Liang-Wei Wang

Low-temperature ordering of (001) granular FePt films by inserting ultrathin SiO2 layers

L10 granular FePt–SiO2 films with a (001) preferred orientation and well-separated grains of 5.14nm were obtained by depositing atomic-scale Fe∕Pt∕SiO2 multilayers (MLs) on glass substrates and subsequently annealing MLs at a temperature of 350°C. Large out-of-plane coercivity of 7700Oe and a high ordering factor of 0.83 were achieved. Alternate atomic-scale depositions promoted the formation of (001) textures. Furthermore, because of the low surface energy of SiO2 layers, SiO2 tended to diffuse into grain boundaries of FePt during annealing, which may accelerate diffusion of Fe and Pt atoms, resulting in the low-temperature ordering.

(001) FePt nanoparticles with ultrahigh density of 10 T dots/in.2 on amorphous SiO2 substrates

Highly ordered single-crystalline (001) FePt nanoparticles (NPs) with controllable sizes and a well-aligned easy axis were obtained by tuning the layer numbers and the SiO2-layer thickness of atomic-scale [Fe/Pt/SiO2]n multilayers deposited on SiO2∥Si substrates. After 700 °C annealing, quasi-self-assembled NPs with ultrahigh areal density of 1.0×1013 dots/in.2 and large out-of-plane coercivity (Hc,⊥) of 31 kOe were achieved. All particles were embedded into the SiO2 substrates due to the low surface energy of SiO2, which significantly prevented the coarsening during annealing and resulted in a reduced particle size of 5.6 nm and small size distribution of 14.1%.

Tuning magnetic anisotropy in (001) oriented L10 (Fe1−xCux)55Pt45 films

We have achieved (001) oriented L10 (Fe1−xCux)55Pt45 thin films, with magnetic anisotropy up to 3.6 × 107 erg/cm3, using atomic-scale multilayer sputtering and post annealing at 400 °C for 10 s. By fixing the Pt concentration, structure and magnetic properties are systematically tuned by the Cu addition. Increasing Cu content results in an increase in the tetragonal distortion of the L10 phase, significant changes to the film microstructure, and lowering of the saturation magnetization and anisotropy. The relatively convenient synthesis conditions, along with the tunable magnetic properties, make such materials highly desirable for future magnetic recording technologies.

Promotion of [001]-oriented L10-FePt by rapid thermal annealing with light absorption layer

Highly [001]-oriented L10-FePt grown on SiO2‖Si is achieved by using rapid thermal annealing (RTA) at 400 °C. Due to the dramatic divergence of light absorption ability between Si and FePt films, Si behaves as the light absorption layer to absorb more light emitted from RTA system, which gives rise to larger thermal expansion on Si and induces in-plane tensile stress on FePt films. By raising heating rate during RTA, the transient light intensity is increased; therefore, higher in-plane tensile stress on FePt films is generated, which effectively suppresses the opening-up of in-plane hysteresis loops.

Control of microstructure in (001)-orientated FePt–SiO2 granular films

Highly ordered L10 FePt–SiO2 granular films with a (001)-preferred orientation were obtained by depositing atomic-scale [Fe∕Pt∕SiO2(tox)] multilayers on glass substrates and subsequently annealing multilayers at the temperature of as low as 350°C. The tox value plays an important role on both microstructure and magnetic properties. The average grain sizes of samples A, B, and C (tox=0.11,0.28,0.56) are 12.12, 15.46, and 5.14nm, respectively, and the corresponding perpendicular coercivities are 6800, 5550, and 7700Oe. Due to different microstructures, samples show distinct coercivity dependence on annealing time.

Evolution of granular to particulate structure of (001) FePt on amorphous substrates (invited)

Well-aligned (001)-oriented granular or particulate L10 FePt films on amorphous SiO2 or glass substrates were obtained by depositing atomic-scale Fe∕Pt∕SiO2 multilayers followed by various postannealing conditions. After 350°C annealing, (001) FePt–SiO2 granular films were obtained with an average grain size of 5.14nm, perpendicular coercivity of 7.7kOe, and squareness of 0.95. The diffusion of SiO2 to grain boundaries and free surface of FePt accelerated the diffusion of Fe and Pt atoms to form the L10 phase; therefore, low-temperature ordering of FePt was achieved. By further raising the annealing temperature to 700°C, isolated particles of aligned (001) single crystals were achieved. The nucleation of voids occurred at low temperatures due to the existence of low-surface-energy SiO2, and then an agglomeration process proceeded, leading to a particulate structure. The isolated particles showed a perpendicular coercivity of 60kOe and squareness of unity.

Probing the A1 to L10 transformation in FeCuPt using the first order reversal curve method

The A1-L10 phase transformation has been investigated in (001) FeCuPt thin films prepared by atomic-scale multilayer sputtering and rapid thermal annealing (RTA). Traditional x-ray diffraction is not always applicable in generating a true order parameter, due to non-ideal crystallinity of the A1 phase. Using the first-order reversal curve (FORC) method, the A1 and L10 phases are deconvoluted into two distinct features in the FORC distribution, whose relative intensities change with the RTA temperature. The L10 ordering takes place via a nucleation-and-growth mode. A magnetization-based phase fraction is extracted, providing a quantitative measure of the L10 phase homogeneity.

High thermal stability and low Gilbert damping constant of CoFeB/MgO bilayer with perpendicular magnetic anisotropy by Al capping and rapid thermal annealing

We demonstrate that the magnetic anisotropy of the CoFeB/MgO bilayer can be manipulated by adding an aluminum capping layer. After rapid thermal annealing, we can achieve large perpendicular magnetic anisotropy of CoFeB with a high thermal stability factor (Δ = 72) while the Gilbert damping constant can be reduced down to only 0.011 simultaneously. The boron and residual oxygen in the bulk CoFeB layer are properly absorbed by the Al capping layer during annealing, leading to the enhanced exchange stiffness and reduced damping. The interfacial Fe-O bonding can be optimized by tuning annealing temperature and thickness of Al, resulting in enhanced perpendicular anisotropy.

Highly (001)-oriented thin continuous L10 FePt film by introducing an FeOx cap layer

We demonstrate a thin and continuous L10 FePt film with a well-aligned (001) texture directly grown on Si || SiO2 substrates by introducing an FeOx cap layer. The agglomeration of capped FePt films is greatly suppressed by inhibiting the surface diffusion. This, in turn, yields a continuous and smooth film, which significantly promotes the (001) out-of-plane orientation and perpendicular anisotropy. The reduction of Fe oxides occurs during annealing, which not only promotes interdiffusion of Fe and Pt for L10 ordering but also removes the cap layer simultaneously. Therefore, additional etching for the cap layer is not required for further fabricating bit patterned media.

001) FePt graded media with PtMn underlayers

(001)-oriented FePt graded media are obtained by using PtMn underlayers. The PtMn underlayer not only behaves as the (001) structural template but provides the diffusion source of Mn. The diffusion of Mn into FePt reduces its anisotropy but, on the other hand, the exchange coupling between antiferromagnetic PtMn and ferromagnetic FePt enhances the anisotropy. Hysteresis loops taken from x-ray magnetic circular dichroism confirm the competition between these two effects, leading to the lowest anisotropy in the middle of FePt.