Patent Publication Number: US-2011058408-A1

Title: Memory cell arrangements; memory cell reader; method for determining a memory cell storage state

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Singapore Patent Application No. 200905930-4, which was filed Sep. 7, 2009, and is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     Various embodiments relate generally to memory cell arrangements, to a memory cell reader, and to a method for determining a memory cell storage state. 
     BACKGROUND 
     In 2007, the world-wide Hard Disk Drive (HDD) industry distributed more than 500 million HDD units and factory revenues reached 32 billion US dollars. Such strong demand for HDD units comes not only from traditional PC and enterprise storage markets but also from new storage demands such as entertainment, digital video storage and mobile devices. Low cost over storage capacity of HDD units is a key factor behind the huge numbers of shipments and demands. The HDD industry roadmap predicts a 40% annual areal density growth to maintain its domination in the storage market. Technically, areal density growth requires higher linear and track densities. Thus, data playback requires reader geometry size to shrink accordingly in order to detect a magnetic signal. However, smaller reader size often results in higher resistance and impedance mismatch problems, which are serious issues for high speed data transfer. In addition, noise from thermal fluctuation and spin momentum torque are other effects concerned with reader shrinking. To meet the areal density requirements beyond 2 Tbit/in 2 , future reader designs should target performances characterised by low resistance area product (RA&lt;0.1 Ωm 2 ) and high magnetoresistance signal (MR&gt;15%). 
     Current commercial readers adopt MgO based magnetic tunneling junctions (MTJ), which produce large tunneling magnetoresistance (TMR) signals (TMR 30˜70%) with relatively low RA (0.4 to 1 Ωμm 2 ). However, MTJ readers are not expected to exceed 1 Tbit/in 2  as TMR signals significantly decay with decreasing RA. 
     Current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) readers, on the other hand, provide an additional option for high areal density recording. CPP-GMR readers using metal spin valve films have a much lower RA value. One of the challenges facing CPP-GMR technology is low measured GMR signals (GMR&lt;2%) due to high parasitic resistance from the layers in the CPP-GMR reader (R para A˜30 to 50 mΩμm 2 , ΔRA only ˜1-2 mΩμm 2 ). Although many alternative CPP spin valve structures such as half-metal spin valves and current-confined-path (CCP) spin valves have been proposed and demonstrated, the achieved GMR versus RA performance does not satisfy the requirement for future high areal density recording. In TMRC 2007, HGST announced ˜17% GMR signal with RA˜0.2 Ωμm 2  in a CCP-CPP-GMR head. Although the GMR signal reported was greatly improved compared with the previous reports, lower RA values are still needed to achieve areal densities up to 2 Tbit/in 2  and beyond. 
     Another challenge for CCP GMR heads is uniformity control, which becomes a more serious concern with shrinking reader size. Progress in half-metal spin valve heads is relatively slow due to difficulties in the fabrication of GMR films. Band matching between the reference layer and spacer free layer is very critical to obtaining high spin polarization and therefore high GMR signal according to theoretical calculation. Current thin film sputtering techniques cannot avoid defects in the GMR thin film stack and this results in band mismatch. Further, although high spin polarization ratios have been observed in bulk half metal material, it is very difficult to repeat the same behavior with thin films due to poor surface spin polarization. The reported GMR signal of half metal spin valves with metal spacers is normally less than 3% at room temperature. 
     In addition to the issues concerning magnetoresistive signals and RA values, magnetic noise induced by spin momentum transfer (SMT) is another major concern with reader size shrinking. In GMR heads, a small sensing current (I sens ) is used to detect the magnetization status of the free layer. However, the magnetization status of the free layer can be disturbed by I sens  due to the SMT effect. The SMT noise can be ignored when the GMR reader is large; however, such magnetization fluctuations becomes more severe with shrinking GMR readers because the sensing current density is increased. 
     The HDD industry has directed more resources towards developing GMR readers with low RA and high magnetoresistive signals. Although encouraging progress has been achieved in recent years, current reader technology for detecting magnetoresistive signals will eventually face its limitations in increasing areal density. 
