High Resolution Magnetic Force Microscopy Research Articles

Improvement of Magnetic Force Microscope Resolution and Application to High-Density Recording Media

Magnetic force microscope (MFM) tips are prepared by coating magnetic materials on nonmagnetic Si tips with 4 nm radius. The effects of magnetic material and coating thickness on the MFM resolution and the switching field are investigated. MFM resolutions better than 8 nm have been confirmed with tips coated with soft magnetic or hard magnetic materials with optimized thicknesses. The switching field varies in a wide range 0.1-3.0 kOe depending on the coating material and the coating thickness (10-80 nm). High-resolution MFM tips are applied to the observations of magnetization structures of perpendicular and bit-patterned media samples. Magnetization structures of less than 20 nm in scale are clearly observed.

Fabrication of Magnetic Tips for High-Resolution Magnetic Force Microscopy

Effects of magnetic material, coating thickness, and tip radius on magnetic force microscope (MFM) spatial resolution have been systematically investigated. MFM tips are prepared by using an UHV sputtering system by coating magnetic materials on non-magnetic Si tips employing targets of Ni, Ni-Fe, Co, Fe, Fe-B, and Fe-Pd. MFM spatial resolutions better than 9 nm have been confirmed by employing magnetic tips coated with high magnetic moment materials with optimized thicknesses.

High-Resolution Magnetic Force Microscope Tip Coated with Co Film Prepared by Ultra-High Vacuum Evaporation

High-Resolution Magnetic Force Microscope Tip Coated with Co Film Prepared by Ultra-High Vacuum Evaporation

Preparation of high-resolution magnetic force microscope tips coated with Co and FeCo films

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 4 nm radius with Co and Fe65 Co35 (at. %) films by employing an ultra-high vacuum evaporation system. The effect of coating film thickness on MFM spatial resolution is investigated. With increasing the thickness from 10 to 20 nm, the resolutions of Co- and FeCo-coated tips improve from 8.4 to 6.9 nm and from 7.5 to 6.6 nm, respectively. Better resolutions are obtained for the FeCo-coated tips, which is due to enhanced sensitivities related with the higher magnetic moment of FeCo material. As the coating thickness further increases, the resolutions deteriorate due to increase of tip radius. The resolution is affected by the detection sensitivity and the tip radius. Magnetic bits of a perpendicular medium recorded at 1800 kFCI (bit length: 14.1 nm) and 1900 kFCI (13.4 nm) are respectively distinguishable in the MFM images observed by using tips coated with Co and FeCo films.

Characteristics of magnetic force microscopy magnetics on high moment perpendicular magnetic recording writers with high coercivity probes

High resolution magnetic force microscopy (MFM) imaging with high coercivity probes on perpendicular magnetic recording (PMR) writers directly characterizes magnetic field contour for the writer main pole as well as its shields’ magnetic state. Evolution of write bubble and return field was analyzed by MFM imaging in dynamic phase detection scheme. Different write field components and their out of plane second order derivatives were calculated via finite element modeling. The MFM imaged write field distribution correlates well with the PMR writer out of plane write field component. Magnetic responses of the PMR writer main pole, trailing and side shields are quantified. The trailing and side shields exhibit complicated magnetic saturation behaviors comparing with the PMR writer pole. The side shield’s magnetic response is dependent upon its initial equilibrium state.

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

Magnetization precession and domain-wall structure in cobalt-ruthenium-cobalt trilayers

The magnetization dynamics of Co(5 nm)/Ru/Co(5 nm) trilayers with Ru thicknesses from 0.3–0.6 nm is experimentally and theoretically investigated. The coupling between the Co layers is antiferromagnetic (AFM) and yields a stable AFM domain structure with frozen domain walls. Comparing high-resolution magnetic force microscopy (MFM) and pump-probe measurements, we analyze the behavior of the films for different field-strength regimes. For moderate magnetic fields, pump-probe measurements provide dynamic characterization of the coupled precessional modes in the GHz range. The dynamics at small fields is realized by the pinning of AFM domain walls at inhomogeneities. The MFM images yield a domain-wall width that varies from about 150–60 nm. This behavior is explained in terms of a micromagnetic local-anisotropy model.

Individual bit island reversal and switching field distribution in perpendicular magnetic bit patterned media

The switching of single bit magnetic islands in bit patterned media (BPM) for two cases with 10 times difference in coercivity, as well as the switching field distribution (SFD) of the islands, has been studied using magnetic force microscopy (MFM) measurements. The intrinsic SFD is measured to be ∼9–11% of the remanence coercivity ( H cr), which contributes only ∼20–50% of the total SFD broadening (∼23–41% of H cr). High resolution MFM observations clearly showed the influence of surrounding islands on the switching behaviour and switching fields of individual bit islands, resulting in significant contributions in SFD broadening due to non-intrinsic dipolar interactions. It was further observed that single magnetic islands could be switched within a very narrow switching field range as small as 4 Oe, which indicates very sharp and uniform switching for each individual island of BPM.

