Sunita Gangopadhyay

NiMn‐pinned spin valves with high pinning field made by ion beam sputtering

Spin valves of film layer structure, Ta/NiMn/NiFe/Co/Cu/Co/NiFe/Ta/Substrate were fabricated by ion beam sputtering. Optimization of the processes of deposition and posthermal treatment yields highly (111) oriented spin valve films with a giant‐magnetoresistance ratio of above 4% and pinning field of 650 Oe. This is the strongest pinning field ever observed. It stays constant up to 180 °C, then decreases to zero at a blocking temperature of 380 °C. These spin valves are highly thermally stable and, thus, suitable for the application of high density recording heads.

Thermomechanical head performance

Mismatch of thermal expansion of various materials used in the transducer and slider of giant magnetoresistive (GMR) recording heads causes, at higher operating head temperature, mechanical stresses and, in particular, protrusion toward the media [thermal pole tip recession (T-PTR)]. Low T-PTR is necessary for low head-media spacing without mechanical contact. Impact of the magnitude of the Poissons ratio of a photoresist coil insulator on thermal protrusion is shown to be large, due to large variation of the volume compressibility. Three-dimensional FE (3-D FE) thermomechanical modeling shows that the distribution of thermal stress in shields caused by mismatch of coefficient of thermal expansion changes completely close to the air bearing surface due to protruded head surface. During head operation, the primary heat source arises from the writer coil. The maximum temperature and the particular temperature distribution depends on the ability of the head components to dissipate effectively the generated heat. As the transducer continues to scale down in size with increasing areal density and data rate, the power dissipated per unit volume grows due to the larger coil resistance in the core region.

Exchange tab stabilized readback transducers for areal densities exceeding 20 Gb/in/sup 2/

We present results from a high-density giant magnetoresistive magnetic recording reader using exchange bias stabilization. This novel reader design approach reduces the amount of parasitic resistance, as the sense current is not delivered through high resistivity permanent magnets. Heads were demonstrated to deliver areal densities in excess of 24 Gb/inch/sup 2/. The electrical performance of these heads, in particular, amplitude sensitivity, microtrack profiles and areal density capability are presented. Reader film properties and manufacturability of this approach are discussed in detail.

Microstructural study of ion-beam deposited giant magnetoresistive spin valves

Detailed microstructural investigation of ion beam deposited giant magnetoresistance (GMR) spin valves has been carried out using various techniques of transmission electron microscopy (TEM) and x-ray diffraction. Two fcc phases, i.e., FeMn and NiFe/Co/Cu/Co/NiFe have been identified in ion-beam deposited Ta (50 A)/FeMn (80 A)/NiFe(30 A)/Co(15 A)/Cu(33 A)/Co(15 A)/NiFe(60 A)/Ta(25 A)/Si(001) spin valves. The Ta buffer layer is amorphous, while the 50-A-thick Ta cap layer consists of a 25-A-thick amorphous layer and on top of which a Ta oxide layer. The lattice constants of the fcc FeMn and the fcc NiFe/Co/Cu/Co/NiFe increase with the ion-beam voltage. Both the FeMn and the NiFe/Co/Cu/Co/NiFe layers are (111) textured. The misfit strain between the FeMn layer and the pinned NiFe layer is released by the formation of dome shape FeMn surface rather then by the formation of misfit dislocations at the interface between the two layers. The peak to valley height of the domes seems to have little effect on the GM...

Temperature variation of the magnetoresistance in cobalt-enhanced spin-valve structures

We have conducted a survey of the temperature variation of the magnetoresistance in a series of FeMn exchange-biased spin-valve structures. These permalloy-based samples were prepared in an ion-beam sputtering system and feature Co layers inserted at the interfaces with the Cu spacer layer to enhance the interfacial spin-dependent scattering. Typical values for the MR are 4.0% at 295 K and 8.4% at 8 K (sample with 15 A Co thickness). A control sample with no cobalt showed MR values of 1.3% and 4.3% for those same temperatures. Both the MR ratio and the un-normalized magnetoresistive change ΔR are plotted vs temperature. The MR ratio for the cobalt-enhanced samples exhibits nearly linear decrease with rising temperature. The sample with no cobalt exhibits a temperature variation deviating substantially from linearity, with an upward curvature. The temperature dependence for the MR in these spin valves is examined in the light of interchannel spin-mixing and intrachannel scattering.