     To meet the requirement of 40% annual areal density growth rate, a new reader sensor design for magnetic signal detection is proposed. An areal density of 10 Tb/in 2  areal density with approximate bit size 6×11 nm 2  is targeted. To meet the areal density, a linear density of up to 4000 kbit/in, a track density of up to 2500 kt/in (linear density will increase with low track density) and a read bit transfer rate of up to 2 Gbit/s is required. Such high bit transfer rate corresponds to high frequency reader response, which is proportional to 1/RC (product of resistance and capacitance). The HDD roadmap has predicted resistance area product, RA˜0.1 Ωμm 2  for 1 Tb/in 2  areal density with bit transfer rates of approximately 1 Gbit/s. Thus, targeted data transfer rates of 2 Gbit/s for 10 Tb/in 2  areal density ought to achieve an RA value of approximately 0.026 Ωμm 2 . All reader sensor designs under development are far below this specification requirement. 
     New magnetic sensing methods should therefore be in the roadmap to provide a solid base for continual areal density growth. The present disclosure generally relates to a signal detection method for magnetic recording, and magnetic field detection methods on magnetic recording media, e.g. to a perpendicular magnetic recording media. 
     SUMMARY 
     An embodiment is a memory cell arrangement including a magnetoresistive memory cell; and a frequency determiner configured to determine a spin precession frequency provided by the magnetoresistive memory cell; and a storage state determiner configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. 
     Another embodiment is a memory cell reader, including a frequency determiner configured to determine a spin precession frequency provided by a magnetoresistive memory cell; and a storage state determiner configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. 
     Another embodiment is a memory cell arrangement, including a magnetic memory cell; a magnetoresistive cell configured to generate a spin precession frequency under a magnetic field from the magnetic memory cell; a frequency determiner configured to determine a spin precession frequency provided by the magnetoresistive cell; and a storage state determiner configured to determine the magnetic memory cell storage state based on the determined spin precession frequency. 
     Another embodiment is a memory cell arrangement, including a magnetoresistive memory cell array which may include a plurality of magnetoresistive memory cells; and a frequency determiner configured to determine a spin precession frequency provided by one magnetoresistive memory cell of the plurality of magnetoresistive memory cells; and a storage state determiner configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which: 
         FIG. 1  shows a memory cell arrangement in accordance with one embodiment; 
         FIG. 2  shows a memory cell arrangement in accordance with an alternative embodiment; 
         FIG. 3  shows a memory cell reader in accordance with an embodiment; 
         FIG. 4  shows a memory cell arrangement which may include a magnetoresistive memory cell array in accordance with an embodiment; 
         FIG. 5  shows a memory cell arrangement which may include a magnetoresistive memory cell array in accordance with an alternative embodiment; 
         FIG. 6  shows a method for determining a memory cell storage state of a magnetoresistive memory cell in accordance with various embodiments; 
         FIG. 7  shows an illustration of spin torque transfer effect in nanomagnetic devices in accordance with various embodiments; 
         FIG. 8  shows an illustration of spin precession under applied DC current due to spin torque τ in accordance with various embodiments; 
         FIG. 9A  shows an illustration of a spin precession mode excited by a biased DC current through the spin torque transfer effect (STT) effect in accordance with various embodiments; 
         FIG. 9B  shows an illustration of spin precession frequency excited by a biased DC current through the spin torque transfer effect (STT) effect in accordance with various embodiments; 
         FIG. 10  shows a graph of spin oscillation frequency versus media field in accordance with various embodiments; 
         FIG. 11A  shows a magnetoresistive memory cell stack configuration in accordance with various embodiments; 
         FIG. 11B  shows a magnetoresistive memory cell stack configuration in accordance with various embodiments; 
         FIG. 11C  shows a magnetoresistive memory cell stack configuration in accordance with various embodiments. 
         FIG. 12  shows a memory cell arrangement in accordance with an embodiment for determining a magnetic memory cell storage state. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration”. Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. 
     Various embodiments provide alternative methods for detecting magnetic field signals, which need not rely on the GMR values of the magnetic reader sensors. In current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) readers, for example, a DC current is typically applied through the CPP direction in a GMR element. Due to the spin torque transfer (STT) effect, applying a DC bias current through the sensor excites free layer spin precession at a fixed frequency f 0 , which depends on the magnetic field. When a DC current is applied through the CPP direction in a GMR reader, the DC current excites free layer spin precession at a fixed frequency f 0  due to the STT effect. The fixed frequency f 0  may depend on the effective field (H eff ) of the free layer. When the GMR element flies on a magnetic media, H eff  is changed due to a magnetic field (H m ). For example, H eff  may be changed in such a way that H eff =H eff0 +H m  at bit “0” and H eff =H eff0 −H m  at bit “1”, where H eff0  is the effective field without an external media field. 
