Magnetic Force Microscopy Images Research Articles

Nanofabrication, characterisation and modelling of soft-in-hard FeCo–FePt magnetic nanocomposites

Advanced nanofabrication exploiting e-beam lithography has been used to prepare nanocomposites consisting of periodic arrays of soft magnetic FeCo-based nano-inclusions of variable dimensions, embedded in a hard magnetic FePt matrix. Nanocomposites with non-magnetic (Pt) inclusions and single phase hard magnetic FePt microstructures were prepared as reference samples. The formation of Kirkendall voids in the soft-in-hard nanocomposites, because of annealing-induced diffusion, has been identified through high resolution imaging and chemical analysis. Replication of the μm-scaled nanocomposite and hard magnetic units over mm-scaled surfaces allowed global magnetic characterisation of magnetisation reversal using standard magnetometry. Magnetic force microscopy imaging was used for spatially resolved magnetic characterisation in different remanent magnetic states. Both of these experimental techniques were used to study the influence of the size, volume content and nature of the nano-inclusions on magnetisation reversal. The well-defined geometry and nano-scaled size of the inclusions render these nanocomposites as model systems for micromagnetic simulations and very good agreement was achieved between measured and simulated magnetisation reversal curves. This accordance motivates future in-silico optimisation of such soft-in-hard nanocomposites, which may serve as model systems to guide the design of new high performance bulk magnets which are less dependent on critical elements.

Increasing the Q-factor of resonant cantilevers in magnetic force microscopy through helium gas flow

To obtain high-resolution magnetic force microscopy (MFM) images, it is essential to have a cantilever with a high-quality factor. However, conventional vibrating cantilevers typically have quality factor values in the range of a few hundred, which limits their sensitivity for MFM measurements. To address this limitation, numerous studies have explored methods to enhance the quality factor in different environments, including vacuum, air, and liquid. This study introduces a novel approach for improving the quality factor using flowing helium gas. By selecting helium gas with a low viscosity coefficient, we successfully achieved a higher quality factor (Q-factor) of MFM microcantilever oscillations at room temperature in one atmosphere compared with the Q-factor in air. This provides a potential approach for achieving high-resolution MFM measurements under room temperature conditions. By optimizing the gas flow rate at room temperature in one atmosphere, we successfully obtained a higher MFM cantilever oscillation Q-factor and clearer MFM images compared with the air. The experimental results revealed a long and narrow resonant curve, and the quality factor significantly increased to 778.2, which is 3.8 times higher than that observed in air 205.4. Furthermore, systematic investigations demonstrated the capability of this approach to produce high-resolution MFM images of videotape track patterns under the optimized helium gas flow rate of 60 mm/s.

Microwave Field-Induced Changes in Raman Modes and Magnetic Force Images of Antiferromagnetic NiO Films

Effective control of domain walls or magnetic textures in antiferromagnets promises to enable robust, fast, and nonvolatile memories. The lack of net magnetic moment in antiferromagnets implies the need for creative ways to achieve such a manipulation. We conducted a study to investigate changes in magnetic force microscopy (MFM) imaging and in the magnon-related mode in Raman spectroscopy of virgin NiO films under a microwave pump. After MFM and Raman studies were conducted, a combined action of broadband microwave (0.01–20 GHz, power scanned from −20 to 5 dBm) and magnetic field (up to 3 kOe) were applied to virgin epitaxial (111) NiO and (100) NiO films grown on (0001) Al2O3 and (100) MgO substrates, following which the MFM and Raman studies were repeated. We observed a suppression of the magnon-related Raman mode subsequent to the microwave exposure. Based on MFM imaging, this effect appeared to be caused by the suppression of large antiferromagnetic domain walls due to the possible excitation of antiferromagnetic spin oscillations localized within the antiferromagnetic domain walls.

Some Methods for Improving the Quality of Magnetic Force Microscopy Images

Some Methods for Improving the Quality of Magnetic Force Microscopy Images

Some Methods for Improving the Quality of Magnetic Force Microscopy Images

Some Methods for Improving the Quality of Magnetic Force Microscopy Images

Direct Polyphenol Attachment on the Surfaces of Magnetite Nanoparticles, Using Vitis vinifera, Vaccinium corymbosum, or Punica granatum.

