Magnetic Force Microscopy Tip Research Articles

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.

Nanosized magnetization measurement of an isolated Co–Fe circular dot using a MFM tip

We used our newly proposed measurement method, which employs a magnetic force microscopy (MFM) tip as a detector, to investigate the magnetic state of an isolated Co–Fe dot. As the magnetic field is changed from positive to negative or vice versa, the phases (stray fields) change for all points within the dot that are perpendicular to the magnetic filed. In particular, a sharp jump is observed at the edge of the dot. These results are attributed to the movement of the vortex core in the dot and the annihilation of the vortex core in the dot edge.

Simulation of high-resolution MFM tip using exchange-spring magnet

The transfer function of magnetic force microscope (MFM) tips using an exchange-spring trilayer composed of a centered soft magnetic layer and two hard magnetic layers was calculated and the resolution was estimated by considering the thermodynamic noise limit of an MFM cantilever. It was found that reducing the thickness of the centered soft magnetic layer and the magnetization of hard magnetic layer are important to obtain high resolution. Tips using an exchange-spring trilayer with a very thin FeCo layer and isotropic hard magnetic layers, such as CoPt and FePt, are found to be suitable for obtaining a resolution less than 10 nm at room temperature.

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.

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.

Field-dependent behavior of a magnetic force microscopy tip probed by means of high coercive nanomagnets

Single domain uniformly magnetized CoPt dots are used to probe magnetic force microscopy (MFM) tip characteristics. Magnetic contrasts obtained in MFM experiments are exploited to get the tip hysteresis loop in the influence of the dot. Coercivity, remnant magnetization, as well as details about the magnetization reversal of the tips are obtained. Well defined dot geometries allow a quantitative determination of the stray field of the dots acting on the tip. The tip’s hysteresis loop refers to an effective tip height, involved in the imaging, which is much larger than the extension of a magnetic domain of the CoCr tip coating. When MFM tips and sample dots have widely different characteristics, it is possible to get separate magnetic information, such as hysteresis loops, on both tip and sample, depending on the magnitude of the out-of-plane external field.

Investigation of tip-induced magnetic ripples on La0.7Sr0.3MnO3 surface

As demagnetized La1−xSrxMnO3 (LSMO) thin films are magnetized by a local magnetic field from a scanning magnetic tip without an external magnetic field, a magnetic ripple pattern appears on the LSMO surface. The magnetic ripple pattern, which consists of magnetic ripples with varying magnetic periods, is different from the conventional pattern magnetized by an external field. The magnetic ripples with longer magnetic periods have larger magnitude of magnetic moment. The magnitude linearly varies as a function of magnetic period. However, the magnitude of the remanent magnetization induced by the magnetic force microscopy tip is close to that induced by the external field.

Spacer dependence effect in synthetic magnetic force microscopy tips

A series of magnetic force microscopy tips with the synthetic structure consisting of two CoCrPt layers separated by a nonmagnetic Ru layer, which have the same magnetic layers but different thickness of the Ru layer, have been fabricated by sputtering. By analyzing the magnetic force microscopy images taken from the magnetic patterns recorded on longitudinal media, the performance of the tips was found to vary with the Ru thickness in an alternate fashion between enhanced and weakened responses. This phenomenon can be explained by the Ru thickness dependence of the exchange coupling between the two ferromagnetic layers and the corresponding frequency response of the trilayer tip. Synthetic tips with superior performances have been obtained after the Ru thickness was optimized.

Focused ion beam deposition ofCo71Cr17Pt12 and Ni80Fe20 on tips for magnetic force microscopy

We demonstrate that a focused ion beam can deposit magnetic coatings on cantilevers used foratomic force microscopy, thereby producing a sensor for magnetic force microscopy. This techniqueis highly versatile, allowing the convenient deposition of complex or expensive materials, such asCo71Cr17Pt12. A second material chosen for this demonstration was the permalloy(Ni80Fe20). We show magnetic images acquired with these cantilevers to illustrate their excellentproperties and the differences between coatings. In principle, multilayer coatings could beeasily made with this technique.