     The spin precession frequency induced by the DC current may be f 1  at bit “0” and f 2  at bit “1”. The spin precession frequency difference between different bit types may be expressed as f 1 −f 2 . 
     The reader sensor according to various embodiments may detect free layer spin precession frequency instead of linear changes in GMR. 
       FIG. 1  shows an illustration  100  of a memory cell arrangement in accordance with an embodiment. In this embodiment, a memory cell arrangement  102  includes a magnetoresistive memory cell  104 , and a frequency determiner  106  configured to determine a spin precession frequency provided by the magnetoresistive memory cell  104 , and a storage state determiner  108  configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell  104  based on the determined spin precession frequency. The magnetoresistive memory cell  104  may be connected to the frequency determiner  106  and the storage state determiner  108  via a connection  110 . Frequency determiner  106  and the storage state determiner  108  may be connected via the connection  110  to allow the storage state determiner  108  to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell  104  based on the determined spin precession frequency. 
       FIG. 2  shows an illustration  200  of a memory cell arrangement in accordance with an alternative exemplary embodiment. In this embodiment, a memory cell arrangement  202  may include a magnetoresistive memory cell  204 , and a frequency determiner  106  configured to determine a spin precession frequency provided by the magnetoresistive memory cell  204 , and a storage state determiner  108  configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell  204  based on the determined spin precession frequency. 
     The memory cell arrangement  202  may further include a current source  216  coupled to the magnetoresistive memory cell  204  to provide a current to the magnetoresistive memory cell. The current source  216  of memory cell arrangement  202  may include comprises a DC current source to provide a DC current to the magnetoresistive memory cell  204 . The magnetoresistive memory cell  204  of memory cell arrangement  202  may include a free layer structure  210 , a spacer layer structure  212  and a reference layer structure  214 . The frequency determiner  106  of memory cell arrangement  202  may further include a spectrum analyzer. 
     In another exemplary embodiment, storage state determiner  108  of memory cell arrangement  202  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell  204  based on a comparison of the determined spin precession frequency with at least one of a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. 
     In another exemplary embodiment, the storage state determiner  108  of memory cell arrangement  202  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with a frequency threshold, which is arranged between a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. Memory cell arrangement  202  may further include a magnetic field generator  206  configured to apply an external magnetic field to the magnetoresistive memory cell. The magnetic field generator  206  may be configured to apply a fixed external magnetic field to the magnetoresistive memory cell  204 . Magnetoresistive memory cell  204  may be connected to frequency determiner  106  and a storage state determiner  108  via connection means  208 . Current source  216  may include a DC current source  218  to provide a DC current for example, in a CPP direction, to magnetoresistive memory cell  204 . 
     One of the effects of the described embodiments lies in that because the magnetic field signal detection method determines spin precession frequency instead of conventional linear GMR signals, the magnetoresistive memory cell does not require an anti-ferromagnetic (AFM) layer, which has the effect of narrowing shield-to-shield spacing in a the magnetoresistive memory cell. 
       FIG. 3  shows an illustration  300  of a memory cell reader  302  in accordance with an embodiment. In this embodiment, the memory cell reader  302  includes frequency determiner  304  configured to determine a spin precession frequency provided by a magnetoresistive memory cell; and a storage state determiner  306  configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. Frequency determiner  304  may be connected via connection means  308  to storage state determiner  306 . The frequency determiner  304  of memory cell reader  302  may further include a spectrum analyzer. In an exemplary embodiment, storage state determiner  306  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with at least one of a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. 
     In another exemplary embodiment, storage state determiner  306  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with a frequency threshold, which is arranged between a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. 
       FIG. 4  shows an illustration  400  of a memory cell arrangement  402  which may include a magnetoresistive memory cell array  404  in accordance with an embodiment. In this embodiment, memory cell arrangement  402  may include a magnetoresistive memory cell array  404  which may include a plurality of magnetoresistive memory cells  404   a ,  404   b ,  404   c , and a frequency determiner  406  configured to determine a spin precession frequency provided by one magnetoresistive memory cell of the plurality of magnetoresistive memory cells  404   a ,  404   b ,  404   c ; and a storage state determiner  408  configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. 
