Magnetic Force Microscopy Tip Research Articles

An Inverse Problem for the ac Magnetic Force Microscopy of Superconductors

A basic inversion problem for the magnetic force microscopy (MFM) of a semiinfinite superconductor in the Meissner state is formulated, ac Detection is assumed, with the MFM tip oscillating at angular frequency Ω. The coupling of all electrodynamic fields is treated, including a normal-current density in the superconductor. Under certain assumptions on the tip and superconductor geometry, a unique penetration depth λ(z) can be recovered from onedimensional force gradient measurements. This development opens new possibilities for the nondestructive evaluation of superconducting crystals and films.

A basic inverse problem in the magnetic force microscopy of linear magnetic materials

A fundamental inversion problem for the magnetic force microscopy (MFM) of a semi-infinite linear magnetic material is formulated. Magnetostatic conditions are assumed so that a magnetic scalar potential can be employed. Using a point-dipole MFM tip, a unique layer-dependent magnetic permeability can be recovered from one-dimensional force measurements. The inversion procedure uses a wavenumber-dependent kernel function K(k). These results describe the use of MFM data to extract a material property as a function of depth.

Writing Bits of Longitudinal Quantized Magnetic Disk Using Magnetic Force Microscope Tip

Longitudinal quantized magnetic disks (QMDs) consisting of discrete single domain Ni bars on a silicon substrate with a density of nearly 2 Gbis/in.2 were fabricated. A magnetic force microscope (MFM) tip with a large magnetic moment has been used to successfully write individual bits of the QMD by simply positioning the tip near one end of a Ni bar. It was found that the MFM tip can write each individual bit perfectly without writing the neighboring bits. This is attributed to the fact that the magnetization of each QMD bit has only two stable quantized values and therefore the writing process is quantized.

Magnetic nanostructures studied by scanning probe microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy (STM/STS) were applied to study the nanostructural and nanoelectronic properties of ultrathin magnetic films, from individual adatoms and clusters (zero-dimensional systems) to three-dimensional bulklike islands. Complementary investigations of the nanomagnetic structure of ultrathin films were performed by magnetic force microscopy (MFM). Quantum magnetic structures were prepared by electron-beam lithography and nanosphere lithography and analyzed by STM/STS as well as by MFM. Magnetization switching of single-domain Co dots induced by the MFM tip is demonstrated, and its potential application for quantum magnetic storage is shown. Finally, tunneling spectroscopy performed with magnetic probe tips yields new information about the spin-resolved nanoelectronic properties of magnetic nanostructures.

Measurement of the effects of the localized field of a magnetic force microscope tip on a 180° domain wall

Opposite polarity magnetic force microscope (MFM) profiles of domain walls (DWs) in magnetite were measured with a commercial MFM tip magnetized in opposite directions perpendicular to the sample surface. The influence of the tip field on a DW resulted in an overall more attractive interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip. The dependence of the measured alteration on tip-sample separation was fit with a power law at different positions across the DW. The rate of decay of the alteration with tip-sample separation, quantified by the exponent of the power law fit, varied across the DW and was much slower than expected from a simple model.

Evolution of magnetic microstructure in high-coercivity permanent magnets imaged with magnetic force microscopy

Magnetic force microscopy (MFM) has been shown to give high-resolution imaging of magnetic domain structures in a variety of high-coercivity permanent magnets [Folks et al., J. Magn. Magn. Mater. (in press)]. We show that this technique can be extended by the application of external fields during imaging, thus allowing direct observation of submicron microstructural evolution as a function of field. Electromagnets mounted on the MFM supplied fields up to 7 kOe laterally and 3 kOe vertically. In sintered materials, submicron processes such as depinning of domain walls at grain boundaries, domain fragmentation, and hysteresis were observed. MFM tips having very low coercivity highlighted domain walls, whereas higher-coercivity tips suffered unpredictable rotation of their magnetic moment due to both the sample and applied fields, leading to images which are difficult to interpret. For imaging of the finer-grained melt-quenched magnets, however, relatively high-coercivity tips were superior. These results show promise for the direct observation of the submicron-scale processes that dictate bulk magnetic properties, and the quantification of their field dependence.

Study of magnetic properties of magnetic force microscopy probes using micronscale current rings

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip’s effective magnetic charge, q, and effective magnetic moment along the tip long axis, mz, can be determined by the current flowing in the ring. The magnetic moments in the directions transverse to the tip’s long axis were estimated by a straight current wire. It was found that for a silicon tip coated with 65 nm thick cobalt on the side, q=2.8×10−6 emu/cm, mz=3.8×10−9 emu, and mx=my=10−13 emu, which are negligible compared with mz. Furthermore, the tip’s sensitivity to the second derivative of the magnetic field was found to be about 0.1 Oe/nm2.

