Magnetoresistive sensor employing nitrogenated Cu/Ag under-layers with (100) textured growth as templates for CoFe, CoFeX, and Co2(MnFe)X alloys

A magnetoresistive sensor that has a free layer with a face centered cubic, 100 crystal orientation formed on an underlayer structure that has been deposited in the presence of nitrogen. The free layer can be constructed of CoFe, Co2(Mn(1-y)Fey)X (where 0≦y≦1 and X is Si, Ge, Sn, Al, Ga, or a combination thereof), CoFeX (where X is Si, Ge, Sn, Al, Ga, or a combination thereof). The under-layer can include a layer of Ta, a Cu layer formed over the layer of Ta and deposited using a process gas comprising about 20 percent nitrogen and a layer of Ag deposited over the layer of Cu and deposited using a process gas comprising about 50 to 100 percent nitrogen.

FIELD OF THE INVENTION

The present invention relates to magnetoresistive sensors and more particularly to a sensor having an under-layer that promotes a desired grain structure in above deposited sensor layers.

BACKGROUND OF THE INVENTION

The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating, but when the disk rotates air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.

The write head includes at least a coil and first and second pole piece layers separated by a gap layer at an air bearing surface (ABS) of the write head. Current conducted through the coil induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.

In recent read head designs, a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for passing an electrical sense current through the sensor. Magnetic shields are positioned above and below the sensor stack and can also serve as first and second electrical leads, so that the electrical current travels perpendicularly to the plane of the free layer, spacer layer and pinned layer (current perpendicular to the plane (CPP) mode of operation). The magnetization direction of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetization direction of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.

When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering of the conduction electrons is minimized and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. In a read mode the resistance of the spin valve sensor changes about linearly with the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.

In the push to increase data density and sensor performance, researchers have sought to develop magnetoresistive sensors having ever increased sensitivity and reduced size. Certain alloys have shown promise for increasing the performance of such magnetoresistive sensors. However, depositing such layers over available shield lead structure has resulted in less than optimal crystalline grain growth in these magnetic alloys. Thus what is needed is a spin-valve sensor structure that allows favorable alloys to be used with optimal crystalline grain structure and orientation.

SUMMARY OF THE INVENTION

The present invention provides a CPP magnetoresistive sensor that includes an under-layer structure that includes a first sub-layer, a second sub-layer comprising Cu formed over the first sub-layer and a third sub-layer comprising Ag formed over the second sub-layer, and a magnetic free layer structure formed over and in contact with the under-layer structure.

The second copper (Cu) and third silver (Ag) sub-layers are face-centered cubic and thus typically grow with a <111> texture. However, a strong <100> crystalline texture can be achieved by depositing the layers in an Argon (Ar) plus nitrogen (N2) process gas mixture. The second sub-layer is deposited utilizing a process gas mixture that contains about 10-25 percent nitrogen (the balance of gas being Ar) and the third sub-layer is deposited utilizing a process gas mixture that contains about 50-100 percent nitrogen with the balance of gas being Ar.

The ferromagnetic free layer, formed over the under-layer structure is for example CoFe, a ferromagnetic Heusler alloy such as Co2(Mn(1-y)Fey)X (where 0≦y≦1 and X is Si, Ge, Sn, Al, Ga, or a combination thereof), or ternary magnetic alloys CoFeX alloy (where X is Si, Ge, Sn, Al, Ga, or a combination thereof). These materials can provide excellent properties for increased magnetoresistive effect when used in a free layer of a magnetoresistive sensor. Since they are body-centered cubic they ordinarily grow with a <110> texture. However, by growing the free layer on top of an under-layer according to the invention, due to the good in-plane lattice matching to the third sub-layer of Ag the ferromagnetic free layer will also have a <100> texture. This texture is preferred to obtain high magneto-resistance values.

These and other features and advantages of the invention will be apparent upon reading of the following detailed description of preferred embodiments taken in conjunction with the Figures in which like reference numerals indicate like elements throughout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

During operation of the disk storage system, the rotation of the magnetic disk112generates an air bearing between the slider113and the disk surface122which exerts an upward force or lift on the slider. The air bearing thus counter-balances the slight spring force of suspension115and supports slider113off and slightly above the disk surface by a small, substantially constant spacing during normal operation.

With reference toFIG. 2, the orientation of the magnetic head121in a slider113can be seen in more detail.FIG. 2is an ABS view of the slider113, and as can be seen the magnetic head including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system and the accompanying illustration ofFIG. 1are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders.

With reference now toFIG. 3, the invention can be embodied in a current-perpendicular to the plane (CPP) magnetoresistive sensor302such as a giant magnetoresistive sensor (GMR) or tunnel junction magnetoresistive sensor (TMR) that includes a sensor stack304sandwiched between first and second magnetic shields306,308that can be constructed of an electrically conductive material such as NiFe that can serve as electrical leads as well as magnetic shields. The sensor stack304can be constructed to include a magnetic free layer310, a pinned layer structure312and a non-magnetic layer314sandwiched between the free layer310and pinned layer structure314. The non-magnetic layer310can be an electrically conductive, non-magnetic spacer layer such as Cu or Ag if the sensor302is constructed as a giant magnetoresistive sensor (GMR). Alternatively, the layer314can be a thin, electrically insulating, non-magnetic barrier layer such as MgO or Al2O3if the sensor302is constructed as a tunnel junction magnetoresistive sensor (TMR).

