Bharat B. Pant

Commercial TMR heads for hard disk drives: characterization and extendibility at 300 gbit/in/sup 2/

Tunneling magnetoresistive (TMR) reading heads at an areal density of 80-100 Gbit/in/sup 2/ in a longitudinal magnetic recording mode have for the first time been commercialized for both laptop and desktop Seagate hard disk drive products. The first generation TMR products utilized a bottom TMR stack and an abutted hard bias design. These TMR heads have demonstrated three times the amplitude of comparable giant magnetoresistive (GMR) devices, resulting in a 0.6 decade bit error rate gain over GMR. This has enabled high component and drive yields. Due to the improved thermal dissipation of current-perpendicular-to-plane geometry, TMR runs cooler and has better lifetime performance, and has demonstrated the similar electrical static discharge robustness as GMR. TMR has demonstrated equivalent or better process and wafer yields compared to GMR. The TMR heads is proven to be a mature and capable reader technology. Using the same TMR head design in conjunction with perpendicular recording, 274 Gbit/in/sup 2/ has been demonstrated. Advanced design can reach 311 Gbit/in/sup 2/.

Spin transfer stimulated microwave emission in MgO magnetic tunnel junctions

In this work, current-driven precession of magnetization in multilayers with Co-Fe-B/MgO/Co-Fe-B MTJ in the form of dots has been investigated.

High-frequency magnetic-force microscopy characterization of magnetic recording writer poles

Measurements of longitudinal writer poles at the air bearing surface were performed using high-resolution magnetic-force microscopy (MFM) with low coercivity tips. Two-dimensional MFM maps were obtained for various write currents. The modeling results indicate that the MFM maps are related more to the field than to its second derivative. Two techniques were used, dc MFM and high-frequency (HF) MFM. The results show that the HF-MFM technique can distinguish between different writer designs. The writers with the best high-frequency performance showed gradual decrease of the maximum MFM signal with frequency up to 1.5GHz.

Paramagnetic to antiferromagnetic phase transformation in sputter deposited Pt-Mn thin films

Sputter-deposited, equiatomic Ni-Mn thin films were observed to possess a metastable, nanocrystalline, chemically disordered, fcc (A1) structure. Grain growth and a phase change to a chemically ordered, antiferromagnetic L10 structure were identified by x-ray diffraction (XRD) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) experiments revealed exothermic signals that correspond to the grain growth and phase transformation reactions. The enthalpy of transformation for the A1 to L10 phase change was calculated as −3.5 kJ/mol, which agress with thermodynamic modeling. An activation energy of 139 kJ/mol was calculated for the phase transformation by the Kissinger method.

Interdiffusion in CoFe/Cu multilayers and its application to spin-valve structures for data storage

Spin-valve structures might be exposed to higher temperatures in future disk drive applications and might thus degrade faster than it does today if proper materials and methods are not used. In order to determine whether this degradation is due to interdiffusion between constituent layers or is dominated by other phenomena, the interdiffusion coefficients for all layers in the spin valve have to be determined. For diffusion driven degradation it would then be possible to predict lifetimes based on a maximum allowed reduction in ΔR/R where R is the resistivity. Here we report the initial results for a CoFe/Cu interface, common to many spin-valve structures. Interdiffusion in (111) textured polycrystalline CoFe/Cu multilayers has been measured and quantified by x-ray reflectometry. Bulk diffusion is dominant at temperatures above ∼540 °C and is described by an activation energy of Ea=2.41 eV and a prefactor of D0=2.92×10−8 m2/s. Below temperature of 540 °C grain boundary diffusion dominates and is character...

