Data Storage Systems Center

Poster slides

Co-7%Ir+SiOx Soft Magnetic Intermediate Layer for Perpendicular Media

We present an experimental method that enables quantitative assessment of the intergranular exchange coupling through oxide grain boundaries in perpendicular thin film recording media used in hard disk drives. A tri-layer thin film structure consisting of a high coercivity CoPt unicrystal film and a relatively low coercivity CoPt layer separated by a thin oxide interlayer is used to model the perpendicularly magnetized grains separated by oxide grain boundaries. The coupling field between the two CoPt layers is obtained by measuring the hysteresis loop shift of the soft magnetic layer. The exchange coupling energy density as a function of oxide layer thickness has been obtained for a number of oxides used in present perpendicular thin film media. The effect of Cr segregation in CoCrPt grains to the grain boundaries is studied by ‘dusting’ Cr on the oxide/magnetic layer interfaces. It is found that the critical thickness below which the exchange coupling increases exponentially is surprisingly large, especially for SiOx. However, the addition of Cr to the interfaces can significantly reduce this critical thickness.

Magnetic properties of 3D-transition metal alloys have been extensively studied for the last 70 years, however little is known about the metastable extension of the FCC gamma phase in the iron-rich region of the iron-nickel (Fe-Ni) phase diagram at low temperatures. Permalloy alloys, with the FCC gamma-phase or ordered Fe3Ni structures, are known to be interesting soft magnetic materials for many data storage applications. The investigation of alternative iron-rich phases possessing similar properties may be more cost effective for some applications. By extending the known compositional dependence of Curie temperature (Tc) for the FCC g-phase at high Ni concentrations, we expect metastable g-FeNi alloys to have a low Tc in the iron-rich region of the phase diagram, making it suitable for applications in magnetocaloric cooling and self-regulated RF heating. We report the synthesis of metastable g-phase FeNi magnetic nanoparticles possessing low Tcs that are suppressed by varying nickel and iron content, and investigate their magnetic and structural properties.

In order to improve the recording efficiency in current perpendicular recording media, it is important to reduce the distance from the head to SUL. In this project, a soft magnetic intermediate layer is proposed to partially replace the presently used Ru intermediate layer. The new soft magnetic intermediate layer serves two purposes: (1) to decrease the thickness of the nonmagnetic Ru intermediate layer by providing a proper crystalline texture and surface morphology for the optimal crystalline grain growth of the magnetic storage layer on top and (2) to act as an additional soft magnetic underlayer that is closer to the head air bearing surface to enhance the recording performance.

Co-7%Ir alloy was chosen as in soft magnetic intermediate layer due to the hcp structure and zero inherent crystalline K1. X-ray diffraction showed that the film grows as stable hcp(00.2) texture on amorphous Ta. The oxide was added to create intergranular exchange decoupling and deposited on fcc(111) NiW seedlayer. The hcp(00.2) texture was confirmed and a transmission electron micrograph displayed domed structure uniformly surrounded by oxide and the grain size is about 8nm. The measured inplane coercivity Hc was 21.5Oe and inplane permeability μ was 72. It is in progress to achieve better crystalline texture and softer magnetic properties by optimizing the sputtering process parameters.

Nanoparticles of composition (Fe50 Co50)97 V2 Nb_1 were synthesized in an induction plasma torch. These particles were then oxidized isochronally at varying temperatures, ranging from room temperature to 900 °C. The progress of oxidation and the oxidation products were then analyzed by Vibrating Sample Magnetometry (VSM) and powder X-Ray Diffraction (XRD). The particles were also viewed using the Transmission Electron Microscope (TEM) to observe any faceting of the particles, both natively and after oxidation, and the formation of the oxide layer with increasing temperature. The magnetization of the nanoparticles was observed to decrease with increasing oxidation temperature, beginning most notably above 400 °C. As the oxidation temperature approached 600 °C, however, an inflection point was noted in the magnetization versus oxidation temperature plot. This indicates a change in the mode of oxidation, either by altering the oxide species or a change in the diffusion rate due to the building up of an oxide layer. Through XRD analysis, one can see the rapid sharpening of peaks after 400 °C along with the introduction of new oxide peaks. There is also some shifting of peaks, which can either be attributed to the introduction of strains due to the growing Oxide-FeCo interface or the changing of phases during oxidation between Magnetite, Hematite, Maghemite, and Wustite. As observed through TEM, minimally oxidized particles are faceted along the preferred crystallographic faces, while the more heavily oxidized particles become more rounded. The oxidizing species appears to be iron and cobalt ions, which leave the observed void in the center of the particles.

For the current industry standard — perpendicular magnetic recording (PMR) — it has been brought to attention that there is a fundamental limit to this recording scheme. Heat assisted magnetic recording (HAMR), bit patterned media (BPM) and microwave assisted magnetic recording (MAMR) are widely considered to be promising candidates to push the areal density of magnetic recording further. However, due to economic and technical reasons, it is still of great importance to probe the extendability of the current PMR scheme.

In this study, various aspects of the PMR scheme will be investigated. Factors such as the effect of linear density vs. track density and capping layer dimension vs. granular layer dimension are studied in a systematic way via FEM analysis and micro-magnetic modeling. Recording head field is calculated by FEM method, and media hysteresis modeling is performed to imitate the present day media. Recording properties are then simulated by micro-magnetic modeling. Finally, the paths for recording density to reach 1Tb/in2 will also be discussed.

Hua Yuan_DSSC spring review 2009 poster