MFM Tip Research Articles

MFM Tip With a Ferromagnetic Disk-Shaped Apex for Large Domain Scanning

MFM Tip With a Ferromagnetic Disk-Shaped Apex for Large Domain Scanning

Improved durable vortex core MFM tip

• Vortex-Core probe offers long life time and stable magnetic moment. • Stability of vortex core in the disk increased for smaller diameter and larger disk thickness. • The stability of vortex core is improved by VC pinning at the disk center. • Asymmetry pinning center better pins VC than symmetrical one. • Probe with pinned VC gives 15 nm spatial resolution. Vortex-Core magnetic force microscopy (VC MFM) promises magnetic tip durability and long-term calibration of its magnetic moment that is crucial for quantitative scanning. It uses the vortex core of a ferromagnetic disk located at the tip apex as a magnetic probe. However, our experience showed that MFM images might be distorted when scanning magnetic objects larger than 100 nm due to a VC shift generated by the stray field of the scanned sample. We addressed the problem by changing the tip geometry and introducing artificial VC pinning at the disk center. These steps are essential for improving the VC tip for quantitative scanning. The scanning experiments with the optimized VC tip showed improved characteristics of the magnetic images – spatial resolution of the tip was on the level of commercial MFM tip with certain details depicted even better.

Room-temperature zero field and high-density skyrmions in Pd/Co/Pd multilayer films

Magnetic skyrmions are topological chiral spin textures, which have potential application in storage-memory and other spintronic devices. Zero-field skyrmion and high-density skyrmion are the two most important aspects for the future skyrmion-based device applications. Here, we experimentally study the room temperature zero-field skyrmion and the evolution of high density skyrmion in [Pd/Co/Pd]N multilayer films from structure symmetry to asymmetry. The results show that the density of zero-field skyrmion can be adjusted by the number of cycles at the constant thickness of Co 0.4 nm. The maximum density ηSk, determined by Lorentz TEM observation, reaches 60 μm−2 in [Pd(3)/Co(1)/Pd(2)]15 multilayer film. We also found that the maximum density ηSk mainly occur in the reversible region of magnetization reversal by the first-order reversal curve (FORC) analysis. Interestingly, the polarity of zero-field skyrmion can be tuned by different MFM tips. The simulated results well confirm the experimental result. Our findings provide a pathway for designing the zero-field and high density skyrmion in magnetic multilayer films, which may apply for the development of skyrmion-based spintronic devices.

Magnetic-field imaging using vortex-core MFM tip

We have developed a vortex-core magnetic force microscope (VC MFM) for magnetic field imaging at the nanoscale for many research fields—physics, biology, materials science, and metrology. The method solves principally quantitative scanning by increasing magnetic tip durability and introducing its calibration. We show that nature itself gives us a sharp, durable, and calibrated magnetic probe. It is represented by a narrow magnetic vortex core located in the center of a ferromagnetic disk placed at the apex of a scanning tip. Such a tip offers potentially high spatial resolution—the vortex core is magnetically sharp (the vortex diameter is < 20 nm for Permalloy), but at the same time, the disk is geometrically blunt and therefore durable. The magnetic moment of the vortex core is independent of the disk diameter and can be tuned smoothly by the disk thickness. We describe here the basic properties of the VC tip, its technology, and sensitivity to the magnetic field and show its durability. The first results obtained on hard disk drive are promising—from the analysis of data tracks, the spatial resolution of the VC tip is only a bit worse than the one of the standard MFM tips. We believe that the VC tip could be a sensor of choice for magnetic field imaging for scientific areas mentioned above.

Nanoscale probing of asymmetric magnetization reversal in perpendicularly exchange biased Pt/Co/Pt/IrMn multilayers

Abstract Asymmetric magnetization reversal in perpendicularly exchange biased Pt/Co/Pt/IrMn multilayers was studied in nanometer scale by non-contact magnetic force microscopy with variable highly localized bipolar magnetic fields of the MFM tip. The hysteresis process of domain nucleation and pinned domain wall motion has been triggered and mapped simultaneously through MFM. Unstable magnetization reversal of submicron domains has been directly observed as well as exchange bias induced asymmetry in the depinning fields for domain wall motion. The current results demonstrated a possible way to locally mapping and manipulating novel magnetic nano-structures such as vortices and Skyrmions.

