at 140 ° C.
And the direction of this magnetic field is rotated by 90 ° around the normal to the substrate surface with respect to the magnetic field applied during the sputtering of the two ferromagnetic layers. As a result, the anisotropic directions of the two ferromagnetic layers formed an angle of approximately 90 ° with each other.

FIG. 3a shows the magnetoresistance curve measured with the conventional magnetic field sensor described above. The resistance (R), which is a function of the applied magnetic field (H), is determined in the range -1.5kA / m to 1.5kA / m. FIG. 3b shows the magnetoresistance curve measured with the magnetic field sensor of the invention described above. From the drawing, the magnetic resistance is kA / m
It can be calculated that each is about 9.4%. Further, these graphs show that the hysteresis of the magnetic field sensor of the present invention is significantly smaller than that of the known magnetic field sensor.
Moreover, the hysteresis curve of the magnetic field sensor of the invention has a more regular tendency than the hysteresis curve of known magnetic field sensors.

FIG. 4a shows a top view of the microstructure of an experimental assembly that allows the performance of magnetic field sensors to be investigated. A single Au conductive strip 22 is formed on the ferrite substrate 21.
The conductive strip 22 is covered with an insulating layer 23 of SiO ₂ , on which two horizontal Au electrical contact strips 24, 25 are formed. The multilayer magnetic field sensor 26 is arranged so as to cross the gap between the strips 24 and 25. The multilayer sensor 26 has the following nominal components. 3nm Ta / 8nm Ni ₈₀ Fe ₂₀ /2.5nm Cu / 6nm N
The order of _{i 80 Fe 20 / 8nm FeMn /} 3nm Ta layer mentioned here is also a sequence was deposited these layers.

FIG. 4b shows an enlarged sectional view of the experimental assembly shown in FIG. 4a. In this figure, the cross section is taken along line 27. The same members in FIGS. 4a and 4b are designated by the same reference numerals. In the experiment for determining the performance of the multilayer magnetic field sensor 26, a test current I _{T is} passed along the conductive strip 22. The test current I _T has an amplitude of 2 mA RMS and a frequency of 520 Hz.
Of the superposed sine component and the constant bias component I _B. The presence of the oscillating current I _T induces an alternating magnetic flux of the multilayer magnetic field sensor 26, and the accompanying resistance change of the magnetic field sensor 26 becomes an experimental output, and this resistance change can be measured via the contact strips 24, 25. is there. This experimental output typically contains a number of spectral harmonics superimposed on the background noise level.

Comparative Example In this example, in the conventional magnetic field sensor 26 (see FIG. 1), the crystal magnetic anisotropy direction of the bias-free NiFe layer of the magnetic field sensor 26 is the exchange anisotropy of the bias NiFe layer of the magnetic field sensor 26. It is illustrated that the direction is parallel (or non-parallel). The spectral decomposition of the experimental output measured in this example is shown in [Table 1]. In this table, h _n represents the intensity of the n-th spectral harmonic relative to the intensity of the first spectral harmonic, and N is the first spectral harmonic. The strength of the background noise against the strength of the wave is shown, and all these strengths are shown in decibels (dB). The intensities shown in the table are listed for three different values of DC bias current I _B.

[Table 1]

EXAMPLE In this example, in the magnetic field sensor 26 of the present invention (see FIG. 2), the crystal magnetic anisotropy direction of the bias-free NiFe layer of the magnetic field sensor 26 is the exchange anisotropy of the bias NiFe layer of the magnetic field sensor 26. It is illustrated that it is orthogonal (perpendicular) to the direction. The spectral decomposition of the experimental output measured in this example is shown in [Table 2], in which h _n and N have the same meanings as in [Table 1].

[Table 2] From the viewpoint of the present invention, the noise level (N) in [Table 2]
And intensity of the average of the second and subsequent spectral harmonics (h _n) is significantly lower than that of Table 1.

FIG. 5a shows a perspective view of a magnetic head equipped with the magnetic field sensor of the present invention. The magnetic head comprises a magnetic field sensor 11 having an electrical connection 12. The magnetic head also comprises a magnetic substrate 13 of, for example, ferrite. Alternatively, the magnetic substrate 13 may be formed of a non-magnetic material in which a magnetic material such as NiFe is deposited. The magnetic head further comprises magnetic flux guides 14, 15. The magnetic substrate 13, the magnetic flux guides 14 and 15, and the magnetic field sensor 11 are arranged in relation to each other so that they form a magnetic circuit. The end surfaces 16 and 17 form a part of the magnetic pole surface of the magnetic head. A magnetic gap is formed between the end faces 16 and 17.

