Yu-Nu Hsu

Effects of Ag underlayers on the microstructure and magnetic properties of epitaxial FePt thin films

In this work Ag underlayers, with a slightly larger unit cell than FePt, were found not only to induce epitaxial growth of the FePt films but also to reduce the FePt ordering temperature. Without using the Ag underlayer, the FePt film deposited onto the Si substrate was fcc disordered. By the use of the Ag underlayer, it was observed that the FePt unit cells were expanded in the film plane. This has caused the shrinkage of the FePt unit cells along the film normal direction and resulted in the in situ ordering of the FePt thin film at reduced temperatures. The microstructural and magnetic properties of the FePt/Ag films at varied substrate temperature and FePt thickness were studied to investigate the L10 FePt ordering.

Magnetic properties of nanostructured CoPt and FePt thin films

Polycrystalline MgO underlayer films lead to different preferred orientations of L1/sub 0/ CoPt and FePt films after the annealing process, depending on the thickness of the L1/sub 0/ films. L1/sub 0/ films with a thickness greater than 20 nm revealed mostly L1/sub 0/ [100] fiber texture and consequently in-plane anisotropy as determined by magnetic hysteresis. Strong perpendicular anisotropy due to the L1/sub 0/ [001] fiber texture was obtained for thicknesses below 10 nm. The /spl delta/M curve showed strong intergranular exchange coupling. The angular variation of coercivity showed the possibility of both domain wall motion and a rotational mechanism of magnetization reversal.

Atomic ordering and coercivity mechanism in FePt and CoPt polycrystalline thin films

40 nm thick CoPt and FePt films were prepared on oxidized Si substrates with 10 nm MgO underlayers. The maximum coercivity (H/sub c/) for CoPt films was found to be /spl sim/10 kOe after annealing at 700/spl deg/C for /spl sim/20-30 minutes (min). Structural analysis showed a significant amount of FCC phase as well as the ordered L1/sub 0/ phase in these films. FePt films showed an abrupt increase of ordered volume fraction and H/sub c/ in the initial stage of annealing and predominance of the tetragonal L1/sub 0/ phase after 10 min. at 700/spl deg/C. The maximum H/sub c/ reached /spl sim/16 kOe after annealing at 700/spl deg/C for more than 20 minutes. Dark field (DF) images of the annealed CoPt films showed individual grains which exhibited a possibility of several variants or disordered phase with dimensions similar to the exchange correlation length, b/sub cm/. The temperature dependence of H/sub c/ seems to indicate a weak pinning mechanism in the highly ordered FePt films. Magnetic force microscopy indicated a complex domain structure consisting of clusters with dimensions of several hundred nanometers.

The effects of Ag underlayer and Pt intermediate layers on the microstructure and magnetic properties of epitaxial FePt thin films

Abstract In this work, Ag underlayers, which have a slightly larger lattice parameter than FePt, were found not only to induce epitaxial growth of the FePt films but also to reduce the temperature at which the atomic ordering occurred. Without using the Ag underlayer, the FePt film deposited onto the hydrofluoric acid etched-Si substrate was FCC disordered. When Ag underlayers were used the FePt unit cells were expanded in the film plane. This caused the shrinkage of the FePt unit cells along the film normal direction and resulted in the in situ ordering of the FePt thin film at reduced temperatures. Furthermore, it was found that the degree to which the FePt unit cells contracted along the film normal varied when a Pt intermediate layer was added. This resulted in FePt thin films with different textures and magnetic properties. The microstructural and magnetic properties of the FePt films prepared at various substrate temperatures, FePt thicknesses, Ag underlayer thicknesses and with the use of the Pt intermediate layer were studied to investigate the L10 FePt ordering.

In situ ordering of FEPT thin films by using AG/SI and AG/MN/sub 3/SI/AG/SI templates

In situ ordering of FePt thin films by sputtering onto Ag/Si and Ag/Mn/sub 3/Si/Ag/Si templates has been demonstrated. Due to the evolution from island-like to a continuous film structure as a function of Ag thickness, the ordering and orientation of the FePt films both change with Ag thickness. A continuous [002] Ag film results in a greater L1/sub 0/ phase formation with c-axis oriented both perpendicular to and in the film plane while the island-like Ag template results in less L1/sub 0/ phase formation, but a preferential c-axis orientation in the film plane. On the other hand, the FePt films deposited onto the Ag/Mn/sub 3/Si/Ag/Si template have their c axes aligned perpendicular to the film plane.