Oxidation of tunnel barrier metals in magnetic tunnel junctions

The oxidation of an ultrathin metal layer (<1nm) to form an oxide tunnel barrier is of critical importance for the fabrication of magnetic tunnel junctions (MTJs) with low product of resistance and area (R×A). Nonuniform and excessive or insufficient oxidation will occur by using conventional plasma, air, or O2 and noble gas mixtures as oxidation methods. An oxidation method was investigated to oxidize only an ultrathin layer of metal (such as Y) without oxidizing adjacent ferromagnetic thin film layers. We have now demonstrated that a gas mixture of H2O∕H2 with a fixed chemical potential of oxygen determined by the relative amounts of the two gases can oxidize Y and Ta thin layers while simultaneously keeping a Co ferromagnetic layer completely unoxidized. This universal method can be used to preferentially oxidize a host of other metals with high tendency to form oxides, such as Zr, Hf, Nb, rare earth metals, etc. and may allow us to access the feasible lower limit of barrier thickness in MTJs.

Intermixing and phase separation at the atomic scale in Co-rich (Co,Fe) and Cu multilayered nanostructures

Despite the fact that Co-rich (Co,Fe) alloys and Cu are immiscible materials in bulk form, evidence of thermally induced mixing at the atomic scale has been observed in thin-film multilayers of (Co,Fe) and Cu. However, long term anneals at lower temperatures produced a breakup of the multilayers into a two-phase mixture of (Co,Fe) and Cu particles. The observations were made with the use of the three-dimensional atom probe technique, with supporting evidence from differential scanning calorimetry and x-ray diffraction. Besides their scientific importance, these results are of interest where these (Co,Fe) and Cu thin films are used to produce the giant magnetoresistive effect.

Diffusion in Co90Fe10/Ru multilayers

Signal degradation in spin-valve structures is today a concern for long-term stability of data storage devices. One of the possible degradation mechanisms of spin-valve structures in disk drive applications could be thermally activated diffusion between constituent layers. In order to predict and control performance degradation, the interdiffusion coefficients for all bilayers in the spin-valve structure will have to be determined. Here we report results from a Co90Fe10/Ru interface, common in many spin-valve structures. The diffusion in (0002) oriented polycrystalline Co90Fe10/Ru multilayers has been measured and quantified by x-ray reflectivity in the temperature range of 450–540 °C. The bulk diffusion in this case is described by an activation energy of Ea=4.95 eV and a prefactor of D0=6.43×10−9 m2/s. No grain boundary diffusion was detected in the large-grain structure dominated by high symmetry grain boundaries at the temperature interval in this study. For a spin-valve structure that contains Co90Fe...

Thermodynamic Evaluation of the Interface Stability Between Selected Metal Oxides and Co

For an interface to be considered thermodynamically stable, the phases in contact must be in equilibrium with each other (connected by a stable tie-line) and have negligible mutual solubility on the phase diagram. The stability of Co based magnetic tunnel junctions (MTJs), with Co/M x O 1- x /Co structures (M = Al, Gd, Hf, La, Mg, Si, Ti, Ta, Y and Zr), were evaluated with regard to these two conditions. Specifically, low temperature ternary isothermal phase diagrams were calculated and evaluated for the Co–M–O systems. All of these systems have at least one oxide in equilibrium with Co and thus have at least one thermodynamically stable tunnel barrier candidate for use in Co based MTJs. In light of the assumptions made in this analysis, along with the uncertainty in applying bulk enthalpy data to thin films, the current evaluation of interfacial stability serves as a first step in identifying suitable stable tunneling barrier materials in MTJs for detailed study.

Locating magnetic noise sources in TMR and GMR recording heads using scanning probe microscopy

A scanning magnetic microscope was designed and built to pinpoint the location of magnetic noise induced by magnetic instabilities in submicrometer-sized giant magnetoresistive (GMR) and tunneling magnetoresistive (TMR) recording heads. A scanning nanometer-sized magnetic tip was used to generate a localized magnetic field and excite the free-layer magnetic moment at the air-bearing surface (ABS). The spectral response of the GMR and TMR recording heads is due to this localized oscillating excitation and is recorded as a function of the tip location. By mapping out the magnetic noise as a function of position, the trouble spots that indicate magnetic instabilities inside the recording head are identified.