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Three-terminal magnetoresistive elements with soft-magnetic thin film biasing
Journal of Magnetism and Magnetic Materials 114 (1992) 213-216 North-Holland
Three-terminal film biasing Masahiro
Kitada
magnetoresistive and Noboru
Central Research Laboratory,
elements with soft-magnetic
thin
Shimizu
Hitachi, Ltd., Kokubunji,
Tokyo 185, Japan
Received 11 October 1991; in revised form 27 January 1992
Three-terminal magnetoresistive (MR) elements with soft-magnetic thin film biasing have been evaluated for a highly sensitive magnetic sensor. Permalloy thin films are used as both MR and soft-magnetic biasing films. The thickness is 45 nm for MR, and 45 and 90 nm for biasing films. The size of the soft film is the same as the MR film. The cross-sectional structure is soft film(Ni-Fe)/insulator/MR(Ni-Fe) films. The soft films are separated from the MR films by insulator films. The distance between the MR and the soft films is 50 nm. MO/AI bilayer thin films are used as electrodes and leads. The MR response curves were observed to evaluate the element characteristics. The prepared elements show a complete differential response. The MR response curve form is noiseless. The bias strength depends on the element width and soft film thickness. The output voltage of the element initially increases linearly with increasing sensor current and then saturates. The effective output is about ten times that of the conventional shunt biasing element, when the sensor current is the same.
1. Introduction
Magnetoresistive sensors using permalloy thin films have been developed for the magnetic tape recording systems. Various biasing methods were proposed and evaluated to linearize the output wave form [l-3]. The outputs of the permanent magnet, current, and soft-magnetic film biasing sensors are effectively higher than those of shunt and barber-pole elements [4]. This is caused by the fact that the sensor current for the shunt biasing element is divided into the MR and shunt films. In contrast to this, the current is fully induced into the MR film- for the permanentmagnet and soft-magnetic film biasing elements. To increase the effective MR sensor output, the current handling should be increased. Therefore, the permanent magnet, current, and soft-magnetic film biasing sensors are suitable for highoutput heads. On the other hand, a differential to: Dr. M. Kitada, Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan.
Correspondence
0304-8853/92/$05.00
drive sensor is desirable to eliminate output wave-form distortion. In this research, a differential MR sensor biasing with soft-magnetic films [5] has been evaluated to develop a high-output MR sensor. This paper describes the basic characteristics of the elements.
2. Experimental
details
2.1. Element structure A schematic example of a differential MR sensor biasing with soft-magnetic films is shown in fig. 1. The soft-magnetic thin film for magnetic biasing is placed adjacent to the magnetoresistive thin film and separated from it by an insulating thin film. Three electrical leads are formed on the layered MR thin film to drive differentially. Opposing magnetic fields are induced by an electrical current. The induced magnetic fields are biased towards the soft-magnetic thin film side.
0 1992 - Elsevier Science Publishers B.V. All rights resewed
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
214
film
-6
-4
-2
0
2
4
6 kA/m
(b)
Fig. 1. Schematic view of a differential magnetorekive ment using soft magnetic thin films.
ele-
Therefore, the two MR sensor sections are biased in opposite directions to each other. The bias field necessary to differentially drive the element is l-l.5 kA/m. This can be obtained by a current density of the order of lo6 A/cm*. Half-channel MR response curves for the soft-magnetic film biasing magnetoresistive sensor, obtained by the finite element method [6-81, are shown in fig. 2(a). The differential MR responses are shown in fig. 2(b), when the soft-magnetic films are magnetized on both sides in opposite direction, as shown in fig. 1. 2.2. Element preparation The cross section of the differential MR sensor used in the present study is shown in fig. 3(a). The substrate used is Corning No. 7059 glass. The MR thin films are permalloy (Ni-19 wt%Fe) with nearly zero magnetostriction [S]. The film was prepared using a conventional electron beam deposition system and the thickness was 45 nm. The soft-magnetic film was also permalloy with
Fig. 2. Magnetoresistive response curves for a three-terminal element obtained by the finite-element method, (a) half-channel responses, (b) differential responses.
the same composition. The soft-film thickness was 45 and 90 nm, so the influence of soft-film thickness on the bias field strength could be established. The planar shape of the soft film was the same as that of the MR sensor film. The electrodes and leads of the differential elements were MO/AI [9,10]. The appearance of the element is shown in fig. 3(b). In the figure the soft film lies beneath the sensor film. The coercivity and anisotropic magnetic field of the permalloy thin films in a sheet condition were 160 and 368 A/m for the sensor film. The magnetoresistivity of the sensor film was approximately 2.7%. The MR responses were measured at a current density of the order lo6 A/cm* under an alternating magnetic field at 50 Hz.
3. Results and discussion 3.1. Magnetoresistive response The typical magnetoresistive responses of the prepared soft-magnetic film (thickness 45 nm)
Fig. 3. Differential magnetoresistive element, (a) cross-sectional structure, (b) optical micrograph.
215
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
OY
1
I
I
I
5
10
15
20
25
Current Fig. 5. Relationship between output and current of soft-magnetic film biasing elements, w indicates element width.
Fig. 4. Differential magnetoresistive response curves of softfilm (a) and shunt-biasing elements (b), the current is 10 mA.
biasing element with three terminals are shown in fig. 4(a). The curves show a complete differential response. The bias-field strength of the differential response increased with decreasing sensor width. It is thought that this is due to the effective anisotropic magnetic-field increasing with the decreasing width of the layered permalloy film. For example, the values were about 500 A/m for a 20 p,rn width and 700 A/m for a 12 km width element. The linearity of the differential response curve became less when the current decreased. This is due to the low linearity of half-channel response curves. The typical MR response curve of a conventional shunt-biasing element driven at the same current is shown in fig. 4(b). These were measured under optimal conditions. The change in voltage of the soft-biasing element was much higher than that of the shunt-biasing one.
is thought that the saturation and drop of output originated from the temperature increase of the MR film. Therefore, the saturation output may be dependent on the heat capacity and/or heat conductivity of the substrate used. In fact, the saturation behavior changed according to the substrate materials. In the figure, the output saturated to a relatively low current range because a glass substrate is used. 3.3. Bias-field strength The bias field as a function of soft-film thickness for the elements is shown in fig. 6. The bias
A',
w=l2pm / /
3.2. Differential magnetoresistive output
/
/
/
The relationship between the output and the current of the soft-magnetic film biasing element is shown in fig. 5. The soft-magnetic film thickness is 45 nm. The output increased initially linearly with increasing current. The value reached a maximum and then dropped quickly. It
\,
.@’ 0’
/” . /O
/
w=20pm
calculation 50
100
J 150
Soft-f i Im thicknesdnm) Fig. 6. Bias field as a function of soft-magnetic film thickness for differential elements, w indicates element width.
216
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
field increased with increasing soft-magnetic film thickness. The results agreed with the calculation. On the other hand, the value decreases with decreasing element width. This is due to an increase in the effective isotropic magnetic field of the permalloy sensor film.
pared and evaluated, to increase output. The elements showed a linear differential MR response. The obtained output voltage is approximately ten times that of the conventional shunt biasing element. This result was confirmed by calculation.
3.4. Discussion Permanent-magnet and current-biasing elements have been proposed for a high output MR sensor. Although a current-biasing element can be used as a differential element, a permanentmagnet biasing element cannot be used. That is, if the permanent-magnet film is formed on one side of the permalloy sensor film, an opposite biasing field is not obtained. To prepare differential element biasing with a permanent-magnet film, permanent-magnetic films for two half-channels must be formed on opposite sides of each other. For the current-biasing element, the device can be prepared in a similar manner to the shunt-biasing element. However, the device temperature rises due to biasing and sensor currents. This thermal effect enhances electromigration and thermal noise. For this reason, high-output elements biasing with current and permanent magnets are not suitable for use in a differential magnetoresistive sensor.
4. Conclusions A magnetoresistive sensor with three terminals, biasing with soft-magnetic films, was pre-
Acknowledgements We thank Dr. Yoshito Tsunoda and Ryo Suzuki, and Mr. Yoshikazu Tsuji, Kazuhiro Momata and Yoshihisa Kamo, for valuable discussions.
References [l] C. Tsang, J. Appl. Phys. 55 (1984) 2226. [2] Y. Kamo, M. Kitada and H. Tsuchiya, J. Appl. Phys. 57 (1985) 3797. [3] C. Tsang, M. Chen, T. Yogi and K. Ju, IEEE Trans, Magn. MAG-26 (1990) 1689. [4] M. Kitada, Y. Kamo and H. Tanabe, J. Appl. Phys. 58 (1985) 1667. [S] M. Kitada and N. Shim& Japanese patent application No. 60-38726 (19851, USP-4814919. [6] R. Potter, IEEE Trans. Magn. MAG-110974) 502. [7] K. Shinagawa, H. Fujiwara, F. Kugiya, T. Okuwaki, and M. Kudo, J. Appl. Phys. 53 (1982) 2585. [8] M. Kitada and N. Shimizu, Thin Solid Films 158 (1988) 167. [9] M. Kitada, H. Yamamoto and H. Tsuchiya, Thin Solid Films 122 (1984) 173. [lo] M. Kitada and N. Shimizu, J. Mater. Sci. 19 (1984) 1339.
