Magnetic sensor

The magnetic sensor comprise a multi-layer structure 10 including a ferromagnetic layer 12 of FeCo alloy, an insulation layer 14 of Al.sub.2 O.sub.3 and a compound semiconductor layer 16 of GaAs. Circularly polarized light is irradiated to the compound semiconductor layer 16 to generate electrons. A dc voltage is applied to the ferromagnetic layer 12 and the compound semiconductor layer 16 by a dc power source 20 while circularly polarized light is irradiated to the compound semiconductor layer 16. When a direction of an external magnetic field changes, a magnetization direction of the ferromagnetic layer 12 accordingly changes, and a magnetoresistance between the ferromagnetic layer 12 and the compound semiconductor layer 16 changes. Changes of the magnetoresistance are measured by a voltmeter 22.

BACKGROUND OF THE INVENTION 
The present invention relates to a magnetic sensor for detecting magnetic 
fields, more specifically to a magnetic sensor which is suitably used in a 
magnetic read head of a magnetic recording apparatus. 
With the recent high densities of the magnetic recording technique, 
relative speeds between magnetic recording media and read heads have much 
decreased. The conventional induction type read heads have found it 
difficult to have sufficient reading signals. 
Then, in order that high reading signals are available at even the low 
relative speed, anisotropic magnetoresistive (AMR) effect magnetic heads 
for detecting magnetic fields themselves are developed. The anisotropic 
magnetoresistive effect is a phenomena that an electric resistance changes 
by some percentages corresponding to magnetized states of a magnetic 
substance. As materials of the anisotropic magnetoresistive effect 
magnetic heads (AMR magnetic heads), NiFe alloy (permalloy) is dominantly 
used. 
However, AMR magnetic heads can have some percentage changes in the reading 
signals, and magnetic sensors having reading output changes of higher 
percentages. As such magnetic sensors are proposed giant magnetoresistive 
(GMR) effect magnetic sensors using giant magnetoresistive effect, 
ferromagnetic tunnel junction magnetic sensor using ferromagnetic tunnel 
junction, and others. 
A ferromagnetic tunnel junction magnetic sensor includes a multi-layer body 
of a ferromagnetic layer/an insulating layer/ferromagnetic layer in which 
the insulating layer is sandwiched between the ferromagnetic layers. When 
a voltage is applied between the ferromagnetic layers to tunnel electrons, 
a tunneling probability of electrons changes depending on a relative angle 
of both ferromagnetic layers to a magnetization direction. This is because 
electron spin of one of the ferromagnetic layers, which supplies electrons 
is polarized, and electrons tunnel in the polarized state. 
A change of the tunneling probability is given by a product of 
polarizabilities of both ferromagnetic layers. That is, a difference 
.DELTA.R of a total resistance R between a maximum value and a minimum 
value is expressed by 
EQU .DELTA.R/R=2.times.P1.times.P2. 
Accordingly, in the ferromagnetic tunnel junction magnetic sensor, one of 
the ferromagnetic layer is formed of a material having a larger coercive 
force, and the other ferromagnetic layer is formed of a material of a 
smaller coercive force. When an external magnetic field changes, a 
magnetization direction of the ferromagnetic layer of the larger coercive 
force does not change, but a magnetization direction of the ferromagnetic 
layer of the smaller coercive force changes to agree to a direction of the 
external magnetic field. As a result, when a magnetization direction of an 
external magnetic field changes, a relative angle between magnetization 
directions of both ferromagnetic layers changes, whereby a tunnel current 
changes, and the external magnetic field can be detected. 
As described above, in the above-described ferromagnetic tunnel junction 
magnetic sensor it is necessary that one of the ferromagnetic layers is 
formed of a material having a large coercive force so that a magnetization 
direction does not change even when an external magnetic field changes. 
Accordingly, to use the ferromagnetic tunnel junction magnetic sensor as a 
magnetic read head, a material of the ferromagnetic layer is limited to a 
material, such as Fe, Co, Ni or others, a magnetization direction of which 
does not change. However, the polarizabilities of these materials are 
10-40%, and in principle changes of tunneling probabilities have upper 
limits. The coercive forces of these materials are tens to hundreds Oe, 
and when a magnetic field of a higher than such coercive force is applied 
due to a cause, characteristics are adversely changed. 
