Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-07-25
2003-12-02
Miller, Brian E. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C324S252000
Reexamination Certificate
active
06657830
ABSTRACT:
PRIORITY TO FOREIGN APPLICATIONS
This application claims priority to Japanese Patent Application No. P2001-013958.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information recording-reproducing apparatus and a magnetic recording-reproducing head to be mounted thereon, and, more specifically, the present invention relates to an information recording medium that retains information by means of inverted magnetic domains on a magnetic recording film formed on a substrate and an information reproducing apparatus designed to reproduce information by detecting leakage magnetic fluxes from the recording medium.
2. Description of the Background
In recent years, magnetic disk apparatuses have greatly increased in recording density, with the track size for recording bits becoming smaller and smaller. These smaller domains require the magnetic reproducing head to have a higher sensitivity than past devices. One such reproducing head is reported in “Nikkei Electronics” No. 774, Jul. 17, 2000, pp. 177-184. The device disclosed in this reference employs a tunnel magnetoresistive film as a next-generation super-sensitive magnetic sensor.
This first conventional example is characterized by a patterned laminate structure consisting of a lower magnetic shield layer, an electrode layer, a soft magnetic free layer, a non-magnetic insulating layer, a ferromagnetic pinned layer, an antiferromagnetic layer to fix the direction of magnetization of the ferromagnetic pinned layer, and an electrode layer. The laminate film has, at both ends thereof, a hard magnetic layer to stabilize the direction of magnetization of the non-magnetic free layer and also has an insulating film to insulate the upper and lower magnetic shields.
In the above-mentioned example, the soft magnetic free layer is formed from a CoFe alloy; the non-magnetic insulating film is formed from aluminum oxide; and the ferromagnetic pinned layer is formed from a CoFe alloy. The sensor film has a very low resistance and a very high MR ratio (calculated by dividing the maximum resistance change due to applied magnetic field by the initial resistance) at room temperature. For instance, a sensor film having a resistance per area of 33.5 &OHgr;·&mgr;m
2
has an MR ratio of 31.6%. A sensor film having a resistance per area of 14.2 &OHgr;·&mgr;m
2
has an MR ratio of 24.4%. A sensor film having a resistance per area of 5.6 &OHgr;·&mgr;m
2
has an MR ratio of 12.3%.
A second known example of sensor film is disclosed in Physical Review Letters, Vol. 82, No. 21, pp. 4288-4291. This sensor film employs a laminate film consisting of CoFe alloy, SrTiO
3
, and La
0.7
Sr
0.3
MnO
3
. It gives a high MR ratio (50% maximum) with a bias voltage (Vs) of −0.4 V at 4.2K.
A third known example of sensor film is disclosed in Physical Review Letters, Vol. 82, No. 3, pp. 616-619. This sensor film employs a laminate film consisting of Ni
0.8
Fe
0.2
, Ta
2
O
5
, Al
2
O
3
, and Ni
0.8
Fe
0.2
. It gives an MR ratio of 4% with a bias voltage (Vs) of −0.2 V at room temperature.
The above-mentioned known examples may be characterized by one or more of the following disadvantages. First, with respect to a sensor film having a low resistance and a high MR ratio at room temperature, such a tunnel magnetoresistive sensor in the form of CoFe/Al oxide/CoFe laminate film may include an MR ratio which steeply decreases (as shown in
FIG. 11
) when a bias voltage is applied across the two CoFe layers. A bias voltage (Vh) of approximately 0.4 V may decrease the MR ratio by up to half from that without bias voltage. When applied to a magnetic read head, this sensor film may have a decreased output in proportion to the bias voltage, unlike the known giant magnetoresistive magnetic read in practical use. Additionally, the tunnel magnetoresistive head, unlike the conventional giant magnetoresistive magnetic read head, may have a decreased signal-to-noise ratio because of its inherent shot noise proportional to the bias voltage.
In order to address one or more of these potential problems and to realize a practical tunnel magnetoresistive head suitable for magnetic recording-reproducing apparatuses with very high recording density, it is preferred to reduce the head resistance. Toward this end, it may be preferable to reduce the thickness of the aluminum oxide film used as the non-magnetic insulating film.
The second known example preferably only needs to meet less stringent requirements for head resistance than the conventional head of the first known example because the MR ratio, which is measured at 5K, reaches the maximum in the vicinity of the head-operating voltage (Vs=−0.5 V). However, a problem may ensure because La
0.7
Sr
0.3
MnO
3
is a substance which undergoes phase transition from ferromagnetic material to paramagnetic material in the neighborhood of room temperature. In other words, its MR ratio becomes almost zero at 0-60° C. (which is the operating temperature of the magnetic recording apparatus).
In the case of the third known example, the MR ratio reaches a maximum in the vicinity of the head-operating voltage (Vs=−0.2 V). This MR ratio, however, is smaller than that of the giant magnetoresistive head in practical use at the present. Therefore, the third known example may not be suitable for future magnetic recording-reproducing apparatuses with very high recording densities.
SUMMARY OF THE INVENTION
At least one embodiment of the present invention is directed to a magnetic sensor which preferably comprises a soft magnetic layer and a ferromagnetic layer, with a non-magnetic layer interposed between them such that the magnetization of said ferromagnetic layer is fixed with respect to the magnetic field to be detected. The invention may also include a magnetoresistive film that changes in magnetoresistance accordingly as the magnetization of the soft magnetic layer rotates in response to the external magnetic field, thereby changing the relative angle with the magnetization of the ferromagnetic layer. The magnetoresistive film may show a change in magnetoresistance upon the application of a detecting current across the soft magnetic layer and the ferromagnetic layer through the non-magnetic layer, with the ratio of change (in absolute value) in magnetoresistance of the magnetoresistive film having a maximum value greater than 20% at a temperature in the range from 0° C. to 60° C. and with a bias voltage (Vs) applied across the ferromagnetic layer and the soft magnetic layer being in the range from +0.2 to +0.8 V and from −0.8 to −0.2 V.
The present invention is also preferably directed to a magnetic sensor of tunnel junction laminate structure comprising a soft magnetic free layer, a non-magnetic insulating layer, and a ferromagnetic pinned layer, wherein the ferromagnetic pinned layer has a spin valve layer whose magnetization is fixed with respect to the magnetic field to be detected. The soft magnetic free layer may permit its magnetization to rotate in response to the external magnetic field, thereby changing the relative angle with the magnetization of the ferromagnetic pinned layer and producing the magnetoresistive effect, with the absolute value of the magnetoresistive effect having a peak at a temperature in the range from 0° C. to 60° C. and for a bias voltage Vs (applied across the ferromagnetic pinned layer and the soft magnetic free layer) in the range from +0.2 to +0.8 V and from −0.8 to −0.2 V.
In the tunnel magnetoresistive magnetic sensor, the ferromagnetic pinned layer may be formed from Fe
3
O
4
or at least one oxide or compound of Cr and Mn. Additionally, in the tunnel magnetoresistive magnetic sensor, the nonmagnetic insulating layer may be formed from at least one oxide of Sr, Ti, and Ta. Moreover, in the tunnel magnetoresistive magnetic sensor, the soft magnetic free layer may be a layer of Co/Fe alloy formed on the nonmagnetic insulating layer or a laminate layer consisting of a layer of Co/Fe alloy and a layer of Ni/Fe
Hayakawa Jun
Ito Kenchi
Hitachi , Ltd.
Miller Brian E.
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