FIG. 5b shows a cross-sectional view of the magnetic head of FIG. 5a. This sectional view is a sectional view in which the end faces 16 and 17 are cut along a plane orthogonal to each other. The same members in FIGS. 5a and 5b are designated by the same reference numerals. Also shown in FIG. 5b is a current coil 18 which acts to adjust the magnetic field sensor 11.

The operation of the magnetic head according to the present invention will be described with reference to FIG. 5b. When the magnetic pole surface of the magnetic head passes through an information carrier 19 such as a magnetic tape, magnetic flux is generated in the above-mentioned magnetic circuit by the magnetic tape due to the information magnetically stored in the magnetic tape. This magnetic flux is also supplied to the magnetic field sensor. The magnetic flux changes are converted by the magnetic field sensor into changes in the resistance of the magnetic field sensor. Such a resistance change is converted into an electrical signal that can be measured by the electrical connection 12.

The magnetic head of the invention having the structure shown in FIGS. 5a and 5b is used only for reading the magnetic information on the magnetic carrier. FIG. 5c is a cross-sectional view of another magnetic head having a magnetic field sensor that can be used for reading as well as recording magnetic information on magnetic tape. The same members in FIGS. 5a, 5b and 5c are designated by the same reference numerals. In the magnetic head of FIG. 5c, the magnetic field sensor 11 is not incorporated in the magnetic circuit formed by the substrate 13 and the magnetic flux guide 14. For this purpose, the current coil 1
It can also be used to record eight electrical signals on the information carrier 19. With respect to the read function of the magnetic head shown in FIG. 5c, the magnetic circuits 13, 14 serve to protect the magnetic field sensor 11.

[Brief description of drawings]

1A and 1B show a part of a conventional magnetic field sensor.

2 a, b and c show part of a magnetic field sensor according to the invention, FIG.

3A shows a magnetic resistance curve of a conventional magnetic field sensor, and FIG. 3B shows a magnetic resistance curve of a magnetic field sensor of the present invention.

FIG. 4a shows a plan view of the microstructure of an experimental assembly with which the performance of a magnetic field sensor can be investigated, and b shows an enlarged sectional view of the experimental assembly shown in a.

FIG. 5 shows a magnetic head including the magnetic field sensor of the present invention.

[Explanation of symbols] 1 non-magnetic substrate 2 first ferromagnetic layer 3 intermediate layer 4 second ferromagnetic layer 11 magnetic field sensor 12 connection part 13 magnetic substrate 14, 15 magnetic flux guide 16, 17 end face 18 current coil 19 information carrier 21 ferrite substrate 22 conductive strips 23 insulating layers 24 and 25 contact strips 26 multilayer magnetic field sensor 27 along line I _T test current

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Rheiner Kuh Hoorn The Netherlands 5621 Beer Aindow Fenflunewachwech 1 (72) Inventor Adrianas Michael Johannes Martinus Honken The Netherlands 5621 Beer Aindow Fenflune Wachwech 1

Claims (6)

[Claims] 1. A magnetic field sensor comprising a magnetoresistive element having uniaxial in-plane anisotropy and having a substrate on which two ferromagnetic layers separated by a nonmagnetic intermediate layer are formed. A magnetic field sensor, wherein the anisotropic direction of the magnetic layer is orthogonal to the anisotropic direction of the other ferromagnetic layer. 2. A magnetic field sensor according to claim 1, wherein one ferromagnetic layer is made of NiFe and the other ferromagnetic layer is made of NiFe magnetically coupled to the antiferromagnetic layer. 3. The magnetic field sensor according to claim 2, wherein the antiferromagnetic layer is made of FeMn. 4. The magnetic field sensor according to claim 1, wherein the intermediate layer is made of Cu or a Cu alloy. 5. The ferromagnetic layer has a thickness of 2.5 to 10 nm, preferably 4 to 9 nm, and the intermediate layer has a thickness of 1.5 to 5 nm, preferably 2 to 3 nm. , 3
Alternatively, the magnetic field sensor according to item 4. 6. A magnetic head comprising the magnetic field sensor according to claim 1. Description: