Magnetic Force Microscopy Tip Research Articles

Customized MFM probes based on magnetic nanorods.

Focused Electron Beam Induced Deposition (FEBID) for magnetic tip fabrication is presented in this work as an alternative to conventional sputtering-based Magnetic Force Microscopy (MFM) tips. FEBID enables the growth of a high-aspect-ratio magnetic nanorod with customized geometry and composition to overcome the key technical limitations of MFM probes currently on the market. The biggest advantage of these tips, in comparison with CoCr coated pyramidal probes, lies in the capability of creating sharp ends, nearly 10 nm in diameter, which provides remarkable (topographic and magnetic) lateral resolution in samples with magnetic features close to the resolution limits of the MFM technique itself. The shape of the nanorods produces a very confined magnetic stray field, whose interaction with the sample is extremely localized and perpendicular to the surface, with negligible in-plane components. This effect can lead to a better analytical and numerical modelling of the MFM probes and to an increase in the sensitivity without perturbing the magnetic configuration of soft samples. Besides, the high-aspect ratio achievable in FEBID nanorod tips makes them magnetically harder than the commercial ones, reaching coercive fields higher than 900 Oe. According to the results shown, tips based on magnetic nanorods grown by FEBID can be eventually used for quantitative analysis in MFM measurements. Moreover, the customized growth of Co- or Fe-based tips onto levers with different mechanical properties allows MFM studies that demand different measuring conditions. To showcase the versatility of this type of probe, as a last step, MFM is performed in a liquid environment, which still remains a challenge for the MFM community largely due to the lack of appropriate probes on the market. This opens up new possibilities in the investigation of magnetic biological samples.

Artifacts in magnetic force microscopy under in-plane applied magnetic field: Magnetic bubble as a case study

The study of magnetic bubble configuration in Co/Ni multilayers circular dots under in-plane magnetic field enlightens two magnetic force microscopy (MFM) measurement artifacts. We demonstrate that any tilt of the magnetic field producing out-of-plane field component due to the inhomogeneity of the MFM set up strongly affects the shape and size of the magnetic bubble independently of the in-plane field component intensity. Furthermore, MFM signal variations for in-plane magnetic field larger than 0.1 Tesla can only be understood considering MFM tip magnetization rotation. These two artifacts can have strong impact on MFM images and need to be carefully checked for reliable results in imaging skyrmionic structures with MFM under-field.

Quantitative magnetic force microscopy using calibration on superconducting flux quanta

We present a new procedure that takes advantage of the magnetic flux quantization of superconducting vortices to calibrate the magnetic properties of tips for magnetic force microscopy (MFM). Indeed, a superconducting vortex, whose quantized flux is dependent upon Plank constant, speed of light and electron charge, behaves as a very well defined magnetic reference object. The proposed calibration procedure has been tested on new and worn tips and shows that the monopole point-like approximation of the probe is a reliable model. This procedure has been then applied to perform quantitative MFM experiments on a soft ferromagnetic thin film of permalloy, leading to the determination of the local out-of-plane component of the canted magnetization, together with its spatial variations across a few μm2 scan area.

Detection of nonmagnetic metal thin film using magnetic force microscopy

Magnetic force microscopy (MFM) allows detection of stray magnetic fields around magnetic materials and the two-dimensional visualization of these fields. This paper presents a theoretical analysis of the oscillations of an MFM tip above a thin film of nonmagnetic metal. The results show good agreement with experimental data obtained by varying the tip height. The phenomenon analyzed here can be applied as a “metal detector” at the nanometer scale and for contactless measurements of sheet resistivity. The detection sensitivity is obtained as a function of oscillation frequency, thus allowing the determination of the best frequency for the phase-shift measurement. The shift in resonance frequency due to the presence of a nonmagnetic metal is also discussed.

Magnetization characterization of MnAs nanoclusters at close range in bended MnAs/InAs heterojunction nanowires

The authors report on the magnetization and magnetic domains of ferromagnetic MnAs nanoclusters located at relatively close range in a bended MnAs/InAs heterojunction nanowire, which are occasionally observed in the nanowires with multiple MnAs nanoclusters, on the basis of the structural and magnetization characterization results. The magnetization of MnAs nanoclusters is oriented along the external magnetic field directions, in particular, in the case of MnAs nanoclusters formed in the almost straight (or slightly bending) heterojunction nanowires. However, the magnetization of a MnAs nanocluster at the markedly bending position of heterojunction nanowires is occasionally directed by the magnetic force microscope tip magnetization. The decrease in the coercivity of nanoclusters may affect the magnetization directions of nanoclusters at the bending position of heterojunction nanowires.