The pinned layer structure312can be constructed as an antiparallel coupled structure having first and second magnetic layers AP2316(reference layer) and AP1318(pinned layer), both of which are antiparallel coupled with one another across a non-magnetic AP coupling layer320sandwiched there-between. The AP coupling layer320can be a material such as Ru, Cr, Ir, or Rh and typically is about 4-12 Angstroms thick, where the optimal thickness depends on the AP1, AP2and AP coupling layer materials used. The AP1layer318can be exchange coupled with a layer of antiferromagnetic material (AFM layer)322. This exchange coupling strongly pins the magnetization of the AP1layer318in a first direction substantially perpendicular the air bearing surface (ABS) as indicated by arrow tail symbol324. Here and in the following, substantially perpendicular to the ABS has the meaning of closer to perpendicular than parallel to the ABS. Similarly, substantially parallel to the ABS has the meaning of closer to parallel than perpendicular to the ABS. The strong antiparallel coupling between the AP1and AP2layers318,316pins the magnetization of the AP2layer in a second direction (opposite to magnetization324) as indicated by arrowhead symbol326. A capping layer328can be provided at the top of the sensor stack304to protect the layers of the sensor stack from damage during manufacture.

First and second magnetic hard bias layers330,332can be provided at either side of the sensor stack304to provide a magnetic bias field to bias the magnetization of the free layer310in a desired direction parallel with the ABS as indicated by arrow symbol334. The first and second hard bias layers330,332are insulated from the sensor stack304and from at least one of the shield/leads306by thin insulation layers336,338in order to prevent sensor current from being shunted across through the bias layers330,332.

The electrical resistance across the sensor stack304varies depending upon the relative orientation of the magnetizations334and326of the free layer310and AP2layer316, respectively. The closer these magnetizations334,326are to being parallel with one another the lower the resistance will be, and the closer they are to being antiparallel the higher the resistance will be. Because the magnetization334of the free layer310is free to move in response to an external magnetic field, this resulting change in electrical resistance across the sensor stack332can be measured to detect the presence and strength of a magnetic field.

Certain materials have the potential to provide excellent magnetoresistive properties when used in a free layer and/or reference layer, such as the free layer310and reference layer316ofFIG. 3. These materials include highly spin-polarized ferromagnetic Heusler alloys such as Co2(Mn(1-y)Fey)X (where 0≦y≦1 and X is Si, Ge, Sn, Al, Ga, or a combination thereof), or CoFeX alloy (where X is Si, Ge, Sn, Al, Ga, or a combination thereof). It should be understood that here and in the following atomic compositions may vary slightly (to about 5 at. %) from the stoichiometric ratios in Co2(Mn(1-y)Fey)X and CoFeX. These deviations are often found to be advantageous and desired to optimize parameters such as magnet-resistance and thermal phase stability. Thus, for example alloys like Co49(Mn0.5Fe0.5)23Ge28and Co34Fe34Ge28are included in the groups of Co2(Mn(1-y)Fey)X and CoFeX, respectively. These exhibit advantageous properties such as high spin-polarization or short spin-diffusion length to increase the magnetoresistive effect. Depending on their chemical order these alloys also are of L21, B2, or A2 type order. For example, fully ordered Co2MnX Heusler alloys are of L21order. If there is disorder among the Mn and X atoms only, then the alloy is of B2 order, if it is disordered it is A2. Band structure calculations and magneto-transport experiments indicate that ferromagnetic Co2MnX type alloys need to be at least B2 ordered to exhibit high spin-polarization and thus yield high magnetoresistive values. Similarly CoFeX alloys are found to order in a B2-like structure that yields high magnetoresistive values. Both Co2(Mn(1-y)Fey)X and CoFeX alloys are based on a body centered cubic cell and thus ordinarily grow with a <110> texture.

In order to increase the performance of the sensor302, it is preferable that active layers of the magnetic sensor (the free layer310and/or reference layer316) be constructed of an alloy such as those described above, but that they also be grown with a <100> crystalline texture rather than a <110> texture, to increase the magnetoresistive effect. The free layer310and reference layer316can also include a layer of CoFe, for example in between the seed layer structure and the Co2(Mn(1-y)Fey)X or CoFeX layer to further improve texture of the Co2(Mn(1-y)Fey)X or CoFeX layer and the spacer layer and to inhibit atomic diffusion upon annealing.

With reference still toFIG. 3, the free layer310is formed on top of an under-layer340, which is preferably a multi-layer structure comprising a first sub-layer342, and a second sub-layer344formed on top of the first sub-layer342and a third sub-layer346formed on top of the second sub-layer344.

