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 kΩ-μm2), 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 (Ni81 Fe19).  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 applications.  For example, in U.S. Pat.  No.  5,729,410 (and later, in U.S. Pat.  No.  6,005,753), Fontana, Jr. et al.  describe a MTJ device where the sensing (free) FM layer magnetic moment is longitudinally biased by a layer of hard FM material located near but separated slightly from the side edges thereof (and later, from the back edge thereof for added transverse biasing) by an intervening layer of electrically insulating material.  The insulating layer isolates the hard biasing material from the electrical leads and the sensing FM layer to prevent shunting of the sense current to the hard biasing material without interfering with the perpendicular sense current flow through the layers in the stack.  Similarly, in U.S. Pat.  No.  6,097,579, Gill proposes sandwiching a permanent magnet layer between two thin dielectric layers to provide longitudinal baising of the MTJ free layer.  However, the Fontana, Jr, et al.  and the Gill approaches are problematic to manufacture because they generally rely on extremely thin insulation layers to allow sufficient magnetostatic coupling to reduce Barkhausen noise in the free FM layer without shunting the sense current. 
FIG.  1A shows an illustrative embodiment of a magnetic tunnel junction (MTJ) sensor 10 from the prior art.  Sensor 10 is viewed from the air bearing surface (ABS) so that, in operation, the magnetic medium (not shown) moves in the image plane vertically with respect to MTJ sensor 10.  MTJ sensor 10 includes an MTJ stack 12 disposed between a first shield (S1) layer 14 and a second shield (S2) layer 16.  MTJ stack 12 may be characterized as an upper electrode 18 separated from a lower electrode 20 by a tunnel barrier 22.  Upper electrode 18 includes a ferromagnetic (FM) pinned layer 24 having a magnetic moment that is pinned by an exchange-coupled antiferromagnetic (AFM) layer 26, and a second lead (L2) layer.  The lower electrode 20 includes a FM free layer 30 and a first lead (L1) layer 32.  MTJ stack 12 operates in the usual manner known in the art except that the stabilization biasing of free layer 30 is provided by a hard magnetic (HM) layer 34 disposed on each side of MTJ stack 12.  To prevent a loss of sensitivity from undesired sense current shunting, HM layers 34 are sandwiched between two insulating layers 36 and 38 substantially as shown.  Practitioners in the art can readily appreciate that the several layers outside of MTJ stack 12 should be precisely created in a series of steps following an initial etching procedure.  The usual processes known in the art give rise to misalignment between the narrow ends of the various layers at the edges of MTJ stack 12, leading to unit performance variations and high unit rejection rates. 
FIG.  1B shows an air bearing surface (ABS) view of another illustrative embodiment of a MTJ sensor 40 from the prior art.  MTJ sensor 40 can be considered to include the end regions 42 and 44 separated from each other by a central region 46.  The active region of MTJ sensor 40 is the MTJ stack 48 formed in the central region 46.  MTJ stack 48 has a generally rectangular shape with a front face (shown) at the ABS, a back edge (not shown) opposite to the front edge and two opposite side edges 50 and 52.  MTJ stack 40 includes a first electrode 54 and a second electrode 56 between which is disposed a tunnel barrier layer 58.  First electrode 54 includes a pinned layer 60, an AFM layer 62 and a seed layer 64, where pinned layer 60 is disposed between tunnel barrier layer 58 and AFM layer 62, which is disposed between pinned layer 60 and seed layer 64.  Second electrode 56 includes a free layer 66 and a cap layer 68, where free layer 66 is disposed between tunnel barrier layer 58 and cap layer 68.  AFM layer 62 is exchange coupled to pinned layer 60 providing an exchange field to pin the magnetization direction of pinned layer 60 perpendicular to the ABS.  The magnetization of free layer 66 is oriented parallel to the ABS (absent other external magnetic fields) and is free to rotate in the presence of a signal magnetic field.  As with MTJ sensor 10 (FIG.  1A), free layer stabilization bias is provided by the HB layers 70, which are sandwiched between the insulation layers 72 and 74 to prevent sensitivity losses through shunting of MTJ stack 48. 
In U.S. Pat.  No.  5,930,087, Brug et al.  disclose a flux-guide MTJ sensor having a FM free layer that extends beyond the active region (MTJ stack) to the sides and also to the rear and the front (to the ABS).  They suggest in passing that the longitudinal biasing layer placed on the flux guide adjacent to each side of a flux-guide MTJ stack may consist of antiferromagnetic materials such as terbium-iron or nickel-oxide (a nonconductor), but Brug et al.  appear to prefer using antiferromagnetic (AFM) manganese compounds or permanent magnetic layers and neither consider nor suggest specific solutions to the MTJ sense current shunting problem arising from such MTJ sensor geometries. 
There is accordingly a need in the MR sensor art for an effective MTJ longitudinal biasing technique that can be implemented using simpler, more reliable fabrication methods leading to higher yields and more consistent unit performance without the sensitivity loss arising from sense current shunting.  These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.