B.C. Lim

Granular L10 FePt:TiO2 (001) nanocomposite thin films with 5nm grains for high density magnetic recording

FePt:TiO2 (001) thin films were deposited on CrRu∕MgO underlayers by dc magnetron cosputtering. The effects of TiO2 content on microstructure and magnetic properties of the epitaxial FePt (001) films were studied. Experimental results showed that even with 20vol% TiO2 doping, FePt films could keep a (001) preferred orientation and perpendicular anisotropy. The FePt:TiO2 (001) film with 5nm grain size, adjustable coercivity, and reduced exchange coupling was obtained by doping 20vol% TiO2 into FePt film, showing great potential as low noise ultrahigh density perpendicular magnetic recording media.

Controlling the crystallographic orientation and the axis of magnetic anisotropy in L10 FePt films

L10 ordered FePt films with (200) or (001) preferred orientations and longitudinal or perpendicular anisotropy have been prepared by dc magnetron sputtering. Different base pressures for sputtering were controlled by air feeding. The fct-FePt film exhibits (200) preferred orientation and has longitudinal anisotropy at the base pressure of 4×10−6 Torr. As the base pressure is improved (below 9×10−7 Torr), the fct-FePt films exhibit (001) preferred orientation and has perpendicular anisotropy. When the CrRu underlayer thickness decreases from 80 to 30 nm, an improved fct-FePt (001) orientation is obtained. The atmospheric constituents such as oxygen and nitrogen play an important role in controlling the crystallographic orientation and axis of magnetic anisotropy.

Large magnetic moment observed in Co-doped ZnO nanocluster-assembled thin films at room temperature

Co-doped ZnO nanocluster-assembled films were deposited by nanocluster-beam deposition. Zn0.986Co0.014O nanoclusters remained wurtzite in structure with size of 5nm. Compared with bulk ZnO, a blueshift of 0.28eV was observed in the absorption edge of the film. Two photo-luminescence bands at 378 and 510nm were detected. Room-temperature ferromagnetism was observed in doped ZnO nanocluster-assembled film. Moreover, it exhibited a large saturated magnetization of 1.4μB∕Co and increased to 3.65μB∕Co after the film was annealed. The possible mechanisms on the observed ferromagnetism and enhanced magnetic moment were discussed.

Exchange coupling assisted FePtC perpendicular recording media

Magnetic hard and soft phases FePtC thin films based exchange coupling assisted (ECA) perpendicular recording media were prepared. The coercivity of the FePtC film decreased after introducing a top magnetic soft FePtC layer. The cross-sectional transmission electron microscopy image illustrated that the soft FePt grains were located both on top of the bottom FePt grains and the grain boundaries, where the hard and soft grains were separated by a C layer accumulated on the hard FePtC layer surface. The reduction of coercivity was attributed to the exchange coupling between FePt hard and soft grains. For hard and soft grains separated by the accumulated C layer, the strength of the exchange coupling depended on the carbon layer thickness.

Microstructural and magnetic properties of L10 FePt–C (0 0 1) textured nanocomposite films grown on different intermediate layers

The FePt : C films with different volume fractions of carbon and different thicknesses were epitaxially grown on a CrRu(2 0 0) underlayer with Pt and MgO intermediate layers. The magnetic properties and microstructure of these FePt : C films were investigated. The FePt : C films grown on the Pt intermediate layer consisted of a continuous layer of FePt, with overlying granular FePt grains, while the FePt : C films grown on the MgO intermediate layer consisted of granular FePt : C layers with overlying granular grains. The formation of the overlying granular FePt grains was attributed to carbon diffusion to the surface which resulted in the second nucleation of FePt. The different interface energies and surface energies of FePt on Pt and MgO intermediate layers caused the formation of an initial continuous FePt layer on the Pt intermediate layer and initial granular FePt layers on the MgO intermediate layer. The coupling between the continuous FePt layer or the granular FePt layer and the overlying granular FePt grains resulted in simultaneous magnetization reversal and thus strong exchange coupling in FePt : C films.