Three-dimensional simulation of the disturbance of magnetic domain walls by magnetic force microscope tips

The micromagnetics of the interaction of a strong 3D magnetic force microscope tip with a 180° asymmetric Bloch domain wall in an 80 nm {100} iron film has been simulated numerically using cubic elements on a 3D lattice. The distortion of the wall is found to be quite well localized on the scale of the tip dimensions, 60×60 nm2. The force of attraction between tip and film and the changes in self-energy of the wall and in the energy of the tip-sample system have been calculated for various positions of the tip. The strong attraction between tip and sample is slightly reduced (∼5%) when the tip is directly over the central vortex in the wall.

Micromagnetic model for magnetic force microscopy tips

A micromagnetic model for simulation of magnetic force microscope (MFM) tips is described. It incorporates a mesh of general polyhedra to allow magnetic films of arbitrary overall shape to be considered. A hierarchical method is employed for rapid evaluation of demagnetizing fields. Results for permalloy coatings of pyramidal tips are presented which show that the precise configuration of the magnetic film can produce differing magnetic behavior in comparison with experiment.

Quantification of magnetic force microscopy using a micronscale current ring

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip’s effective magnetic charge, q, and effective magnetic moment along the tip’s long axis, mz, can be determined from the MFM signal of the ring at a different scan height and a different electric current in the ring. The magnetic moments in the directions transverse to the tip’s long axis were estimated by a straight current wire. It was found that for a Si tip coated with 65 nm cobalt on one side, q is 2.8×10−6 emu/cm, mz is 3.8×10−9 emu, and mx and my are in the order of 10−13 emu, which are negligible compared with mz. Furthermore, the MFMs sensitivity to the second derivative of the magnetic field was determined from the minimum ring current for a measurable MFM signal to be 0.1 Oe/nm2.

Magnetic properties of electrodeposited nanowires

Electrodeposited multilayered nanowires grown within a polycarbonate membrane constitute a new medium in which giant magnetoresistance (GMR) perpendicular to the plane of the multilayers can be measured. These structures can exhibit a perpendicular GMR of at least 22% at ambient temperature. We performed detailed studies both of reversible magnetization and of irreversible remanent magnetization curves for CoNiCu/Cu/CoNiCu multilayered and CoNiCu pulse-deposited nanowire systems with Co:Ni ratios of 6:4 and 7:3 respectively in the range 10 - 290 K, allowing the magnetic phases of these structures to be identified. Shape anisotropy in the pulse-deposited nanowire and inter-layer coupling in the multilayered nanowire are shown to make important contributions to the magnetic properties. Dipolar-like interactions are found to predominate in both nanowire systems. Magnetic force microscope (MFM) images of individual multilayered nanowires exhibit a contrast consistent with there being a soft magnetization parallel to the layers. Switching of the magnetic layers in the multilayered structure into the direction of the MFM tip's stray field is observed.

High resolution imaging of thin-film recording heads by superparamagnetic magnetic force microscopy tips

We have used superparamagnetic magnetic force microscopy (MFM) tips to obtain high spatial resolution MFM images of recording heads. Profiles of the magnetic field gradient above a thin-film recording head under 3 mA bias current to the head and various tip-head distance conditions are presented. At a low tip-head distance, the gap width, gap location, and gap-field structure can be well resolved in these MFM images. Superparamagnetic tips show promise for the magnetic imaging of recording heads with gap widths below 200 nm.

Resonant torque magnetometry: a new in-situ technique for determining the magnetic properties of thin film MFM tips

A new high sensitivity Resonant Torque Magnetometry (RTM) technique has been developed enabling the determination of the magnetic properties of Magnetic Force Microscope (MFM) tips in-situ. A sensitivity of better than 10/sup -12/ emu (10/sup -15/ Am/sup 2/) has been achieved at ambient temperature and pressure. We have studied the micromagnetic properties of a variety of tip coatings including hard CoCr and CoPt films and soft permalloy. In-situ determination of the magnetic properties of MFM tips is vital if quantitative interpretation of MFM images is to be achieved especially when images are made in the presence of bias fields. The m(/spl mu//sub 0/H) loops obtained for CoPt tips have a shape similar to a standard magnetisation loop when the field is applied parallel to the tip axis. With the field applied perpendicular to the tip both CoPt and CoCr demonstrate behaviour consistent with partial demagnetisation of the axial moment, to which the RTM is sensitive.