Investigation of Spatial Resolution in Current-Induced Magnetic Field Detection by Magnetic Force Microscopy

The magnetic field distribution around sub-µm-wide current paths was investigated by magnetic force microscopy (MFM) as a candidate for the current mapping in fine structures. In particular, an undesirable electrostatic force working between an MFM tip and the current path was dynamically eliminated utilizing an extra ac bias to observe the magnetic field correctly. We observed magnetic force signals around current paths consisting of branching or closely aligned metal wires, and the results were compared with results of the numerical simulation of the magnetic field. We found that spatial resolution of magnetic force detection by our method was better than 0.2 µm. The calculation results also indicate that the oscillatory motion of the MFM tip in tapping operation influences spatial resolution.

Surface magnetic ripples induced by a local magnetic field from a scanning magnetic tip

When demagnetized La /sub 1-x/Sr/sub x/MnO/sub 3/ (LSMO) thin films are magnetized by a local magnetic field from a scanning magnetic tip without an external magnetic field, a magnetic ripple pattern appears on the LSMO surface. The magnetic ripple pattern, consisting of magnetic ripples with varying magnetic periods, is different from the conventional pattern magnetized by an external field. The magnetic ripples with longer magnetic periods have a larger magnitude of magnetic moment. However, the magnitude of the remanent magnetization induced by the magnetic force microscopy (MFM) tip is close to that induced by the external field. Since the MFM images of the magnetic ripples with varying periods can be converted to inverse images and spectra by a fast Fourier transformation (FFT), the inverse FFT images and spectra are also used to probe the magnitude of magnetic moment of the periodic magnetic ripples.

Fabrication and magnetic force microscopy (MFM) observation of nano scale ferromagnetic nanodot arrays

A novel electron beam lithography technique was utilized to fabricate nano scale magnetic dots array with high periodicity. C60 film (fullerine) was used as an electron beam sensitive resist for electron beam patterning carried out under a high accelerating voltage of 200 kV in a transmission electron microscope with a scanning mode. 50 nm sized permalloy (Py) nanodot arrays were successfully fabricated using the method followed by ion-milling. An array of Co nanodots having a size of 100×70 nm was alternatively prepared by liftoff instead of ion milling so as to prevent surface damage by ion milling. Lift-off does not yield any surface roughness, which is important for device fabrication. Magnetic force microscopy (MFM) observations were carried out on the fabricated Co nanodot arrays. A single domain state could be realized in patterned Co nanodots. MFM tip induced magnetization effects were also clearly demonstrated in the array of Co nanodots.

Manipulation and sorting of magnetic particles by a magnetic force microscope on a microfluidic magnetic trap platform

We have integrated a microfluidic magnetic trap platform with an external magnetic force microscope (MFM) cantilever. The MFM cantilever tip serves as a magnetorobotic arm that provides a translatable local magnetic field gradient to capture and move magnetic particles with nanometer precision. The MFM electronics have been programmed to sort an initially random distribution of particles by moving them within an array of magnetic trapping elements. We measured the maximum velocity at which the particles can be translated to be 2.2mm∕s±0.1mm∕s, which can potentially permit a sorting rate of approximately 5500particles∕min. We determined a magnetic force of 35.3±2.0pN acting on a 1μm diameter particle by measuring the hydrodynamic drag force necessary to free the particle. Release of the particles from the MFM tip is made possible by a nitride membrane that separates the arm and magnetic trap elements from the particle solution. This platform has potential applications for magnetic-based sorting, manipulation, and probing of biological molecules in a constant-displacement or a constant-force mode.

Metal-coated carbon nanotube tips for magnetic force microscopy

We fabricated cantilevers for magnetic force microscopy with carbon nanotube tips coated with magnetic material. Images of a custom hard drive demonstrated 20 nm lateral resolution, with prospects for further improvements.