17 Gb/in/sup 2/ areal density demonstration at 214 Mb/s

We have simultaneously demonstrated high areal density and high data rate, in an experiment that closely mimics the operation of disc drive products. The system involves thin-film media, GMR merged heads, broad bandwidth electronics, and an EPR4 channel with post-processing. The results presented reflect a statistical sample of components, rather than one-of-a-kind devices. In order to determine the areal density accomplished, we demanded that the bit error rate performance be insensitive to significant deviations of the head position from the recorded track center. We present a thorough description of the components and the SNR budget. The areal density accomplished varies between 15.1 Gb/in/sup 2/ at 193 Mb/s, and 17.1 Gb/in/sup 2/ at 214 Mb/s. A series of areal density capability assessments was obtained by applying various margin conditions. This was done to demonstrate robust experimental results. The outcome of this work may be applied to product development.

Media Roughness and Head-Media Spacing in Heat-Assisted Magnetic Recording

Heat-assisted magnetic recording involves the transfer of energy to the recording medium via optical means. To enable high areal density, the recorded track must be smaller than the diffraction limit of focused light, which is accomplished by using a near-field transducer (NFT) with a corner or peg with small dimension. Energy transfer using such a transducer is a near-field effect, and therefore is highly sensitive to the spacing between the NFT and the medium. Since the recording medium has some surface roughness, there will be a variation in the NFT-to-medium spacing and this will impact the amount of energy transferred from the NFT. We model the effect of Gaussian surface roughness on NFT energy transfer and predict surface temperature variations for a rough surface. In addition, we illustrate how changing the head-medium spacing changes the impact that roughness has on surface temperature variation. We combine these modeled predictions with spinstand measurements of recorded data and conclude that the effect of media roughness results in only limited temperature excursions above the nominal recording medium temperature.

Domain control in magnetic shields using patterned permanent magnet underlayer

The domain state of a magnetic shield in a recording head can be controlled by an adjacent patterned permanent magnet layer. A 1.1-μm-thick electroplated Ni80Fe20 (NF) film with slight uniaxial magnetic anisotropy was patterned into rectangular magnetic shields with various dimensions over patterned thin film made from a 0.1-μm-thick CoCrPt permanent magnet (PM). The shape of the adjacent biasing PM layer should be the shape of a desired final domain in NF. Domain structure in the NF layer and the process of magnetic saturation were imaged using wide-field Kerr microscopy. The NF and PM layers are magnetically coupled and, therefore, a magnetic state with parallel magnetization is preferred. The PM direction of magnetization is set in high magnetic field and the final NF domain state is controlled by the shape of PM features. The simplest stable domain structure in a rectangular thin shield is of an “envelope” type. Using a PM underlayer, either clockwise or counterclockwise domain structure is preferred....

23.8 Gb/in.2 areal density demonstration

We have demonstrated 23.8 Gb/in.2 areal density using a merged read-write grant magnetoresistive head, with an oriented thin film medium tested with broadband electronics and enhanced EPR4 channels. The medium had high signal to noise ratio metrics that was robust unto temperatures as high as 75 °C. A unique aspect of the head design at such a narrow track width is the simultaneous enhancement of the transducer sensitivity while keeping product and system manufacturability in the forefront. The areal density was demonstrated at a track density of 45.8 k tracks/in., using photolithographically defined poles and linear density of 520 k bits/in.

A study of the induced anisotropy in a ferromagnetic grain from an exchange coupled antiferromagnetic grain with uniaxial anisotropy

Approximate analytic solutions for the energy of an antiferromagnetic (AF) grain, which experiences an external exchange torque from a ferromagnetic grain have been obtained for a wide range of AF thickness. The accuracy of the analytic expression is within 2.0% of the exact solution, which do not have a closed analytical form. The model predicts that there are two critical AF grain thickness for each particular exchange energy strength. Below the first critical thickness, the induced anisotropy energy is well approximated by an uniaxial anisotropy term. Above the second critical thickness the induced anisotropy is unidirectional. In the intermediate range the induced anisotropy can not be expressed simply as uniaxial or unidirectional. The exchange bias and coercivity in NiFe/IrMn films have been studied as a function of the IrMn thickness, and the results are consistent with the proposed theory.