NiFe Elements Research Articles

Core-Shell Structured SiO2@NiFe LDH Composite for Broadband Electromagnetic Wave Absorption.

In this work, a novel core-shell structure material, NiFe layered double hydroxide (NiFe LDH) loaded on SiO2 microspheres (SiO2@NiFe LDH), was synthesized by a one-step hydrothermal method, and the spontaneous electrostatic self-assembly process. The morphology, structure, and microwave absorption properties of SiO2@NiFe LDH nanocomposites with different NiFe element ratios were systematically investigated. The results show that the sample of SiO2@NiFe LDH-3 nanocomposite has excellent microwave absorption properties. It exhibits broadband effective absorption bandwidth (RL < -10 dB) of 8.24 GHz (from 9.76 GHz to 18.0 GHz) and the reflection loss is -53.78 dB at the matched thickness of 6.95 mm. It is expected that this SiO2@NiFe-LDH core-shell structural material can be used as a promising non-precious, metal-based material microwave absorber to eliminate electromagnetic wave contamination.

Microstructure investigation of new iron-based FeNiCoAlTiNb shape memory alloys

The microstructure of Fe41Ni28Co17Al11.5Ti1.25Nb1.25 (at.%) polycrystalline alloy was investigated for this study. The homogeneous heat treatment condition is 1277 ​°C for 24 ​h. When the heat treatment temperature is above 1300 ​°C, second phases generate along the grain boundary and contain high Ni, Al, Ti, Nb and low Fe and Co element compare to nominal composition. From the aging heat treatment results, the size of precipitates is around 5–10 ​nm when the sample is aged at 600 ​°C for 72 ​h. From the thermo-magnetization results, the saturation magnetization is 140 emu/g. The magnetization loop is shifted to higher temperature when the aging times increase from 24 to 96 ​h. The result indicates that the transformation temperature tends to increase with increasing the aging times.

Vortex motion in amorphous ferrimagnetic thin film elements

Amorphous Fe64Gd36 thin film square elements are investigated by imaging in the Fresnel mode of a transmission electron microscope (TEM). The equilibrium state without an applied field shows the well-known four-domain flux closure pattern with in-plane magnetization. However, the vortex is displaced from the center of the square element and the domain walls are curved. In a reference measurement of a thin Ni81Fe19 element, the vortex core is perfectly centered and the domain walls straight. When an increasing external field is applied in-plane, the vortex core can be moved. While this motion of the vortex core is linear in NiFe elements, in the ferrimagnetic FeGd squares the vortex core moves by sudden jumps. Micromagnetic simulations show that the asymmetry of the domain patterns as well as the vortex core pinning and depinning can be attributed to random anisotropy and a patchy microstructure in amorphous films.

Translocation of magnetic beads using patterned magnetic pathways for biosensing applications

We have designed, fabricated, and demonstrated a novel system for translocation of magnetic beads at specific sites of the sensor surface on a single chip for biosensor applications. The soft NiFe elliptical (9×4×0.1 μm3) elements are arranged as magnetic pathways connected to the model sensor surface. The patterned NiFe elements can generate different stray magnetic fields when they are subjected to the external rotating magnetic field. The inhomogeneity in stray magnetic fields can govern the magnetic bead motion on the pathways. We demonstrated the motion of Dynabead® M-280 magnetic bead on patterned pathways by controlling the external rotating magnetic field in clockwise and counterclockwise directions. The magnetic beads that were placed on the magnetic elliptical pathways are shown to be transported to the sensor surface, as well as be pulled out away from the surface. This technique enables microtranslocation of the magnetic beads coated with biomolecules to the specific binding sites of the sensor surface and as well as drive off the nonspecific binding biomolecules from the surface in performing number of sequential bead detection experiments for future integrated lab-on-a-chip systems.

Spin configuration of hexagonal shaped ferromagnetic elements arranged in different structures

We report on magnetic imaging study of regular hexagonal shaped NiFe elements arranged in different configurations. By making use of stray field with moderate strength and well-defined symmetry, which is lacking in vortices formed in circular elements, it is possible to control the relative chirality of vortex in a ring network of hexagons unlike in an array network. However, the alternating chirality of the hexagons disappears when the interelemental spacing becomes large, an element is missing from the ring network, or they are arranged in linear chains. The results can be understood by the symmetry and direction of the stray field generated by the individual elements.

