Resolution Magnetic Force Microscopy Research Articles

A Mathematical Algorithm and Custom Nanoprobes to Improve the Resolution of Magnetic Force Microscopy (MFM) Images

We are working on developing special nanoprobes and techniques to improve the resolution of Magnetic Force Microscopy, MFM. In this paper we describe the custom probes fabricated at our lab and a mathematical algorithm to improve the resolution of the images obtained with these probes. The algorithm is based on physical principles and not on statistical nor on digital signal processing techniques. Our method consists in reversing the non-uniform interaction that takes place between the magnetized probe tip and the perpendicular magnetic stray field produced by the magnetized sample. We developed a compensation field that when scanned over the originally data points produces the effect of imaging the magnetic field with a flat response probe tip. Using this method we obtained images of higher resolution than those provided by conventional MFM.

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.

Different types of ferrite thin films as magnetic cantilever coating for magnetic force microscopy

Different types of ferrites are employed in the form of thin films as magnetic coating on cantilevers for magnetic force microscopy (MFM) use. This is especially needed for cantilevers employed in high-frequency MFM (HF-MFM), where stray fields of hard disk recording heads are investigated. Our experiments show that we can operate HF-MFM successfully at carrier frequencies up 2 GHz using such ferrite-coated cantilevers. Thin films of two ferrites, NiZnFe 2O 4 spinel ferrite and Co 2 Z-type hexaferrite (Ba 3Co 2Fe 24O 41, BCFO) were prepared by RF sputtering. As a basis for these probes, we employ commercial micromachined silicon cantilevers. Additionally, films on Si (1 0 0) and Si (1 1 1)-oriented substrates with a thickness up to 100 nm were prepared for analysis purposes, enabling the optimization of the sputter process. For a high spatial resolution of MFM, however, thinner magnetic coatings are required. Therefore, the third type, c, was prepared by laser-ablation with a thickness of 30 nm, also directly onto the Si without additional buffer layer.

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.

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.

Mapping of Magnetic Domains by MFM in Ni<sub>2</sub>MnGa

Influence of structural transition in the evolution of the magnetic domains in the ferromagnetic shape memory alloy system Ni2+xMn1-xGa is reported here using Magnetic Force Microscopy (MFM) studies. Studies reported are with two samples with their martensite transition temperature TM less than and greater than the Curie temperature Tc. Present results show an evolution of MFM across the Tc with a clear twin domains and sub domain structures inside the twins. The higher spatial resolution of MFM (~50nm) as compared to optical microscope (400nm) is useful in probing the domain walls. Force derivative of the MFM signal that may be used as an order parameter seems to scale the onset of magnetic order in the system. One can clearly see the vanishing of the MFM patterns for T>Tc. Results are discussed in the light of models available for tip-sample interactions that track the local magnetization.

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.

Domain overlap in antiferromagnetically coupled [Co∕Pt]∕NiO∕[Co∕Pt] multilayers

Antiferromagnetically coupled magnetic thin films with perpendicular anisotropy exhibit domain overlap regions originating from magnetostatic stray fields localized in the vicinity of the domain walls. Using high resolution magnetic force microscopy, the authors investigate these overlap regions in [Co∕Pt]∕NiO∕[Co∕Pt] multilayers with various strengths of the interlayer exchange coupling. They develop a simple model that provides a quantitative explanation of the formation of these regions and the relationship between the domain overlap width and the coupling strength. Their results are important for application of magnetic layered structures with perpendicular anisotropy in advanced magnetoresistive devices.

GISAXS studies of self-assembling of colloidal Co nanoparticles

In this work we report on the formation of ordered monolayers (2-D) and arrays of rods (3-D) of magnetic Co nanoparticles in magnetic field perpendicular to the substrate surface. Samples were prepared by drying a droplet of colloidal solution of Co nanoparticles (10 nm diameter) on Si/Si 3N 4 substrates in magnetic field between 0.2 and 0.9 T. The samples were characterized by high resolution scanning electron microscopy (SEM), atomic and magnetic force microscopy (AFM/MFM) and grazing incidence small angle X-ray scattering (GISAXS). SEM studies of monolayers show well-ordered 2-D arrays with hexagonal symmetry of 200 nm × 500 nm in size forming a mosaic structure. Rods, about 500 nm in diameter, aligned with the field direction and forming a hexagonal pattern were obtained when higher concentration of colloid and low evaporation rate of the solvent were used. The ordering of nanoparticles in the monolayer analyzed by GISAXS is described by the local order with hexagonal symmetry. The model of close packing of hard spheres is used for ordering of particles inside the rods. Magnetic features corresponding to the 3-D arrays have been observed by MFM pointing out that all magnetic moments in the rod are oriented along the field direction.

Magnetic nanowire based high resolution magnetic force microscope probes

We report an efficient process for controlled fabrication of high-resolution magnetic force microscope probes using preformed magnetic nanowires. Nickel and cobalt nanowires produced by electrodeposition were directly assembled onto the tip of a commercial atomic force microscope cantilever with controlled orientation and length by dielectrophoresis. The properties of these nanowire-based probes are characterized.

Low-temperature high-resolution magnetic force microscopy using a quartz tuning fork

We have developed a low-temperature high resolution magnetic force microscope (MFM) using a quartz tuning fork that can operate in a magnetic field. A tuning fork with a spring constant of 1300N∕m mounted with a commercial MFM cantilever tip was used. We have obtained high-resolution images of the stray magnetic fields exerted from grains with a spatial resolution of 15 nm and force resolution of 2 pN at 4.2 K. Tuning fork-based magnetic force microscopes have the potential to be used at millikelvin temperatures due to their low power dissipation and high force sensitivity.