Dependence of the absolute value of the penetration depth in (Ba1−xKx)Fe2As2 on doping

We report magnetic force microscopy (MFM) measurements on the iron-based superconductor $\mathrm{Ba_{1-x}K_xFe_2As_2}$. By measuring locally the Meissner repulsion with the magnetic MFM tip, we determine the absolute value of the in-plane magnetic penetration depth ($\lambda_{ab}$) in underdoped, optimally-doped, and overdoped samples. The results suggest an abrupt increase of $\lambda_{ab}$ as doping is increased from $x_\mathrm{opt}$, which is potentially related to the presence of a quantum critical point. The response of superconducting vortices to magnetic forces exerted by the MFM tip for $x=0.19$ and $0.58$ is compatible with previously observed structural symmetries at those doping levels.

High magnetization Co-GdOx superparamagnetic granular films as magnetic coating materials for high-sensitivity alternating magnetic force microscopy tip

The Coy(GdOx)1−y, CoyAg1−y, (Fe70Co30)y(AlOx)1−y superparamagnetic (SP) granular films were fabricated by magnetron co-sputtering technique and compared for their magnetic properties. The effect of the different matrix (GdOx, Ag, AlOx) is investigated for refining and separating magnetic particles in non-magnetic matrix by calculating the magnetic volume fraction (y), and the results indicate that the GdOx matrix can more effectively refine and separate magnetic particles. The Co0.44(GdOx)0.56 SP film with the maximum Co volume fraction (y = 0.44) approaches the highest magnetization of 608 emu/cm3 at 20 kOe, and initial susceptibility (1.74 × 10−5 H/m) as compared with Co-Ag, Fe70Co30-AlOx films and some systems of other researchers at room temperature (RT). The magnetic particle size and size distribution in these SP films is calculated by fitting with Langevin function and the results of calculation are consistent with the high-resolution transmission electron microscopy (HRTEM) results. To further confirm the SP behavior, the blocking temperature of the Co0.44(GdOx)0.56 film with the maximum Co volume fraction is measured as ∼145 K, which is below RT. Finally, the Co0.33(GdOx)0.67 thin film having a linear magnetization curve is used for the fabrication of the SP tip and further used for the A-MFM measurement on the bulk Sr-Ferrite (SrF) magnet. The fabricated Co0.33(GdOx)0.67 SP tip along with A-MFM shows an excellent result as compared to the state of art FePt hard magnetic tip using conventional MFM, and it can directly detect the intensity and identify the polarity of the stray field originating from a bulk Sr-Ferrite (SrF) permanent magnet without topography crosstalk in ambient atmosphere.

Preparation and Characterization of Magnetic Force Microscopy Tips Coated with Nickel Films by e-Beam Evaporation

In this work, magnetic force microscopy (MFM) tips coated with a nickel thin-film were prepared and characterized for applications in the measurement of the magnetic write field. Nickel films with various thicknesses in a range of 20 – 80 nm were deposited on silicon substrates and silicon atomic force microscopy (AFM) tips by electron beam evaporation. Film surface morphologies and magnetic properties of the coated nickel films were investigated by using AFM and vibrating sample magnetometry (VSM). The rms roughness increased with the film thickness and was in a range between 0.1 and 0.3 nm. VSM results revealed that the mean coercive field of the nickel films was 20 Oe and there was an increase in the coercivity as the film thickness increased. In addition, the prepared MFM tips were evaluated for the tip response to the dc and ac magnetic field generated from perpendicular write heads. It was found that the MFM tip had the best response to the write field when coated with 60 nm thick nickel film. The coating thickness over 60 nm was inapplicable due to the cantilever bending caused by the film stress.

Direct writing of room temperature and zero field skyrmion lattices by a scanning local magnetic field

Magnetic skyrmions are topologically protected nanoscale spin textures exhibiting fascinating physical behaviors. Recent observations of room temperature skyrmions in sputtered multilayer films are an important step towards their use in ultra-low power devices. Such practical applications prefer skyrmions to be stable at zero magnetic fields and room temperature. Here, we report the creation of skyrmion lattices in Pt/Co/Ta multilayers by a scanning local field using magnetic force microscopy tips. We also show that those newly created skyrmion lattices are stable at both room temperature and zero fields. Lorentz transmission electron microscopy measurements reveal that the skyrmions in our films are of Néel-type. To gain a deeper understanding of the mechanism behind the creation of a skyrmion lattice by the scanning of local fields, we perform micromagnetic simulations and find the experimental results to be in agreement with our simulation data. This study opens another avenue for the creation of skyrmion lattices in thin films.

Individually grown cobalt nanowires as magnetic force microscopy probes.

