Jai-Lin Tsai

Magnetization reversal process in Fe/FePt films

A soft/hard Fe/FePt bilayer with perpendicular magnetization was prepared on a glass substrate. Controlling the Fe layer thickness allowed modification of the hysteresis loops from rigid magnet with perpendicular magnetization to exchange-spring like magnet with parallel magnetization due to the nanoscale soft/hard interface coupling. For rigid magnetic films, the magnetization was reversed at a single switching field and interpreted by the two-spin model. In an exchange-spring like film, the in-plane magnetization reversal process was in two-steps and resulted from domain wall nucleation and propagation from the Fe layer into the FePt layer.

Multifunctional doxorubicin/superparamagnetic iron oxide-encapsulated Pluronic F127 micelles used for chemotherapy/magnetic resonance imaging

Polymeric micelles are frequently used to transport and deliver drugs throughout the body because they protect against degradation. Research on functional polymeric micelles for biomedical applications has generally shown that micelles have beneficial properties, such as specific functionality, enhanced specific tumor targeting, and stabilized nanostructures. The particular aim of this study was to synthesize and characterize multifunctional polymeric micelles for use in controlled drug delivery systems and biomedical imaging. In this study, a theranostic agent, doxorubicin/superparamagnetic iron oxide (SPIO)-encapsulated Pluronic F127 (F127) micelles, was developed for dual chemotherapy/magnetic resonance imaging (MRI) purposes, and the structure and composition of the micellar SPIO were characterized by transmission electron microscopy and magnetic measurements. Our results revealed that the micellar SPIO with a diameter of around 100 nm led to a significant advantage in terms of T2 relaxation as compar...

Magnetic properties and microstructure of graded Fe/FePt films

A soft/hard Fe/FePt bilayer with perpendicular magnetization was prepared on a glass substrate. Annealed Fe/FePt film allowed modification of the Fe/FePt sharp interface to Fe/(Fe-rich FePt)/FePt graded interface with rigid magnetization due to the nanoscale soft/hard interface coupling. The magnetization was reversed at a single switching field and interpreted by the two-spin model. When the annealed temperature of the Fe/FePt film increased, the remanence magnetization decreased continuously but the out-of-plane coercivity increased obviously at 600–700 °C which was interpreted by the graded magnetic anisotropy. The coercivity can be tuning in the exchange coupled composite film.

Low-temperature in-situ growth of high-coercivity Fe-Pt films

FePt films with high coercivity, large energy product and corrosion resistance, have high potential in applications such as MEMs and ultra-high density magnetic recording. Much attention has been paid to the preparation of high quality films at substrate/annealing temperatures as low as possible to accommodate processing adopted in the manufacture of MEMs or sensors. In this study Fe/sub x/Pt/sub 1-x/ thin films were deposited by DC magnetron sputtering onto Si[100] and CrMo-seeded glass substrates at a substrate temperature (Ts) 30 to 600/spl deg/C with or without further post-annealing. Optimum magnetic properties of our studied films were obtained at the Fe/sub 53/Pt/sub 47/ composition as-deposited at Ts 300/spl deg/C. The in-plane coercivity (Hc) is 8 kOe for as-deposited films on both Si[100] and glass substrates. The films on Si[100] showed a maximum magnetization (M2T) and a maximum energy product of 950 emu/cm/sup 3/ and 15.8 MGOe, respectively. The squareness ratio (M/sub r//M/sub s/) of the as-deposited FePt films on glass substrate increases with increasing Ts. Minor hysteresis loops reveal that domain-wall-pinning dominates the magnetic hardening of the FePt films. Post annealing at 400/spl deg/C substantially enhances coercivity and energy product for the Fe/sub 53/Pt/sub 47/ films on glass substrate. Ordering parameters were studied by XRD and quantitatively correlated to magnetic properties.

Magnetoresistance study in thin zig zag NiFe wires

Thickness dependence on the domain wall resistivity of zig zag thin permalloy wires was studied from 10 to 300 K. The maximum domain wall resistivity was obtained in wire with 100-nm-film thickness. The multidomain state resistivity was 14.29 μΩ cm, while single-domain state resistivity was 14.36 μΩ cm at 10 K. The ratio of domain wall magnetoresistance was measured to be 0.034%, 0.112%, and 0.258%, and the magnetic field where the domain wall started to switch was measured as −70, −40, and +80 Oe for wires with thicknesses of 20, 40, and 100 nm, respectively, at 250 K. Domain wall resistivity was nearly independent of temperature for wire with 40-nm-film thickness but varied significantly with temperature for 100-nm-thick wire between 10 and 300 K.