     In an exemplary embodiment, memory cell arrangement  402  may include a magnetoresistive memory cell array  404  which may be connected to a frequency determiner  406  and storage state determiner  408  via coupling connection  410 . Although only three magnetoresistive memory cells are shown within the magnetoresistive memory cell array  404  in  FIG. 4 , the number of magnetoresistive memory cells within magnetoresistive memory cell array  404  is not limited to three and may contain two, three or more than three. 
       FIG. 5  shows an illustration  500  of a memory cell arrangement which may include a magnetoresistive memory cell array  504  in accordance with an alternative embodiment. In this embodiment, memory cell arrangement  502  may include a magnetoresistive memory cell array  504  which may include a plurality of magnetoresistive memory cells  504   a ,  504   b ,  504   c , and a frequency determiner  406  configured to determine a spin precession frequency provided by one magnetoresistive memory cell of the plurality of magnetoresistive memory cells  504   a ,  504   b ,  504   c ; and a storage state determiner  408  configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on the determined spin precession frequency. 
     In an exemplary embodiment, memory cell arrangement  502  may include a magnetoresistive memory cell array  504  which may be connected to a frequency determiner  406  and storage state determiner  408  via coupling connection  508 . Although only three magnetoresistive memory cells are shown within the magnetoresistive memory cell array  504  in  FIG. 5 , the number of magnetoresistive memory cells within magnetoresistive memory cell array  504  is not limited to three and may contain two, three or more than three. 
     In an embodiment, memory cell arrangement  502  may include a current source  516  coupled to at least one magnetoresistive memory cell of the plurality of magnetoresistive memory cells to provide a current to the magnetoresistive memory cell. Current source  516  may include a DC current source to provide a DC current to the magnetoresistive memory cell. In another exemplary embodiment, at least one magnetoresistive memory cell of the plurality of magnetoresistive memory cells  504   a    504   b    504   c  of memory cell arrangement  502  may include a free layer structure  510   a , a spacer layer structure  512   a  and a reference layer structure  514   a . Although free layer structure  510   a , spacer layer structure  512   a  and reference layer structure  514   a  are only shown in magnetoresistive memory cells  504   a , the presence of a free layer structure, a spacer layer structure and a reference layer structure is not limited to being present only in magnetoresistive memory cell  504   a  but may be present in a plurality of magnetoresistive memory cells for example  504   b  and  504   c . The frequency determiner  406  of memory cell arrangement  502  may further include a spectrum analyzer. 
     In another embodiment, storage state determiner  408  of memory cell arrangement  502  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with at least one of a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. 
     In another embodiment, storage state determiner  408  of memory cell arrangement  502  may be configured to determine the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with a frequency threshold, which is arranged between a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. In another exemplary embodiment, the memory cell arrangement  502  may further include a magnetic field generator  506  configured to apply an external magnetic field to at least one magnetoresistive memory cell of the plurality of magnetoresistive memory cells. In another exemplary embodiment, the magnetic field generator  506  may be configured to apply a fixed external magnetic field to the at least one magnetoresistive memory cell of the plurality of magnetoresistive memory cells. 
     In another embodiment, the spin precession frequency provided by one magnetoresistive memory cell of the plurality of magnetoresistive memory cells  504   a ,  504   b ,  504   c  may be determined by using a spectrum analyzer as a frequency determiner  406  through a coplanar circuit design, which is already widely used in wireless communication devices. Current source  516  may include a DC current source  518  to provide a DC current for example, in a CPP direction, to at least one magnetoresistive memory cell of the plurality of magnetoresistive memory cells  504   a ,  504   b ,  504   c.    
       FIG. 6  shows a method  600  for determining a memory cell storage state of a magnetoresistive memory cell in accordance with an embodiment. In  602 , the spin precession frequency provided by the magnetoresistive memory cell is determined. In  604 , the magnetoresistive memory cell storage state of the magnetoresistive memory cell is determined based on the determined spin precession frequency. In another exemplary embodiment, method  600  may include providing a current to the magnetoresistive memory cell. Providing the current to the magnetoresistive memory cell may include providing a DC current to the magnetoresistive memory cell. The spin precession frequency of method  600  may be determined using a spectrum analyzer. The method  600  may include determining the memory cell storage state which may include determining the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with at least one of a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. The method  600  may include determining the memory cell storage state which may include determining the magnetoresistive memory cell storage state of the magnetoresistive memory cell based on a comparison of the determined spin precession frequency with a frequency threshold, which is arranged between a first predefined spin precession frequency associated with a first magnetoresistive memory cell storage state and a second predefined spin precession frequency associated with a second magnetoresistive memory cell storage state. The method  600  may include applying an external magnetic field to the magnetoresistive memory cell. The method  600  may include applying an external magnetic field to the magnetoresistive memory cell which may include applying a fixed external magnetic field to the magnetoresistive memory cell. 