Three-terminal film biasing Masahiro
Kitada
magnetoresistive and Noboru
Central Research Laboratory,
elements with soft-magnetic
thin
Shimizu
Hitachi, Ltd., Kokubunji,
Tokyo 185, Japan
Received 11 October 1991; in revised form 27 January 1992
Three-terminal magnetoresistive (MR) elements with soft-magnetic thin film biasing have been evaluated for a highly sensitive magnetic sensor. Permalloy thin films are used as both MR and soft-magnetic biasing films. The thickness is 45 nm for MR, and 45 and 90 nm for biasing films. The size of the soft film is the same as the MR film. The cross-sectional structure is soft film(Ni-Fe)/insulator/MR(Ni-Fe) films. The soft films are separated from the MR films by insulator films. The distance between the MR and the soft films is 50 nm. MO/AI bilayer thin films are used as electrodes and leads. The MR response curves were observed to evaluate the element characteristics. The prepared elements show a complete differential response. The MR response curve form is noiseless. The bias strength depends on the element width and soft film thickness. The output voltage of the element initially increases linearly with increasing sensor current and then saturates. The effective output is about ten times that of the conventional shunt biasing element, when the sensor current is the same.
1. Introduction
Magnetoresistive sensors using permalloy thin films have been developed for the magnetic tape recording systems. Various biasing methods were proposed and evaluated to linearize the output wave form [l-3]. The outputs of the permanent magnet, current, and soft-magnetic film biasing sensors are effectively higher than those of shunt and barber-pole elements [4]. This is caused by the fact that the sensor current for the shunt biasing element is divided into the MR and shunt films. In contrast to this, the current is fully induced into the MR film- for the permanentmagnet and soft-magnetic film biasing elements. To increase the effective MR sensor output, the current handling should be increased. Therefore, the permanent magnet, current, and soft-magnetic film biasing sensors are suitable for highoutput heads. On the other hand, a differential to: Dr. M. Kitada, Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan.
Correspondence
0304-8853/92/$05.00
drive sensor is desirable to eliminate output wave-form distortion. In this research, a differential MR sensor biasing with soft-magnetic films [5] has been evaluated to develop a high-output MR sensor. This paper describes the basic characteristics of the elements.
2. Experimental
details
2.1. Element structure A schematic example of a differential MR sensor biasing with soft-magnetic films is shown in fig. 1. The soft-magnetic thin film for magnetic biasing is placed adjacent to the magnetoresistive thin film and separated from it by an insulating thin film. Three electrical leads are formed on the layered MR thin film to drive differentially. Opposing magnetic fields are induced by an electrical current. The induced magnetic fields are biased towards the soft-magnetic thin film side.
0 1992 - Elsevier Science Publishers B.V. All rights resewed
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
214
film
-6
-4
-2
0
2
4
6 kA/m
(b)
Fig. 1. Schematic view of a differential magnetorekive ment using soft magnetic thin films.
ele-
Therefore, the two MR sensor sections are biased in opposite directions to each other. The bias field necessary to differentially drive the element is l-l.5 kA/m. This can be obtained by a current density of the order of lo6 A/cm*. Half-channel MR response curves for the soft-magnetic film biasing magnetoresistive sensor, obtained by the finite element method [6-81, are shown in fig. 2(a). The differential MR responses are shown in fig. 2(b), when the soft-magnetic films are magnetized on both sides in opposite direction, as shown in fig. 1. 2.2. Element preparation The cross section of the differential MR sensor used in the present study is shown in fig. 3(a). The substrate used is Corning No. 7059 glass. The MR thin films are permalloy (Ni-19 wt%Fe) with nearly zero magnetostriction [S]. The film was prepared using a conventional electron beam deposition system and the thickness was 45 nm. The soft-magnetic film was also permalloy with
Fig. 2. Magnetoresistive response curves for a three-terminal element obtained by the finite-element method, (a) half-channel responses, (b) differential responses.
the same composition. The soft-film thickness was 45 and 90 nm, so the influence of soft-film thickness on the bias field strength could be established. The planar shape of the soft film was the same as that of the MR sensor film. The electrodes and leads of the differential elements were MO/AI [9,10]. The appearance of the element is shown in fig. 3(b). In the figure the soft film lies beneath the sensor film. The coercivity and anisotropic magnetic field of the permalloy thin films in a sheet condition were 160 and 368 A/m for the sensor film. The magnetoresistivity of the sensor film was approximately 2.7%. The MR responses were measured at a current density of the order lo6 A/cm* under an alternating magnetic field at 50 Hz.
