Abstract:
Improved sensitivity GMR sensors useful for thin film magnetic read heads are disclosed. Spin transfer induced destabilization of the magnetic free layer is suppressed through the application of Tb containing alloys in the free layer. Sense currents can be increased by a factor of five in comparison to prior art designs without an increase in spin transfer induced noise.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to the structure of thin film magnetic read heads. More specifically, the invention relates to the improvement of sensitivity in giant magnetoresistive sensors.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  (Prior Art) is a partial cross sectional view of a thin film read/write head combination. A read head  104  employing a giant magnetoresistive sensors  106  (hereinafter referred to as a “GMR sensors”) is combined with an inductive write head  102  to form a combined magnetic head  100 . In a magnetic disk or tape drive an air bearing surface (ABS) of the combined magnetic head is supported adjacent to the moving magnetic media to write information on or read information from a surface of the media. In a write mode, information is written to the surface by magnetic fields that fringe across gap  114  between first  112  and second  116  pole pieces of the write head  102 . Write head  102  also comprises yoke  120 , coil  118 , and backgap  122 . In a read mode, the resistance of the GMR sensor  106  changes proportionally to the magnitudes of the magnetic fields from the moving magnetic media. When a sense current is conducted through the GMR sensor  106 , resistance changes cause potential changes that are detected and processed as playback signals.  
         [0005]      FIG. 2  (Prior Art) is an air bearing surface view of read head  104  of  FIG. 1 . GMR sensor  106  includes a nonmagnetic conductive layer  206 , also called a spacer layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned reference layer  208 , and a free layer  204 . The magnetization of the pinned reference layer  208  is maintained (“pinned”) at 90 degrees to the magnetization of the free layer  204  by exchange coupling with pinned layer  212  and anti-ferromagnetic layer  214 . The magnetization of the free layer  204  changes freely in response to magnetic fields from the moving magnetic media at the air bearing surface. When the directions of magnetization of the pinned and free layers are parallel, scattering of conduction electrons passing through the layers is minimal, and when the directions are antiparallel, scattering is maximized. Changes in the scattering of the conduction electrons change the resistance of the GMR sensor in proportion to sin θ, where θ is the angle between the magnetizations of the pinned and free layers. Sense DC current I DC    150  is conducted through the GMR sensor for detecting a change in resistance of the layer structure. This configuration of GMR sensor is typically known as a CPP-GMR sensor, which employ a sense current perpendicular to the plane of film layers. The change of resistance of the layer structure produces a voltage V sense    152  which is directed to the monitoring electronics.  
         [0006]     The anti-ferromagnetic layer  214  interfacially engages the pinned layers  212  and  208  in order to pin the magnetization of the pinned layers in a predetermined direction by magnetic exchange coupling. Since the anti-ferromagnetic pinning layer is not magnetized, it exerts no magnetic influence on the free layer  204 . This is advantageous since the magnetization of the free layer should be free to rotate about a bias point in response to magnetic fields from the moving magnetic media. Advantageously, the magnetization of the pinned layers  212  and  208  can be strongly pinned by the anti-ferromagnetic pinning layer  214  so that their orientation cannot be easily changed by stray magnetic fields.  
         [0007]     Structurally, the read head  104  includes a GMR sensor  106  sandwiched between shield layers  108  and  110 . GMR sensor  106  typically comprises a 30 A (angstrom) NiFeCr seed layer  216 ; a 150 A PtMn anti-ferromagnetic pinning layer  214  grown on the seed layer  216 ; a CoFe pinned layer  212  grown on layer  214 ; a 4-8 A Ru layer  210 ; a second CoFe pinned reference layer  208  grown on layer  210 ; a Cu 20-40 A spacer layer  206  grown on pinned layer  208 ; a CoFe/NiFe 30 A free layer placed on spacer layer  206 ; and, a Ta interface layer  202  grown between the free layer  204  and Shield  2  ref  110 .  
         [0008]     U.S. Pat. No. 5,695,864 discloses a device in which electrons flow through a free or excitable magnet, or reflect from it, to make its magnetization respond. To accomplish this, the spin vectors of the flowing electrons are preferentially polarized by an auxiliary ferromagnet, whose moment orientation is fixed. The electrons flow between the fixed and free ferromagnets through a non-magnetic metallic spacer which is thick enough to make the static inter-magnetic exchange coupling negligible. While transmitting through or reflecting from the free ferromagnet, the spins of the moving electrons interact by quantum-mechanical exchange with the local, permanently present, spontaneously-polarized electron spins of the free magnet. This interaction causes a transfer of vectorial angular momentum between the several metallic layers in the device which causes the magnetization vector of the free magnet to change its direction continually with time. Thus excited, the magnetization vector will precess about its original axis. The precession cone angle will either attain a new equilibrium value which will be sustained by the current or will increase beyond 90 degrees and precess with decreasing amplitude until the magnetization vector has reversed by 180 degrees from its initial direction.  
