R. Ranjan

Effect of microstructural features on media noise in longitudinal recording media

High‐resolution transmission‐electron microscopy (HRTEM) has been used to analyze CoCrTa, CoCrPt, and CoNiCrPt alloy thin films deposited under similar conditions onto NiP plated Al substrates with a Cr underlayer. Apparent ∼50 nm Co alloy grains observed by bright field TEM are found to be clusters of smaller 10–20 nm grains with a common growth orientation. Subtle differences have also been observed in the stacking fault density, Co alloy fcc phase concentration, lattice mismatch, and grain separation in the films. In particular, HRTEM shows that the CoCrTa film has clear physical separation between the 10 and 20 nm grains, which is not observed in the other media. Despite similar hysteresis loop properties, the signal‐to‐noise ratio of CoCrTa is 2.3 dB higher than CoCrPt and 4.6 dB higher than CoNiCrPt. Small isolated grains have been shown to improve media noise, as is observed in CoCrTa media.

Transmission electron microscopic analysis of microstructural features in magnetic recording media

A study of the relationships between the microstructure and magnetic properties of cobalt alloy magnetic thin films and chromium underlayers deposited onto circumferentially textured NiP/Al substrates is presented. The variation of the chromium microstructure as a function of argon pressure and substrate temperature during sputtering is analyzed using high-resolution scanning electron microscopy and transmission electron microscopy (TEM). This combination shows that the layers grow in columns which have increasing physical separation with increasing pressure and decreasing temperature. Crystallographic grain separation, however, follows a different trend because intercolumnar crystallographic alignment decreases with increasing temperature. TEM is also used to show how the growth of the chromium underlayer affects the cobalt alloy layer and to analyze the resulting microstructures in different magnetic alloy layers. >

Crystallographic orientation of textured CoCrTa/Cr sputtered thin film media for longitudinal recording

We have studied the influence of deposition conditions on the structure and properties of Co84Cr14Ta2 alloy magnetic thin films and Cr underlayers sputtered onto circumferentially textured and smooth polished NiP/Al substrates. Suitable processing conditions provide in‐plane magnetic anisotropy in the textured media, with enhanced coercivity (Hc) and coercivity squareness (S*) in the circumferential direction and reduced Hc and S* in the radial direction. The crystallography in these films was studied by transmission electron microscopy (TEM) to address the origin of the observed magnetic anisotropy. The CoCrTa alloy grains are found to grow semicoherently on the chromium underlayer, resulting in vertical <200≳ Cr and <1120≳ CoCrTa axes. This requires an in‐plane orientation of the CoCrTa c axis, which is observed to be nearly random by high‐resolution TEM. Microdiffraction shows apparent preferred c‐axis alignment locally at texture lines, but this observation is primarily attributable to asymmetric til...

Magnetic Clusters. Intergranular Exchange Interaction And Their Microstructural Basis in Thin Film Longitudinal Media

We have experimental examined the micromagnetic structure in CoCrTa/Cr films deposited onto NiP/Al substrates at 25, 100, 150 200 and 250/spl deg/C. We prepared all the films in an ac-erased magnetic state. The Lorentz transmission electron microscopy images demonstrate a monotonic decrease of magnetic cluster size with increase of deposition temperature and the magnetic force microscopy results show a decrease of image contrast. This indicates a decrease of intergranular exchange coupling with increase of deposition temperature. Nanoprobe energy dispersive spectroscopy (EDS) reveals an increase of Cr segregation at CoCrTa grain boundaries with increase of deposition temperature. The correlation of micromagnetic and microcompositional observation supports that Cr segregation at grain boundaries is responsible for decoupling CoCrTa grains. Our nanoprobe EDS results also indicate that the Cr diffuses to the grain boundaries from the diffusion process involving the Cr underlayer.

The role of NiAl underlayers in longitudinal thin-film recording media

Longitudinal recording media made with chromium and with nickel-aluminum underlayers are compared. NiAl films have smaller grains and random crystallographic orientation, while Cr films have larger grains and either a {110} or a {200} preferred orientation depending on the deposition conditions. Cobalt alloy films grown on NiAl have nearly random crystallographic orientation and significant c-axis out-of-plane component, and lower hysteresis loop squareness and coercivity than films made on a heated Cr underlayer. NiAl-underlayer media do not exhibit in-plane magnetic anisotropy induced by the substrate texture lines. CoCrTaPt/Cr/NiAl media had higher overwrite than CoCrTaPt/Cr media due to higher coercive squareness and a sharper hysteresis loop closure.

Lorentz transmission electron microscopy study of micromagnetic structures in real computer hard disks

We have studied micromagnetic structures in real computer hard disks, with emphasis on magnetic vortices and ripples at bit-transition regions and track edges, using Lorentz transmission electron microscopy (LTEM). A combined dimpling and chemical etching was used to produce LTEM specimens of Co alloy/Cr with large, uniformly thin areas from the typical C/Co alloy/Cr/NiP/Al(substrate) hard disk structures.