This study presents an alternative approach to directly synthesizing magnetite nanoparticles (MNPs) in the presence of Vitis vinifera, Vaccinium corymbosum, and Punica granatum derived from natural sources (grapes, blueberries, and pomegranates, respectively). A modified co-precipitation method that combines phytochemical techniques was developed to produce semispherical MNPs that range in size from 7.7 to 8.8 nm and are coated with a ~1.5 nm thick layer of polyphenols. The observed structure, composition, and surface properties of the MNPs@polyphenols demonstrated the dual functionality of the phenolic groups as both reducing agents and capping molecules that are bonding with Fe ions on the surfaces of the MNPs via -OH groups. Magnetic force microscopy images revealed the uniaxial orientation of single magnetic domains (SMDs) associated with the inverse spinel structure of the magnetite (Fe3O4). The samples' inductive heating (H0 = 28.9 kA/m, f = 764 kHz), measured via the specific loss power (SLP) of the samples, yielded values of up to 187.2 W/g and showed the influence of the average particle size. A cell viability assessment was conducted via the MTT and NRu tests to estimate the metabolic and lysosomal activities of the MNPs@polyphenols in K562 (chronic myelogenous leukemia, ATCC) cells.

Magnetization reversal of [Co/Pd] perpendicular magnetic thin film dot on (Bi,La)(Fe,Co)O3 multiferroic thin film by applying electric field

A multilayer structure with a high-quality (Bi,La)(Fe,Co)O3 multiferroic thin film/[Co/Pd] perpendicular magnetic thin film dots was fabricated for demonstrating magnetization reversal of [Co/Pd] dots under an applied electric field. Although the magnetization direction of the multiferroic thin film was reversed under the electric field, the magnetic properties of the multiferroic thin films were generally low. If the multiferroic thin film in this structure can control the magnetization direction of the highly functional magnetic thin film under an electric field, high-performance magnetic devices with low power consumption are easily obtained. The magnetic domain structure of the [Co/Pd] dots fabricated on the (Bi,La)(Fe,Co)O3 thin film was analyzed by magnetic force microscopy (MFM). The structure was de-magnetized before the local electric-field application and magnetized after applying the field, showing reduced magnetic contrast of the dot. The line profile of the MFM image revealed a downward magnetic moment of 75%, which reversed to upward under the local electric field. Magnetic interaction between the (Bi,La)(Fe,Co)O3 and [Co/Pd] layers was also observed in magnetization hysteresis measurements. These results indicate that the magnetization direction of the [Co/Pd] dots was transferred through the magnetization reversal of the (Bi,La)(Fe,Co)O3 layer under a local electric field. That is, the magnetization of [Co/Pd] dots were reversed by applying a local electric field to the multilayer structure. This demonstration can potentially realize high-performance magnetic devices such as large capacity memory with low power consumption.

Large ordered moment with strong easy-plane anisotropy and vortex-domain pattern in the kagome ferromagnet Fe3Sn

We report the magnetic anisotropy of kagome bilayer ferromagnet Fe3Sn probed by the bulk magnetometry and magnetic force microscopy (MFM) on high-quality single crystals. The dependence of magnetization on the orientation of the external magnetic field reveals strong easy-plane magnetocrystalline anisotropy and anisotropy of the saturation magnetization. The leading magnetocrystalline anisotropy constant shows a monotonous increase from K1≈−1.0×106 J/m3 at 300 K to −1.3×106 J/m3 at 2 K. Our ab initio electronic structure calculations yield the value of total magnetic moment of 7.1 μB/f.u. and a magnetocrystalline anisotropy energy density of −0.57 meV/f.u. (−1.62×106J/m3) both being in reasonable agreement with the experimental values. The MFM imaging reveals micrometer-scale magnetic vortices with weakly pinned cores that vanish at the saturation field of ∼3 T applied perpendicular to the kagome plane. The observed vortex-domain structure is well reproduced by the micromagnetic simulations, using the experimentally determined value of the anisotropy and exchange stiffness.

Numerical Characterization of Magnetic Vortex Probe Imaging for Magnetic Force Microscopy

We study theoretically the performance and limitations of the magnetic vortex probe for magnetic force microscopy (MFM). In the ideal case, the only magnetically active part of the probe is the magnetic vortex core (VC) existing in the center of a Permalloy (Py) disk located at the apex of a non-magnetic tip. Such VC can be effectively characterized as a point dipole with the magnetic moment of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> Am <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> embedded about 20 nm inside the disk, which is similar to the commercial low-momentum MFM probes. In addition to the standard probes, the ideal VC probe offers high durability and a well-controlled magnetic moment suitable for quantitative MFM. However, since the VC probe is made of magnetically soft material its magnetization profile and the resulting MFM image can be deformed by the stray field of the sample. Therefore, the VC probe is suited for imaging the samples with small domain sizes which generate low stray fields. We numerically examine VC imaging of typical magnetic samples, i.e. domain arranged as a single circular dot, stripe patterns, and the chessboard. We determine the limiting dimensions of the domains that can be correctly imaged by the VC tip of optimal parameters. In general, we can conclude that MFM imaging by VC probes gives reasonable results for the samples with domains with lateral dimensions up to 100 nm.

