Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2000-11-01
2004-05-25
Cao, Allen (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06741434
ABSTRACT:
TECHNICAL FIELD
The invention relates to a magnetic sensor and a method of producing the same and, particularly, relates to a magnetic sensor having a ferromagnetic tunnel junction used for a read head for high density magnetic record or a sensor for sensing a magnetic field and a method of producing the same.
BACKGROUND ART
In a junction of a laminate structure in which a metal layer, an insulation layer, and a metal layer are laminated in this sequence (in this description, such a laminate structure is denoted as “metal/insulator/metal”), it is known that in the case where the thickness of the insulation layer is substantially small (of the order of several hundreds picometers to several thousands picometers), a small electric current is passed when a voltage is applied between the metal layers on both sides. This phenomenon is called “tunnel effect”, and can be explained quantum-mechanically. Also, the current is called a tunnel current, and such a junction is called a tunnel junction.
As an insulation layer in such a tunnel junction, an oxidized metal film is conventionally used. For instance, a film of aluminum oxide formed by oxidizing a surface layer of aluminum by natural oxidation, plasma oxidation, or thermal oxidation is used as an insulation layer. The aluminum oxide film can have a thickness of the order of several hundreds picometers to several thousands picometers, which is needed for a tunnel effect, by control of the oxidation conditions.
A junction having a structure of ferromagnetic metal/insulator/ferromagnetic metal, which is formed to have metal layers of ferromagnetic material on both sides of a tunnel junction, is called a ferromagnetic tunnel junction. In this case, it is known that a-magnitude of tunnel current depends on the magnetization conditions of both ferromagnetic metal layers. The largest tunnel current is passed when the directions of magnetization of both layers are oriented in the same direction, and a small tunnel current is passed when the directions of magnetization of both layers are opposite to each other. It is explained that conductive electrons are polarized in a ferromagnetic material, and that this phenomena is caused by the electrons tunneling while retaining the polarization. The electron polarizing in one direction can only tunnel to the state in which electrons are polarized in that direction, and the electron polarizing in the opposite direction can, only tunnel to the state in which electrons are polarized in the opposed direction. When ferromagnetic substances of both metal layers sandwiching an insulation layer have the same direction of magnetization, tunneling can occur between the same states, and a tunnel current is large (a tunnel probability is high). When they have opposite directions of magnetization, tunneling cannot occur unless an electron in a state in which the electron polarizes in one direction and an electron in a state in which the electron polarizes in the opposite direction respectively find vacancies in states in the layer to which they are to tunnel and, in general, a tunnel current is small (a tunnel probability is small).
Thus, in the ferromagnetic tunnel junction, a tunnel probability (tunnel resistance) depends on the magnetization states of magnetic layers in both sides and, on this account, the tunnel resistance can be controlled by applying an external magnetic field to change the magnetization states of the magnetic layers.
In other words, a change in external magnetic field can be detected by the change in tunnel resistance.
A tunnel resistance R can be represented by the following equation:
R=R
s
+(½)&Dgr;
R
(1−cos &thgr;) (1)
wherein &thgr; is a relative angle of magnetization between both magnetic layers, R
s
is a tunnel resistance when the relative angle &thgr; is 0°, i.e. the directions of magnetization of both magnetic layers are parallel, and &Dgr;R is a difference between a tunnel resistance in the case where the directions of magnetization of both magnetic layers are parallel and a tunnel resistance in the case where the directions of magnetization of both layers are anti-parallel.
As is understood from equation (1), a tunnel resistance is smallest when the directions of magnetization of both magnetic layers are parallel, and is largest when they are anti-parallel. This is caused by that electrons in a ferromagnetic substance being polarized in spin. An electron is commonly in either an upward-spinning state or a downward-spinning state. The electron in the upward-spinning state is called an up-spin electron, and the electron in the downward-spinning state is called a down-spin electron.
In a non-magnetic substance, the numbers of up-spin electrons and down-spin electrons are equal to each other. On this account, a non-magnetic substance does not show magnetic properties, as a whole. On the other hand, in a ferromagnetic substance, the numbers of up-spin electrons and down-spin electrons are different from each other. Accordingly, the ferromagnetic substance has an upward or downward magnetization, as a whole.
It is known that when electrons tunnel in a tunnel junction, respective electrons tunnel retaining their spin state. An electron can tunnel to a magnetic layer if the layer has a vacancy in energy level corresponding to the spin state of the tunneling electron, and cannot tunnel if there is no vacancy in energy level.
A rate of change in tunnel resistance, &Dgr;R/R
s
, is represented by the following equation using a product of a polarizability of a magnetic layer of electron source and a polarizability of vacant energy level in a magnetic layer to which electrons are to tunnel:
&Dgr;
R/R
s
=2
P
1
P
2
/(1
−P
1
P
2
) (2)
wherein P
1
denotes of a spin polarizability of electron of an electron source, and P
2
denotes a spin polarizability of vacant energy level in a magnetic layer to which electrons are to tunnel. Further, P
1
and P
2
are represented as follows:
P
1
, P
2
=2(
N
up
−N
down
)/(
N
up
+N
down
) (3)
wherein N
up
denotes the number of up-spin electrons or the number of levels for up-spin electrons, and N
down
denotes the number of down-spin electrons or the number of levels for down-spin electrons.
The polarizabilities P
1
, P
2
depend the type of ferromagnetic material, and some materials may show a polarizability close to 50%. In this case, a rate of change in resistance of the order of several tens of percent can be expected, which is larger than a rate of change in resistance obtained by anisotropic magnetoresistance effect (AMR) or giant magnetoresistance effect (GMR). For example, it is theoretically predicted that a rate of change in resistance having a value of the order of 20 to 50% can be obtained when a ferromagnetic metal, such as Co, Fe, and Ni, is used in a magnetic layer, and values close thereto have been obtained experimentally. Thus, since a rate of change in resistance in tunnel effect is larger compared to that in conventional anisotropic magnetoresistance effect or giant magnetoresistance effect, an element using a ferromagnetic tunnel junction is expected to be applied to a magnetic sensor in the next generation of devices.
In a tunnel conjunction element, defects, such as pinholes, tend to be generated when a tunnel insulation film sandwiched between two magnetic layers has a small thickness. If a tunnel insulation film has an increased thickness in order to prevent the generation of pinholes, however, there is a problem of reduced rate of change in magnetic resistance.
On the other hand, when a ferromagnetic tunnel conjunction element is used as a magnetic sensor, in general, a magnetic field is applied while passing a constant current (sense current), and change in value of resistance is detected and is converted to a voltage to be output. The ferromagnetic tunnel effect is known to have dependency on applied voltage, and its rate of change in resistance varies dependent on the applied voltage. In
FIG. 1
, a representative result of measurements of
Kikuchi Hideyuki
Kobayashi Kazuo
Sato Masashige
Cao Allen
Fujitsu Limited
Greer Burns & Crain Ltd.
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