MFM studies of microstructure and magnetic properties of iron thin films prepared by sputtering

We have studied the microstructure and magnetic domain structure of thin Fe film by magnetic force microscopy (MFM). Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single domain. Two series of iron thin films have been grown on microcrystalline glass substrate by DC magnetron sputtering. They were prepared at different Ar pressures and annealing time. The results by MFM show characters of both surface morphology of the films and their local magnetic properties. It is suggested that the pin effect by single domain wall influences magnetic properties of the iron thin films.

Magnetic nanowire based high resolution magnetic force microscope probes

Magnetic nanowire based high resolution magnetic force microscope probes

Composite Nanowire-Based Probes for Magnetic Resonance Force Microscopy

We present a nanowire-based methodology for the fabrication of ultrahigh sensitivity and resolution probes for atomic resolution magnetic resonance force microscopy (MRFM). The fabrication technique combines electrochemical deposition of multifunctional metals into nanoporous polycarbonate membranes and chemically selective electroless deposition of optical nanoreflector onto the nanowire. The completed composite nanowire structure contains all the required elements for an ultrahigh sensitivity and resolution MRFM sensor with (a) a magnetic nanowire segment providing atomic resolution magnetic field imaging gradients as well as large force gradients for high sensitivity, (b) a noble metal enhanced nanowire segment providing efficient scattering cross-section from a sub-wavelength source for optical readout of nanowire vibration, and (c) a nonmagnetic/nonplasmonic nanowire segment providing the cantilever structure for mechanical detection of magnetic resonance.

Deconvolution processing for increasing the resolution of magnetic force microscopymeasurements

Magnetic force microscopy (MFM) is a widely used form of scanning probe microscopy(SPM) that is used for obtaining a magnetic image from a surface with nanoscaleresolution. Currently, the resolution of MFM is limited to approximately 20 nm due to thelong-range nature of magnetic interactions between the MFM probe’s tip and themagnetic medium. This paper describes an optimal signal processing solutionto the problem of achieving high resolution MFM with the goal of significantlyexceeding the level at which MFM metrology currently performs, thus providingmetrologists with a means of resolution increase that is believed will enable, forexample, the rapid development of high-density magnetic recording media(>100 Gbit in−2). This is achieved by using focused-ion beam trimming of a conventional tip to create aprobe tip of predictable magnetic characteristics and then, provided that the sample couldbe approximated as a thin-film, using knowledge of this tip’s sensitivity field for performinga deconvolution on the measured signal to better estimate the magnetic state of the surfaceunder study. Stated differently, as the MFM measured signal is modelled as the convolutionof the magnetized surface and the tip’s sensitivity field, we exploit knowledge of the tip’sproperties to increase the resolution of the MFM image. Details of the deconvolutionapproach as well as images resulting from this processing are the focus of this paper.

Noninvasive observation of magnetization reversal in micron-sized Co bars

We demonstrate how room temperature scanning Hall probe microscopy (SHPM) can be applied to noninvasively probe the (meta-)stable magnetic domain states of individual mesoscopic Co bars as a function of applied field. These data are correlated with magneto-optical measurements of the hysteresis loop as well as higher spatial resolution magnetic force microscopy imaging. In particular, we show how a combination of visualization techniques is essential for fully understanding the mechanism of magnetization reversal in such microstructures. Micromagnetic calculations of the magnetic states and their corresponding stray fields are correlated with the observations. These experiments prove the usefulness of a noninvasive and quantitative technique such as room temperature SHPM in an applied magnetic field to study magnetic domain stability and switching field distributions.

Magnetic Properties and Magnetic Domain Structure of Nd-Fe Based Metallic Glasses

The transition from hard to soft magnetic behavior with increasing quenching rate is shown for Nd60Al10Fe20Co10 rods and melt-spun ribbons with different thickness. Microstructures characterized by high resolution transmission electron microscopy (HRTEM) show that clusters with a size from 1 to 5 nm randomly distribute in the amorphous matrix in the samples prepared by different quenching rate. By compared the magnetic behaviors with those of fully crystalline Nd70Fe30 alloy, it is found that the clusters with a local ordering similar to the Al metastable Nd-Fe binary phase is responsible for the hard magnetic properties in samples subjected to relatively low quenching rate. Magnetic domain structure of Nd60Al10Fe20Co10 and Nd70Fe30 alloys were studied by high resolution magnetic force microscopy (MFM). The magnetic hardening mechanism of Nd60Al10Fe20Co10 bulk metallic glasses is discussed.

High resolution magnetic force microscopy using focused ion beam modified tips

Atomic force microscope tips coated by the thermal evaporation of a magnetic 30 nm thick Co film have been modified by focused ion beam milling with Ga+ ions to produce tips suitable for magnetic force microscopy. Such tips possess a planar magnetic element with high magnetic shape anisotropy, an extremely high aspect ratio of greater than 30:1, and an end radius of less than 25 nm. These tips have been used in a commercial atomic force microscope under ambient conditions to obtain 30 nm resolution magnetic images of an established CoNi/Pt multilayer reference sample.

Magnetic force microscopy using focused ion beam sharpened tip with deposited antiferro–ferromagnetic multiple layers

The resolution of magnetic force microscopy can be improved by sharpening the tip using a focused ion beam (FIB). However, when the tip is too sharp it is difficult to deposit continuous films on the tip, and the magnetization of a certain portion of the tip may become unstable due to the small thickness or particle size of the magnetic coating. This in turn may degrade the signal to noise ratio of the detection signal. In this work, a novel tip coated with antiferromagnet–ferromagnet exchanged coupled multiple layers was developed. It was found that the newly developed tip exhibits a much higher resolution than the conventional trimmed and nontrimmed tips.