Time- and vector-resolved magneto-optical Kerr effect measurements of large angle precessional reorientation in a 2×2μm2 ferromagnet

The precessional dynamics of a 2×2μm2 CoFe∕NiFe (4.6nm) element stimulated by an in-plane pulsed magnetic field have been investigated using time- and vector-resolved Kerr microscopy measurements and micromagnetic simulations. The time-resolved signals were normalized to in-plane hysteresis loops obtained from the patterned material, and suggest that the magnetization reorients through an angle of 100°±10°. The simulations reveal that only the magnetization of the center region undergoes large angle reorientation, while the canted magnetization at the edges of the element remains pinned. An enhanced Gilbert damping parameter of 0.1 was required to reproduce the experimentally observed Kerr signals.

Microwave-assisted Magnetization Reversal of Sub-micrometer Wide NiFe Pattern

Microwave-assisted magnetization reversal of 1.3×0.3×0.035 μm3 NiFe element with a natural ferromagnetic resonance frequency of 8.3 GHz was experimentally investigated by means of vector network analyzer ferromagnetic resonance measurement. The irreversible switching of magnetization after the application of ac magnetic field with an appropriate frequency was detected from a discontinuous increase of ferromagnetic resonance frequency in the NiFe element at a non-zero dc bias field. The switching field can be reduced from 300 to 192 Oe when the ac magnetic field with the frequency of 7GHz and the amplitude of 130 Oe is applied along the short axis of the NiFe element.

Saturation field direction dependence of domain structures in NiFe elements

The domain structures in NiFe elements were studied by magnetic force microscopy measurement and micromagnetic modeling. The remanent states in the elements were dependent on the direction of the saturation field. The “S” and “U” states were observed at remanence by applying the saturation field at different directions. The “S” and “U” states are metastable: magnetic force microscopy tip field-induced switching from the “S” and “U” states to the flux closure configuration was observed.

Submicron patterning of Ta, NiFe, and Pac-man type Ta/NiFe/Ta magnetic elements

Submicron patterning of Ta, NiFe, and Pac-man type magnetic elements of Ta/NiFe/Ta has been carried out in inductively coupled plasmas (ICPs) of Cl2/Ar. Etch behavior was quite dependent on materials and plasma parameters. An ion-enhanced etch mechanism played a critical role for desorption of metal chloride etch products. Side-wall contamination with etch products was observed at a higher Cl2 concentration (>50%). Compared to relatively damaged surfaces and profiles by the ion milling method, the ICP etching technique produced clear, smooth, and well-defined Pac-man type elements.

Analysis of magnetic properties in patterned micro-NiFe elements with micro-MOKE and micromagnetics

Magnetic properties of a patterned micro-NiFe element were analyzed with micro-MOKE and micromagnetics. Patterned 8 μm × 16 μm NiFe elements have higher coercive field, Hc = 1.9 Oe, for the hard axis than that, Hc = 0.2 Oe, of a 5 mm diameter disk NiFe sample. The magnetization reversal processes have been investigated with the micromagnetic simulation for the various applied field directions. The increase of coercive field in patterned micro-NiFe elements can be attributed to the large shape anisotropy and pinned spins along the edges. Total anisotropy of a 8 μm × 16 μm NiFe sample was about 50 Oe. The magnetization reversal field of pinned spins along the edges in patterned micro-NiFe sample was about 500 Oe. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Magnetization configuration and switching behavior of submicron NiFe elements: Pac-man shape

Two types of submicron permalloy element, namely Pac-man, were investigated by a magnetic force microscope for magnetization configuration and switching behaviors. Two distinct domain configurations, bidomain for Pac-man type I and single domain for Pac-man type II, were observed in arrays of Pac-man elements. The domain configuration depends on the slot angle for the Pac-man type I, but is independent of the slot angle for the Pac-man type II. Array of Pac-man elements with a slot angle of 180° shows the highest switching field and the narrowest switching field distribution, as compared to rectangular and hexagonal elements of the same overall dimensions.