Correlation of Interface Interdiffusion and Skyrmionic Phases.

Magnetic skyrmions are prime candidates for the next generation of spintronic devices. Skyrmions and other topological magnetic structures are known to be stabilized by the Dzyaloshinskii-Moriya interaction (DMI) that occurs when the inversion symmetry is broken in thin films. Here, we show by first-principles calculations and atomistic spin dynamics simulations that metastable skyrmionic states can also be found in nominally symmetric multilayered systems. We demonstrate that this is correlated with the large enhancement of the DMI strength due to the presence of local defects. In particular, we find that metastable skyrmions can occur in Pd/Co/Pd multilayers without external magnetic fields and can be stable even near room temperature conditions. Our theoretical findings corroborate with magnetic force microscopy images and X-ray magnetic circular dichroism measurements and highlight the possibility of tuning the intensity of DMI by using interdiffusion at thin film interfaces.

Effects of nanoindents on the martensitic transformation of Ni-Mn-Ga shape-memory Heusler films: A study by high-resolution imaging as a function of temperature

Nanoindentation was used to analyze the effect of localized plastic deformation on the martensitic transformation of epitaxial Ni-Mn-Ga films on MgO(001) substrate. Atomic and magnetic force microscopy imaging at elevated temperature was applied to study the martensitic transformation route from the nanometer to the micrometer scale. We analyzed the cooling and heating curves for the martensitic transformation of the nanoindented areas as a function of the applied loads as well as the distance from the material pile-ups around the residual impressions. We observe a thermodynamically governed local increase of the martensitic transformation temperature (i.e. martensite stabilization) as a function of the applied loads. The local increase of the transformation temperature vs. the applied load (P) follows a non-linear regime: reducing the slope by increasing the applied load and showing a plateau for P ≥ 5 mN. The observed effect is local and almost disappears for distances larger than 500 nm from the pile-ups around the residual impressions, where the material transforms similar to the pristine sample. The local increase of transformation temperature as a function of nanoindentation loads is the dominant effect and occurs in both the cooling and the heating curves. Therefore, no considerable thermal hysteresis variation is observed in the transformation of the material. Moreover, we report in average ∼12% higher relative areal shift of the transformation curves over cooling than heating close to the indents reflecting an unequal impact of indents on TM and TA in Ni-Mn-Ga.

Artifacts in magnetic force microscopy of histological sections

Detection of iron-oxide nanoparticles in biological samples has widespread applications. Iron can be naturally present in biological tissues as physiological ferrihydrite particles of ferritin(iron) or as biogenic magnetite nanoparticles in certain neurodegenerative pathologies. In our previous studies, we had reported how magnetic force microscopy (MFM) can be used to map iron deposits in tissue sections in a label-free manner. In this study, to improve the efficiency of MFM for histological analysis, we explored the effect of increasing scan rate. Our results show how MFM images of tissue sections can be contaminated with artifacts due to topographical cross-talk, especially at higher scan rates. These artifacts were observed in rodent spleen as well as in sections of brain tissue obtained from patients with Alzheimer’s Disease. Our results show how topographical cross talk can make it challenging to unambiguously detect histological iron via MFM. Multimodal approaches can help overcome some of these limitations.

Direct Visualization of Surface Spin-Flip Transition in MnBi_{4}Te_{7}.

We report direct visualization of spin-flip transition of the surface layer in antiferromagnet MnBi_{4}Te_{7}, a natural superlattice of alternating MnBi_{2}Te_{4} and Bi_{2}Te_{3} layers, using cryogenic magnetic force microscopy (MFM). The observation of magnetic contrast across domain walls and step edges confirms that the antiferromagnetic order persists to the surface layers. The magnetic field dependence of the MFM images reveals that the surface magnetic layer undergoes a first-order spin-flip transition at a magnetic field that is lower than the bulk transition, in excellent agreement with a revised Mills model. Our analysis suggests no reduction of the order parameter in the surface magnetic layer, implying robust ferromagnetism in the single-layer limit. The direct visualization of surface spin-flip transition not only opens up exploration of surface metamagnetic transitions in layered antiferromagnets, but also provides experimental support for realizing quantized transport in ultrathin films of MnBi_{4}Te_{7} and other natural superlattice topological magnets.