Confirmation of spatial coexistence of magneto-electric coupling in Bi0.7Dy0.3FeO3 thin films integrated with Si/ZnO film for MEMS and memory applications

Spatial presence of ferroelectric and magnetic domain structure of the multiferroic dysprosium (Dy)-modified BiFeO3 (Bi0.7Dy0.3FeO3 or BDFO) deposited on ZnO at the macroscopic level is investigated in this paper. BDFO thin film is deposited on Si/ZnO using pulsed laser deposition (PLD) technique. Magnetic properties are observed by saturated magnetic hysteresis at room temperature. Ferroelectric hysteresis loop (P–E) is used to compare the response of magnetic field on ferroelectric properties at room temperature of BDFO and BDFO/ZnO thin films. The changes in ferroelectric hysteresis loops with magnetic field ensures about the magnetoelectric (M–E) coupling in BDFO/ZnO films. A well-saturated ferroelectric hysteresis loop with remarkable improvement in remanent polarization (∼1.72μC/cm2) is observed. The obtained results confirm the coexistence of ferromagnetic and ferroelectric ordering with significant coupling at room temperature. The coupling behavior and magnetic transition are also verified using multimode atomic force microscope by applying bias between sample and MFM tip. The leakage current density is measured in the order of 10−5A/cm2. Integration of BDFO films with ZnO piezoelectric thin film suggests in principle its potential in applications of micro electro mechanical systems (MEMS) as well as in memory devices and in strong history dependent systems.

Magnetic energy flow imaging of magnetic recording head with enhanced resolution and sensitivity by A-MFM using high magnetic moment superparamagnetic tip

In this report, the magnetic energy flow of AC magnetic field is imaged from a perpendicular magnetic recording head by alternating magnetic force microscopy (A-MFM) with high magnetic moment Co0.43(GdOx)0.57 superparamagnetic (SP) tip. The present magnetic energy flow imaging phenomenon is able to detect the flow direction of AC magnetic field energy from the recording head. A simple model for the flow direction of the AC magnetic energy is given and revealed that when the AC magnetic field energy from the surface of main pole increases, the energy from the side cross section decreases due to the periodical change of magnetization direction at the main pole area. The A-MFM magnetic energy imaging performance by 100 nm Co0.43(GdOx)0.57 SP tip is compared with the magnetic field imaging with the 25 nm Co0.80Zr0.05Nb0.15 soft magnetic (SM) tip. The spatial resolution measured by Fourier analysis for Co0.43(GdOx)0.57 SP tip is enhanced dramatically to ∼10 nm with low thermal noise, as compared to ∼13 nm by Co0.80Zr0.05Nb0.15 SM tip as well as the previously reported MFM tip. Moreover, the sensitivity and resolution are investigated for a range of head current to confirm the suitability of the magnetic energy flow imaging by Co0.43(GdOx)0.57 SP tip to low as well as high magnetic field source. Further, the A-MFM measurements are performed with the 100 nm Au coated conducting tip to investigate the possibility of electrostatic contribution in magnetic energy imaging by Co0.43(GdOx)0.57 SP tip and revealed that the present A-MFM measurement is free from any electrostatic artifacts. The present magnetic energy flow imaging phenomena with the high magnetic moment Co0.43(GdOx)0.57 SP tip provides an avenue for the analysis of the magnetic field energy component from the head field profile.

Epitaxial hard magnetic SmCo5 MFM tips – a new approach to advanced magnetic force microscopy imaging

Cantilever based scanning force sensors, which probe a specific tip-sample interaction through a functional tip coating, are limited by the material performance achievable in the coating process. In case of the wide spread magnetic force microscopy (MFM) technique, the magnetic performance of MFM tips, especially the response to magnetic fields and the coercivity, fall far behind the quality known from permanent magnet films prepared with optimized process conditions on appropriate substrates. We resolve this limitation by starting from an optimized thin film architecture - a highly anisotropic SmCo5 film grown epitaxially on MgO(110) substrates - from which a tip is separated by focused ion beam and is attached to a cantilever. Not compromising on resolution and sensitivity, we demonstrate an unrivaled rigidity in magnetic fields, which will largely advance quantitative microscopic investigation of magnetic materials with strong stray fields and allows MFM measurements in external magnetic fields of currently up to 0.7 T. The material optimization for a specific sample - cantilever interaction without restrictions in substrate, film architecture, film preparation conditions and tip shape, is not limited to MFM but offers new opportunities also for other scanning force microscopy modes.