Thermal fluctuation effects on quasistatic magnetic switching of patterned Ni81Fe19 elements (invited)

We have studied quasistatic magnetic switching of electron-beam patterned Ni81Fe19 (NiFe) elements over a wide range of temperatures. Switching properties depend on both lateral and thickness dimensions of the patterned structures. In large aspect-ratio elements, the switching between the two bistable states occurs through thermally activated nucleation and the switching field depends linearly on temperature. As the aspect ratio decreases, a third stable remanent state-trapped vortex state develops, but the switching fields between different states also linearly depend on temperature. As the film thickness decreases, the trapped vortex population clearly becomes thermally activated.

Magnetostatic interactions between sub-micrometer patterned magnetic elements

We have investigated the effect of the magnetostatic interaction field between submicrometer patterned magnetic elements in arrays (/spl sim/10/sup 8/ elements) using Alternating Gradient Force Magnetometry. Single layer NiFe elements were studied over a range of width (0.12 to 0.48 /spl mu/m), length to width aspect ratio (1.5 to 4) and thickness (30 /spl Aring/ to 60 /spl Aring/). The arrays were patterned using e-beam lithography in a rectangular lattice with bit separation equal to element width. Effective interaction fields were obtained using a novel application of /spl Delta/M plots to these patterned arrays. The /spl Delta/M plot is derived by subtracting remanent magnetization curves that had initial states of full magnetization from those that were initially demagnetized. Interaction fields were quantified using H/sub int/=/spl Delta/M//spl chi//sub irr/, where /spl Delta/M is the difference in these remanent loops and /spl chi//sub irr/ is the derivative of the remanent magnetization with respect to field. We found H/sub int/ to increase with element aspect ratio, thickness and inverse width. For structures with the largest aspect ratio, thickness and smallest width, H/sub int//spl ap/6 Oe. We calculated the actual dipolar interaction fields using micromagnetic simulation and used these fields in a simple 2-d Ising model to simulate the experiment. We found H/sub int/ that is a good measure of the difference in interaction fields tending to magnetize or demagnetize the sample.

Imaging magnetic domain structure in sub-500 nm thin film elements

Magnetic imaging in the transmission electron microscope (TEM) has been used to examine submicron elements with the aim of discovering down to what element size complex domain patterns can form. The elements were squares, circles, triangles, and pentagons in the size range 100–500 nm and were made from 36 nm Co films or 8 nm Ni80Fe20 (NiFe) with in-plane magnetization. The magnetic domain structures in these elements were imaged at high resolution using the differential phase contrast imaging mode in a TEM. Nonuniform magnetization structures were seen in the images. Vortices were present at remanence in all shapes of 36-nm-thick Co elements down to 100 nm size and in circular NiFe elements down to 116 nm diameter. Triangular NiFe elements did not have a vortex state at remanence, instead the magnetization curved round within the element but did not achieve complete flux closure. In simulations of square and circular NiFe elements, it was found that defects at the edges of the elements encouraged reversal by a vortex mechanism, whereas for simulated elements with no defects, reversal was by rotation and occurred at much lower fields.

Interactions and switching field distributions of nanoscale magnetic elements

Magnetic nano-elements made from NiFe and Co have been investigated using magnetic imaging in the transmission electron microscope. Nano-elements like these have possible uses for in-plane patterned media or solid state memory. In both cases the elements will need to be patterned into closely spaced arrays and magnetostatic interactions between the elements will begin to become significant. Arrays must therefore be designed so that an element’s interactions with its neighbors will be small compared to its coercivity. Arrays of NiFe elements 300 nm long, 50–100 nm wide, and 26 nm thick, were fabricated by electron beam lithography and lift-off patterning. Their switching behavior and the interactions between them were studied in detail. Magnetization sequences were recorded and hysteresis loops constructed. For rows of NiFe elements with the gap between elements the same as the element width or larger, the interactions turn out to be small, suggesting that denser arrays would be possible.