Self-Assembled Magnetic Nanohead-FeSi Nanowire Epitaxial Heterojunctions by Chemical Vapor Deposition

Self-assembled magnetic nanohead-FeSi nanowire (NW) epitaxial heterojunctions by chemical vapor deposition (CVD) are reported. Free-standing FeSi NWs, ∼30 nm in diameter and several micrometers long, were grown by reduction of Fe2O3 in a H2 atmosphere on a heated silicon substrate at 750 °C. These NWs shows very uniform dimensions, which is thought to be due to a higher diffusion activation energy for the nanostructure. A magnetic Fe3O4 head with a diameter of ∼30 nm was observed on each FeSi NW. The epitaxial relationship of these junctions is Si[001]//Fe3O4[2̅01], Si(001)//Fe3O4(102). Ferromagnetic behavior of the nanoheads was identified, and the coercivity was characterized as ∼220 Oe at 10 K. This one-step CVD approach for self-assembling epitaxially magnetic heads could enable device integration for spintronics and sensors. Particularly, such FeSi nanowires with highly localized magnetic nanoheads are ideal for high resolution magnetic force microscopy (MFM).

Analysis of Shingle-Write Readback Using Magnetic-Force Microscopy

Shingle recording experiments were conducted on a precision spin-stand using conventional recording components. Although both head and medium correspond to a 250 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (Gigabits/sq. inch) product, the raw written densities were pushed beyond 1 Tb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (Terabits/sq. inch). High-resolution Magnetic-force microscopy (MFM) was used to examine the resulting written patterns. The MFM images have an apparent resolution down to 16 nm. From these images we are able to characterize the signal and noise in two dimensions up to the highest densities. Two-dimensional signal-processing was applied to the "waveforms" recovered from these MFM images. At the highest written densities, very poor raw error-rates were encountered and customer data could only be recovered successfully by employing relatively low rate codes. Although the experiments pushed the raw recording density to 1039 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , the highest customer information density at which data was successfully recovered was 623 Gb/in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Ferromagnetism in Zn 1−xCr xTe ( x = 0.05, 0.15) films grown on GaAs(1 0 0) substrate

Zn 1− x Cr x Te ( x = 0.05, 0.15) films were grown on GaAs(1 0 0) substrate by thermal evaporation method. X-ray diffraction analysis showed the presence of ZnCrTe phase without any secondary phase. The surface was analyzed by high resolution magnetic force microscope and profile measurements showed orientation of magnetic domains in the range of 0.5–2 nm with increase of Cr content. Magnetic moment–magnetic field measurements showed a characteristic hysteresis loop even at room temperature. The Curie temperature was estimated to be greater than 300 K. From the electron spin resonance spectra, the valence state of Cr in ZnTe was found to be +2 with d 2 electronic configuration. Hall effect study was done at room temperature and the result showed the presence of p-type charge carriers and hole concentration was found to increase from 5.95 × 10 12 to 6.7 × 10 12 m −3 when Cr content increases. We deduce the origin of ferromagnetic behavior based on the observed experimental results.

Demonstration of ultra-high-resolution MFM images using Co 90Fe 10-coated CNT probes

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co 90Fe 10 films (20, 30, 40, and 50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40 nm-Co 90Fe 10-coated tip (50 nm tip diameter) about a lateral detect density of 1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.

Assembly and magnetic properties of nickel nanoparticles on silicon nanowires

The directed assembly of magnetic Ni nanoparticles at the tips of silicon nanowires is reported. Using electrodeposition Ni shells of thickness from 10 to 200 nm were selectively deposited on Au catalytic seeds at the ends of nanowires. Magnetic characterization confirms a low coercivity (≈115 Oe) ferromagnetic behavior at 300 K. This approach to multifunctional magnetic-semiconducting nanostructure assembly could be extended to electrodeposition of other materials on the nanowire ends, opening up additional approaches to device integration. Such magnetically functionalized nanowires offer an approach to developing highly localized magnetic probes for high resolution magnetic resonance force microscopy.

High-resolution and high-coercivity FePtL1 magnetic force microscopy nanoprobes to study next-generation magnetic recording media

A cylindrical probe with almost perfectly flat plateaulike surface was focused ion beam (FIB) milled from an atomic force microscopy probe in order to create the required surface conditions for thin film deposition with finely controlled deposition/growth parameters. A composition of Pd(5 nm)/MgO(8 nm)/FePt(10 nm)/MgO(8 nm) was sputter deposited on the plateau probe, followed by deposition of a Pd (5 nm) protective layer. The plateau probe was then FIB-milled to produce a tip with a curvature radius of ∼25 nm. After annealing the probe at 650 °C for ∼15 min to generate an ultrahigh anisotropy L10 phase, magnetic force microscopy (MFM) imaging was performed with the probe on magnetic tracks with linear densities ranging from 200 to 1200 KFCI. The results show sub-20-nm lateral resolution in ambient conditions and magnetic tracks, which are otherwise invisible to standard MFM probes, are clearly evident with the FIB-fabricated FePt probe. With relatively high spatial resolution and coercivity values higher than 1 T, among other applications, this type of probe may be ideal for high-quality MFM study of next-generation recording media.

