J. J. Qiu

Perspectives of Read Head Technology for 10 Tb/in

For a read head design, two of the most important parameters are the resolution and the signal to the noise ratio (SNR). For 10 Tb/in2 recording, the sensor dimensions required to achieve the resolutions are derived for various bit aspect ratio (BAR) designs. It is shown that the todays shielded read head technology is unable to scale down to meet the linear density requirement. By analyzing the data rate requirements at various BAR values, it is argued that the current tunnel magnetoresistance head is no longer suitable to provide high SNR due to its high impedance. A differential dual spin valve (DDSV) , which does not need to have magnetic shields, is proposed to achieve the required SNR and linear resolution of the read head for 10 Tb/in2. A systematical analysis on the effect of stiffness field, magnetic shields and media field shows that thermally activated magnetic noise may not necessarily be a crucial limiting factor for 10 Tb/in2. The conventional course to increase SNR by increasing GMR ratio is still applicable. Assuming realistic constraints and requirements, we come out with the DDSV reader specifications for 10 Tb/in2 recording. With a stiffness field of 300 Oe and an operating current density of 4 ? 107 A/cm2, a SNR of 20 dB can be achieved under a media field of 300 Oe for an unshielded DDSV reader having a GMR ratio of 12%. Preliminary studies show that DDSV is very promising for the application in 10 Tb/in2. The differential effect in DDSV is demonstrated using two types of dual spin valve. For a dual spin valve with two reference layer (RL) magnetizations in the same direction, the total resistance change under uniform field is the sum of two spin valves as expected, while for another similar dual spin valve but with the two RL magnetizations in the opposite direction, no resistance change is observed in the dynamic field region from 300 Oe to - 300 Oe.

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A spin valve magnetoresistive random access memory with a flux-closed structure is presented. The flux-closed structure prevents the disruption to magnetization of the recording layer and increases the thermal stability. Simulation results show that the switching field increases under the uniform external field, however, the switching field has no change under the bit current field. Only the bit current that flows mostly at the center is used to switch the cell. Experimental results show that the bit current of 10 mA is sufficient to switch the 1 μm×4 μm cell.

Recording

A simple methodology to fabricate soft magnetic FeCoNi granular films from cheap salt solutions viaelectrodeposition at room temperature is demonstrated. With the addition of a small quantity of organic and inorganic additives into the solutions, the FeCoNi nano-granular films possess ultra-high permeability, large resistivity, and other desirable magnetic properties for gigahertz microwave applications. Typically, the films have a coercivity of less than 10 or 20 Oe along the hard or easy axis, respectively, with a saturation flux density of up to 2.43 T. The magnetic permeability and resistivity are correspondingly up to a magnitude order of about 103 and 10−4 Ω cm. These soft FeCoNi films also show a big anisotropic field of more than 50 Oe and a very small magnetostriction of <10−5. They can be potentially applied to microwave absorption as well as other applications.

Low switching current flux-closed magnetoresistive random access memory

Abstract Ultrathin metallic films like CoFe, Ta, Cu, Cr, and NiFe are widely used in magnetic devices such as magnetic random access memory (MRAM) and magnetic recording heads. Dewetting corrosions were often observed after O2 plasma ashing in MRAM fabrications. The surface stability of these films was then examined. The results show that dewetting takes place when CoFe or Cu films are exposed to air after an O2 plasma process. In contrast to the dewetting reported so far in organic or metallic liquid films on solid substrates, the observed dewetting does not occur in a liquid state but in a solid state. Several in situ and ex situ process methods were examined to control the dewetting. It is found that after ashing, the immediate immersion of wafer into acetone and ultrasonic cleaning some minutes after opening chamber can greatly suppress the occurrence of dewettings. Process examinations show that the heating is unimportant for the formation of the dewetting, while moisture in air may play an important role in the formation of the dewetting, acting as a necessary catalyst. Several dewetting patterns were observed, and the pattern shape depends not only on the thickness of the film, but also on the plasma parameters. Possible mechanisms responsible for the formation of these patterns are discussed.

