Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
1998-10-30
2002-01-22
Ometz, David L. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
Reexamination Certificate
active
06341053
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetic tunnel junction device usable as a playback magnetic head of a high density magnetic disc device or a memory cell of a magnetic random access memory (MRAM) or an external magnetic field sensor.
BACKGROUND OF THE INVENTION
A magnetic tunnel junction device has a tunnel magnetoresistance response as a function of applied magnetic field. Each magnetic tunnel junction in the device is formed of a ferromagnetic pinned layer, a ferromagnetic free layer, and an insulating tunnel barrier between and in contact with the two ferromagnetic layers. Magnetization direction of the ferromagnetic pinned layer is prevented from rotating, while magnetization of the ferromagnetic free layer is free to rotate between states parallel and antiparallel to the fixed magnetization of the ferromagnetic pinned layer. In the parallel state, the tunneling current is at a maximum and the tunneling resistance at a minimum. On the other hand, in the antiparallel state, the tunneling current is at a minimum and the tunneling resistance is at a maximum. The direction of the magnetization can be altered by an external magnetic field. Thus, the tunneling resistance is sensitive to magnetic field.
In “Microstructured Magnetic Tunnel Junctions”, Journal of Applied Physics 81 (8), Apr. 15, 1997, pp. 3741-3746, Gallagher, et al. reported on magnetic tunnel junctions using a tunneling barrier formed by plasma oxidizing an Al layer, which demonstrated large magnetoresistance (MR) ratios (15% to 22%) at room temperatures in low field. They reported on processes used to make magnetic tunnel junction devices with dimensions on the micron-to-submicron scale. For achieving two stable states in zero magnetic field, they employed an antiferromagnetic layer to exchange bias one of the electrode layers. This electrode layer is referred to as the pinned layer. The other electrode layer is referred to as the free layer.
FIG. 5
illustrates, a typical example of a magnetic tunnel junction having a tunneling barrier formed by oxidizing an Al layer, which provides a maximum MR ratio of 21%. In
FIG. 5
, the magnetic tunnel junction is on a substrate
54
of silicon (Si) and is formed of a series of layers of material stacked one on top of the other. The magnetic tunnel junction, in this example, comprises a bottom electrode
55
of platinum (Pt) (20 nm thick), an initial ferromagnetic layer
56
of a nickel-iron (Ni—Fe) alloy (4 nm thick), an antiferromagnetic layer
57
of an iron-manganese (Fe—Mn) alloy (10 nm), a fixed or ferromagnetic pinned layer
58
of Ni—Fe (8 nm thick), a tunneling barrier layer
59
of aluminum oxide (Al
2
O
3
) formed by exposing a surface of an aluminum layer with 1.0 to 3.0 nm thick to an oxygen glow discharge, a ferromagnetic free layer
60
of cobalt (Co) (8 nm thick), and a top electrode of Pt (20 nm thick).
Magnetic tunnel devices including a tunneling barrier layer of Al
2
O
3
formed by exposing a surface of an aluminum layer to the ambient atmosphere are described in JP-A 6-244477, JP-A 5-63254, JP-A 8-70148, JP-A 8-70149 and JP-A 8-316548. Which are laid-open publications of Japanese Patent Applications relating to inventions by T. Miyazaki and M. Etsumura.
In “Relationship between the Barrier and Magnetoresistance Effect in Ferromagnetic Tunneling Junctions”, Journal of Japanese Applied Magnetism Society, Vol. 21, No. 4-2, 1977, pp. 493-496, N. Tesuka et al. reported on Fe/Al oxide/Fe junctions under varying oxidation conditions of the Al layer.
Fabrication of magnetic tunnel junctions at sizes below several microns is needed for their application to a playback magnetic head of a high-density magnetic disc device or a memory cell of a high density MRAM. In this case, the magnetic domain instabilities taking place in magnetic layers after a magnetic field has been applied cause a smaller signal to noise ratio. There remains a need, therefore, to fabricate magnetic tunnel junctions at sizes below several microns, which provide a larger signal to noise ratio in a magnetic field.
An object of the present invention is to accomplish the above-mentioned need.
SUMMARY OF THE INVENTION
A magnetic tunnel junction device according to one implementation of the present invention comprises a first pinning layer, a ferromagnetic free layer, a tunneling barrier layer, a ferromagnetic pinned layer, and a second pinning layer, which are stacked one on top of the other in this order. The first and second pinning layers may be in the form of antiferromagnetic layers, respectively. The ferromagnetic free layer is adjacent to the first pinning layer. Exchange coupling between the ferromagnetic free layer and the first pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic free layer along a track width direction. In other words, the first pinning layer has a pinning field, which pins a magnetization of the free layer in the track width direction. Exchange coupling between the ferromagnetic pinned layer and the second pinning layer develops a magnetic anisotropy, which aligns magnetization of the ferromagnetic pinned layer along a MR height direction. In other words, the second pinning layer has a pinning field, which pins a magnetization of the pinned layer In the MR height direction.
In the present application, a direction, in the plane of stalked layers of a magnetic tunnel junction, along the applied external magnetic field direction is called a MR height direction. A traverse direction, in another plane of the layers, forming right angles to the applied external magnetic field direction is called a track width direction.
A process of fabricating a magnetic tunnel junction comprises the step of forming a series of layers one on top of the others, the series of layers including a first antiferromagnetic pinning layer, a ferromagnetic free layer, a tunneling barrier layer, a ferromagnetic pinned layer, and a second antiferromagnetic pinning layer. The process also comprises the step of heating the layers at a temperature higher than a blocking temperature T
B2
of the material of the second antiferromagnetic pinning layer in a magnetic field directed parallel to a MR height direction. The process further comprises the step of heating the layers at a temperature higher than a blocking temperature T
B1
of the material of the first antiferromagnetic pinning layer in a magnetic field directed parallel to a track width direction.
It is required for suppressing noise upon sensing an external magnetic field to continuously vary the direction of magnetization in a ferromagnetic free layer of a magnetic tunnel junction after application of the field. For accomplishing this continuous variation, it is effective to develop a unidirectional magnetic domain, in the free layer, having a magnetic anisotropy, which aligns magnetization of the free layer along a track width direction that form right angles to the applied external magnetic field direction. There is a relation between noise and the magnetization direction of the free layer, which direction rotates upon application of external magnetic field. If the magnetization of the free layer is aligned in a MR height direction that is parallel to the applied external field direction, variation in the direction of magnetization due to the applied external field is in discontinuous magnetic domain displacement mode, thus providing a magnetoresistance (MR) curve with hysteresis. In the case where the magnetization of the free layer is aligned in the track width height direction, variation in the direction of magnetization due to the applied external field is in continuous magnetic domain rotation mode, thus providing a magnetoresistance (MR) curve without hysteresis.
According to one implementation of the present invention, therefore, the antiferromagnetic pinning layer is employed to exchange bias the free layer to induce a unidirectional magnetic anisotropy. The magnetic field induced due to the exchange coupling between the pinning layer and the free layer is larger in ma
Ishiwata Nobuyuki
Kamijo Atsushi
Matsutera Hisao
Nakada Masafumi
Tsuge Hisanao
NEC Corporation
Ometz David L.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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