Switching of nanoscale magnetic elements

We have investigated the magnetic properties of ultra-small-patterned elements of Co and NiFe thin films. The elements were rectangular with an aspect ratio in the range 3.75–20. The smallest were 200×40 nm2 with 50 nm gaps between them, corresponding to an areal density of 27 Gbit/in2 if used as discrete-patterned media for magnetic recording. The elements were fabricated by electron-beam lithography and lift-off patterning and high-resolution magnetic images were obtained by Lorentz microscopy in a transmission electron microscope. In situ magnetization reversal experiments showed that the strong dependence of the switching field on element width extended to the smallest elements of both materials. The switching field for 40-nm-wide Co elements was 1200 Oe and for 40-nm-wide NiFe elements was 800 Oe. Element length and aspect ratio had little effect.

Finite elements in numerical micromagnetics: Part II: patterned magnetic elements

Lithographically patterned magnetic elements may be the basic structural units of future quantized magnetic disks and nanoscale magnetic sensors. A hybrid finite element/boundary element method is developed, in order to solve the Gilbert equation of motion for an array of interacting magnetic elements. The calculated time evolution of the magnetic domain structure provides further understanding regarding how structural properties like the element shape or the polycrystalline microstructure influence the magnetization reversal mechanism. Small-scale zig-zag domains experimentally observed in NiFe element have to be attributed to a transverse anisotropy. In bars with one pointed end, the formation of the domains starts from the flat ends. Narrow elements with a width smaller than 200 nm remain in a nearly single domain state. Pointed ends suppress the formation of domains and thus increase the switching field by about a factor of two in Co elements. The polycrystalline microstructure and edge irregularities significantly affect the magnetization reversal mechanism of small Co elements.

Micromagnetic modelling of nanocrystalline magnets and structures

Numerical micromagnetic 3D simulation of nanocrystalline magnetic materials and structures determine the microstructural factors controlling the magnetisation reversal process. Predictions for size and shape of grains and elements in order to obtain optimum magnetic properties are made. The micromagnetic predictions of coercivity and remanence for composite hard magnets cover the range of (H c , J r )=(0.34, 1.4 T) to (0.61 MA/m, 1.1 T), assuming a grain size of 20 nm and an Nd 2Fe 14B content of 40%. The calculations reveal a linear trade off of remanence and coercivity as a function of the α-Fe to Fe 3B ratio. Increasing Fe 3B content improves the coercive field, but reduces the remanence and deteriorates the squareness of the hysteresis loop. The coercive field shows a maximum at a grain size of about D=15–20 nm. Intergrain exchange interactions override the magnetocrystalline anisotropy of the ND 2Fe 14B grains for smaller grains, whereas exchange hardening of the soft phases becomes less effective for larger grains. The magnetisation distribution shows a vortex-like structure within soft magnetic regions with an extension greater than 80 nm. The vortices have zero net magnetisation and act as nucleation sites, reducing remanence and coercivity. The shape of the grains considerably influences the squareness of the demagnetisation curve. Our simulation shows that the magnetisation reversal starts with reversible rotations of the magnetisation within the soft magnetic phase followed by irreversible switching of the hard magnetic grains. The minimum size and the domain wall jaggedness of nanocrystalline Co/Pt multilayered films strongly depend on the average size and misorientation of grains. Switching fields in patterned NiFe and Co elements are also influenced and the formation of a multidomain structure is suppressed on the type of anisotropy and the size and shape of the elements containing nanocrystalline grains.

Micromagnetic simulation of the long-range interaction between NiFe nano-elements using the BE-method

A higher-order hybrid finite element/boundary element method has been developed to simulate the time evolution of the domain structure of NiFe elements arranged in a closely packed array. The centre-to-centre spacing was 250 nm. The elements have two pointed ends are 200 nm wide, 3.5 μm long and 26 nm thick. Different magnetisation reversal mechanisms occur determined by the strength and the direction of the interaction field. The simulation predict a spread in the switching field in the order of 8 kA/m which has to be attributed to magnetostatic interactions.

Domain configurations of nanostructured Permalloy elements

The magnetization distributions of an array of small NiFe elements were studied using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The dependence of the domain configurations at zero field as a function of the aspect ratio was observed using MFM, and confirms the earlier observations using LTEM. Comparison of the images of similar islands using both techniques elucidate the complementarity between the LTEM and MFM measurements which individually show different facets of the magnetization distributions on soft magnetic thin films.