Microstructure and crystallography of textured CoCrTa/Cr recording media

Abstract Transmission electron microscopy (TEM) has been applied to recording media having a 60 nm CoCrTa film and a 75 nm chromium underlayer, which were DC magnetron-sputtered onto a circumferentially textured NiP/Al substrate. Suitable processing conditions provide in-plane magnetic anisotropy with the preferred direction parallel to the texture lines. This paper focuses on TEM techniques which address the preferred crystallographic orientations purported to be responsible for this important magnetic anisotropy. Elongated probe microdiffraction (EPMD) and standard selected-area diffraction (SAD) patterns from cross-section samples show that the bcc chromium underlayer and the hcp cobalt alloy have strong 〈110〉 and 〈11 2 0〉 out-of-plane growth directions, respectively, which result in in-plane orientations of the Cr 〈110〉 axis and the magnetically easy Co alloy 〈0001〉 c -axis. However, the angular spreads of the growth directions are greater perpendicular to the texture grooves than parallel to them, causing similarly greater out-of-plane components of the Cr 〈110〉 and Co alloy 〈0001〉 axes perpendicular to the texture lines than parallel to them. High-resolution plan-view TEM shows that the Co alloy c -axis has nearly random in-plane orientation. Dark field imaging and EPMD of the plan-view sample show locally altered crystallography near texture lines, but suggest that the change is due in part to the growth orientation tilt, not a dramatic, local, preferred in-plane orientation. Combined, these data suggest that the decreased c -axis in-plane component perpendicular to the texture lines is caused primarily by the 〈11 2 0〉 growth direction following the local grooved surface, not the bulk surface, causing the c -axis to be locally titled out of the bulk film plane near the grooves. This crystallographic difference may have implications upon the observed magnetic anisotropy in these films.

Noise properties and microstructure of oriented CoCrTa/Cr media

Magnetic and noise properties of oriented CoCrTa/Cr media sputtered at temperatures from ambient to 250 °C were studied and correlated with microstructural data from HRSEM and HRTEM micrographs. Coercivity increases monotonically and loop squareness decreases with substrate temperature. Transition and uniform magnetization noise power are highest for media sputtered at 150 °C. Micrographs and noise powers show that the media sputtered at 150 °C is relatively highly exchange‐coupled. Media sputtered below 150 °C shows topographic isolation, and those sputtered above 150 °C exhibit increasing intergranular isolation of about 10 A and thus lower exchange. Coercivity varies linearly but exchange drops abruptly at high temperature. Other mechanisms besides exchange contribute to the coercivity variation.

The role of NiAl underlayers in longitudinal recording media (abstract)

NiAl has been proposed as a replacement for the Cr underlayer in longitudinal recording media.1 NiAl has a B2 ordered structure with a lattice parameter close to that of Cr. Hence the same epitaxial relationships that exist in CoCrPt/Cr could also exist in CoCrPt/NiAl. Although high coercivity media have been made using a NiAl underlayer, a detailed microstructural study of the CoCrPt/NiAl is lacking. We have investigated the crystal orientation and epitaxial relationships in CoCrPt/NiAl bilayers deposited by DCM sputtering at a range of deposition temperatures up to 250 °C and sputter rates of 0.5–10 nm/s. Samples were examined by high resolution transmission electron microscopy in cross section and plan view. None of the samples showed the (002) NiAl/(1120)Co orientation that would be expected by analogy to CoCrPt/Cr films deposited at high temperatures. The CoCrPt crystals in the CoCrPt/NiAl are oriented with their c axes at a range of angles to the film plane, accounting for the lower squareness (Sq)...

The Effect of Microstructure on the Magnetic Properties of Thin Film Magnetic Media

Magnetic recording media used today are based on sputtered cobalt alloy films with thicknesses in the order of 50 nm. As recording density is increased, the microstructure of the film must be controlled with increasing level of sophistication to achieve the magnetic properties necessary for good recording performance. Recording density has been increasing at the compound annual rate of 30%, and in recent years at a higher rate. Already, 1 Gb/in 2 has been achieved in the laboratory [1], and 10 Gb/in 2 is being contemplated [2]. In order to achieve such densities, microstructural characteristics such as grain morphology, size distribution, crystallographic orientation, and grain separation must be controlled with great precision, and their relationship to magnetic properties must be understood. The paper will describe the effect of sputter process conditions and the selection of magnetic alloys on the film microstructure and describe what might be required to achieve high recording densities. Particular attention will be paid to grain size distribution and grain separation. Grain separation is important for low noise performance of the media. Alloy selection and sputtering conditions can be manipulated to achieve different levels of separation between the grains.