       FIG. 7  shows an illustration  700  of the spin torque transfer effect in nanomagnetic devices in accordance with one exemplary embodiment. In this exemplary embodiment, ferromagnetic (FM) layers  704  and  702  may form a FM/non-magnetic/FM structure. Current applied along CPP direction  706  may induce spin torque inside a FM layer. The applied current may be polarized (spin up) when flowing from a first FM layer  704  to a second FM layer  702 . If the first FM layer  704  and second FM layer  702  have different magnetization directions  708 ,  712 , the spin polarized electron current may interact with local spins inside second FM layer  702  through exchange coupling. Such spin torque may try to align the local spin direction  714  to the local spin direction in first FM layer  710 . Since spin torque may be proportional to the spin polarized current, the magnetization of first FM layer  704  may be switched by the spin torque if the spin polarized current is high enough. 
       FIG. 8  shows an illustration  800  of spin precession under an applied DC current due to spin torque τ in accordance with one exemplary embodiment of the invention. 
     The modified Landau-Lifshitz-Gilbert equation with spin torque may be determined by 
     
       
         
           
             
               
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     where γ may be the gyromagnetic ratio and a may be the damping parameter, H eff  may be the effective magnetic field, α j  may be proportional to applied current amplitude and spin polarization ratio, m (M) may be the magnetization vector of free (fixed) layer. The polarization ratio may depend on material properties of first FM layer  704 . 
     In this exemplary embodiment, a traceable path of the precessional motion of a particle is illustrated by oscillation path  812 . From the modified Landau-Lifshitz-Gilbert equation the time-dependent precessional motion may depend on the cross product result  806  of the magnetization vector of the free layer  810  and the vector of effective magnetic field H eff . Directions of spin torque τ  804  and damping torque  806  are also illustrated in  FIG. 8 . 
     Calculations suggest that the spins may start to precess at high frequency even at small applied DC current. For small angle elliptical precession of a thin-film ferromagnet, precession frequency f may be determined by 
     
       
         
           
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     where M effe  may be the effective saturation magnetization of free layer, H app  may be the applied magnetic field. H k  may be the anisotropy field. As applied field H app  may change either in direction or amplitude, the precession frequency f may change accordingly. 
       FIG. 9A  shows an illustration  900  of a spin precession mode  908  excited by a biased DC current through the STT effect in accordance with an embodiment. The illustration shows that a spin precession mode  908  may be excited by a biased DC current (4 mA) through the STT effect. 
       FIG. 9B  shows an illustration  910  of spin precession frequency  916  which may be excited by a biased DC current through the spin torque transfer effect (STT) effect in accordance with an embodiment. The spin precession frequency may be fixed (for example at f=29.8 GHz in  FIG. 9B ) when the bias current and magnetic field are fixed. The three smaller peaks in  FIG. 9B  may be from harmonic frequencies and may be filtered. With fixed bias current and external magnetic field, the spin precession frequency may follow only one mode ( FIG. 9A ) and only one frequency ( FIG. 9B ). 
       FIG. 10  shows a graph  100  of spin oscillation frequencies versus media field in accordance to one aspect of the invention. In this exemplary embodiment, preliminary results of spin precession frequency versus media field are shown for the case where a DC current may be applied in a CPP direction in a GMR element. The graph  1006  shows that when magnetic field increases from −200 Oe to 200 Oe, the spin precession frequency may shift linearly from 15 GHz to 30 GHz. 
     As typical magnetic fields from magnetic media at the memory cell reader flying height range from approximately −200 Oe to +200 Oe, spin precession at spin precession frequencies from 15 GHz to 30 GHz may be present in this range. A recording bit “0” at −200 Oe and recording bit “1” at 200 Oe, may result in a spin precession frequency change Δf of up to 15 GHz, which is large and easily measurable. The spin precession frequency may be further increased by tuning the material of the free layer material and the bias field. 
