The continued exponential growth in areal density for longitudinal magnetic-recording devices places ever more stringent demands on disk performance. The design of materials and processes must provide the required advances in technology. The magnetic properties are controlled through the choice of underlayers, magnetic alloys, and the deposition processes that control crystallographic orientation and magnetic isolation between grains. The requirement of lower head-disk spacing places increasing stress on the tribological performance of the disks, controlled by a very thin overcoat and lubricant layer. This article reviews the various materials issues relevant to magnetic-disk technology. The major obstacle for achieving high areal density in thin-film media is transition noise. This noise arises from the zig-zag transition boundaries that occur due to cooperative switching of the magnetic grains. Both exchange coupling between grains and magnetostatic interactions cause magnetic-cluster sizes larger than the grain size. The goal is to magnetically isolate the grains and keep the grain size small. As the dimensions of the bit cell shrink, smaller grain size is required to obtain enough grains per bit cell to maintain the required signal-to-noise ratio (SNR). In the 10–40-Gbits/in. 2 areal-density range, the issue of thermal stability of small ( In addition to good SNR, a narrow transition width is needed in order to pack the transitions closer together. The objective is to minimize interactions between transitions to reduce nonlinear amplitude loss and superlinear noise.

Materials Issues in Magnetic-Disk Performance

The magnetic properties and microstructure of barium ferrite films (BaFe12O19) coated by a sol‐gel technique involving a spin coating of metal‐organic compounds are investigated. The in‐plane magnetic properties and the grain size are controlled by a Fe/Ba atomic ratio in the coating solution and annealing conditions. The in‐plane coercivity increases as the decrease of the Fe/Ba ratio and the increase of the annealing temperature. Maximum value of 4–6 kOe is obtained at the Fe/Ba ratio of 2–8 with the annealing at 900 °C for 10 min. At these preparation conditions, the c axis of the hexagonal barium ferrite is slightly oriented parallel to the film plane. The grain size decreases as the decrease of the Fe/Ba ratio. The films with the Fe/Ba ratio of 4–6 show a grain size of almost one‐tenth of that of the sputtered films without additives such as Cr2O3 or CoTi. The smaller grain size of the sol‐gel‐coated films is probably due to the high nucleation density at the annealing process.

Magnetic and structural properties of Ba–ferrite films prepared by sol‐gel processing

Temperature dependencies of magnetic properties were measured in barium-ferrite thin films with grains having in-plane aligned as well as randomly oriented easy axes. The temperature dependence of H/sub c/ was shown to be similar to that of the effective anisotropy field, which is determined by the crystalline and shape anisotropies. The measured H/sub c/ values in the easy-axis aligned and randomly oriented films are significantly smaller than those predicted by the Stoner-Wohlfarth (SW) coherent rotation model. Such discrepancies are believed to be caused by incoherent rotation. Deviations from the SW model were also found within a wide temperature range in the angular dependence of coercivity and remanent coercivity, the field dependence of torque and rotational hysteresis. These led to the conclusion that temperature has only weak affects on the rotation mode of the magnetization. The discrepancies between the thermal activation volume and the grain size which were found previously at room temperature, and were believed to be yet another signature of incoherent rotation, were shown to also exist at a low temperature of 173 K.

Temperature dependent magnetic properties of barium-ferrite thin-film recording media

The effect of employing various sputtered underlayers and commercially available substrates in order to optimize the characteristics of barium ferrite (BaM) thin films for magnetic recording media have been studied. Our results show that the choice of underlayer or substrate controls the resultant surface morphology and magnetic properties of the BaM film. In particular, it was found that barium ferrite thin films with high coercivity, good squareness, low switching field distribution (>5000 Oe, 0.92, and 0.58, respectively) and possessing a c-axis in-plane orientation texture were achieved when deposited on an amorphous sputtered underlayer of Al/sub 2/O/sub 3/. In addition, the use of an Al/sub 2/O/sub 3/ underlayer results in equiaxial barium ferrite grains which may allow a favorable reduction in intergranular interaction as evidenced by the characteristic /spl Delta/M curves.

Underlayer And Substrate Effects In RF-magnetron Sputtered Barium Ferrite Thin Film Media

Magnetic properties of La–Co substituted barium ferrite thin films with large magnetic anisotropy

The high density recording characteristics of barium ferrite thin film media have been investigated. High levels of overwrite are achieved. Media microstructures have been identified to have a close relation to the noise behavior in the barium ferrite thin films. A linear increase in noise spectra is observed, indicating the existence of transition noise. In addition, positive /spl Delta/M peaks are seen in all longitudinal barium ferrite samples. Magnetostatic interactions rather than exchange interactions among neighboring grains are believed to be the cause of the positive /spl Delta/M. To reduce media noise, a reduction in both grain size and grain aspect ratio is necessary.

Xiaoyu Sui

Papers

Recording performance of longitudinal barium ferrite thin film media