An object of the present invention is to provide a magnetic sensor which 
can overcome the above-described disadvantages, and has a high 
magnetoresistance ratio and is hard against disturbing magnetic fields. 
SUMMARY OF THE INVENTION 
The above-described object is achieved by a magnetic sensor comprising: a 
multi-layer body including a ferromagnetic layer, an insulation layer, and 
a GaAs-based compound semiconductor layer laid one on another; and 
irradiating means for irradiating circularly polarized light to the 
GaAs-based compound semiconductor layer of the multi-layer body, external 
magnetic fields being detected based on tunnel resistances between the 
ferromagnetic layer of the multi-layer body and the GaAs-based compound 
semiconductor layer thereof. 
In the above-described magnetic sensor, it is preferable that the 
ferromagnetic layer of the multi-layer body comprises: a first 
ferromagnetic layer contacting the insulation layer; and a second 
ferromagnetic layer contacting the first ferromagnetic layer and having a 
smaller coercive force than the first ferromagnetic layer. 
In the above-described magnetic sensor, it is preferable that a conducting 
layer is disposed on at least one side of the insulation film of the 
multi-layer body. 
In the above-described magnetic sensor, it is preferable that the second 
ferromagnetic layer is formed of permalloy or zero magnetostrictive NiFeCo 
alloy. 
In the above-described magnetic sensor, it is preferable that the first 
ferromagnetic layer is formed of Fe, Co, FeCo alloy or iron nitride. 
In the above-described magnetic sensor, it is preferable that the 
conducting layer is formed of Al, Cu, Ag or Au. 
In the above-described magnetic sensor, it is preferable that the 
insulation layer is formed of Al.sub.2 O.sub.3, SiO.sub.2, AlN, NiO or 
CoO. 
As described above, according to the present invention, the compound 
semiconductor layer supplies polarized electrons, whereby the magnetic 
sensor can have high detectivity. It is unnecessary to use a ferromagnetic 
material having a large coercive force, whereby the magnetic sensor can be 
hard against disturbances. 
According to the present invention, a first ferromagnetic layer having high 
polarizability is disposed in contact with the insulation layer which much 
influences magnetoresistance ratios, whereby high magnetoresistance ratios 
can be obtained. On the other hand, a second ferromagnetic layer is 
disposed on the first ferromagnetic layer having a small coercive force, 
whereby sufficient sensitivity can be obtained to small external magnetic 
fields.

DETAILED DESCRIPTION OF THE INVENTION 
The magnetic sensor according to a first embodiment of the present 
invention will be explained with reference to FIGS. 1 to 3. 
FIG. 1 is a view of the basic structure of the magnetic sensor according to 
the present embodiment. FIG. 2 is a graph of the result of the calculation 
of magnetic field dependence of magnetoresistance. FIG. 3 is a view of the 
magnetic sensor according to the present embodiment. 
As shown in FIG. 1, the basic structure of the magnetic sensor according to 
the present embodiment is a multi-layer structure 10 comprising a 
ferromagnetic layer 12 of FeCo alloy, an insulation layer 14 of Al.sub.2 
O.sub.3, and a compound semiconductor layer 16 of GaAs. 
The ferromagnetic layer 12 is formed of an about 10 nm-thick FeCo alloy 
layer of FeCo alloy, whose coercive force is 10 Oe, which is relatively 
low. The polarizability of the FeCo alloy is about 46%. When an external 
magnetic field changes, a magnetization direction of the ferromagnetic 
layer 12 changes to agree to a direction of the external magnetic field. 
The insulation layer 14 is about 5 nm-thick Al.sub.2 O.sub.3 layer which 
insulates the ferromagnetic layer 12 from the compound semiconductor layer 
16. 
The compound semiconductor layer 16 is an about 10 nm-thick GaAs layer. 
When circularly polarized light is irradiated to the GaAs layer, polarized 
electrons are generated as in the conventional ferromagnetic layer. The 
polarizability of the GaAs layer obtained when circularly polarized light 
is irradiated is 50%. 