     These results present significant improvements over traditional GMR (or TMR) memory cell readers. Traditional GMR (or TMR) memory cell readers use a different detection scheme. For example, traditionally a free layer may be pre-set to be perpendicular to a reference layer through a bias field. Bit “0” or “1” in the media field induces a magnetic field on free layer and force it to rotate from its original position. Since GMR depends on the relative angle between free and reference layers, bit “0” and “1” in the media field induces a GMR difference (AGMR) due to free layer rotation. This detection method is limited by the GMR value of the element (for example, ΔTMR&lt;20%, ΔGMR&lt;5%), which is much smaller than the proposed method (Δf≈100%). 
     In exemplary embodiments of this invention, the memory cell reader may determine the spin precession frequency so the requirement for high GMR output is no longer as critical a factor in reader design in this invention as it is in traditional GMR (or TMR) reader designs using traditional GMR detection schemes. 
       FIG. 11A  shows an illustration  1100  of a magnetoresistive memory cell stack configuration according to one exemplary embodiment of the invention. In this exemplary embodiment, a stack configuration  1102  of a reader sensor may include a seed layer (not shown), a tri-layer spin valve GMR structure (reference layer-spacer layer-free layer  1118 - 1120 - 1116 ), sandwiched by top and bottom electrodes (not shown). The material of the spacer layer may be but is not limited to Cu. The current may flow along the CPP direction  1104 . The dimensions of each layer the tri-layer spin valve GMR structure may be defined by a stripe height  1108  and track width  1106 . The free layer  1116  may border an air bearing surface  1110 . The magnetization direction of the free layer  1114  may be nearly antiparallel to the magnetization direction of the reference layer  1112 . The media field may lie along the direction of the free layer magnetization. Electron flow from reference layer to the free layer and excited spin torque may drive the free layer precession at a frequency f 0 , which may vary depending on the media field. 
     Materials of the reference layers and free layers may be but are not limited by common material candidates, such as CoFe, NiFe. The stack thicknessess may be significantly reduced to meet the narrow shield-to-shield-spacing (SSS) requirements. 
       FIG. 11B  shows an illustration  1122  of a magnetoresistive memory cell stack configuration according to an alternative exemplary embodiment of the invention. In this exemplary embodiment, a stack configuration  1122  of a reader sensor may include reference layer-spacer layer-free layer  1130 - 1120 - 1128 . The magnetization direction of the free layer  1126  may be perpendicular to the magnetization direction of the reference layer  1124 . The media field may lie along the direction of the free layer magnetization. The reference layer  1130  may have out-of-plane magnetization. Hence, the magnetization of the free layer or reference layer may be out-of-plane with perpendicular anisotropy. The effect of the magnetization direction of the free layer  1126  being perpendicular to the magnetization direction of the reference layer  1124  may result in an increase in the spin torque effect. The reference layer may be of a high anisotropy K u  material, which is more stable under media field. The material candidates of the free layer and reference layer may be but or not limited to CoPt/FePt and high K u  materials. Other possible material and structure candidates can be but are not limited to multilayer structures such as Co/Ni, Co/Pt. 
       FIG. 11C  shows an illustration  1132  of a magnetoresistive memory cell stack configuration according to another alternative exemplary embodiment of the invention. In this exemplary embodiment, a stack configuration  1132  of a reader sensor may include reference layer-spacer layer-free layer  1140 - 1120 - 1138 . The free layer magnetization direction  1136  may be out-of-plane and perpendicular to reference layer magnetization direction  1134  resulting in an increase in the spin torque effect. As the media field may be perpendicular to the direction of the free layer magnetization  1136 , the field dependence of spin precession frequency may not be linear. Possible material and structure candidates can be but are not limited to CoPt, FePt or multilayer structures such as Co/Ni, Co/Pt. 
     Traditonal reader sensor structures using traditional GMR value detection schemes include an anti-ferromagnetic (AFM) layer such as IrMn, for example, to fix magnetization of the reference layer. The typical AFM layer thickness is typically around 6-7 nm to ensure pinning effect and thermal stability. Such thickness almost exceeds the 12 nm SSS budget for 10 Tbit/in 2  areal density and seriously decays the GMR effect due to its large resistance ratio within the whole stack structure. As the exemplary embodiments of  FIGS. 11A ,  11 B and  11 C may have the AFM layer removed, SSS values as small as 12 nm can be achieved. The free layer need not be biased to obtain better sensitivity as in traditional reader sensors and thus allowing reader sensor structures to be simplified due to magnetic hard bias free design. 