Scanning force microspy study of biogenic nanoparticles for medical applications

We present a combined atomic force microscopy (AFM) and magnetic force microscopy (MFM) study of bacterial magnetosomes, consisting of membrane-covered magnetite particles. AFM imaging unambiguously demonstrates the existence of single isolated magnetosomes with a size of about 40nm, in addition to various clusters of magnetosomes. With MFM we have particularly addressed the magnetic properties of single magnetosomes. These nanoparticles behave like single mono-domain nanomagnets, as shown by the comparison of the MFM data to simple simulations. These simulations also provide information on the size of the magnetic kernel of the magnetosomes. Moreover, the invasiveness of the MFM tip is addressed.

Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film

Heat-assisted magnetic probe recording on a CoNi/Pt multilayered film is demonstrated by delivering a current through a magnetic force microscopy tip into the recording medium, in combination with an external magnetic field. Without local heating by the probe, no bits could be written because the external field exceeds a level that demagnetizes the medium in its entirety due to local variations in the coercivity of the medium. In contrast, magnetic bits were successfully written by the heat-assisted magnetic probe recording into a saturated medium even if there was no external field, because of the demagnetization field from the surrounding of the heated area. A magnetic bit as small as 80nm in diameter was obtained by this method.

Simulation of high-resolution MFM tips for high-density magnetic recording media with low bit aspect ratio

To design the high-resolution magnetic force microscopy (MFM) tips suitable for high-density magnetic recording media with low bit aspect ratio (BAR), the transfer functions of tips with various sharpened tip-ends were calculated for a checkered perpendicular magnetization pattern and the resolution of these tips was estimated by considering the resolution limit due to thermal noise at room temperature. The cylindrical tip with a spheroidal tip-end having a large ellipticity is found to be a suitable candidate for high-resolution MFM imaging of magnetic recording media with low BAR. For the tips with ellipticities larger than tan45°, there are no zero-signal frequencies. The sensitivity shows a maximum around an ellipticity of tan70°. The spheroidal tip shows a much smaller tip thickness dependence compared to the bar shape tip with a flat tip-end or an ellipsoidal tip-end, because only the tip-end mainly contributes to signals in case of the spheroidal tip.

Theory of vortex force microscopy in superconducting layers

The interaction between a vortex within a finite-thickness type-II superconductor and a magnetic force microscopy tip is studied. By analyzing the expression of the arising lateral force, we show that the superconducting penetration depth may be recovered from experiment, using the so-called Laplace transform inversion method. This entails a vertical displacement experiment. The consideration of lateral scanning modes has allowed us to extend the theory to the more stable Hankel transform inversion method, which eventually becomes a Fourier analysis application. For the case of vortices in two-layered superconductors, we show that magnetic particle manipulation is possible by tuning the configuration of the layers.

Focused Ion Beam Milled CoPt Magnetic Force Microscopy Tips for High Resolution Domain Images

High-coercivity CoPt magnetic force microscope tips have been modified by focused ion beam milling to improve the resolution of magnetic domain images. The magnetic materials around the apex have been removed, leaving a 30-nm diameter magnetic particle at the tip end. Due to the smaller amount of magnetic material, the stray field from this new tip is significantly reduced, and the spatial resolution of the magnetic domain images is improved. The tip is used to obtain high-resolution domain images of a CoCrPt-SiO/sub 2//Ru perpendicular recording medium with linear recording densities from 800 to 1000 kfci. Magnetic patterns of 900 kfci, corresponding to a bit size of 28 nm, are well resolved. From the analysis of the power spectrum of the track profiles for these images, a spatial resolution as good as 11 nm under ambient conditions with a commercial magnetic force microscope is achieved.

Magnetic force microscopy with work function compensation

Electrostatic interaction due to the work function difference between the tip of a magnetic force microscope (MFM) tip and the sample surface competes with the magnetic interaction at a lower tip–sample distance. This results in the superposition of sample topographic and magnetic images. As a consequence, the interpretation of MFM images becomes difficult, and the spatial resolution is limited. In this work, it is demonstrated that by applying an external voltage to the tip, the electrostatic tip–sample interaction can be compensated and the influence of topography on the magnetic images can be significantly reduced. The minimum tip–sample distance without topography influence is dramatically reduced and the detected signal is increased by 50%. The impact on the spatial resolution of the MFM is discussed.