Dynamic magnetic information storage or retrieval – Head – Magnetoresistive reproducing head
Reexamination Certificate
2001-05-03
2004-12-21
Renner, Craig A. (Department: 2652)
Dynamic magnetic information storage or retrieval
Head
Magnetoresistive reproducing head
C360S324120
Reexamination Certificate
active
06833982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a magnetic tunnel junction (MTJ) device and more particularly to an MTJ device for use as a magnetoresistive (MR) head for reading magnetically-recorded data.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (DASD or disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors (MR heads) are preferred in the art because of their capability to read data at greater track and linear densities than earlier thin film inductive heads. An MR sensor detects the magnetic data on a disk surface through a change in the MR sensing layer resistance responsive to changes in the magnetic flux sensed by the MR layer.
The early MR sensors rely on the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetic moment of the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) changes the moment direction in the MR element, thereby changing the MR element resistance and the sense current or voltage.
The later giant magnetoresistance (GMR) sensor relies on the spin-scattering effect. In GMR sensors, the resistance of the GMR stack varies as a function of the spin-dependent transmission of the conduction electrons between two magnetic layers separated by a non-magnetic spacer layer and the accompanying spin-dependent scattering that occurs at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic (FM) material separated by a layer of non-magnetic conductive material (e.g., copper) are generally referred to as spin valve (SV) sensors.
In 1995, a new class of high magnetoresistive (MR) materials was discovered in which the nonmagnetic layer separating the two FM layers is made with an ultrathin nonconductive material, such as an aluminum oxide layer <20 Å thick. With the switching of magnetization of the two magnetic layers between parallel and antiparallel states, the differences in the tunneling coefficient of the junction and thus the magnetoresistance ratio have been demonstrated to be more than 25%. A distinctive feature of this magnetic tunnel junction (MTJ) class of materials is its high impedance (>100 k&OHgr;-&mgr;m
2
), which allows for large signal outputs.
A MTJ device has two ferromagnetic (FM) layers separated by a thin insulating tunnel barrier layer. MTJ operation relies on the spin-polarized electron tunneling phenomenon known in the art. One of the two FM layers (the reference layer) has a higher saturation field in one direction because of, for example, a higher coercivity, than the other FM layer (the sensing layer), which is more free to rotate in response to external fields. The insulating tunnel barrier layer is thin enough so that quantum mechanical tunneling occurs between the two FM layers. The tunneling phenomenon is electron-spin dependent, making the magnetic response of the MTJ a function of the relative moment orientations and spin polarizations of the two FM layers.
When used as memory cells, the MTJ memory cell state is determined by measuring the cell resistance to a sense current passed perpendicularly through the MTJ from one FM layer to the other. The charge carrier probability of tunneling across the insulating tunnel barrier layer depends on the relative alignment of the magnetic moments (magnetization directions) of the two FM layers. The tunneling current is spin polarized, which means that the electrical current passing from one of the FM layers, for example, the reference layer whose magnetic moment is pinned to prevent rotation, is predominantly composed of electrons of one spin type (spin up or spin down, depending on the reference orientation of the magnetic moment). The degree of spin polarization of the tunneling current is determined by the electronic band structure of the magnetic material composing the FM layer at the interface of the FM layer with the tunnel barrier layer. The FM reference layer thus acts as a spin filter for tunneling electrons. The probability of tunneling of the charge carriers depends on the availability of electronic states of the same spin polarization as the spin polarization of the electrical current in the FM sensing layer. When the magnetic moment of the FM sensing layer is parallel to that of the FM reference layer, more electronic states are available than when the two FM layer magnetic moments are antiparallel. Accordingly, charge carrier tunneling probability is highest when the magnetic moments of both layers are parallel and is lowest when the magnetic moments are antiparallel. Between these two extremes, the tunneling probability assumes some intermediate value, so that the electrical resistance of the MTJ memory cell depends on both the sense current spin polarization and the electronic states in both FM layers. As a result, the two orthogonal moment directions available in the free FM sensing layer together define two possible bit states (0 or 1) for the MTJ memory cell. Serious interest in the MTJ memory cell has lagged for some time because of difficulties in achieving useful responses in practical structures at noncryogenic temperatures.
The magnetoresistive (MR) sensor known in the art detects magnetic field signals through the resistance changes of a read element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux sensed by the read element. The conventional MR sensor, such as that used as a MR read head for reading data in magnetic recording disk drives, operates on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni
81
Fe
19
). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage.
The use of an MTJ device as a MR read head is also well-known in the art. One of the problems with the MTJ read head is the difficulty encountered in developing a sensor structure that generates an output signal that is both stable and linear with respect to the magnetic field strength sensed in the recorded medium. Some means is required to stabilize the magnetic domain state of the MTJ free FM sensing layer to prevent unacceptable Barkhausen noise arising from shifting magnetic domain walls within the free sensing layer. Also, some means for achieving a substantially linear response of the head is necessary for acceptable sensitivity. The longitudinal stabilization problem is particularly difficult in an MTJ MR read head because, unlike an AMR sensor, the MTJ sense current passes perpendicularly through the stack of FM and tunnel barrier layers so that any metallic materials in direct contact with the edges of the FM layers act to shunt (short-circuit) the read head sense resistance.
Practitioners have proposed several methods for resolving these problems to permit the use of MTJ sensors in magnetic read head applica
Hitachi Global Storage Technologies - Netherlands B.V.
Johnston Ervin F.
Renner Craig A.
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