Writing and reading 7.5 Gbits/in/sup 2/ longitudinal quantized magnetic disk using magnetic force microscope tips

Quantized magnetic disks (QMDs) consisting of longitudinal single-domain Ni bars with densities from 3 Gbits/in/sup 2/ to 10 Gbits/in/sup 2/ were fabricated using nanoimprint lithography and a lift-off process. Two different types of MFM tips were used in the writing and reading process, one for erasing and writing, another for reading; both were magnetically 'hard'. The QMDs were first erased by aligning all bars with the write-tip scanning the entire sample, then written using lithography software in NanoScope III. Error-free results were obtained up to 7.5 Gbits/in/sup 2/ despite the poorly-defined magnetic field from the write-tip and despite not having a feedback control for the write-tip position.

Novel 'writing' using magnetic force microscopy in ultrahigh vacuum

Circular magnetic domains have been nucleated on ultrathin Co films by controlled surface touching using magnetic force microscopy tip in ultrahigh vacuum. The process of domain nucleation is due to strain induced perpendicular anisotropy on the surface of Co film. The smallest nucleated domain is 64 nm in diameter. The dependence of the size of the "written" domain on the time period of tip-sample contact has been measured.

Optimizing the NIST magnetic imaging reference sample

We have further developed the NIST magnetic imaging reference sample to include a magnetic pattern which can indicate the magnetic polarity of a magnetic force microscope tip. Several samples cut from the same disk were measured with a single tip. We have also measured a single transition with several tips. Both measurements have shown the variability in images taken with different tips and different instrument configuration which underscores the need for a well calibrated sample.

Localized micromagnetic perturbation of domain walls in magnetite using a magnetic force microscope

Magnetic force microscope (MFM) profiles of domain walls (DWs) in magnetite were measured using commercially available MFM tips. Opposite polarity profiles of a single DW segment were obtained by magnetizing the MFM tip in opposite directions perpendicular to the sample surface. During a measurement, the field of the tip locally magnetized the DW, resulting in a more attractive tip-sample interaction. The difference between opposite polarity DW profiles provided a qualitative measurement of the reversible changes in DW structure due to the localized field of the MFM tip.

Structure of cross-tie wall in thin Co films resolved by magnetic force microscopy

We have studied the magnetic domain structure of a thin polycrystalline Co film by magnetic force microscopy (MFM). Domain walls of the cross-tie type have been observed for a Co film of 50 nm thickness. Due to the high lateral resolution of MFM we have been able to study the magnetic structure of a single cross tie. We have determined locations of Bloch lines within a domain wall comparing the experimental data with a theoretical model of a cross-tie wall. In order to explain our experimental results we have proposed a model for the interaction between a MFM tip and a cross-tie wall.

Magnetic force microscopy of domain wall fine structures in iron films

This work investigated the micromagnetic structure of single crystal iron films by means of magnetic force microscopy (MFM) and Kerr microscopy. The investigated samples were epitaxially grown Fe films on a GaAs substrate. The film thickness varied between 30 and 500 nm. With the Kerr microscope it was possible to localize efficiently the domain walls in iron films. These walls were subsequently imaged by MFM with a high resolution. The MFM was used to observe the fine structure of a Bloch wall. Experimental results were compared with the calculated MFM response that was based on a two-dimensional, micromagnetic model describing the magnetic structure of an asymmetric Bloch wall. Both theory and experiment demonstrated the influence of the stray field of the tip on the wall structure. We could observe symmetric Néel walls and cross tie walls in a 30 nm iron film after reducing the sensitivity of the MFM tip. In order to achieve this state the tip was demagnetized by an external field.

Domain structure of iron single crystals grown on Si(111) investigated by magnetic force microscopy (abstract)

Single-crystal iron films grown epitaxially on (111) silicon were studied using a magnetic force microscope (MFM). The crystalline anisotropy easy axes (〈100〉), directed 35° out of the film plane, result in a significant magnetic charge density at the film surface and a sixfold symmetry in the energy minima of the system. To reduce the magnetostatic energy of this configuration, an alternating, stripe domain pattern formed in samples of thickness ranging from 1240 to 315 nm. Because of the wavelike form of the MFM response across the stripes, two-dimensional Fourier transforms of the MFM images were used to give measures at the domain period, i.e., the wavelength of the stripes, as well as the complexity of the patterns. The stripe domain period was found to be approximately equal to the film thickness which is consistent with previous theoretical predictions. The domain patterns of the films in various remanent states and progressive stages of the magnetization process were investigated using the MFM and an in situ, variable magnetic field. A MFM tip that is magnetically soft compared to the iron is most suitable for this. Measurements of the domain period and pattern complexity as a funciton of applied field were correlated with bulk hysteresis measurements.