AC electric fields were utilized in the growth of individual high-aspect ratio cobalt nanowires from simple salt solutions using the Directed Electrochemical Nanowire Assembly method. Nanowire diameters were tuned from the submicron scale to 40 nm by adjusting the AC voltage frequency and the growth solution concentration. The structural properties of the nanowires, including shape and crystallinity, were identified using electron microscopy. Hysteresis loops obtained along different directions of an individual nanowire using vibrating sample magnetometry showed that the magnetocrystalline anisotropy energy has the same order of magnitude as the shape anisotropy energy. Additionally, the saturation magnetization of an individual cobalt nanowire was estimated to be close to the bulk single crystal value. A small cobalt nanowire segment was grown from a conductive atomic force microscope cantilever tip that was utilized in magnetic force microscopy (MFM) imaging. The fabricated MFM tip provided moderate quality magnetic images of an iron-cobalt thin-film sample.

Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate

Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.

High resolution magnetic field energy imaging of the magnetic recording head by A-MFM with Co-GdOx super-paramagnetic tip

In this letter, the concept of a high-resolution magnetic field energy imaging technique is demonstrated by a high susceptibility superparamagnetic Co-GdOx magnetic force microscopy (MFM) tip for a perpendicular magnetic recording head with alternating magnetic force microscopy (A-MFM). The distribution of the magnetic energy gradient from the perpendicular recording head is imaged by the Co-GdOx superparamagnetic tip and compared with magnetic field imaging by the FePt-MgO hard magnetic tip. The Fourier analysis of the A-MFM amplitude images revealed enhancement in a spatial resolution of 13 nm by the Co-GdOx superparamagnetic tip as compared to 17 nm by the state-of-the-art FePt-MgO hard magnetic tip. The magnetic dipolar nature and short range force character of magnetic energy imaging by the Co-GdOx superparamagnetic tip showed high performance, confirmed by the tip transfer function analysis as compared to the monopole type FePt-MgO hard magnetic tip. The proposed technique opens an opportunity for the development of advanced high-resolution magnetic energy based imaging methods and development of the high-resolution MFM tips.

Switching Field and Resolution of MFM Tips Prepared by Coating Fe/Co50Pt50 Magnetic Films

Magnetic force microscope (MFM) tips are prepared by coating Fe(10 nm)/Co50Pt50(x nm)/Ru(5 nm) (at. %) films at 300 °C on Si-base tips of 4 nm radius. The thickness of CoPt layer, x, is varied in a range of 10 – 200 nm. The effects of coating thickness on spatial resolution and switching field of MFM tip are investigated. With increasing the thickness, the resolution deteriorates from 9.4 to 12.1 nm. The switching field increases from 0.675 to 2.025 kOe with increasing the coating thickness from 10 to 200 nm. A tip prepared by coating 100 nm thick Co50Pt50 film which has high resolution and high switching field property is successfully applied to the observation of magnetization structure of high-Ku magnetic thin film with L10-ordered structure.

Analyses of magnetization switching and magnetic domains in lateral MnAs nanowires in combination with structural characterization

We report on structural and magnetic characterization results of lateral MnAs nanowires formed by selective-area metal–organic vapor phase epitaxy. The lateral MnAs nanowires observed by backscattered and secondary electron microscopies have a high aspect ratio (i.e., 2.6 µm long and 360 nm wide) and well-defined crystal facets. The external magnetic field dependence in magnetic force microscopy observations of the lateral nanowires reveals that single and multiple magnetic domains were formed in the lateral nanowires. This study suggests that magnetic domains are tuned by the structural control of lateral nanowires. However, no magnetization switching is observed under a relatively high external magnetic field of 5000 G, which should be enough for the magnetization switching of the lateral nanowires, possibly owing to the magnetization switching of the magnetic force microscope tip itself used in this study during observations.

A novel method for the injection and manipulation of magnetic charge states in nanostructures.

Realising the promise of next-generation magnetic nanotechnologies is contingent on the development of novel methods for controlling magnetic states at the nanoscale. There is currently demand for simple and flexible techniques to access exotic magnetisation states without convoluted fabrication and application processes. 360° domain walls (metastable twists in magnetisation separating two domains with parallel magnetisation) are one such state, which is currently of great interest in data storage and magnonics. Here, we demonstrate a straightforward and powerful process whereby a moving magnetic charge, provided experimentally by a magnetic force microscope tip, can write and manipulate magnetic charge states in ferromagnetic nanowires. The method is applicable to a wide range of nanowire architectures with considerable benefits over existing techniques. We confirm the method’s efficacy via the injection and spatial manipulation of 360° domain walls in Py and Co nanowires. Experimental results are supported by micromagnetic simulations of the tip-nanowire interaction.