         [0009]     U.S. Pat. No. 5,780,176 discloses an exchange coupling film having a stacked-film-structure consisting of a ferromagnetic film made of at least one material of Fe, Co and Ni, and an anti-ferromagnetic film. The exchange coupling film is made of a ferromagnetic material to which an element is added, provided at the interface between the ferromagnetic film and the anti-ferromagnetic film so as to improve the lattice matching. This results in the enhancement of the exchange coupling force. A magnetoresistance effect element including an exchange coupling film described above, and an electrode for supplying a current to the ferromagnetic film constitutes the exchange coupling film.  
         [0010]     U.S. Pat. No. 5,919,580 discloses a spin valve device containing a chromium or chromium and aluminum anti-ferromagnetic layer, which acts as a pinning layer for a magnetoresistive ferromagnetic layer, by exchange coupling. The anti-ferromagnetic layer has a tunable Neel temperature and anisotropy constant, and is corrosion resistant.  
         [0011]     U.S. Pat. No. 6,105,237 discloses a spin valve sensor provided with a spacer layer sandwiched between a free layer and a pinned layer. The pinned layer is pinned by a pinning layer constructed of a material having a high coercivity, and a low magnetic moment. The high coercivity is employed for pinning the pinned layer, and the low moment assures that stray fields from the pinning layer do not affect the coercivity of the free layer. The magnetic moment is preferably less than 300 emu/cc and the coercivity is preferably greater than 500 Oe. The magnetic orientation of the pinning layer is set by a magnetic field at room temperature that may be applied at the suspension level. The materials with which the pinning layer may be formed are amorphous materials TbFeCo and CoSm, and a non-amorphous material CoPtCr, provided the Cr is of sufficient proportion to minimize the moment of the CoPtCr material.  
         [0012]     US Patent Application Publication US 2003/0151407 discloses a structure and method for forming a magnetic-field sensor device comprising depositing a first electrode onto a substrate. Then, an electrically insulating layer is deposited on the first electrode. Next, a portion of the insulating layer is removed to expose a region of the first electrode, thereby creating an empty space. After this, at least one layer of chemically-synthesized nanoparticles is deposited on the insulating layer and within the empty space. Next, a second electrode is deposited on both the layer of nanoparticles and the insulating layer. Alternatively, multiple layers of nanoparticles may be deposited, or only a single nanoparticle may be deposited. The substrate is either conducting or non-conducting, and the first and second electrodes are electrically conducting and may be magnetic or non-magnetic. Additionally, a metallic layer of magnetic material may be first deposited on the substrate.  
         [0013]     US Patent Application Publication US 2004/0161636 discloses a structure and method of fabricating a magnetic read head, comprising forming a fill layer for a magnetic read head gap using atomic layer deposition (ALD). The fill layer comprises an insulator, preferably aluminum oxide, aluminum nitride, mixtures thereof and layered structures thereof. Materials having higher thermal conductivity than aluminum oxide, such as berylium oxide and boron nitride, can also be employed in layers within an aluminum oxide structure. The thickness of the ALD-formed head gap fill layer is between approximately 5 nm and 100 nm, preferably between approximately 10 nm and 40 nm.  
         [0014]     In an article entitled “Control of Magnetization Dynamics in Ni 81 Fe 19  Thin Films Through the Use of Rare Earth Dopants”, by Bailey et al., (IEEE Transactions on Magnetics, Vol. 37, No. 4, July 2001, pg 1749), the magnetization dynamics of soft ferromagnetic thin films tuned with rare earth dopants is disclosed. Low concentrations (2 to 10%) of Tb in 50 nm Ni 81 Fe 19  films are found to increase the Gilbert magnetic damping parameter alpha over two orders of magnitude without great effect on easy axis coercivity or saturation magnetization.  
         [0015]     One way to increase the sensitivity of the GMR sensors described above is to increase I DC , which increases V sense  for a given magnetic signal amplitude. However, increasing I DC  beyond a certain point creates spin transfer torques which produce gross instability in the magnetization of the free layer. This instability is manifest as oscillations in the magnetization of the free layer, which are large enough in amplitude to obscure the magnetization changes induced by the moving media. The instability of the free layer induced by the spin transfer torques of the sense current, produces a type of noise which can be called spin transfer induced noise. It is noise because it produces a signal containing random fluctuations that can obscure the measurement signal of interest. It is to be distinguished from other types of noise such as thermal noise. The spin transfer induced noise significantly limits the sensitivity of prior art CPP-GMR sensors. What is needed is an improved GMR sensor having improved sensitivity and stability.  