To generate a large polarization in the compound semiconductor layer 16, it 
is preferable to make circularly polarized light incident at an angle as 
small as possible, because a polarization takes place in the compound 
semiconductor layer in the direction of advance of irradiated circularly 
polarized light. 
When an external magnetic field is detected, as shown in FIG. 1, a dc 
voltage is applied to the ferromagnetic layer 12 and the compound 
semiconductor layer 16 by a dc power source 20 while circularly polarized 
light is irradiated to the compound semiconductor layer 16. A voltage 
between the ferromagnetic layer 12 and the compound semiconductor layer 16 
is measured by a voltmeter. When a direction of an external magnetic field 
changes, a magnetization direction of the ferromagnetic layer 12 
accordingly changes, and a magnetoresistance between the ferromagnetic 
layer 12 and the compound semiconductor layer 16 changes. This 
magnetoresistance change is measured by the voltmeter 22. 
FIG. 2 shows changes of a magnetoresistance ratio .DELTA.R/R with changes 
of an external magnetic field H. As apparent in FIG. 2, a 
magnetoresistance ratio .DELTA.R/R is 0% when an external magnetic filed H 
is -40 Oe, but when the external magnetic field H changes from -40 Oe to 
+40 Oe, the magnetoresistance ratio .DELTA.R/R abruptly increases, and is 
46% when the external magnetic field H is above 40 Oe. 
The characteristics of the magnetoresistance ratio .DELTA.R/R relative to 
the external magnetic field H slightly exhibit hysteresis. 
FIG. 3 shows a specific structure of the magnetic sensor using the basic 
structure of FIG. 1. 
For example, the multi-layer body 10 of FIG. 1 is buried partially in the 
surface of a support substrate 30 of Al.sub.2 O.sub.3 --TiC with the 
compound semiconductor layer 16 exposed on the surface. 
An optical waveguide 32 is formed on the surface of the support substrate 
30 with one end faced to the multi-layer body 10. The end of the optical 
waveguide is cut slant so that light emitted from the optical waveguide 32 
enters the surface of the compound semiconductor layer 16. A quarter-wave 
plate 34 is disposed on the end so that linearly polarized light is 
changed to circularly polarized light. 
To detect an external magnetic field by the magnetic sensor, linearly 
polarized light enters the other end of the optical waveguide 32 and is 
changed to circularly polarized light by the quarter-wave plate to be 
irradiated to the compound semiconductor layer 16, and electric 
resistances between the compound semiconductor layer 16 of the multi-layer 
body 10 and the ferromagnetic layer 12 thereof is measured during the 
irradiation. 
By preparing a structure in which the ferromagnetic layer 12 of the 
multi-layer body 10 of the magnetic sensor of FIG. 3 passes near a 
magnetic storage medium, a magnetic read head for reading magnetic states 
stored in the magnetic storage medium can be realized. 
As described above, according to the present embodiment, the compound 
semiconductor layer can supply electrons having higher polarizabilities 
than the usual ferromagnetic materials do, whereby the magnetic sensor can 
have higher sensitivities than the conventional magnetic sensors. It is 
unnecessary to use a ferromagnetic material having coercive force, which 
makes the magnetic sensor hard against disturbances. 
The magnetic sensor according to a second embodiment of the present 
invention will be explained with reference to FIG. 4. FIG. 4 is a view of 
the basic structure of the magnetic sensor according to the present 
embodiment. Common or the same members of the present embodiment with or 
as the first embodiment are represented by the same reference numerals not 
to repeat or simplify the description. 
The basic structure of the present embodiment is characterized in that, as 
shown in FIG. 4, a ferromagnetic layer 12 is a two-layer structure 
including an about 2 nm-thick FeCo alloy layer 12a and an about 8 nm-thick 
NiFe alloy layer 12b. The FeCo alloy layer 12a is positioned on the side 
of an insulation layer 14. 
FeCo alloy has an about 46% polarizability, which is high, and a 10 Oe 
coercive force, which is relatively small, and it cannot be said that FeCo 
alloy is sufficiently sensitive to small external magnetic fields. On the 
other hand, NiFe alloy has an about 18% polarizability, which is low, and 
a 0.1 Oe coercive force, which is very small, and is sufficiently 
sensitive to small external magnetic fields. 