Quantitative analysis of magnetization reversal in submicron S-patterned structures with narrow constrictions by magnetic force microscopy

We present a quantitative study of magnetization reversal and domain wall pinning phenomenon in an S-patterned ferromagnetic wire with narrow constrictions by magnetic force microscopy (MFM). The variation of domain structures with magnetic field is evidenced by electron transport measurements, simulation, and MFM images. The switching behaviors are strongly affected by narrow constrictions connecting the half-ring wires. The proposed MFM analysis effectively separate the local hysteresis loops at the arc section and the narrow constriction, which provide quantitative data, including relative magnetization intensity, local switching field, and domain wall depinning field for the multidomain wire. The MFM results correlate with other experimental data well, and provide direct evidence that the domain state at constriction dominates the magnetoresistance behavior.

Study of domain wall magnetoresistance by submicron patterned magnetic structure

Through the proper design of submicron permalloy (Ni80Fe20) wires, domain wall magnetoresistance is investigated. A positive contribution to magnetoresistance (MR) was found for domain walls in these wires after extracting the anisotropic MR effect. Some theoretical models are used to discuss our results.

MAGNETIC PROPERTIES AND GROWTH BEHAVIOR OF ND-FE-B FILMS ON SI(111)

NdFeB films were prepared by dc-magnetron sputter-deposition onto Si(111) at 600 to 700°C. Anisotropic films with remanence ratio larger than 0.67 and intrinsic coercivity greater than 7 kOe were obtainable at a substrate temperature of 650°C on Si(111) using one of W, Mo, or Pt as the underlayer. The mechanical polishing can reduce the film thickness from about 850 nm to 150 nm, and the coercivity remains 2–5 kOe. The coercivity of being strongly dependent on film thickness can be explained by the domain-wall motion in films through suitable modeling. The depth-profile and microstructure of the films on different underlayers were explored by secondary ion mass spectroscopy (SIMS) and high resolution transmission electron microscopy. The Pt underlayer, being in fact a Pt/Ti/SiO2/Si(111), is a sound barrier layer between the NdFeB film and Si(111) so that there is negligible interdiffusion as evidenced by SIMS depth profile. The formation of silicide was found in films with Mo or W underlayer sputtered at 650°C. This can be observed from the overlap region between the underlayer and Si(111) in the SIMS depth profile. Diffusion coefficients of the Nd, Fe, B in Si(111) were also quantitatively estimated.

Magnetic properties and microstructure of perpendicular FePt(B-Ag) granular films

Multilayers [FePt(1 nm)/(BxAg1-x)(0.1 nm)]10 (x = 0 - 1) were alternately deposited on a glass substrate and subsequently annealed by a rapid thermal process (RTP) at 800 °C for 3 min. After RTP, FePt and immiscible (B, Ag) layers intermix to form (FePt)90(B-Ag)10 granular. The intermixed (B, Ag) atoms with high mobility diffuse among FePt grain boundaries to isolate and refine FePt grains uniformly with average grain size of 10 - 12 nm. The c-axis alignment in original multilayers remains unaltered during intermixing. With increasing Ag content in (BxAg1-x) (x = 0.3 - 0.7), c-axis orientation and grain-refinement are further improved. The optimal composition is (Fe0.48Pt0.52)90(B0.7Ag0.3)10.

Room-temperature anomalous Hall effect in amorphous Si-based magnetic semiconductor

Here, we show that Mn-doped amorphous hydrogenated Si reveals room-temperature ferromagnetism. Various characterization techniques rule out the formation of magnetic clusters. In particular, anomalous Hall-effect is found even at 300 K in annealed Si89.5Mn10.5 samples. The observed anomalous Hall-effect provides direct evidence that the ferromagnetic order is coupled to the itinerant carriers, making these samples workable magnetic semiconductors. This work demonstrates the great potential for Si-based semiconductor spintronics at room temperature, which is readily integrated with the current information technology.