Machine learning estimation of magnetic parameters and classification of magnetic vortex states

Analysis of properties related to spin textures, such as the magnetic vortex state, is mainly based on spin configuration data, which is directly related to magnetic parameters involved in the system's Hamiltonian. Here, we focus on magnetic parameter estimation by implementing the machine learning (ML) approach, especially on magnetic force microscopy (MFM) images of vortex states within nanodots generated by micromagnetic simulation. The exchange constant Aex and saturation magnetization Ms as well as exchange length as a reduced parameter Lex(Aex, Ms) are estimated by different convolutional neural network (CNN) models. We also evaluated the CNN models, trained on simulated MFM images with non-zero temperature, on a reference experimental MFM image and found the performance to a satisfactory level of accuracy. Moreover, the same CNN models, trained for binary classification of vortex states based on helicity from MFM images, successfully identified the vortex helicity from simulated as well as experimental MFM images. These findings show the possible application of ML in magnetic parameter estimation and the analysis of magnetic vortex states simply with images obtained from this commonly used imaging technique that is significant in efficient investigation of material properties based on intrinsic parameters for spintronic device applications.

Optimizing Magnetic Force Microscopy Resolution and Sensitivity to Visualize Nanoscale Magnetic Domains.

Magnetic force microscopy (MFM) enables mapping local magnetic fields across a sample surface with nanoscale resolution. To perform MFM, an atomic force microscopy (AFM) probe whose tip has been magnetized vertically (i.e., perpendicular to the probe cantilever) is oscillated at a fixed height above the sample surface. The resultant shifts in the oscillation phase or frequency, which are proportional to the magnitude and sign of the vertical magnetic force gradient at each pixel location, are then tracked and mapped. Although the spatial resolution and sensitivity of the technique increases with decreasing lift height above the surface, this seemingly straightforward path to improved MFM images is complicated by considerations such as minimizing topographical artifacts due to shorter range van der Waals forces, increasing the oscillation amplitude to further improve sensitivity, and the presence of surface contaminants (in particular water due to humidity under ambient conditions). In addition, due to the orientation of the probe's magnetic dipole moment, MFM is intrinsically more sensitive to samples with an out-of-plane magnetization vector. Here, high-resolution topographical and magnetic phase images of single and bicomponent nanomagnet artificial spin-ice (ASI) arrays obtained in an inert (argon) atmosphere glovebox with <0.1 ppm O2 and H2O are reported. Optimization of lift height and drive amplitude for high resolution and sensitivity while simultaneously avoiding the introduction of topographical artifacts is discussed, and detection of the stray magnetic fields emanating from either end of the nanoscale bar magnets (~250 nm long and <100 nm wide) aligned in the plane of the ASI sample surface is shown. Likewise, using the example of a Ni-Mn-Ga magnetic shape memory alloy (MSMA), MFM is demonstrated in an inert atmosphere with magnetic phase sensitivity capable of resolving a series of adjacent magnetic domains each ~200 nm wide.

XPEEM and MFM Imaging of Ferroic Materials

AbstractThe authors describe and compare two complementary techniques that are habitually used to image ferromagnetic and ferroelectric materials with sub‐micron spatial resolutions (typically 50 nm, at best 10 nm). The first technique is variable‐temperature photoemission electron microscopy with magnetic/antiferromagnetic/polar contrast from circularly/linearly polarized incident X‐rays (XPEEM). The second technique is magnetic force microscopy (MFM). Focusing mainly on the authors’ own work, but not exclusively, published/unpublished XPEEM and MFM images of ferroic domains and complex magnetic textures (involving vortices and phase separation) are presented. Highlights include the use of two XPEEM images to create 2D vector maps of in‐plane (IP) magnetization, and the use of imaging to detect electrically driven local reversals of magnetization. The brief and simple descriptions of XPEEM and MFM should be useful for beginners seeking to employ these techniques in order to understand and harness ferroic materials.