Electrodeposition of granular FeCoNi films with large permeability for microwave applications

In this work, a new MRAM structure with guided SAF layers is presented. This new structure addresses the issues regarding additional read operation, reduction of saturation field, write operation margin, and unintentional switching.

Dewetting observations of ultrathin metallic films

As the recording density of the hard disk increases, the smaller reader track width is required. The tunneling GMR reader which has been used in the hard disk drive has the potential for lTb/in2 due to its large MR ratio. To maintain the SNR and match the impedance of the preamplifier, the low RA less than 0.5 Omegamum2 is required. However, the low RA less than 1 Omegamum2 is difficult to achieve due to both the thickness and barrier height limitation. On the other hand, as the track width reduces, the side reading effect will increase the magnetic track width. In this paper, a track width reduction scheme is presented in order to reduce the resistance.

Spin-flop switching of the guided synthetic anti-ferromagnet MRAM

A series of polycrystalline full-Heusler Co2MnAl thin films were deposited on Si (100) coated with thermo SiO2 by using different types of seed layers such as Cr, Mg, MgO∕Cr, and MgO. The properties of the Co2MnAl thin films such as the coercivity, atomic composition, and crystalline structure strongly depend on the deposition conditions and seed layers. Very soft Co2MnAl thin films with coercivity of 8.3Oe and small magnetostriction coefficient λS=1.43×10−5 had been obtained when MgO was used as seed layer. Magnetic tunnel junctions with magnetoresistance ratio of 12%–19% by utilizing the Co2MnAl as bottom ferromagnetic electrode have been successfully fabricated.

Side-shielded TGMR reader with track width reduction scheme

The magnetostriction coefficient (λs) of ultrathin sputtered polycrystalline as-deposited and annealed Ta∕Ni81Fe19(t)∕Ta films was studied as a function of the thickness. λs and magnetoresistance (MR) of bottom-type spin valves (SVs) with nano-oxide layer (NOL) added in the pinned layer were investigated by using NiFe, Co90Fe10, and CoFe∕NiFe∕CoFe layers as free layer (FL), respectively. λs of SV with NOL increased slightly except that of CoFe FL. NOLs were added at different positions to study the effects of NOL on λs of CoFe FL. All λs of CoFe FL change from negative to positive and its absolute value also increases significantly with CoFeOx related NOL added below. Our λs and surface roughness results indicated that the structure of the film not the roughness dominates λs of ultrathin FL in SVs.

Structural and magnetoresistive properties of magnetic tunnel junctions with half-metallic Co2mnAl

Two sets of processes for sub-100-nm current-perpendicular-to-plane (CPP) spin-valves (SVs) were developed. In the first method, a CPP SV was fabricated by using via-hole instead of small cell patterning. This method makes it easy to characterize the CPP spin valve films. The shunting effect has been analyzed in this method. In the second method, a resist layer for planarization, ion-milling, and wet etching for self-opening via-hole in a self-aligned fashion by making use of the height difference between the cell and surrounding regions was introduced. This method is suitable for the actual recording heads with small dimension even without an ion-beam-deposition system. CPP sensors with size from 0.05 /spl mu/m/spl middot/0.05 /spl mu/m to 0.4 /spl mu/m/spl middot/ 0.5 /spl mu/m with different free- layer structures have been investigated by means of the first method.

The influence of nano-oxide layer on magnetostriction of sensing layer in bottom spin valves

A methodology to fabricate ultrasoft CoFe nano-/microfilms directly via electrodeposition from a semineutral iron sulfate solution is demonstrated. Using boron-reducer as the additive, the CoFe films become very soft with high magnetic moment. Typically, the film coercivity in the easy and hard axes is 6.5 and 2.5 Oersted, respectively, with a saturation polarization up to an average of 2.45 Tesla. Despite the softness, these shining and smooth films still display a high-anisotropic field of ~45 Oersted with permeability up to 1 0 4 . This kind of films can potentially be used in current and future magnetic recording systems as well as microelectronic and biotechnological devices.