Chih-Huang Lai

Influence of La doping in multiferroic properties of BiFeO3 thin films

The influence of La doping on the crystal structure and multiferroic properties of BiFeO3 (BFO) thin films was investigated. BFO-based films of pure perovskite phase were deposited on BaPbO3∕Pt∕TiOx∕SiO2∕Si substrates by rf-magnetron sputtering. The increased dielectric constant and remanent polarization of the La-doped BFO films were due to the increased lattice parameters and an improved crystallinity. The polarization switching and the fatigue behavior of the BFO films were significantly enhanced by the La doping. The in-plane magnetization-field curves revealed that the room-temperature saturated magnetization of the BFO films increased with La doping.

Ion-irradiation-induced direct ordering of L10 FePt phase

A highly ordered L10 FePt phase was directly achieved by using 2 MeV He-ion irradiation without conventional postannealing. The coercivity greater than 5700 Oe can be obtained after disordered FePt was irradiated at the beam current of several μA/cm2 with the ion doses of 2.4×1016 ions/cm2. The high beam-current-density results in direct beam heating on samples. In addition, the irradiation-induced heating process provides efficient microscopic energy transfer and creates excess point defects, which significantly enhances the diffusion and promotes the formation of the ordered phase. Consequently, the direct ordering of FePt took place by using ion-irradiation heating at temperature as low as 230 °C. Due to the feasibilities of the fast temperature rise and the focused beam size, this ion-irradiation heating treatment can be potentially used as an alternative technique for the rapid thermal annealing and for the magnetic patterning.

Nanoporous Gyroid Nickel from Block Copolymer Templates via Electroless Plating

H.-Y. Hsueh , Prof. R.-M. Ho Department of Chemical EngineeringNational Tsing Hua UniversityHsinchu 30013, Taiwan E-mail: rmho@mx.nthu.edu.tw .-C. Y Huang , Prof. C.-H. Lai Department of Materials Science and EngineeringNational Tsing Hua UniversityHsinchu 30013, Taiwan Makida , . T Prof. H. Hasegawa Department of Polymer ChemistryGraduate School of EngineeringKyoto UniversityNishikyo-ku, Kyoto, 615-8510, Japan

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.

Dynamic stress-induced low-temperature ordering of FePt

The ordering temperature of FePt was significantly reduced to 275°C by introducing a Cu underlayer on the HF-cleaned Si(001) substrate. A coercivity Hc as high as 6200Oe can be achieved after postannealing at 275°C, and Hc can be further increased to 7000Oe after 300°C postannealing. During the formation of copper silicide Cu3Si, the expanded volume induces a dynamic in-plane tensile stress on FePt films, which accelerates the formation of the ordered FePt phase at low temperature. Different from the static stress induced by the lattice mismatch between films and underlayers, the dynamic stress is relaxed after the formation of Cu3Si is completed; therefore, the low-temperature ordering of FePt mainly takes place simultaneously with the evolution of dynamic stress during the formation of Cu3Si. The coercivity of FePt depends on the amount of Cu3Si and on the stress developed by Cu3Si.

(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%.

Nonpolar resistive switching in the Pt/MgO/Pt nonvolatile memory device

Nonpolar resistive switching (RS), which is the coexistence of unipolar and bipolar RS characteristics, in the Pt/MgO/Pt memory device with the nonforming nature is demonstrated. The nonforming nature is ascribed to the relatively high defect density of the MgO film deposited by using the ion beam sputtering in Ar atmosphere. The results of Auger electron spectroscopy and x-ray photoelectron spectroscopy analyses combing with the temperature dependence of resistance suggest that metallic Mg filaments are formed in the low resistance state. The voltage-polarity-independent RESET process implies that filaments may be ruptured by local Joule heating, leading to nonpolar characteristics.

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.

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.