The first sub-layer342is for example 20-40 Å thick Ta or another metallic layer promoting adhesion and good crystallinity, the second sub-layer344comprises 50-80 Å Cu. The third sub-layer346comprises 30-60 Å Ag. Going forward, thinner under-layers may be advantageous in order to keep the magnetic shield-to-shield separation defining the magnetic read gap as small as possible since a decreasing read gap increases magnetic resolution and thus is desired for higher recording densities. Therefore, in such a case, the under-layers340,342,344can have a combined thickness of 80 Å or less. The second and third sub-layers344and346of under-layer340have a <100> texture that is a result of a unique deposition process that will be described below, the deposition process being compatible with the formation of a GMR sensor, without any extreme high temperature annealing that might otherwise damage the sensor.

FIG. 4illustrates an alternate embodiment of the invention.FIG. 4illustrates a magnetoresistive sensor402that is similar to that ofFIG. 3, except that the sensor402includes an AP coupled or “synthetic” free layer404. The synthetic free layer404includes first magnetic layer408(FL1) and second magnetic layer (FL2)406that are antiparallel coupled across a non-magnetic AP coupling layer410sandwiched there-between. The AP coupling layer410can be a material such as Ru, Cr, Ir, or Rh and is about 4-12 Angstroms thick, where the optimal thickness depends on the AP1, AP2and AP coupling layer materials used. The antiparallel coupling of the layers406,408causes the layers406,408to have magnetizations that are biased in opposite directions substantially parallel with the ABS as indicated by arrow symbols412,414. In response to an external magnetic field, the magnetizations412,414rotate, but remain substantially antiparallel with one another.

Magnetic free layers406,408can both be constructed of a magnetic material such as a Co2(Mn(1-y)Fey)X (where 0≦y≦1 and X is Si, Ge, Sn, Al, Ga, or a combination thereof), a Heusler alloy or CoFeX (where X is Si, Ge, Sn, Al, Ga, or a combination thereof) as described in the previous embodiment. Alternatively, the first magnetic free layer408can be one of the above alloys and the second free magnetic layer406can be another type of magnetic material such as CoFe.

As with the previously described embodiment, the sensor402can include the under-layer340, including sub-layers342,344,346, where the layer342is for example a 20-30 Å thick layer of Ta or another metallic material promoting adhesion and good crystallinity, the layer344comprises 50-80 Å thick Cu and the layer346comprises 30-60 Å thick Ag. Also, as described above, the second and third sub-layers344and346of underlayer340have a desired <100> crystalline texture. Also, as before, this desired <100> crystalline texture is formed by a novel manufacturing process that will be described herein below.

The under-layer can be formed by a standard reactive thin film deposition process such as sputter deposition. A first sub-layer342such as Ta or another metallic layer promoting adhesion and good crystallinity is deposited on first lead and shield306(FIG. 3). Then second sub-layer344formed of Cu is deposited on first sub-layer342in an Ar—N2process gas. The amount of N2in the process gas is chosen to optimize <100> texture, for example 10-25 percent N2for magnetron sputtering. Then, the third sub-layer346formed from Ag is deposited on second sub-layer344in an Ar—N2process gas. Again, the amount of N2in the process gas is chosen to optimize <100> texture, for example 50-100 percent N2for sputtering. Ordinarily, Ag grows in a <111> texture. However, the inventors have found that when Ag is grown on Cu with a <100> texture utilizing a process gas described above, it grows with a desired <100> texture. Similarly, Cu has a face-centered cubic lattice. Ordinarily it grows in a <111> texture. However, when grown on Ta utilizing a process gas described above, it grows with a desired <100> texture. The Cu layer is important as a pre-seed layer for the Ag, which by itself would not grow in a <100> texture. Au grown in the <100> direction has a very good in-plane lattice match to Co2(Mn(1-y)Fey)X and CoFeX alloys (where 0≦y≦1 and X is Si, Ge, Sn, Al, Ga, or a combination thereof) and also to CoFe grown in the <100> direction since there is a close square root 2 relationship between the lattice constants of these materials and that of Ag. Ag has a face-centered cubic lattice.

Following the growth of under-layer340, the free layer structure of spin-valve or tunnel valve as described inFIGS. 3 and 4is grown. As described above, the free layer310or404grows with a desired <100> texture on underlayer340having a third sub-layer of Ag <100> texture.

FIGS. 5 and 6show an atomic level illustration of the interface and the close to square-root(2) relationship between the lattice spacings of the <100> Ag layer346and the <100> free layer310. Due to the square-root(2) relationship the lattices are well matched when the lattice of the free layer310or404is rotated 45 degrees from the lattice of the underlying Ag layer346. Although the free layer could be the free layer310ofFIG. 3or the free layer404ofFIG. 4, for purposes of simplicity the free layer will be referred to as310inFIG. 5.FIG. 5shows a top down view of atoms of the deposited layers346and310, whileFIG. 6shows a side cross sectional view as viewed from line6-6ofFIG. 5. As seen inFIG. 5, the relationship between the lattice sizes of layers346and310is such that the crystal lattice of the free layer310is rotated at an angle602of 45 degrees with respect to the lattice of the layer346. While it is rotated 45 degrees, the layer310maintains the desired <100> texture.