       SUMMARY OF THE INVENTION  
       [0016]     It is an object of the present invention to provide a GMR sensor comprising an anti-ferromagnetic layer, a first pinned layer coupled to the anti-ferromagnetic layer, an interface layer formed over the first pinned layer, a second pinned layer formed over the interface layer, a spacer layer formed over the second pinned layer, and a free layer comprising a NiFeTb alloy formed over the spacer layer.  
         [0017]     It is another object of the present invention to provide a dual GMR sensor comprising a first anti-ferromagnetic layer, a first pinned layer formed over the first anti-ferromagnetic layer, a first spacer layer formed over the first pinned layer, a second anti-ferromagnetic layer, a second pinned layer formed over the second anti-ferromagnetic layer, a second spacer layer formed over the second pinned layer, and a free layer interposed between the first and second spacer layers, said free layer comprising a NiFeTb alloy.  
         [0018]     It is yet another object of the present invention to provide a dual GMR sensor comprising a first anti-ferromagnetic layer, a first pinned layer formed over the first anti-ferromagnetic layer, a second anti-ferromagnetic layer, a second pinned layer formed over the second anti-ferromagnetic layer, and a free layer interposed between the first and second pinned layers, said free layer comprising a NiFeTb alloy.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will be better understood when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:  
         [0020]      FIG. 1  (Prior Art) is a partial cross sectional view of a thin film read/write head combination;  
         [0021]      FIG. 2  (Prior Art) is an air bearing surface view of read head  104  of  FIG. 1 ;  
         [0022]      FIG. 3  is an air bearing surface view of a GMR sensor in accordance with an embodiment of the present invention;  
         [0023]      FIG. 4  is an air bearing surface view of a dual GMR sensor in accordance with an embodiment of the present invention; and,  
         [0024]      FIG. 5  is an air bearing surface view of a compact dual GMR sensor in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     Spin transfer induced instability of the free layer magnetization is one major limiting factor for increasing the sensitivity of GMR sensors. As the DC current I DC  is increased, the moving electrons spin gets polarized, which can destabilize the free layer via a spin induced torque effect. The origin of this phenomenon is the systematic absorption of energy from the spin-polarized conduction electron current into the spin-system which comprises what is typically referred to as the magnetization. If the rate at which the energy is absorbed (or pumped into) the spin/magnetization system is greater than that by which the magnetization can loose energy to the mechanical motion of the lattice (of constituent atoms), then the magnetization can start to gyrate. The gyrations are symptomatic of a destabilization of the free layer magnetization. This destabilization manifests itself as a type of noise voltage that is added to the signal voltage. It can be described as noise because it produces a signal containing random fluctuations that obscures the measurement signal of interest. It should not be confused with other types of noise such as thermal noise, which may also be present. Suppression of this spin transfer induced noise voltage, that occurs as a result of the destabilization of the free layer magnetization, is required to improve the magnetic sensitivity of the GMR sensor. It is an object of the present invention to provide a free layer structure that enhances the stability of the free layer magnetization at higher I DC  current levels and reduces spin transfer induced noise. The enhanced stability allows higher I DC  current levels and subsequently improved sensor sensitivity.  
         [0026]      FIG. 3  is an air bearing surface view of a GMR sensor  300  in accordance with an embodiment of the present invention. Seed layer  216 , anti-ferromagnetic layer  214 , pinned layer  212 , referenced pinned layer  208 , and spacer layer  206  are constructed as previously described in  FIG. 2 , as is well known to those skilled in the art. However, free layer  204  of the prior art is replaced with layers  308 - 314  of the present invention, which may be described as a synthetic free layer  302 . The synthetic free layer comprises a CoFe layer  314 , a NiFe layer  312 , a Ru layer  304 , a CoFe layer  306 , and NiFeTb layer  308 . The atomic components of each layer  308 - 314 , as designated in  FIG. 3 , are for identification only. The specific stoichiometry is specified below. For example, CoFe layer  314  actually comprises preferably about 10 atomic % Fe, 90 atomic % Co, but can vary from about 5 atomic % Fe, 95 atomic % Co to 15 atomic % Fe, 85 atomic % Co. Layer  314  may be between 10 and 30 angstroms thick, preferably about 20 angstroms thick. NiFe layer  312  comprises about 20 atomic % Fe and 80 atomic % Ni. Layer  312  may be between 20 and 60 angstroms thick, preferably about 40 angstroms thick. Ru layer  304  may be between 4 and 8 angstroms in thickness. CoFe layer  306  is between 2 to 10 angstroms in thickness, preferably about 5 angstroms in thickness. The specific composition is the same as layer  314 . Layer  308  contains Ni, Fe, and Tb. The Tb in layer  308  is present in concentrations from about 2 to 10 atomic %, preferably between 2 and 5 atomic %. The remainder is Fe and Ni, in a atomic ratio of about 4:1 (Ni:Fe). For example, for a Tb concentration of 10 atomic %, Fe would be about 18%, and Ni about 72 atomic %. For 2% Tb, Fe would be about 19.6%, and Ni 78.4%. Layer  308  is between 10 and 40 angstroms thick, preferably between 20 and 30 angstroms thick.  
