Sangki Jeong

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.

Growth and characterization of L1/sub 0/ FePt and CoPt [001] textured polycrystalline thin films

Thin [001] textured FePt and CoPt polycrystalline films (5 nm) were deposited with an MgO polycrystalline underlayer on an oxidized Si substrate followed by a Rapid Thermal Annealing (RTA) process. Structural analysis indicated the samples to be fully ordered after 10 minutes (min) of RTA at 700/spl deg/C. The magnetic hysteresis exhibited strong perpendicular anisotropy and a coercivity (H/sub c/) between 6 and 8 kOe. The measured and simulated angular dependence of H/sub c/ and remanent coercivity (H/sub re/) showed the possibility of domain wall motion or incoherent rotation. /spl delta/M curves demonstrated strong intergranular exchange coupling. MFM observations showed domain sizes to be 100-200 nm. Activation volumes (V/sub act/) were found to be /spl sim/0.4-0.5/spl times/10/sup -18/ cm/sup 3/. The temperature dependence of the coercivity (H/sub c/) indicated a weak pinning mechanism.

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 ordered polycrystalline FePt L10 (001) nanostructured films and the effect of CrMn and Zn top layer diffusion

The nanostructure and magnetic properties of in situ ordered polycrystalline FePt thin films (9 nm) have been studied. Films were deposited with a MgO underlayer onto Si and glass substrates using conventional rf diode sputtering. Perpendicular anisotropy was obtained when the substrate temperature exceeded 490 °C. The maximum order parameter (S) was estimated to be ∼0.9. The coercivities (Hc) ranged from 5 to 12 kOe. Observed high values of the coercive squareness (S*) indicate the existence of exchange coupling. High resolution transmission electron microscopy indicates a defective region which might be associated with defect-related localized incoherent nucleation, thereby inducing low values of Hc. Zn and CrMn top layers at ambient temperature were deposited after the deposition of FePt films at elevated temperatures. Rapid thermal annealing of these samples at 180–450 °C for 1 min resulted in a decrease of S* from 0.8–0.85 to ∼0.5–0.6.The nanostructure and magnetic properties of in situ ordered polycrystalline FePt thin films (9 nm) have been studied. Films were deposited with a MgO underlayer onto Si and glass substrates using conventional rf diode sputtering. Perpendicular anisotropy was obtained when the substrate temperature exceeded 490 °C. The maximum order parameter (S) was estimated to be ∼0.9. The coercivities (Hc) ranged from 5 to 12 kOe. Observed high values of the coercive squareness (S*) indicate the existence of exchange coupling. High resolution transmission electron microscopy indicates a defective region which might be associated with defect-related localized incoherent nucleation, thereby inducing low values of Hc. Zn and CrMn top layers at ambient temperature were deposited after the deposition of FePt films at elevated temperatures. Rapid thermal annealing of these samples at 180–450 °C for 1 min resulted in a decrease of S* from 0.8–0.85 to ∼0.5–0.6.

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...

Electronic, magnetic, and structural properties of L10FePtxPd1−x alloys

We present theoretical and experimental results on the electronic, magnetic, and structural properties of L10 FePd, FePt, and FePtxPd1−x alloys. These alloys have large magnetizations and magnetocrystalline anisotropies and as such are potentially technologically important for applications as permanent magnets or high-density storage media. Results of first principles electronic structure calculations show that the system is a strong ferromagnet with an almost full majority Fe band, and that magnetization and magnetocrystalline anisotropy remain large over the range of composition. Total energy calculations predict phase separation on the disordered Pd/Pt layer at low temperatures. Experimental studies using energy dispersion x-ray line scans confirm regions that are Pt rich and Pd rich forming at low temperatures.

Nanostructure and magnetic properties of polycrystalline FePdPt/MgO thin films

Atomic ordering, nanostructure and the magnetic properties of polycrystalline FePdPt thin films deposited with MgO underlayers have been investigated. The film thickness was fixed at 5 nm in all films, a thickness where perpendicular anisotropy is dominant. Films deposited at ambient temperature were postannealed at various temperatures to study the kinetics of atomic ordering. The Pd additions to FePt were effective in reducing the temperature needed for atomic ordering. The mean grain size determined by transmission electron microscopy is about 10 nm in annealed films. Within the limits of our observations, FePdPt films do not show phase separation into more than one phase of the L10 structure. The maximum coercivity (Hc) for annealed FePdPt films is about ∼3000–4000 Oe. In situ ordered FePdPt films at 530 °C show no indication of a lower temperature for atomic ordering.