Thomas P. Nolan

Microstructure and Exchange Coupling of Segregated Oxide Perpendicular Recording Media

The magnetic properties and corresponding microstructure of (Co ₈₀Pt₂₀)_x (metal oxide)_1-x perpendicular recording media have been studied as a function of the volume percentage (vol%) of metal oxide sputtering into the magnetic film. The exchange coupling field (Hex) estimated from the coercivity (H _c) and nucleation field (H_n) decreases rapidly between 0-20 vol% of metal oxide. The analytical transmission electron microscope composition analysis of (Co₈₀Pt₂₀)_x(TiO₂)_1-x media confirms that the microstructure includes crystalline grain cores in an apparently amorphous oxide matrix. The grain cores comprise only Co and Pt in a nearly constant ratio independent of the vol% of oxide addition. The amorphous matrix contains Co, Ti, and O, but no Pt. The Co concentration is nearly constant in grain boundary and core regions, unlike high-temperature longitudinal recording media wherein Co segregates to form a concentration gradient in the grain core. Perpendicular media thus maintain fairly high anisotropy of the grain core phase, even for very high-oxide concentrations that significantly decrease remanant magnetization (Mrt), Hc, and thermal stability (K_uV/kT)

Chromium segregation in CoCrTa/Cr and CoCrPt/Cr thin films for longitudinal recording media

Analytical electron microscopy is employed to correlate Cr segregation in Co/sub 34/Cr/sub 12/Ta/sub 4//Cr and Co/sub 76/Cr/sub 12/Pt/sub 12//Cr films with specific microstructural features such as grain boundary misorientation. Energy-filtered (EFTEM) chemical maps show that Cr segregation occurs independently of the Cr underlayer, and is highly alloy dependent. The CoCrTa film contained extensive grain boundary Cr enrichment whereas EFTEM images from the CoCrPt media show homogeneous Cr distribution. No statistically significant Ta or Pt segregation was observed. EFTEM elemental maps and energy dispersive spectroscopy (EDS) indicate that grain boundary Cr segregation depends on the type of boundary. Quantitative analysis of the Cr levels using nanoprobe EDS shows that the random angle grain boundaries contain more Cr (23/spl plusmn/4 at%) than 90/spl deg/ boundaries (17/spl plusmn/4 at%). EDS and EFTEM composition profiles show Cr enriched grain boundaries surrounded by regions of Cr depletion.

Effect of Composite Designs on Writability and Thermal Stability of Perpendicular Recording Media

The function of two types of magnetic inhomogeneity employed in perpendicular magnetic recording media designs are analyzed. Continuous granular composite (CGC) media have variable lateral exchange coupling (Hex) through the thickness of the magnetic layers, which can reduce coercivity (Hc) and narrow switching field distribution (SFD). Exchange coupled composite (ECC) media have variable crystalline anisotropy field (Hk) and interlayer exchange coupling (J) through the film thickness, which also reduces Hc. Interactions between the Hk, Hex, and J values of combined ECC+CGC media are presented. The optimum anisotropy profile for conventional ECC media is found to be different than previously modeled. A CGC layer included in the ECC media architecture is shown to change the optimal anisotropy profile and interlayer coupling values.

Chromium Distribution in CoCrTa/Cr Longitudinal Recording Media

The current study of CoCr 12 Ta 4 /Cr longitudinal recording media combines high resolution electron microscopy (HRTEM) with nanoprobe energy dispersive spectroscopy (EDS) and energy-filtered imaging (EFTEM) to correlate the Cr distribution with specific microstructural features. EFTEM images show Cr enrichment at grain boundaries, both random angle boundaries and 90° bicrystal boundaries. Cr segregation within grains is also observed in the elemental maps. This intragrain segregation often occurs at a series of defects that may define separately nucleated grains having 00 misorientation. Nanoprobe EDS measurements indicate that these defects contain localized concentrations of 25 to 30 % Cr. The random angle grain boundary Cr concentration occurs with a wide range, 19 to 36 at% (mean 22%) whereas the more crystallographically related 900 boundaries contain less Cr with less variation, 15 to 21 at% (mean 17% Cr). Composition profiles across grain boundaries using both nanoprobe EDS and EFTEM images show the full-width-half-maximum of the segregation to be approximately 4 nm, with Cr depleted regions next to the grain boundary having less than 7 at% Cr. The Ta concentration revealed no statistical evidence of segregation.

Application of High-Resolution Electron Microscopy to the Study of Magnetic Thin Films and Multilayers

The performance of a wide range of modern magnetic thin-film materials for information storage is found to depend dramatically on grain nucleation and coherence at interfaces. Three such systems, Fe/Pt Multilayers, CO/Pt Multilayers, and CoCrTa/Cr bilayers are considered here. High-resolution electron Microscopy (HREM) shows that ordered CoPt and FePt L1o structures can be formed and oriented, using coherence with the substrate, with their magnetically easy c-axes perpendicular to the film plane. This orientation results in the perpendicular magnetization required for Magneto-optic recording Media. Different sputtering gases used in producing CO/Pt Multilayers result in varying degrees of interfacial intermixing, which is shown to strongly affect the perpendicular magnetic anisotropy energy. The required high in-plane coercivity of longitudinal recording media is also correlated with the observed interface coherence and the resulting oriented growth of CoCrTa with the magnetically hard axis perpendicular to the film plane. Many features of the microstructure can be observed directly at the atomic level by HREM, and so the usefulness of this technique is emphasized in this paper.

Microstructural Characterization Methods for Magnetic Thin Films

Microstructural characterization is key to determining the structure-property-processing relationships required to optimize the performance of magnetic thin films for longitudinal magnetic recording. Since the grain size of modem recording media is on the order of 10 to 20 nm, only high-resolution characterization methods such as transmission electron microscopy (TEM) can accurately describe the microstructure. Complete analysis requires a combination of conventional and high-resolution TEM imaging with analytical methods such as energy dispersivespectroscopy and energy-filtered TEM imaging. This paper provides examples from CoCr(Pt,Ta) alloys that reveal the strengths and limitations of these characterization methods as they apply to microstructural characterization of magnetic thin films.