Jun-Yang Lai

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.

Magnetization patterns of permalloy square frames

Four different magnetization configurations of micron- and submicron-sized permalloy square frames are investigated by numerical simulations and experiments. Beside the pure conventional 90° Neel type wall with zero net magnetic pole, we also obtain numerically another high energy domain wall with positive or negative net magnetic poles in the corner. These three kinds of domain walls constitute four different patterns in square frames. We compare the magnetic pole density distributions derived from the spin configurations of simulation results with the images taken by magnetic force microscopy, and find reasonable agreement between them.

Magneto-Optical Kerr Effect Enhanced by Surface Plasmon Resonance and Its Application on Biological Detection

We investigated the magneto-optical Kerr signals enhanced by surface plasmon resonance (SPR) and its application of the biomolecular detection. We measure the SPR curve obtained in multilayered metallic structures (t nm Co/45 nm Au) with varying cobalt thickness t. The thicker cobalt layer causes the weaker SPR. The magneto-optical signals also receive different magnitude of amplification due to measuring the hysteresis loop on different segments of the SPR curve via magneto-optical Kerr effect (MOKE). It also worth noticing that the magneto-optical signals from the reflectance maximum or reflectance minimum of SPR would be enhanced significantly, if the thickness of Co is within the range of 3 to 28 nm. By measuring and analyzing the magneto-optical signals from samples with different thicknesses of Co, we determined an optimal thickness of 8 nm where the signal amplification is the largest. In biomedical applications, this combined system is able to lead as a sensitive biomolecule sensing. We have verified the feasibility via fetal bovine serum (FBS) attached.

Metastable state before magnetization reversal in single-domain elongated thin films

A metastable state for the elongated Permalloy elements is found to exist within a short field range before magnetization reversal. From the micromagnetic simulation it exhibits a wavelike structure, and the corresponding magnetic pole density distribution shows a pattern with periodic spots along the boundary. We also use magnetic force microscopy imaging and magnetoresistance measurement, which measures the nucleation field and coercive field, to study the noncoherent magnetization reversal properties of Permalloy ellipse with long axis, short axis, and thicknesses of 18μm, 1μm, and 45nm, respectively. The metastable state before switching is found to be repeatable.

As-deposited magnetic states in arrays of rectangular permalloy elements

We study the magnetization configurations of the metastable states including various vortex states and cross-tie states existing in rectangular Permalloy thin films by micromagnetic simulations and by experiments. First, we use the micromagnetic simulation to obtain various stable states in rectangles. In our experiment, we fabricate the array of rectangular thin films with different aspect ratios. Several stable states are obtained in the array, and it is found that when the aspect ratio is close to one, only one vortex state can exist. In large aspect ratios, multivortex and cross-tie states can exist.

Comparing the magnetic property of shell thickness controlled of Ag-Ni core-shell nanoparticles

In this paper, we studied the critical diameter of superparamagnetic core-shell particles. One pot method and microwave-assisted technique were used to synthesize Ag-Ni core-shell nanoparticles with diameters of 10 nm and 30 nm. From the experimental results of hysteresis (M-H curves) and temperature dependent magnetization (M-T curves), the theoretical critical diameter does not agree with the experimental observation of the core-shell structure. Furthermore, the blocking temperature equation should be reconsidered.

Study of polyvinyl alcohol nanofibrous membrane by electrospinning as a magnetic nanoparticle delivery approach

Electrospinning technique was used to fabricate polyvinyl alcohol (PVA)-based magnetic biodegradable nanofibers. PVA solution was mixed with ferrofluid or magnetic nanoparticles (MNPs) powder and formed two individual nanofibrous membranes (PVA/ferrofluid and PVA/MNPs powder) by electrospinning. The surface morphology of the nanofibrous membrane was characterized by scanning electron microscopy and the magnetic properties were measured by vibrating sample magnetometer. Macrophages (RAW 264.7) were co-cultured with the nanofibrous membranes for 12, 24, and 48 h and exhibited good cell viability (>95%). Results showed that the PVA fibers would be degraded and the embedded Fe3O4 nanoparticles would be released and delivered to cells.

Size dependence of switching fields of permalloy rings

The size dependence of switching fields of permalloy rings is studied by micromagnetic simulations and by magnetoresistance measurements. Experimentally, the switching fields can be obtained from the discontinuities of the magnetoresistance curve. From the simulated hysteresis loops and the magnetoresistance curves obtained by experiment it is shown that the switching field from the vortex state to the reverse onion state increases considerably as decreasing of the lateral size or as increasing of the thickness. There is, however, no significant correlation between the ring size and the switching field from the onion state to the vortex state.

Magnetization patterns of permalloy networks

Magnetization patterns in Permalloy network junctions are obtained by use of experiments and micromagnetic simulations. In the as-deposited thin film of the honeycomb network composed of three-arm junctions, only the lowest energy configuration, which possesses /spl plusmn/1 reduced net magnetic pole density in the junction center, is observed. In the as-deposited thin film of the square network composed of four-arm junctions, the configuration with the lowest energy density is the most frequently observed experimentally. The pattern of the independently existent antivortex, which has higher energy density, is for the first time observed experimentally in the square network.

Effects of interparticle dipole interaction on permalloy thin film arrays

The magnetic structures and hysteresis loops of permalloy thin film arrays are investigated here using magnetic force microscopy and vibrating sample magnetometer. The strength of interparticle dipole interaction can be revealed by the number of single-domain pairs with antiparallel magnetizations when the array is relaxed from a strong hard-axis field. Besides, hysteresis loops obtained by vibrating sample magnetometer measurements show that arrays with narrower interparticle spacings have lower coercivities and remanences. The results obtained from vibrating sample magnetometer are in very good agreement with magnetic force microscopy imaging.