A. Johnston

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/.

A model of the magnetic properties of coupled ferromagnetic∕antiferromagnetic bilayers

A granular level model which is capable of predicting the bulk magnetic properties of coupled ferromagnetic and antiferromagnetic layers is described. The model is used in an extensive investigation of the effect of the thermal instability of the antiferromagnetic layer as a function of the layer thickness, grain diameter, temperature, and the grain size distribution σ. The calculations give good qualitative agreement with experiment and provide an understanding of the role of the antiferromagnetic layer in determining the exchange bias field and the coercivity.

A model of the temperature dependence of exchange bias in coupled ferromagnetic∕antiferromagnetic bilayers

A granular level model of the magnetic properties of coupled ferromagnetic∕antiferromagnetic layers is used to calculate the temperature dependence of the exchange bias. The predicted results are in good qualitative agreement with experiment. Agreement with experiment requires the introduction of the temperature dependence of the anisotropy constant of the antiferromagnetic layer.

Transmission electron microscopy study of CoFe films with high saturation magnetization

Transmission electron microscopy (TEM) has been used to study magnetization processes in four high moment CoFe films. While all films were of similar total thickness, 50nm, the differences between them were the inclusion or otherwise of a seed layer and the introduction of nonmagnetic spacers to form laminated films. The detailed reversal mechanism for easy and hard axis reversals of each film was investigated. As expected cross-tie walls were observed in the films with thicker CoFe layers and wall displacements between layers were seen with the introduction of one or more spacer layers. Magnetization dispersion was reduced as multilayering was introduced. In the laminated film with three spacer layers, defect areas where the local magnetization distribution differed markedly from the surrounding film were observed. Cross-sectional TEM showed that layer roughness increased through the stack and this was the probable cause of the localized magnetic anomalies.

XMCD measurements of exchange biased PtMn/Co bilayers

Abstract The magnetic properties of a series of PtMn (t A )/ Co (20 A ) (t=50, 100, 150, 200 A ) exchange biased films are investigated using bulk magnetometry and X-ray magnetic circular dichroism (XMCD). From the bulk magnetometry measurements, the exchange field and coercivity is observed to increase with PtMn thickness. XMCD spectra are used to probe the interface region between the ferromagnetic Co and antiferromagnetic PtMn. The observed Mn XMCD is indicative of uncompensated Mn at the interface. These uncompensated Mn spins may be partially responsible for the increased coercivity observed in this exchange biased system.

The effect of roughness on the micromagnetic properties of high moment multilayer films

Electron microscopy has been used to determine directly the effect of artificially introduced roughness on the micromagnetic processes that occur in soft high moment multilayer films. The nanodefects, introduced to roughen the substrate, and the local magnetic domain structure were identified in the same images, leading to unambiguous information on the role played by the former. Characteristic of the micromagnetic state of the samples was a high density of 360° wall segments that showed quite remarkable resistance to annihilation. Following a description of the domain walls generated in both easy and hard axis magnetization cycles, a model is proposed for the way the observed domain walls respond when their ends are strongly pinned and, using this, we account for their continued existence over an extended number of magnetization cycles. Finally, the implications for device performance are discussed briefly.

Lorentz microscopy investigation of the free layer reversal in CoFe and Co top spin-valves

Abstract The magnetisation reversal of the free layer of two different spin-valve (SV) films with crossed anisotropy was studied using Lorentz microscopy in a modified transmission electron microscope. Both SVs were of similar construction, the principal difference being the replacement of Co by CoFe in the layers surrounding the non-magnetic spacer. Marked differences in the reversal mode were apparent. For CoFe SVs reversal was usually by rotation and experimental results were in good agreement with those predicted by a modified Stoner–Wohlfarth model. By contrast, reversal in Co SVs tended to involve complex domain processes with a high degree of irreversibility. The differences are attributed to the much higher magnetostriction constant of Co which gives rise to substantial local property variation.

Asymmetric magnetization reversal of the free layer of a spin-valve

The reversal of the free layer of a spin-valve with crossed anisotropy was studied by transmission Lorentz electron microscopy. In situ magnetizing experiments were carried out using the Fresnel imaging mode for various applied field orientations. Reversal could be by simple magnetization rotation or by rotation combined with complex domain processes. Asymmetric magnetization reversals whereby the modes on the outward and return paths of a magnetization cycle were seen on occasion. Insight into why the reversal mode varied in the way it did was obtained using a modified Stoner-Wohlfarth model. The model provided a good description of the various modes and there was reasonable agreement between predicted and observed fields at which key stages of the reversals took place. Even though a single variable parameter model of the kind used cannot describe a multi-domain state, its use in inferring the nature of domain configurations that arise is discussed as are its other strengths and weaknesses.

A model of the exchange bias setting process in magnetic read sensors

A model of the acquisition of exchange bias during the high temperature annealing process used to set the bias direction in the antiferromagnet is described. The model is applied to the investigation of the process of setting the bias direction in the antiferromagnetic layer, which comprises a high-temperature anneal in a field sufficiently large to saturate the ferromagnetic layers. It is shown that there is an optimal setting temperature depending on the material parameters. The temperature dependence of the antiferromagnetic anisotropy is shown to be an important factor in achieving maximum exchange bias.

Micromagnetic reversal behavior of multiscale permalloy elements

Lorentz microscopy has been used to study the micromagnetic processes occurring during the reversal of multiscale permalloy elements. The elements, which have similar dimensions to write heads used in magnetic recording, typically have length scales varying from 10μm in the element “core” down to 100nm in the element “tip.” A discussion of the effect of varying the geometry and critical dimensions of the elements on the reversal behavior and switching fields is presented. While the magnetization processes in the core tend to be similar to what is observed in the absence of a tip, the presence of the core strongly influences the tip reversal, even for tips with widths of 100nm. The results demonstrate clearly the role played by shape anisotropy in complex shaped elements fabricated from an isotropic magnetic film.