Then, in the present embodiment, the FeCo alloy layer 12a having high 
polarizability is disposed in contact with the insulation layer 14 which 
much influences the magnetoresistance ratio, whereby high 
magnetoresistance ratios are available. On the other hand, the NiFe alloy 
layer 12b having the small coercive force is disposed on the FeCo alloy 
layer 12a, whereby sufficient sensitivity to even small external magnetic 
fields is available. 
Thus, the magnetic sensor according to the present embodiment can have high 
magnetoresistance ratios and is sufficiently sensitive to external 
magnetic fields. 
The magnetic sensor according to a third embodiment of the present 
invention will be explained with reference to FIG. 5. FIG. 5 is a view of 
the basic structure of the magnetic sensor according to the present 
embodiment. Common or the same members of the present embodiment with or 
as the magnetic sensor of FIG. 4 are represented by the same reference 
numerals not to repeat or simplify the description. 
As shown in FIG. 5, the basic structure of the magnetic sensor according to 
the present embodiment comprises a conducting layer 18 of an about 3 
nm-thick Al layer sandwiched between a compound semiconductor layer 16 of 
GaAs and an insulation layer 14 of Al.sub.2 O.sub.3. 
The conducting layer 18 is included in consideration of the method for 
fabricating the magnetic sensor. 
In fabricating the multi-layer structure of the magnetic sensor, the 
compound semiconductor layer 16 is formed of a GaAs substrate, and the 
insulation layer 14, the ferromagnetic layers 12 (12a, 12b) are laid one 
on another in the stated order. However, it is very difficult to form the 
Al.sub.2 O.sub.3 layer of the insulation film 14 directly on the GaAs 
substrate. In view of this, in the present embodiment, the Al layer of the 
conducting layer 18 is formed on the GaAs substrate, and then the surface 
of the Al layer is oxidized to form the Al.sub.2 O.sub.3 layer of the 
insulation layer 14. The Al.sub.2 O.sub.3 layer having good quality can 
easily form. 
Furthermore, the insertion of the conducting film 18 does not deteriorate 
characteristics of the magnetic sensor. Polarized electrons generated in 
the compound semiconductor layer 16 pass through the conducting layer 18 
as they are and never change their polarized state. Accordingly the 
magnetic sensor is prevented from changing the magnetoresistance to 
deteriorate the characteristics. 
As described above, according to the present embodiment, the magnetic 
sensor can be simply fabricated without affecting the characteristics. 
The present invention is not limited to the above-described embodiments and 
includes various modifications. 
For example, in the present embodiments, the compound semiconductor layer 
is formed of GaAs layer but may be formed of any material as long as the 
material can generate polarized electrons by irradiation of circularly 
polarized light. For example, in place of the GaAs layer, a strained GaAs 
layer, or a superstructure of alternately laid GaAs thin layers can be 
used and AlGaAs thin layers. The polarizability of the strained GaAs layer 
is about 80%, and that of the superstructure is about 75%, which are 
higher than that of the GaAs layer. 
In the above-described embodiments, the ferromagnetic layer contacting the 
insulation layer is formed of FeCo alloy having the high polarizability 
but may be formed of another ferromagnetic material, e.g., Fe 
(polarizability: 44%), Co (polarizability: 34%), iron nitride 
(polarizability: above 50%). 
In the above-described embodiments, the ferromagnetic layer which is not in 
contact with the insulation layer is formed of NiFe alloy having the small 
coercive force but may be formed of another ferromagnetic material, e.g., 
zero magnetostrictive NiFeCo alloy (coercive force: about 1 Oe). Zero 
magnetostrictive NiFeCo alloy means alloys having compositions on the line 
.lambda.s=0 in the graph of FIG. 6. 
In the above-described embodiments, the insulation film is formed of 
Al.sub.2 O.sub.3 but may be formed of another material, e.g., SiO.sub.2, 
AlN, NiO or CoO. 
In the above-described embodiments, the conducting layer is disposed on the 
side of the insulation layer where the compound semiconductor layer is 
disposed but may be disposed on the side of the ferromagnetic substance. 
In the above-described embodiments, the conducting layer is formed of Al 
but may be formed of another material, e.g., Cu, Ag or Au.