         [0027]     As disclosed in the related art by Bailey et al. cited above, small additions of Tb to NiFe can substantially increase the effective magnetic damping of the NiFeTb alloy film without otherwise significantly changing the primary magnetic properties relative to NiFe. This increased damping (by enhanced spin-orbit coupling) acts like a “magnetic-friction” which necessarily increases the rate at which energy in the dynamical magnetization of the NiFeTb can be dissipated into the lattice. Because the composite CoFe/NiFeTb layers ( 306 ,  308 ) are antiferromagnetically coupled strongly to the functional composite NiFe/CoFe free layer ( 314 , 312 ) through the Ru layer  304 , the addition of Tb in layer  308  will also effectively increase the frictional damping of the magnetization motion of the aforementioned functional free layer. This enhanced damping/friction will allow for an increase in IDC prior to where the energy absorbed by the free layer from the spin-polarized dc current IDC exceeds the rate of damping to the lattice and the concomitant onset of spin-torque instability. This increase in IDC under stable operating conditions translates directly into an increase in signal or sensitivity of the GMR sensor  300 .  
         [0028]      FIG. 4  is an air bearing surface view of a dual GMR sensor  400  in accordance with an embodiment of the present invention. This configuration further increases measurement gain by incorporating two sets of anti-ferromagnetic pinning layers  406   a,b ; two sets of pinned and pinned reference layers ( 408   a,b  and  412   a,b ); two spacer layers  414   a,b ; and free layer  402 . Within free layer  402  are a centrally located NiFeTb containing layer  420 , sandwiched between Ru interface layers  418   a,b  and CoFe layers  416   a,b . Layer  420  is between 10 and 40 angstroms thick, preferably between 20 and 30 angstroms thick. The Tb in layer  420  is present in concentrations from about 2 to 10 atomic %, preferably between 2 and 5 atomic %. The remainder is Fe and Ni, in a atomic ratio of about 4:1 (Ni:Fe). Ru interface layers  418   a,b  are about 4-8 angstroms thick. CoFe layers  416   a,b  are between 10 and 50 angstroms thick, preferably about 20 angstroms thick. They have the same composition as previously described in layers  314  and  306  above.  
         [0029]     Seed layers  404   a,b  comprise a first NiFeCr containing layer, nominally 30 angstroms in thickness, covered by a 5-angstrom layer of NiFe. The composition of layers  404   a,b  are well known to those skilled in the art. Anti-ferromagnetic pinning layers  406   a,b  comprise Ir, Mn, and Cr. Layers  406   a,b  are nominally 75 angstroms thick, but may be between 40 and 100 angstroms thick. CoFe layers  408   a,b  and  412   a,b  are nominally 20 angstroms thick, but may be between 10 and 30 angstroms thick. They have the same composition as previously described in layers  314  and  306  above. Copper spacer layers  414   a,b  are nominally about 20 angstroms thick, but may be between 10 and 40 angstroms thick. Ru interface layers  410   a,b  are about 4-8 angstroms thick. Ru/Ta interface layer  422  is placed between anti-ferromagnetic layer  406   b  and shield  2 .  
         [0030]     One disadvantage of the dual sensor of  FIG. 4  is that the additional layers required for the dual sets of pinning and pinned layers can produce a total film stack thicker than that of  FIG. 3 . This may reduce the resolution of the GMR sensor, since the distance between shield  1  and shield  2  is a factor in determining the smallest size magnetic element that can be detected on the media. In order to overcome this potential shortcoming, a preferred embodiment of the low noise dual GMR is provided in  FIG. 5 .  
         [0031]      FIG. 5  is an air bearing surface view of a compact dual GMR sensor  500  in accordance with an embodiment of the present invention. In this embodiment, pinned reference layers  412   a,b  have been removed, along with corresponding Ru interface layers  410   a,b . This alteration provides a shorter film stack and therefore a higher resolution sensor than that of  FIG. 4 . All other layers remain as described previously. A high sensitivity is provided by the noise reduction of the Tb containing layer  420 , coupled with the amplification of the dual sensor structure.  
         [0032]     The present invention is not limited by the previous embodiments heretofore described. Rather, the scope of the present invention is to be defined by these descriptions taken together with the attached claims and their equivalents.