G. C. Han

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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Room temperature ferromagnetic Zn0.95Co0.05O thin film with (002) preferential orientation has been deposited by pulsed laser deposition at different substrate temperatures on n-type (100) silicon substrate. A maximum saturation magnetization of 0.86μB∕Co was achieved at room temperature. High-resolution transmission electron microscope and x-ray photoelectron spectroscopy results indicate that this ferromagnetism behavior is not due to Co microclusters in the thin film. Zinc interstitials, oxygen vacancies, and lattice defects induced by low substrate deposition temperature show a significant effect on ferromagnetic behavior. By changing the substrate deposition temperature to control the amount of zinc interstitials, oxygen vacancies, and lattice defects, ferromagnetism can be tuned significantly as well, which is a direct evidence that these defects contributed to the ferromagnetism in ZnO:Co thin films.

Recording

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.

Effect of substrate temperature on pulsed laser ablated Zn0.95Co0.05O diluted magnetic semiconducting thin films

Using two suitably designed thin film systems, the impact of metallic cobalt nanodots on the mechanism of ferromagnetism (FM) in ZnO thin films was studied. At a relatively higher oxygen partial pressure, Co nanodots embedded ZnO thin films no longer show FM, as compared to Co-doped ZnO dilute magnetic semiconductor (DMS). The structural and magnetic properties of these two systems support the hypothesis that (1) the FM of DMS thin films is due to bound magnetic polarons instead of cobalt nanoclusters and (2) the critical defect concentration is the key parameter which controls the FM properties in DMS thin films.

Low switching current flux-closed magnetoresistive random access memory

Mn1.4Ga films with high perpendicular magnetic anisotropy and high crystalline quality were grown on MgO substrates with Cr buffer layer using molecular beam epitaxy. The crystalline structure and the surface morphology of the films have been systematically investigated as functions of in-situ annealing temperature (Ta) and film thickness. It is found that the magnetic properties can be largely tuned by adjusting Ta. As Ta increases, both saturation magnetization (Ms) and uniaxial perpendicular magnetic anisotropy constant (Ku) increase to the maximum values of 612 emu/cc and 18 Merg/cc at 300 °C, respectively, and then decrease. The morphology also changes with Ta, showing a minimum roughness of 2.2 A at Ta = 450 °C. On the other hand, as the thickness increases, Ms and Ku increase while coercivity decreases, which indicates there is a magnetic dead layer with a thickness of about 1.5 nm at the interfaces. The detailed examination on the surface morphology of the films with various thicknesses shows a co...

Role of metallic cobalt in room temperature dilute ferromagnetic semiconductor Zn0.95Co0.05O1−δ

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.

Annealing temperature and thickness dependence of magnetic properties in epitaxial L10-Mn1.4Ga films

TiN was investigated as a potential spacer material in L10-FePt based pseudo spin valves (PSV). PSVs with the structure MgO/L10-Fe50Pt50 (20 nm)/TiN (5 nm)/L10-Fe50Pt50 (x nm) were fabricated, where x was varied from 5 to 20 nm. The highest giant magnetoresistance (GMR) ratio of 0.61% was obtained for the PSV with a top L10-FePt thickness of 20 nm. Contributions to the GMR arose from both the spin dependent scattering at the FePt/TiN interfaces and domain wall resistivity. Magnon magnetoresistance was also observed in the fabricated PSVs.

Dewetting observations of ultrathin metallic films

Thermal magnetization fluctuations in tunneling magnetoresistive heads with track width of ∼100nm have been studied through high-frequency magnetic noise measurement as a function of both transverse and longitudinal fields. The study suggests that in the dynamic region of the heads, the related power spectral density is determined by the magnetization fluctuations of the free layer. At high fields which drive the sensor to the magnetization saturation, an “abnormal” resonance with substantially high amplitude in comparison with that of the free layer is observed when the magnetization of the free layer is nearly antiparallel to that of the reference layer. This abnormal resonance is excluded from the spin torque effect. The detailed analysis suggests that this resonance is due to the magnetization fluctuations in the reference/pinned layers.

(001) textured L10-FePt pseudo spin valve with TiN spacer

We use lateral superconductor (SC)-ferromagnet (FM)-SC device to probe the magnetization reversal process of micron sized FM disk. Upon decreasing external magnetic field from saturated state, a buckling pattern forms first. The onset of buckling pattern and its switching to vortex state are studied with the help of Andreev conductance of the SC-FM interface below the critical temperature (Tc) of SC and the magnetoresistances both below and above the Tc. In the latter case, micromagnetic modeling has been carried out to interpret the experimental results by taking into account the current distributions when the electrodes are at different states.

Field dependence of high-frequency magnetic noise in tunneling magnetoresistive heads

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.