3. Results and discussion 3.1. Magnetoresistive response The typical magnetoresistive responses of the prepared soft-magnetic film (thickness 45 nm)
Fig. 3. Differential magnetoresistive element, (a) cross-sectional structure, (b) optical micrograph.
215
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
OY
1
I
I
I
5
10
15
20
25
Current Fig. 5. Relationship between output and current of soft-magnetic film biasing elements, w indicates element width.
Fig. 4. Differential magnetoresistive response curves of softfilm (a) and shunt-biasing elements (b), the current is 10 mA.
biasing element with three terminals are shown in fig. 4(a). The curves show a complete differential response. The bias-field strength of the differential response increased with decreasing sensor width. It is thought that this is due to the effective anisotropic magnetic-field increasing with the decreasing width of the layered permalloy film. For example, the values were about 500 A/m for a 20 p,rn width and 700 A/m for a 12 km width element. The linearity of the differential response curve became less when the current decreased. This is due to the low linearity of half-channel response curves. The typical MR response curve of a conventional shunt-biasing element driven at the same current is shown in fig. 4(b). These were measured under optimal conditions. The change in voltage of the soft-biasing element was much higher than that of the shunt-biasing one.
is thought that the saturation and drop of output originated from the temperature increase of the MR film. Therefore, the saturation output may be dependent on the heat capacity and/or heat conductivity of the substrate used. In fact, the saturation behavior changed according to the substrate materials. In the figure, the output saturated to a relatively low current range because a glass substrate is used. 3.3. Bias-field strength The bias field as a function of soft-film thickness for the elements is shown in fig. 6. The bias
A',
w=l2pm / /
3.2. Differential magnetoresistive output
/
/
/
The relationship between the output and the current of the soft-magnetic film biasing element is shown in fig. 5. The soft-magnetic film thickness is 45 nm. The output increased initially linearly with increasing current. The value reached a maximum and then dropped quickly. It
\,
.@’ 0’
/” . /O
/
w=20pm
calculation 50
100
J 150
Soft-f i Im thicknesdnm) Fig. 6. Bias field as a function of soft-magnetic film thickness for differential elements, w indicates element width.
216
M. Kitada, N. Shimizu / Three-terminal magnetoresistive elements
field increased with increasing soft-magnetic film thickness. The results agreed with the calculation. On the other hand, the value decreases with decreasing element width. This is due to an increase in the effective isotropic magnetic field of the permalloy sensor film.
pared and evaluated, to increase output. The elements showed a linear differential MR response. The obtained output voltage is approximately ten times that of the conventional shunt biasing element. This result was confirmed by calculation.
3.4. Discussion Permanent-magnet and current-biasing elements have been proposed for a high output MR sensor. Although a current-biasing element can be used as a differential element, a permanentmagnet biasing element cannot be used. That is, if the permanent-magnet film is formed on one side of the permalloy sensor film, an opposite biasing field is not obtained. To prepare differential element biasing with a permanent-magnet film, permanent-magnetic films for two half-channels must be formed on opposite sides of each other. For the current-biasing element, the device can be prepared in a similar manner to the shunt-biasing element. However, the device temperature rises due to biasing and sensor currents. This thermal effect enhances electromigration and thermal noise. For this reason, high-output elements biasing with current and permanent magnets are not suitable for use in a differential magnetoresistive sensor.
4. Conclusions A magnetoresistive sensor with three terminals, biasing with soft-magnetic films, was pre-
Acknowledgements We thank Dr. Yoshito Tsunoda and Ryo Suzuki, and Mr. Yoshikazu Tsuji, Kazuhiro Momata and Yoshihisa Kamo, for valuable discussions.
References [l] C. Tsang, J. Appl. Phys. 55 (1984) 2226. [2] Y. Kamo, M. Kitada and H. Tsuchiya, J. Appl. Phys. 57 (1985) 3797. [3] C. Tsang, M. Chen, T. Yogi and K. Ju, IEEE Trans, Magn. MAG-26 (1990) 1689. [4] M. Kitada, Y. Kamo and H. Tanabe, J. Appl. Phys. 58 (1985) 1667. [S] M. Kitada and N. Shim& Japanese patent application No. 60-38726 (19851, USP-4814919. [6] R. Potter, IEEE Trans. Magn. MAG-110974) 502. [7] K. Shinagawa, H. Fujiwara, F. Kugiya, T. Okuwaki, and M. Kudo, J. Appl. Phys. 53 (1982) 2585. [8] M. Kitada and N. Shimizu, Thin Solid Films 158 (1988) 167. [9] M. Kitada, H. Yamamoto and H. Tsuchiya, Thin Solid Films 122 (1984) 173. [lo] M. Kitada and N. Shimizu, J. Mater. Sci. 19 (1984) 1339.