Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2002-03-14
2003-11-25
Evans, Jefferson (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06654212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic sensor for detecting a magnetic field by utilizing the tunneling magnetoresistive effect. More particularly, the present invention relates to a magnetic sensor, which is capable of providing a stable resistance change rate and can be formed with good reproducibility, as well as to a method for manufacturing the magnetic sensor.
2. Description of the Related Art
A GMR (giant magnetoresistive) sensor exhibiting the giant magnetoresistive effect is employed as a head mounted on, e.g., a hard disk drive and dedicated to reproduction, and it is known as having a high sensitivity.
Among various types of GMR sensors, there is a spin valve film having a relatively simple structure and having resistance capable of being changed by a weak external magnetic field. The spin valve film is of a four-layered structure in the simplest form.
FIG. 19
schematically partly shows a structure of a spin valve film.
FIG. 19
is a sectional view looking from the side facing a recording medium.
In
FIG. 19
, reference numerals
1
and
3
each denote a ferromagnetic layer made of a NiFe alloy, for example. A nonmagnetic electrically conductive layer
2
made of Cu, for example, is interposed between the two ferromagnetic layers.
In the spin valve film of
FIG. 19
, the ferromagnetic layer
1
is the so-called free magnetic layer, and the ferromagnetic layer
3
is the so-called pinned magnetic layer. In this specification, the ferromagnetic layer
1
and the ferromagnetic layer
3
will be referred to as “free magnetic layer” and “pinned magnetic layer” hereinafter, respectively.
Also, as shown in
FIG. 19
, an antiferromagnetic layer
4
made of a NiMn alloy, for example, is formed on the pinned magnetic layer
3
in contact with each other. With annealing a carried out under a magnetic field, an exchange anisotropic magnetic field is generated between the pinned magnetic layer
3
and the antiferromagnetic layer
4
, whereby magnetization of the pinned magnetic layer
3
is pinned in the height direction (Y-direction as shown).
On the other hand, magnetization of the free magnetic layer
1
is affected by a bias layer (not shown), etc., and. aligned in the track-width direction (X-direction as shown). The magnetized directions of the pinned magnetic layer
3
and the free magnetic layer
1
are thus in a crossing relation.
A pair of electrode layers
5
,
5
are provided, as shown in
FIG. 19
, on both sides of a multilayered film, including from the free magnetic layer
1
to the antiferromagnetic layer
4
, as viewed in the track-width direction (X-direction). The electrode layers
5
,
5
are formed of, e.g., Cu (copper), W (tungsten) or Cr (chromium).
In the spin valve film shown in
FIG. 19
, when the magnetized direction of the free magnetic layer
1
is varied depending on a leakage magnetic field from a recording medium such as a hard disk, electrical resistance is changed based on correlation to the magnetized direction of the pinned magnetic layer
3
, whereby a voltage change is resulted depending on a change in value of the electrical resistance. In accordance with such a voltage change, the leakage magnetic field from the recording medium is detected. A resistance change rate (MR ratio) of the spin valve film is in the range of about several to ten-odd percentages.
Meanwhile, with the recent progress toward higher recording density, the plane recording density of a hard disk drive has been increased more and more. Such a trend raises a problem as to whether GMR sensors primarily used at present are adaptable for higher recording density expected in the future.
In that situation, attention has been focused on a tunneling magnetoresistive sensor as a reproduction head to be employed in place of GMR sensors. The tunneling magnetoresistive sensor has a resistance change rate (TMR ratio) as high as several tens of percent, and hence is able to produce a much greater reproduction output than that obtainable with the GMR sensors.
FIG. 20
schematically partly shows a structure of a conventional tunneling magnetoresistive sensor.
FIG. 20
is a sectional view looking from the side facing a recording medium.
In
FIG. 20
, as with the spin valve film shown in
FIG. 19
, numerals
1
and
3
denote a free magnetic layer and a pinned magnetic layer, respectively. An antiferromagnetic layer
4
is formed on the pinned magnetic layer
3
in contact with each other.
The structure of the tunneling magnetoresistive sensor differs from that of the spin valve film primarily in the following points. An insulation barrier layer
6
made of Al
2
O
3
(alumina), for example, is formed between the free magnetic layer
1
and the pinned magnetic layer
3
. Also, a pair of electrode layers
5
,
5
are provided on both sides of a multilayered film, including from the free magnetic layer
1
to the antiferromagnetic layer
4
, as viewed in the vertical direction (Z-direction as shown) relative to surfaces of the multilayered film.
In the tunneling magnetoresistive sensor, when a voltage is applied to the two ferromagnetic layers (i.e., the free magnetic layer
1
and the pinned magnetic layer
3
), an electric current (tunnel current) flows through the insulation barrier layer
6
based on the tunnel effect.
In the tunneling magnetoresistive sensor, as with the spin valve film, magnetization of the pinned magnetic layer
3
is pinned in the Y-direction as shown, and magnetization of the free magnetic layer
1
is aligned in the X-direction as shown. Then, the magnetized direction of the free magnetic layer
1
is varied under an influence of an external magnetic field.
When the magnetized directions of the pinned magnetic layer
3
and the free magnetic layer
1
are antiparallel to each other, the tunnel current is hardest to flow and the resistance value is maximized. When the magnetized directions of the pinned magnetic layer
3
and the free magnetic layer
1
are parallel to each other, the tunnel current is easiest to flow and the resistance value is minimized.
Upon the magnetized direction of the free magnetic layer
1
being varied under the influence of the external magnetic field, a resulting change in electrical resistance is taken out as a voltage change, whereby a leakage magnetic field from the recording medium is detected.
A resistance change rate (TMR ratio; &Dgr;R
TMR
) of the tunneling magnetoresistive sensor is expressed by 2P
P
P
F
/(1−P
P
P
F
). Herein, P
P
represents a spin polarization rate (i.e., difference in the number of electrons between upward spins and downward spins, which is normalized based on the total number of electrons; referred to simply as a “polarization rate” hereinafter) of the pinned magnetic layer. P
F
represents a polarization rate of the free magnetic layer. As seen from that formula, the resistance change rate is determined depending on the polarization rate of the ferromagnetic layer. Theoretically, the resistance change rate is increased as the polarization rate increases.
In most of conventional tunneling magnetoresistive sensors, the insulation barrier layer
6
is formed of Al
2
O
3
(alumina).
With the recent progress toward higher recording density, however, the following problems have occurred in the conventional tunneling magnetoresistive sensors using Al
2
O
3
as a material of the insulation barrier layer
6
.
(1) The first problem resides in dielectric strength. The insulation barrier layer
6
is formed in a very small thickness as thin as 1 to 2 nm. When the insulation barrier layer
6
is formed in such a very small thickness, the film made of alumina cannot provide a satisfactory level of dielectric withstand voltage. Hence, when a large electric current flows through the tunneling magnetoresistive sensor, the insulation barrier layer
6
is apt to cause breakdown.
In particular, the tunneling magnetoresistive sensor has a large sensor resistance, and when a voltage is applied to the two ferromagnetic layers (i.e., the free magnetic layer
1
an
Alps Electric Co. ,Ltd.
Brinks Hofer Gilson & Lione
Evans Jefferson
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