Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2002-09-26
2004-11-16
Evans, Jefferson (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324200
Reexamination Certificate
active
06819532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exchange coupling film utilizing exchange coupling which acts between a ferromagnetic layer and an antiferromagnetic layer, a magneto-resistance effect device having the exchange coupling film, and a reproducing magnetic head and a magnetic random access memory which use the magneto-resistance effect device.
2. Description of the Related Art
A so-called spin valve type of magneto-resistance effect device is used for a magnetic random access memory (MRAM) which is notable as a reproducing magnetic head and a nonvolatile memory of a hard disk drive (HDD) apparatus corresponding to high magnetic recording density. A basic structure of the spin valve type of magneto-resistance effect device is a multilayer in which a ferromagnetic layer, a nonmagnetic material layer, the ferromagnetic layer, and an antiferromagnetic layer are formed in the order or in reverse. At this point, a magnetic moment of the ferromagnetic layer adjoining the antiferromagnetic layer is fixed by exchange coupling (it is also referred to as exchange bias) which acts between the antiferromagnetic layer and the ferromagnetic layer adjoining the antiferromagnetic layer so as not to rotate the magnetic moment to an external magnetic field, so that the ferromagnetic layer is referred to as a fixing layer or a pinning layer. Generally a multilayered film including the ferromagnetic layer and the antiferromagnetic layer is referred to as an exchange coupling film or an exchange bias film. The magnetic moment of the other ferromagnetic layer isolated from the pinning layer through the nonmagnetic material layer can be rotate responsive to the external magnetic field, so that the ferromagnetic layer is referred to as a free layer.
The spin valve type of magneto-resistance effect device is divided into two kinds by difference in the nonmagnetic material layer: (1) A giant magneto-resistance effect (GMR) device in which the nonmagnetic material layer includes nonmagnetic metal such as Cu and (2) A tunnel magneto-resistance effect (TMR) device in which the nonmagnetic material layer includes an insulating layer (tunnel barrier layer) such as aluminum oxide (AlO
X
). The TMR device is also referred to as a ferromagnetic tunnel junction device. In any of the devices, by utilizing a phenomenon that a relative angle defined by the magnetic moment of the free layer and that of the pinning layer changes as conductance of the device changes, information of a magnetic recording medium is read in case of a reproducing magnetic head of the HDD apparatus and information of stored bits is read in case of the MRAM. With respect to write, the magnetic moment of a recording bit in the magnetic recording medium is reversed by using a fringing field from a recording magnetic head in case of the HDD apparatus, the magnetic moment of the free layer in the device is reversed by a resultant magnetic field which current flown through a bit line and a word line forms in case of the MRAM.
The conductance of the device is dependent on cos &thgr;, where an angle defined by the magnetic moment of the free layer and that of the pinning layer is &thgr;, the conductance becomes a maximum in case that the both magnetic moments are parallel to each other (&thgr;=0°), the conductance becomes a minimum in case that the both magnetic moments are antiparallel to each other (&thgr;=180°).
In the GMR device and the TMR device, the GMR device differs completely from the TMR device in physical origin, however it is the same to utilize an effect that the conductance of the device changes corresponding to a change in the relative angle (&thgr;) of the magnetic moments of the both magnetic layers, namely the magneto-resistance effect. That is to say, magnetic resistance of the GMR device is derived from a difference in scattering length between an electron having spin-up and an electron having spin-down depending on the angle defined by the magnetic moment of the free layer and that of the pinning layer, on the other hand, the magnetic resistance of the TMR device is derived from a difference in tunnel probability between an electron having spin-up and an electron having spin-down depending on the angle.
FIG. 1
is a graph showing hysteresis curves of dependence of magnetization of the spin valve type of magneto-resistance effect device on a magnetic field (M-H curve) and dependence of resistance (inverse number of the conductance) on the magnetic field (R-H curve), where transverse axes are magnetic field strength and longitudinal axes are the magnetization and the resistance. The sharp hysteresis near the zero magnetic field corresponds to a magnetic rotation of the free layer, and the hysteresis appeared in the high magnetic field corresponds to the magnetic rotation of the pinning layer. The shift in the hysteresis of the pinning layer is derived from the exchange coupling (it is also referred to as the exchange bias) acting an interlayer between the pinning layer (ferromagnetic layer) and the antiferromagnetic layer adjoining the pinning layer (hereinafter referred to as ferromagnetic-layer/antiferromagnetic-layer interlayer), shift quantity H
ex
of the hysteresis of the pinning layer from the zero magnetic field is referred to as an exchange coupling magnetic field (it is also referred to as exchange bias magnetic field). A magnetization direction of the free layer is antiparallel to the magnetization direction of the pinning layer within an area between the hysteresis of the free layer and that of the pinning layer, the resistance of the device becomes larger and the conductance of the device becomes smaller within the area.
In the both spin valve type of magneto-resistance effect devices of the GMR and the TMR, it is necessary that the magnetic moment of the pinning layer is fixed in one direction for stable operation of the device, and it is necessary that the strong exchange coupling in the ferromagnetic-layer/antiferromagnetic-layer interlayer is generated and the hysteresis of the free layer is sufficiently separated from the hysteresis of the pinning layer, namely the area where the magnetic moment of the free layer is antiparallel to that of the pinning layer is extended. For the purpose, the exchange coupling magnetic field H
ex
must be increased while expansion of the hysteresis of the pinning layer, namely coercive force H
cp
shown in
FIG. 1
is decreased.
The exchange coupling energy J acting the ferromagnetic-layer/antiferromagnetic-layer interlayer is given as follows:
J=H
ex
×M
s
×t
(FORMULA 1)
where the exchange coupling magnetic field is H
ex
, saturation magnetization of the ferromagnetic layer is M
s
, and a film thickness of the ferromagnetic layer is t.
It is generally recognized that the exchange coupling energy J is decided by a combination of the antiferromagnetic material and the ferromagnetic material. As shown obviously in the formula 1, the exchange coupling magnetic field H
ex
increases as the film thickness of the ferromagnetic layer t decreases when the exchange coupling energy J is constant. However the exchange coupling magnetic field H
ex
may be increased seemingly by decreasing film thickness of the pinning layer, it is limited that the exchange coupling magnetic field H
ex
is increased only by decreasing film thickness of the pinning layer because the coercive force H
cp
is tend to increase with decreasing film thickness of the ferromagnetic layer t. When an oxide material typified by NiO or an ordered alloy typified by PtMn is used as the antiferromagnetic material, the coercive force of the pinning layer becomes larger than the exchange coupling magnetic field H
ex
, which causes a problem in device operation. Accordingly, in order to increase the exchange coupling magnetic field H
ex
, it is necessary that firstly the exchange coupling energy is increased, secondly the film thickness of the pinning layer is decreased within a range where operation of the magneto-resistance devi
Evans Jefferson
NEC Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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