Structure and magnetic properties of L10 CoPt(Ag/MgO,MgO) thin films

Disordered fcc (face-centered-cubic) [100] fiber-textured CoPt thin films with thicknesses of 20–40 nm were deposited on a nonmagnetic sputtered MgO seed layer with and without a Ag intermediate layer. These were subsequently annealed at 600–750 °C for 5–10 min using rapid thermal annealing (RTA). The structural variants were determined to coexist in the in-plane and normal to the plane directions after the RTA process. The MgO underlayer without the intermediate layer revealed strong in-plane anisotropy by magnetic hysteresis measurement. Selected area diffraction (SAD) by TEM and XRD measurements for these samples confirmed the preferential growth of in-plane variants of c axis compared to the perpendicular direction. High remanence and coercive squareness (S*) were obtained due to strong in-plane texture and especially due to exchange coupling among the grains. Evidence for this was provided by ΔM curves. Initial magnetization curves for samples annealed above 600 °C, showed the possibility of assemble...

Microstructural and crystallographic aspects of thin film recording media

Various aspects of the structure of thin film longitudinal recording media are presented and discussed. In particular we discuss the role that the various layers of thin film media play in controlling the microstructure and magnetic properties of the magnetic layer. These include the grain size of the films, the texture of the films, the role of the intermediate layer and the role of chemical segregation.

Soft magnetic properties of nanocrystalline amorphous HITPERM films and multilayers

Amorphous precursors to HITPERM (Fe, Co)-Zr-B-Cu (a-HITPERM) films and a-HITPERM/SiO/sub 2/ multilayer films have been deposited on glass or Si substrates by a rf sputtering system with a target of composition (Fe/sub 0.7/Co/sub 0.3/)/sub 88/Zr/sub 7/B/sub 4/Cu/sub 1/. It was found that the a-HITPERM single layer film with a thickness of 100 mm possessed good soft magnetic properties with a saturation magnetization of 4/spl pi/M/sub s//spl ges/14 kG and a coercivity of H/sub c//spl sim/0.9 Oe at room temperature. Hc increases from 0.9 Oe to 25 Oe when the him thickness increases from 100 to 150 nm. To obtain excellent soft magnetic properties at larger thicknesses, a-HITPERM/SiO/sub 2/ multilayer films have been synthesized. In these, H/sub c/ drops from 25 Oe (single layer) to 0.25 Oe (multilayers) with the same total a-HITPERM thickness. The intervening SiO/sub 2/ layers play an important role in reducing the coercivity. Experimental results show the optimum thickness for SiO/sub 2/ was 2-4 nm. M-H loops for the multilayers films exhibit pronounced two step magnetization reversal processes for temperatures between 5 and 25 K. This behavior can be attributed to magneto-static coupling between a-HITPERM layers and sequential switching of the layers. The coupling exhibits itself in a blocked phenomenon with wide (stepped) hysteresis curves for T 25 K. The effects of magneto-static coupling on the magnetic properties of a-HITPERM/SiO/sub 2/ films will be discussed.

Magnetic and structural characterization and ferromagnetic resonance study of thin film HITPERM soft magnetic materials for data storage applications

HITPERM/SiO2 single and multilayer thin films have been produced using a target composition of (Fe70Co30)88Zr7B4Cu1. The as-deposited HITPERM film contains small bcc (or B2) nanocrystals of volume fraction less than 10% surrounded by an amorphous matrix. The lattice parameter of the nanocrystal is about 5% larger than an equilibrium FeCo phase. The saturation induction determined from FMR measurements (1.53±0.08 T) is consistent with VSM and SQUID measurements (1.45–1.5 T) and also with as-spun amorphous ribbons (1.55–1.62 T). The Lande g-factors (2.15±0.05) are typical of transition metals, particularly, of Fe. The Landau–Lifshitz–Gilbert damping parameters of the single and multilayered films are small (α = 0.0055±0.0004) with each layer acting almost independently. Neither thickness variation of each layer nor the number of stacking significantly affects the damping process in a range of film thicknesses of 50–150 nm, while the coercivities are strongly dependent on those parameters. This supports a no...

Dragging of domains by an electric current in Metglas 2826 MBa)

A steady, unidirectional motion of stripe domains, caused by hydromagnetic forces, has been observed in ribbons of Metglas 2826 MB traversed by an electric current at 300 K. A half‐wave rectified 60 Hz current density of peak value jp≂20 A/mm2 is sufficient to induce this ‘‘domain drag’’ motion. A dc current density with smaller superposed 60 Hz sinewave ac current, does it too. Because of the very low coercivity (Hc=7 mOe), these jp values are ∼103 times smaller than those needed for domain drag in Gd–Co films, but comparable to those in Co crystals at 4 K. A reversal of current reverses the sense of wall motion. As predicted by theory, wall motion is in the same direction as the motion of effective charge carriers, shown by the sign of the Hall effect to be holes.