Magnetic charge distribution and stray field landscape of asymmetric néel walls in a magnetically patterned exchange bias layer system

The 3D stray field landscape above an exchange bias layer system with engineered domain walls has been fully characterized by quantitative magnetic force microscopy (qMFM) measurements. This method is based on a complete quantification of the MFM tip’s imaging properties and the subtraction of its contribution from the measured MFM data by deconvolution in Fourier space. The magnetically patterned Ir17Mn83/Co70Fe30-exchange-bias-multilayers have been designed to contain asymmetric head-to-head (hh)/tail-to-tail (tt) Néel walls between domains of different magnetic anisotropies for potential use in guided particle transport. In the current application, qMFM reveals the effective magnetic charge profile on the surface of the sample—with high spatial resolution and in an absolute quantitative manner. These data enable to calculate the magnetostatic potential and the full stray field landscape above the sample surface. It has been successfully tested against: (i) micromagnetic simulations of the magnetization structure of a comparable exchange-bias layer system, (ii) measurements of the magnetization profile across the domain boundary with x-ray photoemission electron microscopy, and (iii) direct stray field measurements obtained by scanning Hall probe microscopy at elevated scan heights. This approach results in a quantitative determination of the stray field landscape at close distances to the sample surface, which will be of importance for remote magnetic particle transport applications in lab-on-a-chip devices. Furthermore, the highly resolving and quantitative MFM approach reveals details of the domain transition across the artificially structured phase boundary, which have to be attributed to a continuous change in the materials parameters across this boundary, rather than an abrupt one.

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.

Imaging the magnetic nanodomains inNd2Fe14B

We study magnetic domains in Nd2Fe14B single crystals using high resolution magnetic force mi- croscopy (MFM). Previous MFM studies and small angle neutron scattering experiments suggested the presence of nano-scale domains in addition to optically detected micrometer-scale ones. We find, in addition to the elongated, wavy nano-domains reported by a previous MFM study, that the micrometer size, star shape fractal pattern is constructed of an elongated network of nano-domains ~20 nm in width, with resolution-limited domain walls thinner than 2 nm. While the microscopic domains exhibit significant resilience to an external magnetic field, some of the nano-domains are sensitive to the magnetic field of the MFM tip.

Pulsed magnetic field magnetic force microscope and evaluation of magnetic properties of soft magnetic tips

A pulsed magnetic field magnetic force microscope (PMF-MFM) is developed for evaluation of the magnetic properties of nano-scale materials and devices, as well as the characteristics of MFM tips. We present the setup of the PMF-MFM system, and focus on the evaluation of a FeCo soft magnetic tip by PMF-MFM. We find a new theoretical method to calculate tip magnetization curves (M-H curves) using MFM phase signals. We measure the MFM phase and amplitude signals for the FeCo tip during the presence of the pulsed magnetic fields oriented parallel and antiparallel to the initial tip magnetization direction, and acquire the tip coercivity Hc ~ 1.1 kOe. The tip M-H curves are also calculated using the MFM phase signals data. We obtain the basic features of the tip magnetic properties from the tip M-H curves.

Differential magnetic force microscope imaging.

This paper presents a method for differential magnetic force microscope imaging based on a two-pass scanning procedure to extract differential magnetic forces and eliminate or significantly reduce background forces with reversed tip magnetization. In the work, the difference of two scanned images with reversed tip magnetization was used to express the local magnetic forces. The magnetic sample was first scanned with a low lift distance between the MFM tip and the sample surface, and the magnetization direction of the probe was then changed after the first scan to perform the second scan. The differential magnetic force image was obtained through the subtraction of the two images from the two scans. The theoretical and experimental results have shown that the proposed method for differential magnetic force microscope imaging is able to reduce the effect of background or environment interference forces, and offers an improved image contrast and signal to noise ratio (SNR).

3-D Mapping of Sensitivity of Graphene Hall Devices to Local Magnetic and Electrical Fields

We report the response of sub-micron epitaxial graphene Hall devices to localized and inhomogeneous magnetic field produced by an MFM tip. We analyze the magneto-transport properties of epitaxial graphene and report the independent contribution of magnetic and electric fields in the measured transverse voltage maps with respect to the lateral and vertical position of the tip. A finite element model has also been developed to support the experimental results.

Radio Frequency Magnetic Near Field Measurements of Coplanar Waveguide Simulated Power and Ground Lines in Radio Frequency Integrated Circuits Using a MFM Tip

This paper describes our study on high-resolution measurements of the distribution of the radio frequency (RF) magnetic near field from a coplanar waveguide (CPW) with a size similar to power and ground lines in radio frequency integrated circuit chips using a magnetic force microscope tip and amplitude modulation of the line current. Cantilever resonance frequency modulation of approximately 25 kHz supplies the CPW with current and carrier frequencies up to 0.1 GHz. The CPW has a signal line width of 3 μm and a ground line width of 50 μm. The oscillation amplitudes of the tip with and without a DC magnetic field indicates that the distribution of the RF magnetic near field in the proximity of the CPW can be observed for a carrier frequency up to 0.1 GHz. This result also suggests that this method has potential as a new technique for high-resolution RF field measurements.