Improvement of chemical ordering of FePt "001… oriented films by MgO buffer layer

Chemically ordered L10 FePt layer was obtained at temperature as low as 280°C by in situ heating by using MgO buffer layer on CrRu (200) underlayer. The chemical ordering and magnetocrystalline anisotropy, Ku, increased with increasing deposition temperature from 280to400°C. The out-of-plane coercivity Hc⊥ also increased significantly from 2.7to9.0kOe. All the films showed perpendicular anisotropy. The reason for the increase in the coercivity was investigated by reducing the thickness of the FePt layer to 4nm and varying the thickness of the MgO buffer layer at 1, 2, and 4nm. Well isolated FePt islands were formed. When the MgO layer was 1nm, the Hc⊥ of the 4nm FePt layer was as high as 12kOe. However, it reduced to 6.3kOe when the MgO was 4nm. The change in Hc⊥ was due to the improved chemical ordering and thus higher magnetocrystalline anisotropy.

Development of

The FePt:C films with different volume fractions of carbon were epitaxially grown on the CrRu(200) underlayer with Pt and MgO intermediate layers. The magnetic properties and microstructure of these FePt:C films were investigated. The FePt:C films grown on Pt intermediate layer consisted of a continuous layer of FePt, with overlying granular FePt grains. The FePt:C films grown on MgO intermediate layer consisted of a granular FePt:C layer with overlying granular grains. The formation of the overlying granular FePt grains was attributed to carbon diffusion to the surface, which resulted in the second nucleation of FePt. The different interface energy and surface energy of FePt on Pt and MgO intermediate layers caused the formation of initial continuous FePt layer on Pt intermediate layer and initial granular FePt layers on MgO intermediate layer. The out-of-plane coercivity measured at room temperature increased from 10.2 to 19 kOe when C concentrations increased from 0 to 15%. Further increasing the C contents to 20% and 25% caused the decrease of coercivity to 17.2 and 14.8 kOe, respectively. The coercivity of the exchange coupling assisted FePt:C perpendicular media was indeed substantially reduced.

{\rm L}1_{0}

With inserting one or two Ag layers, the FePt [001] preferred orientation is still maintained. The exchange coupling decreases, indicated by the decrease of the slope of the hysteresis loops at coercivity and significant increase of the coercivity as the Ag layer is inserted. The domain wall is effectively pinned by the Ag itself and some structure defects caused by the insertion of the Ag layer. The domain size decreases and the magnetic reversal tends to be a rotational mode as the thickness of the inserted Ag layer increases. The media noise is effectively reduced and the signal-to noise-ratio is remarkably enhanced when one 2-nm Ag is introduced.

FePt:C (001) Thin Films With High Coercivity and Small Grain Size for Ultra-High-Density Magnetic Recording Media

The effects of Ru underlayer on crystallographic structure, microstructures, and magnetic properties of Co72Pt28 films were investigated. It was observed that Ru deposition pressure played a more important role in determining the crystallographic texture compared to Ru thickness. Ru had a preferred (0002) texture when deposited at low Ar pressure and induced perpendicular magnetic anisotropy for Co72Pt28. However, at high Ar pressure, Ru (101¯0) and Ru (101¯1) peaks appeared that changed the easy axes of magnetization of Co72Pt28 film from out-of-plane to in-plane directions. With increasing thickness of Ru top layer in the dual-layer structure of Ru underlayer, coercivities increased due to increased magnetic anisotropy and reduced intergranular magnetic interaction.

Improvement of recording performance in FePt perpendicular media by Ag pinning layer

Nano-granular L10 FePt:C composite films with perpendicular anisotropy were in situ deposited on glass substrates at 350degC by dc magnetron sputtering. The magnetic, microstructural, and recording properties of the FePt:C perpendicular media were investigated. A good FePt (001) texture was maintained after incorporating up to 20 vol.% C, the measured full-width at half-maximum (FWHM) of the rocking curves of the FePt (001) peak slightly increased from 7.2deg to 8.5deg. FePt:C (20 vol.% C) films consisted of a continuous layer of FePt, with overlying granular FePt grains (5.6 nm) that were well separated by carbon. Hc increased from about 2600 to 3700 Oe with increasing C concentration from 0 to 15 vol.%, and then decreased with further increasing C concentration. The exchange coupling did not show obvious change with increased carbon content up to 20 vol.% due to the strong coupling between continuous layer and overlying granular FePt grains, which resulted in the overlap and fluctuation of recorded signals at a linear density of 221 kfci