       FIG. 12  shows an illustration  1200  of a memory cell arrangement in accordance with an embodiment. In this embodiment, a memory cell arrangement  1202 , includes a magnetic memory cell  1212 , a magnetoresistive cell  1204  configured to generate a spin precession frequency under a magnetic field from the magnetic memory cell  1212 , a frequency determiner  1206  configured to determine a spin precession frequency provided by the magnetoresistive cell  1204 , and a storage state determiner  1208  configured to determine the magnetic memory cell storage state based on the determined spin precession frequency. In this embodiment, a magnetoresistive cell reader arrangement  1216  of memory cell arrangement  1202  may detect the storage state of magnetic memory cell  1212 . Frequency determiner  1206  may be connected via connection means  1210  to storage state determiner  1208 . Magnetic memory cell  1212  may be connected to magnetoresistive cell reader arrangement  1216  of memory cell arrangement  1202  via connection means  1214 . The frequency determiner  1206  of memory cell arrangement  1202  may further include a spectrum analyzer. In an exemplary embodiment, storage state determiner  1208  may be configured to determine the magnetic memory cell storage state of the magnetic memory cell based on a comparison of the determined spin precession frequency with at least one of a first predefined spin precession frequency associated with a first magnetic memory cell storage state and a second predefined spin precession frequency associated with a second magnetic memory cell storage state. 
     In another exemplary embodiment, storage state determiner  1208  may be configured to determine the magnetic memory cell storage state of the magnetic memory cell based on a comparison of the determined spin precession frequency with a frequency threshold, which is arranged between a first predefined spin precession frequency associated with a first magnetic memory cell storage state and a second predefined spin precession frequency associated with a second magnetic memory cell storage state. 
     Since spin precession frequency signal-to-noise ratio relies on the frequency difference instead of GMR value, very low values of resistance×area (RA) of the GMR element and narrow shield-to-shield spacing are attainable. These factors are very critical for high density magnetic recording. In addition, there is negligible noise from spin torque transfer, which is a big problem associated with shrinking down of reading element in traditional GMR detection schemes. Exemplary embodiments of the invention thus achieve the following effects. 
     1. Small shield-to-shield spacing (SSS): The simple GMR structure does not require a synthetic anti-ferromagnetic (SAF) or anti-ferromagnetic AFM pinned structure. Hard reference layers used in the preferred exemplary embodiments makes it possible to remove anti-ferromagnetic (AFM) layer. This means that the SSS distance may be greatly reduced to 9 nm or less, resulting in a four fold larger linearly density compared with present technology. 
     2. No spin torque noise: The spin torque transfer (STT) effect is measured as a frequency determined signal source instead of noise. This is a big advantage for small sensor. 
     3. Ultra-low RA: As memory cell readers of exemplary embodiments of the invention typically use an all metal CPP GMR structure, resistance×area RA values below 0.05 Ωμm 2  are typically achieved. Therefore, memory cell readers according to exemplary embodiments of the invention are not no longer restrained by the impedance limit of smaller reader sensor designs. 
     4. No need for high GMR value: As memory cell readers of exemplary embodiments of the invention use the frequency spectrum (from several GHz to 20 GHz) to detect media field, a high GMR output is not as critical as a traditional reader design even though the GMR effect may be used to generate a voltage signal. The required signal-to-noise ratio does not depend on GMR value but on frequency change Δf, providing a suitable solution for 10 Tbit/in 2  or even higher recording technology. 
     5. Low noise and high signal-to-noise ratio (SNR): As memory cell readers of exemplary embodiments of the invention function at high frequency, noise from thermal magnetization fluctuation, Johnson noise/shot noise can be filtered as much as possible. Compared with STT induced signals, the noise voltage output is much lower. In addition, Johnson noise is naturally reduced at low RA value. 
     6. Possible elimination of hard biasing: As the reference layer and free layer may be parallel or anti-parallel to air bearing surface (ABS), a hard bias permanent magnet is not required to set free layer direction, resulting in a great process simplification magnetic cell reader design. 
     Aspects of various embodiments propose magnetic field detection methods to solve issues of RA, output signal and spin torque noise. Small shield-to-shield spacing without requiring an AFM layer and high speed signal detection free from spin torque noise may be attained. 
     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.