Control of Magnetic Domains in Co/Pd Multilayered Nanowires with Perpendicular Magnetic Anisotropy

Magnetic domain wall (DW) motion induced by spin transfer torque in magnetic nanowires is of emerging technological interest for its possible applications in spintronic memory or logic devices. Co/Pd multilayered magnetic nanowires with perpendicular magnetic anisotropy were fabricated on the surfaces of Si wafers by ion-beam sputtering. The nanowires had different sized widths and pinning sites formed by an anodic oxidation method via scanning probe microscopy (SPM) with an MFM tip. The magnetic domain structure was changed by an anodic oxidation method. To discover the current-induced DW motion in the Co/Pd nanowires, we employed micromagnetic modeling based on the Landau-Lifschitz-Gilbert (LLG) equation. The split DW motions and configurations due to the edge effects of pinning site and nanowire appeared.

Measurement of magnetic near field on a coplanar waveguide using a MFM tip

This paper proposes a method to measure the ac magnetic field distribution in close proximity to a coplanar waveguide (CPW) employing a cantilever tip fabricated for magnetic force microscopy as a sensor. Almost the entire surface of the tip is coated with a magnetically hard film (Co–Cr–Pt), which is magnetized vertically. Herein we focus on the accuracy of the tip image distribution in close proximity of the ac fields from downsized CPWs which, for example, have a signal line as fine as 5 μm and ground lines of 50 μm. The results suggest that the system has potential as a micron scale RF field detector, although a few technological problems must be solved.

Quantitative Magnetic Force Microscopy Study of the Diameter Evolution of Bubble Domains in a $(\hbox{Co/Pd})_{80}$ Multilayer

The evolution of bubble domains in a (Co/Pd) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">80</sub> multi-layered thin film with perpendicular anisotropy has been studied by Magnetic Force Microscopy in varying magnetic fields. As expected, the bubble domains show a continuous increase of their size with decreasing field. The diameters have been quantitatively evaluated by means of signal deconvolution. To that end the MFM tip was previously calibrated on the same multilayer in zero field state. The resulting bubble diameters and the values for bubble strip-out and collapse fields agree well with the theoretically predicted values for this system, proving the validity of the applied micromagnetic model.

MFM analysis of magnetic inhomogeneity in recorded area for perpendicular magnetic recording media by simultaneous imaging of perpendicular and in-plane magnetic field gradient

A new MFM method which can measure the perpendicular and in-plane magnetic field gradient simultaneously was developed. The measuring direction of in-plane magnetic field can be easily changed only by selecting the scanning direction of MFM tip with respect to the sample. This method was used to successfully estimate the magnetic domain structure of recorded area on CoCrPt-SiO2 perpendicular magnetic recording media. When the scanning direction is along the down-track direction, the component of strong magnetic field gradient along the down-track direction can be observed with high spatial resolution at the bit transition. On the other hand, when the scanning direction is along the corss-track direction, the new MFM method is effective to mask the component of magnetic field from homogeneous recorded bits, and consequently the components of magnetic field from bended recorded bits at the track edge can be observed. The simultaneous measurement of perpendicular and in-plane magnetic field is useful to evaluate the magnetic inhomogeneity of recorded area such as bit transition and track edge.

Characterization of Magnetic Field Distribution in a Trailing-Edge Shielded Head by Frequency-Modulated Magnetic Force Microscopy

In this paper, the ac magnetic field distribution of a trailing-edge shielded SPT head were successfully investigated by using a newly developed frequency-modulated magnetic force microscopy (FM-MFM) technique with a hard magnetic MFM tip. The results show sharper magnetic field gradient at the trailing edge than at the leading edge of the main pole. The perpendicular component of the magnetic field of the main pole region is opposite to that of the trailing shield/return pole region. As the write current increases, the saturation of magnetic field in the main pole region occurs from the trailing edge first. With increasing write current, the return field of the trailing shield edge increases significantly as well, which may cause return field-induced partial erasure. The double-peak feature of the distribution of ac magnetic field generated from the head only depends on the tip-sample distance and does not depend on the input write current. The FM-MFM technique used in this work was proved to be applicable in the imaging of the ac magnetic field of a recording head with high spatial resolution.