Yun-Chung Wu

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%.

Low-temperature ordering of L10 FePt by PtMn underlayer

The ordering temperature of FePt films grown on PtMn underlayers was reduced due to phase transformation of the PtMn L10 phase. The in-plane coercivity of 10nm FePt on 50nm PtMn underlayers was 7688Oe after annealing at 325°C. An exchange field of 554Oe was also observed, which indicates that PtMn may not only induce the ordering of FePt at a reduced temperature, but also may provide extra anisotropy. A large coercivity of 10500Oe can be achieved for 35nm FePt films on PtMn underlayers after 325°C annealing.

Magnetic multilayers on porous anodized alumina for percolated perpendicular media

Co∕Pt multilayers deposited on anodized alumina substrates are introduced as percolated perpendicular media. The pores act as pinning sites due to variations of thickness and anisotropy direction around the perimeters. Coercivity, domain size, and switching field can be engineered by controlling pore density. The media exhibit strong perpendicular anisotropy and the switching field remains unchanged at its minimum up to an angular deviation of 50° from the easy axis. A better tolerance of switching-field distributions can thus be achieved, which may help to achieve a high signal-to-noise ratio. The thermal stability of the proposed media is investigated by micromagnetic simulations.

Controlling magnetization reversal in Co/Pt nanostructures with perpendicular anisotropy

We demonstrate a simple method to tailor the magnetization reversal mechanisms of Co/Pt multilayers by depositing them onto large area nanoporous anodized alumina (AAO) with various aspect ratios, A = pore depth/diameter. Magnetization reversal of composite (Co/Pt)/AAO films with large A is governed by strong domain-wall pinning which gradually transforms into a rotation-dominated reversal for samples with smaller A, as investigated by a first-order reversal curve method in conjunction with analysis of the angular dependent switching fields. The change of the magnetization reversal mode is attributed to topographical changes induced by the aspect ratio of the AAO templates.

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.

Ultrahigh-density (001)-oriented FePt nanoparticles by atomic-scale-multilayer deposition

Highly ordered (001)-oriented FePt nanoparticles (NPs) with controllable sizes and well-aligned easy axis were obtained by tuning the total layer number n of atomic-scale [Fe/Pt/SiO2]n multilayers deposited on SiO2∥Si substrates. After annealing at 700 °C, the ordered FePt NPs with a small particle size of 5.5 nm (an extremely high areal density of 1.2×1013dots/in.2), a large out-of-plane coercivity of 28.5 kOe, and a narrow size distribution of 13% were attained. All NPs were embedded in the SiO2 substrates, resulting in a flat top surface. Finally, a full-stack film structure of perpendicular recording media composed of FePt NPs (recording layer), SiO2 (nonmagnetic intermediate layer) and CoZrTa (soft underlayer) was demonstrated.