Mei-Feng Lai

Cell patterning using microstructured ferromagnetic thin films

Magnetic cell patterning is demonstrated through controlling the micromagnetic states in microstructured ferromagneticthin films. The number of magnetic nanoparticles entering the cells by endocytosis can be determined by magnetophoresis experiment and is found to be dependent of the cocultured extracellular magnetic nanoparticles concentrations. In zigzag magnetic films the effects of cell patterning differ for magnetic films at as-deposited state and at remanent states after applying fields in different directions. Remanent states of concentric rings are proposed for cell patterning. Cells can be arranged at any positions in sequence by selectively changing the magnetic field directions.

Mechanisms of magnetization reversals in elliptical thin films

The size dependence of reversal mechanisms and the corresponding switching fields of elliptical Permalloy thin films are studied by micromagnetic simulation. From the simulated spin configurations, it is shown that for ellipses with different dimensions there exist three reversal mechanisms: the buckling-instability mode, the vortex-propagation mode, and the mixed mode of buckling-instability and vortex-propagation modes.

Evolution of vortex states under external magnetic field

Abstract The behavior of one-, two-, and three-vortex states of submicron-sized elliptical elements under the influence of external magnetic field is investigated here by means of micromagnetic simulations. The total magnetization curves show abrupt changes at the nucleation or annihilation of vortices. The magnetization curves and critical field show significant dependence on the field direction with respect to the particle’s long axis.

Influences of the aspect ratio and film thickness on switching properties of elliptical Permalloy elements

The size dependence on the switching properties of microstructured Permalloy (Ni/sub 80/Fe/sub 20/) ellipses were investigated by magnetoresistance measurements and magnetic force microscopy. Elements with fixed short axes of 1 /spl mu/m, long axes varying from 2 to 10 /spl mu/m, and film thickness varying from 8 to 55 nm were fabricated by electron beam lithography through a lift-off technique. A single-domain configuration was observed in the elements with the range of aspect ratios (long/short axis) from 5 to 10. More complex domain structures appear in the lower aspect ratio and thicker samples. The switching properties show a strong dependence on the film thickness as well as the aspect ratio. The switching fields of uniform magnetization reversal increase with increasing thickness up to a critical value (24<t/sub c/<40 nm), whereas they decrease with increasing thickness above t/sub c/. Nevertheless, the switching fields only show weak dependency on aspect ratio.

Cells positioning using magnetic domain walls of ferromagnetic zigzag thin film

Magnetic cell positioning is demonstrated by controlling the magnetic domain walls in ferromagnetic zigzag thin films. Magnetophoresis experiment is performed to determine the number of magnetic nanoparticles that enter the cells by endocytosis. It is observed that in the zigzag structure with larger wavelength the magnetic cells are better aligned at the vertices.

Magnetization reversal in nanostructured permalloy rings

Summary form only given. Recently, nanostructured rings have attracted much attention in the application of MRAM design because the shape anisotropy makes it easier to generate a closure flux loop spin configuration, which avoids the field leakage and therefore enhances the data storage density. A Permalloy ring with the size of 187.5 nm outer diameter, 112.5 nm inner diameter, and 30nm thickness has been studied by means of micromagnetic simulation. The authors show the hysteresis loop of the ring from our simulation result, with schematic pictures used to represent the stable states I, II, and III of the plateaus.

Compare analysis for the nanotoxicity effects of different amounts of endocytic iron oxide nanoparticles at single cell level.

Developing methods that evaluate the cellular uptake of magnetic nanoparticles (MNPs) and nanotoxicity effects at single-cellular level are needed. In this study, magnetophoresis combining fluorescence based cytotoxicity assay was proposed to assess the viability and the single-cellular MNPs uptake simultaneously. Malignant cells (SKHep-1, HepG2, HeLa) were incubated with 10 nm anionic iron oxide nanoparticles. Prussian blue stain was performed to visualize the distribution of magnetic nanoparticles. MTT and fluorescence based assay analyzed the cytotoxicity effects of the bulk cell population and single cell, respectively. DAPI/PI stained was applied to evaluate death mechanism. The number of intracellular MNPs was found to be strongly correlated with the cell death. Significant differences between cellular MNP uptake in living and dead cells were observed. The method could be useful for future study of the nanotoxicity induced by MNPs.

Microseparator for magnetic particle separations

In this study, a magnetic particle microseparator using patterned magnetic films to switch on and off separation is proposed, and numerical simulation is performed to analyze the function of the microseparator. Two magnetic films with the same magnetic dipole moment and different magnetization switching fields are used to produce open and closed magnetic fluxes that can exert different forces on the magnetic particles. For the case of open magnetic flux where the separation is on, almost all magnetic particles are trapped by the magnetic force, and the separation rate is 87.5%. For the case of closed magnetic flux where the separation is off, most of the magnetic particles pass through the separation region without being trapped.

Dependence of magnetization processes on vortex helicities in Permalloy nanorings

Two different magnetization reversal processes, which are caused by the helicities of the two small vortex domain walls in the two sides of the onion state, are found numerically to exist in thin film rings. When the two vortex domain walls are in the same helicity, the onion state will transform to flux closure state (vortex state) with increasing of the field, and then transform from the flux closure state (vortex state) to the reverse onion state with further increasing of the field. When the two vortex domain walls are in opposite helicities, however, with increasing of the field, the onion state will transform directly to the reverse onion state skipping the flux closure state (vortex state).

Wheatstone bridge giant-magnetoresistance based cell counter.

A Wheatstone bridge giant magnetoresistance (GMR) biosensor was proposed here for the detection and counting of magnetic cells. The biosensor was made of a top-pinned spin-valve layer structure, and it was integrated with a microchannel possessing the function of hydrodynamic focusing that allowed the cells to flow in series one by one and ensured the accuracy of detection. Through measuring the magnetoresistance variation caused by the stray field of the magnetic cells that flowed through the microchannel above the GMR biosensor, we can not only detect and count the cells but we can also recognize cells with different magnetic moments. In addition, a magnetic field gradient was applied for the separation of different cells into different channels.