Experimental determination of ultra-sharp stray field distribution from a magnetic vortex core structure

The fine magnetic stray field from a vortex structure of micron-sized permalloy (Ni 80Fe 20) elements has been studied by high-resolution magnetic force microscopy. By systematically studying the width of the stray field gradient distribution at different tip-to-sample distances, we show that the half-width at half-maximum (HWHM) of the signal from vortex core can be as narrow as ∼21 nm at a closest tip-to-sample distance of 23 nm, even including the convolution effect of the finite size of the magnetic tip. a weak circular reverse component is found around the center of the magnetic vortex in the measured magnetic force microscope (MFM) signals, which can be attributed to the reverse magnetization around the vortex core. Successive micromagnetic and MFM imaging simulations show good agreements with our experimental results on the width of the stray field distribution.

Diamond and Related Nanomaterials for MEMS/NEMS Applications

Diamond materials offer great potential for electronic and biomedical application. Properties like very high stiffness, thermal conductivity, optical transparency range, chemical stability and erosion resistance for the diamond based materials extend their applicability for several aspects of science and technology. Nanostructures of diamond materials have extraordinary multifunctional properties such as high thermal conductivity and a Youngs modulus value close to the one for single crystal diamond. Controlled nucleation and growth of nanocrystalline diamond along with their applicability for MEMS and NEMS structures are useful of future generation sensor platforms. Researchers around the world have been putting their efforts to achieve high precision in the direction of fabricating the devices based on diamond materials. Nanomanufacturing of such devices requires the controlled design of nanoscale diamond structures. This issue covers various topics related to carbon-based nanomaterials. The first article by Hanada et al. reports deposition process of ultrananocrystalline diamond films. Authors have demonstrated application of pulse laser deposition for diamond growth. The second article (by Ohmagari et al.) in this issue is about atomic bonding configuration of ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C: H) films, where authors have used near-edge X-ray absorption fine structure spectroscopy. The third article (by Teng et al.) is about structural transformation upon nitrogen doping of ultrananocrystalline diamond films. Tanaka et al. report high-resolutionmagnetic force microscopy using carbon nanotube probes in the fourth article of this issue. In the fifth article, Tung et al. report fabrication of carbon nanotubes STM tips by liquid catalyst-assisted microwave plasma-enhanced chemical vapor deposition. In a similar type of study, Sakamoto et al. (the sixth article) report fabrication of Pd-filled carbon nanotubes for SPM applications. In the seventh article (by Katzenmeyer et al.), the authors report Poly (hydridocarbyne) as highly processable insulating polymer precursor to micro/nanostructures and graphite conductors. In the concluding article, I. Chasiotis and K. N. Jonnalagadda report strength and fracture resistance of amorphous diamond-like carbon films for MEMS applications.

Magnetic vortices in tridimensional nanomagnetic caps observed using transmission electron microscopy and magnetic force microscopy

Magnetic domain formation on three-dimensional nanostructures was investigated. Co/Pd multilayers were deposited on polystyrene nanospheres (50--1000 nm) to form a magnetic cap with variable thickness. High resolution transmission electron microscopy and magnetic force microscopy images, allied with micromagnetic simulations, were used to correlate the three-dimensional shape of the caps with their domain structures. For smaller spheres (50--100 nm), the caps are segmented into nanopillars $(\ensuremath{\simeq}10\text{ }\text{nm})$ oriented toward the radial direction. For larger spheres (500--1000 nm), the cap is a continuous film with the magnetization forming a vortex at the top of the cap, with a core well larger than the ones observed in planar disks.

High resolution magnetic force microscopy of patternedL10-FePt dot arrays by nanosphere lithography

High resolution magnetic force microscopy (MFM) has been carried out onL10-FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tipmagnetization reversal experiment is carried out to determine the magnetic domains andverify the imaging stability of MFM and the mutual perturbations between the magnetictip and the sample. We have identified that the critical size for the single domain region isabout 90 nm across. Comparison with MFM image simulation also suggests that themagnetizations of the triangular dots in both single and double domain states are parallelto one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of theL10-FePt phase and the need for decreasing the magnetostatic energy.

High resolution magnetic force microscopy study of e-beam lithography patterned Co∕Pt nanodots

E-beam lithography has been used to pattern a continuous Co∕Pt multilayer film with perpendicular anisotropy into circular islands of various sizes down to 25nm diameter on a 60nm pitch. High resolution magnetic force microscopy with in situ applied field has been used to directly determine the switching field distribution (SFD) and hysteresis loop of the islands. For the smallest islands, the coercivity is reduced and the width of the SFD is increased, indicating that they would not be well suited to data storage applications.

Improving the spatial resolution of a magnetic force microscope tip via focused ion beam modification and magnetic film coating

To fabricate high-resolution magnetic tips by a more convenient method, this study applied focused ion beam milling and magnetic film coating techniques to manufacture high aspect ratio (HAR) magnetic force microscope (MFM) tips. The results showed that a HAR MFM tip with a ratio of 10:1 was successfully fabricated. MFM measurements demonstrated that this probe provided clear high-resolution MFM images. The ultimate lateral resolution of the HAR MFM tip was reduced to a minimum of 20 nm.