Investigations on the structural and magnetic properties of Ba1-xGdxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles

The structural and magnetic properties of Ba1-xGdxFe12O19 (0 ≤ x ≤ 0.15) nanoparticles prepared using the proteic sol-gel process were investigated. Infrared spectroscopy, Raman spectroscopy, andX-ray powder diffraction measurements with Rietveld refinement analysis confirmed the formation of M-type BaFe12O19, whereas extra peaks of secondary phases were also detected for BaFeO3 (2θ = 28°), GdFeO3 (2θ = 38.4°), and Fe2O3 (2θ = 38.6°). Average crystallite size increases with Gd substitution in the range of 61–63 nm. Scanning electron microscopy revealed that the particle morphology of some samples consisted of a tendency for regular hexagonal platelets with inhomogeneous distribution of average grain size in the range of 250–280 nm. The room temperature magnetic studies revealed that considerable large saturation magnetization (Ms), remanence (Mr), and coercivity (Hc) for the Ba1-xGdxFe12O19 nanoparticles monotonically decreased to a minimum value until x = 0.1. However, it increased again toMs≈ 55.5 emu·g−1, Mr≈ 22.7 emu·g−1, andHc≈ 3.7 kOe for the x = 0.15 sample. The squareness ratio (SQR = Mr/Ms) values were lower than 0.5, illustrating the magnetic multidomain nature and uniaxial anisotropy of the samples. These results were investigated deeply with relation to hyperfine parameters and surface morphology of the samples by 57Fe Mössbauer spectroscopy and magnetic force microscopy (MFM). The hyperfine parameters indicated that the substitution of Gd3+ for Ba2+ induces exchange interactions in 2b-O2-4f2 and 12k-O2-2b, resulting in the higher magnetocrystalline anisotropy of the samples. Additionally, MFM images confirm that grain consisting of distributed smaller grains gives rise to a strip-and helix-like magnetic domain structure.

Experimental demonstration of skyrmionic magnetic tunnel junction at room temperature

Chiral magnetic skyrmions are topological swirling spin textures that hold promise for future information technology. The electrical nucleation and motion of skyrmions have been experimentally demonstrated in the last decade, while electrical detection compatible with semiconductor processes has not been achieved, and this is considered one of the most crucial gaps regarding the use of skyrmions in real applications. Here, we report the direct observation of nanoscale skyrmions in CoFeB/MgO-based magnetic tunnel junction devices at room temperature. High-resolution magnetic force microscopy imaging and tunneling magnetoresistance measurements are used to illustrate the electrical detection of skyrmions, which are stabilized under the cooperation of interfacial Dzyaloshinskii–Moriya interaction, perpendicular magnetic anisotropy, and dipolar stray field. This skyrmionic magnetic tunnel junction shows a stable nonlinear multilevel resistance thanks to its topological nature and tunable density of skyrmions under current pulse excitation. These features provide important perspectives for spintronics to realize high-density memory and neuromorphic computing.

Quantum calibrated magnetic force microscopy

We report the quantum calibration of a magnetic force microscope (MFM) by measuring the two-dimensional magnetic stray field distribution of the MFM tip using a single nitrogen vacancy (NV) center in diamond. From the measured stray field distribution and the mechanical properties of the cantilever a calibration function is derived allowing to convert MFM images to quantum calibrated stray field maps. This novel approach overcomes limitations of prior MFM calibration schemes and allows quantum calibrated nanoscale stray field measurements in a field range inaccessible to scanning NV magnetometry. Quantum calibrated measurements of a stray field reference sample allow its use as a transfer standard, opening the road towards fast and easily accessible quantum traceable calibrations of virtually any MFM.

Influence of martensitic configuration on hysteretic properties of Heusler films studied by advanced imaging in magnetic field and temperature

Ferromagnetic-shape-memory (FSM) Heusler compounds are an important class of multifunctional materials having promising applications in a vast variety of areas such as actuating, sensing, energy harvesting, spintronics and multicaloric cooling. Their multifunctionality stems from a reversible martensitic phase transition. However, their full exploitation is prevented by some undesirable characteristics of the martensitic transition: thermal hysteresis and broad transition. We studied here the role of specific martensitic configurations on the transition characteristics. By advanced magnetic force microscopy imaging in a wide temperature (260–350 K) and magnetic field range (up to 14 T) we directly observed the nucleation and the self-accommodation of the martensitic twinning configurations under zero-field, isofield and isothermal conditions. The experiments were performed on Ni-Mn-Ga epitaxial thin films with martensitic twinning configurations made of both X- and Y-type, which are characterized by different orientations of the twinning planes (i.e. at 45° and 90° degrees to the (001) MgO substrate, respectively). We have found that between the two possible twinning configurations, the Y-type, which nucleates first, shows a significantly smaller thermal hysteresis as well as a sharper phase transition with respect to X-type twinning configuration, for all the three investigated conditions.