K. M. Cher

Optical property study of FePt-C nanocomposite thin film for heat-assisted magnetic recording.

Optical properties of the FePt-C nanocomposite thin film that was synthesized by sputtering with MgO/NiTa underlayer on glass substrate have been determined by an approach combining spectroscopic ellipsometry and transmission over the wavelength range of 380 - 1700 nm. It was observed that the refractive index is larger than the extinction coefficient, indicating that free electron absorption is not the dominant optical transition in the FePt-C thin film. Compared with FePt thin film, the FePt-C thin film has smaller optical constants, which lead to better optical performance including smaller optical spot on recording media and higher transducer efficiency for heat assisted magnetic recording.

Room‐Temperature Ferromagnetism in ZnO‐Encapsulated 1.9 nm FePt3 Nanoparticle–Composite Thin Films with Giant Interfacial Anisotropy

As synthesized 1.9-nm FePt3 nanoparticles are superparamagnetic at room temperature. Coating those nanoparticles with ZnO renders them permanently ferromagnetic with coercivity field of 650 Oe at room temperature. First-principles calculations indicate that giant interfacial anisotropy, induced by the strong spin-orbit interaction of enhanced orbit momentum of Fe, overcomes the superparamagnetic limit, leading to exceptional room-temperature permanent ferromagnetism. The findings are important for the understanding of the origin of permanent ferromagnetism at ultrasmall size and critical for ultrahigh density recording and information processing.

Compositional Effects on the Structure and Phase Transition of Epitaxial FeRh Thin Films

Compositional and temperature dependent ferromagnetic/anti-ferromagnetic transition of highly textured Fe100 - xRhx (where x = 35 to 65) thin films deposited epitaxially on MgO (001) single crystal substrates were studied. First-order transition from ferromagnetic to anti-ferromagnetic phase was observed around 48 at. % Rh where x-ray diffraction spectra revealed a sharp discontinued decrease in lattice parameter-c coupled with a sudden decrease in magnetization from 1600 to 200 emu/cm3. The transition was sharp compared to non-textured randomly oriented thin films, bearing similarities to bulk FeRh. Temperature-dependent x-ray diffraction spectra and magnetic measurements revealed the occurrence of temperature induced anti-ferromagnetic to ferromagnetic phase transitions for films with Rh content larger than 47 at. %. The temperatures at which such transitions occurred were dependent on the Rh content, and broadened with increasing amounts of Rh.

Development of Spin-Torque Oscillators and High CoPt Media With Small Grain Size for Microwave-Assisted Magnetic Recording

We report the selection of materials and design of spin-torque oscillators (STOs) which are able to operate in an alternative magnetic field (variable frequency) of magnitude ~8000 Oe, generate large enough in-plane ac magnetic field in the recording media, and have tunable high frequency at low driving current for microwave-assisted magnetic recording (MAMR) based on micromagnetic simulations. The mechanism and integration approaches of STOs with the current recording system to have the least driving current are also presented. This paper also covers the material choice, layer-stack optimization, and process development on Cox Pt1-x alloy films with extremely high coercivity of more than 1.5 T (Ku of about 1.5 × 107 erg/cc), well-isolated small grains of size 6-7 nm, for the extension of perpendicular magnetic recording and MAMR.

FePt-Based HAMR Media With a Function Layer for Better Thermal Control

A function layer with temperature-dependent electrical/thermal conductivity has been integrated into the FePt-based heat-assisted magnetic recording (HAMR) media layout. The Cu2O function layer enables a better thermal control of the HAMR media to use a moderate laser power to heat up the recording media with minimized side effects regarding to the achievable thermal gradient and thermal spot size. The inserted Cu2O layer also solved the surface morphology issue for the HAMR media design using the Cu-based heat sink layer (decomposed from the copper nitride). The developed Cu2O function layer could be applied for a real HAMR application.

Synthesis, Characterization and Hard Ferromagnetism in FePt/ZnO Nanocomposites with Ultra-Small Size

Multi-component hybrid nanostructures containing two nanoscaled components of FePt and ZnO were successfully fabricated through seed mediated growth. The preformed FePt nanoparticles, which were fabricated either by the reduction of Pt(acac)2 and the decomposition of Fe(CO)5 or by simultaneous chemical reduction of Pt(acac)2 and Fe(acac)3 by 1,2-hexadecanediol at high temperature, work as the hetero-nucleation seeds for the preparation of hybrid nanostructures. The end products can be either FePt@ZnO core/shell nanoparticle assembly or FePt/ZnO nanocomposites, depending on the seeding particle size. If the seeding particle size is larger than 3.5 nm, core/shell nanoparticle assembly was formed, while if the seeding particle is smaller than 2 nm, FePt/ZnO nanocomposites were formed. For the FePt@ZnO core/shell, HRTEM showed a quasi-epitaxial growth between the FePt core and the ZnO shell. The ZnO shell was highly deformed. The core/shell nanoparticle assembly exhibits both semiconducting and magnetic properties which is superparamagnetic at room temperature. For the nanocomposites, the as-synthesized ultra-small 1.9 nm FePt3 nanoparticles are superparamagnetic. After embedding into the ZnO matrix, those superparamagnetic nanoparticles become magnetically hard with coercivity field of 650 Oe at room temperature. First-principles calculations indicate a giant interfacial anisotropic energy, induced by the strong spin-orbit interaction between Pt and O at interface, leading to room-temperature permanent ferromagnetism. The findings shed light on research for new material designs with giant interfacial anisotropy for various applications.

Enhanced Exchange Coupling in TiO

We reported the TiO2 doping effects on the microstructure and magnetic properties of FePt-C nanocomposite films (dual doping of C and TiO2). X-ray diffraction measurements revealed that the (001) orientation was well kept and there was no observable deterioration in chemical ordering degree, indicating a negligible change in anisotropy energy with increased TiO2 doping. The coercivity of FePt-C-TiO2 films decreased, while the slope of the hysteresis loops increased, with TiO2 volume fraction, implying an enhanced exchange coupling induced by TiO2 doping. Angular-dependent switching fields of FePt-C-TiO2 films depicted that the reversal mode was changed from coherent-rotation dominated to wall-motion dominated, possibly resulting from the enhanced exchange coupling. Cross-sectional transmission electron microscope images showed that the grain boundaries became less clear and grains either coalesced or contacted each other at increased TiO2 doping. The compositional-depth profile revealed that the C was pushed onto the film surface due to TiO2 doping. A surfactant mediated growth model was proposed to account for the observed enhancement of exchange coupling and the change of microstructure. This study showed that to reduce grain size and promote grain isolation, a proper selection of boundary materials with different surface free energy is needed.