Diamagnetic vortex barrier stripes in underdopedBaFe2(As1−xPx)2

We report magnetic force microscopy (MFM) measurements on underdoped $BaFe_2(As_{1-x}P_x)_2$ ($x=0.26$) that show enhanced superconductivity along stripes parallel to twin boundaries. These stripes of enhanced diamagnetic response repel vortices when we cool in a finite magnetic field and act as barriers when we drag vortices with the magnetic MFM tip. The stripes disappear when we warm the sample towards the superconducting transition temperature. We show that stripes can move when we warm the sample and comment on the relationship between the stripes and similar stripes observed previously in $Ba(Fe_{1-x}Co_x)_2As_2$.

Spatial resolution and switching field of magnetic force microscope tips prepared by coating Fe/Co-Pt layers

Magnetic force microscope tips are prepared by coating Si tips of 4 nm radius with magnetic bi-layer film consisting of soft magnetic material of Fe(x nm) and hard magnetic material Co-Pt alloy (20–x nm), 0 ≤ x ≤ 20. The effects of layer thickness ratio and stacking sequence on the spatial resolution and the switching field are investigated. Higher resolutions are observed for tips prepared by coating Co-Pt alloy followed by Fe film deposition compared with those prepared by employing the opposite deposition sequence. The resolution improves from 11.0 to 8.2 nm whereas the switching field decreases from 1525 to 475 Oe with increasing the x value from 0 to 20 nm. The present study has shown a possibility of tuning MFM tip performance by employing a soft/hard dual layer structure.

Effect of Oxidation Protection Layer on the Performance of Magnetic Force Microscope Tip

Magnetic force microscope (MFM) tips are prepared by coating silicon tips of 4 nm radius with 20-nm-thick metallic magnetic films with and without 2-nm-thick oxidation protection layers. Iron (Fe) is used as the magnetic material, whereas carbon (C), silicon nitride (Si-N), or silicon carbide (Si-C) is employed as the protection layer. The MFM tips are exposed in an environment of high temperature of 70 °C and high relative humidity of nearly 100%. The effect of protection layer on the spatial resolution is investigated as a function of period of exposure to the environment. The resolution of MFM tip without protection layer deteriorates from 7.2 to 21.2 nm in 10 days. The deterioration is attributed to an increase in the tip radius and a loss of the detection sensitivity caused by oxidation of coated Fe material. In contrast, the resolutions of tips with C, Si-N, and Si-C layers are kept almost constant at 12.1 ± 0.5, 12.1 ± 0.5, and 14.8 ± 2.1 nm for a time span of 10 days, respectively. The coating of a very thin protection layer has been shown effective in keeping the MFM tip performance for a long period of time by preventing the oxidation of coated metallic magnetic material.

Exchange Springs in <inline-formula> <tex-math notation="LaTeX">${L} 1_{0}$ </tex-math></inline-formula>-FePt(110)/<inline-formula> <tex-math notation="LaTeX">${A}1$ </tex-math></inline-formula>-FePt Bilayer Films

To investigate the effect of hard/soft exchange spring on magnetic properties, ${L} 1_{0}$ -FePt/ ${A}1$ -FePt bilayer films are fabricated on MgO(110) substrates by using magnetron sputtering. The hard layer annealed at a moderate temperature of 500 °C showed an unseparated granular morphology with an almost full coverage, in spite of an incomplete $A1\to {L} 1_{0}$ transition. The c -axis (easy axis) of ${L} 1_{0}$ -FePt preferred to align in-plane rather than off-plane. For a system of ${L} 1_{0}$ -FePt (10 nm)/ ${A}1$ -FePt (20 nm), the magnetization jumped abruptly at 1.4 kOe (=nucleation field of the soft layer) and −7 kOe (=switching field of the hard layer), respectively. However, a perfectly rectangle-like magnetization curve with a coercivity as high as $\sim$ 10 kOe was observed for a system of ${L} 1_{0}$ -FePt (30 nm)/ ${A}1$ -FePt (20 nm). This indicates that the exchange length might be able to be affected by grain growth in an incompletely $A1\to {L} 1_{0}$ transited FePt film. According to the experimental results, exchange length of the annealed FePt (30 nm) film should exceed 10 nm, which is much larger than that (5 nm) of an ideal ${L} 1_{0}$ -FePt. These bilayer films are suitable for fabricating magnetic force microscopy tips to measure different magnetic objects.

Improvement of Magnetic Force Microscope Performance by Tuning the Coating Material of Sensor Tip

Effects of magnetic material, coating thickness, and tip radius on the magnetic force microscope (MFM) spatial resolution and the switching field have been investigated. MFM tips are prepared by coating soft and hard magnetic materials on non-magnetic Si-base tips. MFM spatial resolutions better than 8 nm are obtained by optimizing the coating layer thickness at around 20 nm on the Si tips with top radius of 3 – 5 nm. The switching field can be increased greater than 1 kOe by coating a high-Ku magnetic material and by increasing the coating thickness. The tips are successfully applied to the observations of magnetic thin films which include magnetization structures less than 20 nm in length.