High Resolution Magnetic Force Microscopy Research Articles

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.

Topological Spin Textures in an Insulating van der Waals Ferromagnet.

Generation and control of topological spin textures constitutes one of the most exciting challenges of modern spintronics given their potential applications in information storage technologies. Of particular interest are magnetic insulators, which due to low damping, absence of Joule heating and reduced dissipation can provide energy-efficient spin-textures platform. Here, it is demonstrated that the interplay between sample thickness, external magnetic fields, and optical excitations can generate a prolific paramount of spin textures, and their coexistence in insulating CrBr3 van der Waals (vdW) ferromagnets. Using high-resolution magnetic force microscopy and large-scale micromagnetic simulation methods, the existence of a large region in T-B phase diagram is demonstrated where different stripe domains, skyrmion crystals, and magnetic domains exist and can be intrinsically selected or transformed to each-other via a phase-switch mechanism. Lorentz transmission electron microscopy unveils the mixed chirality of the magnetic textures that are of Bloch-type at given conditions but can be further manipulated into Néel-type or hybrid-type via thickness-engineering. The topological phase transformation between the different magnetic objects can be further inspected by standard photoluminescence optical probes resolved by circular polarization indicative of an existence of exciton-skyrmion coupling mechanism. The findings identify vdW magnetic insulators as a promising framework of materials for the manipulation and generation of highly ordered skyrmion lattices relevant for device integration at the atomic level.

Long-Term Performance of Magnetic Force Microscopy Tips Grown by Focused Electron Beam Induced Deposition

High-resolution micro- and nanostructures can be grown using Focused Electron Beam Induced Deposition (FEBID), a direct-write, resist-free nanolithography technology which allows additive patterning, typically with sub-100 nm lateral resolution, and down to 10 nm in optimal conditions. This technique has been used to grow magnetic tips for use in Magnetic Force Microscopy (MFM). Due to their high aspect ratio and good magnetic behavior, these FEBID magnetic tips provide several advantages over commercial magnetic tips when used for simultaneous topographical and magnetic measurements. Here, we report a study of the durability of these excellent candidates for high-resolution MFM measurements. A batch of FEBID-grown magnetic tips was subjected to a systematic analysis of MFM magnetic contrast for 30 weeks, using magnetic storage tape as a test specimen. Our results indicate that these FEBID magnetic tips operate effectively over a long period of time. The magnetic signal was well preserved, with a maximum reduction of 60% after 21 weeks of recurrent use. No significant contrast degradation was observed after 30 weeks in storage.

Nano-columnar, self-organised NiCrC/a-C:H thin films deposited by magnetron sputtering

Thin films of NiCr–C were deposited by pulse magnetron sputtering of the NiCr20 alloy cathode in an atmosphere containing a mixture of argon and acetylene. Their structure, chemical composition, and magnetic properties were studied using X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, atomic and magnetic force microscopy, and superconducting quantum interference magnetometry methods. Film amorphisation observed upon increasing carbon content was followed by amorphous carbon segregation. As a result of the self-assembly processes during the coating growth, nanostructures were obtained consisting of metallic columns surrounded in carbon sheaths, with axes parallel to the growth direction. The resulting films exhibited weak ferromagnetism in a direction parallel to the axis of the nanocolumns, while the system’s response was superparamagnetic for the direction of magnetisation perpendicular to the nanocolumns.

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.

Direct imaging of distorted vortex structures and magnetic vortex annihilation processes in ferromagnetic/antiferromagnetic disk structures

Topological objects, such as magnetic vortices in soft-magnetic thin films, have attracted continued attention over the past 20 years for their unique magnetic reversal behavior and compelling potential applications. In exchange-coupled ferromagnetic/antiferromagnetic microdisks, high-resolution magnetic force microscopy imaging reveals a distorted ferromagnetic vortex structure due to randomly distributed (nonplanar) uncompensated spins at the antiferromagnetic/ferromagnetic interface. Further, in-field imaging and magneto-optical Kerr effect magnetometry demonstrate an unexpected increase in the stability of an otherwise somewhat volatile, intermediate vortex-antivortex state in exchange-coupled microdisks.

Microscopic Origin of Magnetization Reversal in Nanoscale Exchange-Coupled Ferri/Ferromagnetic Bilayers: Implications for High Energy Density Permanent Magnets and Spintronic Devices.

Giant exchange bias shifts of several Tesla have been reported in ferrimagnetic/ferromagnetic bilayer systems, which could be highly beneficial for contemporary high energy density permanent magnets and spintronic devices. However, the lack of microscopic studies of the reversal owing to the difficulty of measuring few nanometer-wide magnetic structures in high fields precludes the assessment of the lateral size of the inhomogeneity in relation to the intended application. In this study, the magnetic reversal process of nanoscale exchange-coupled bilayer systems, consisting of a ferrimagnetic TbFeCo alloy layer and a ferromagnetic [Co/Ni/Pt]N multilayer, was investigated. In particular, minor loop measurements, probing solely on the reversal characteristics of the softer ferromagnetic layer, reveal two distinct reversal mechanisms, which depend critically on the thickness of the ferromagnetic layer. For thick layers, irreversible switching of the macroscopic minor loop is observed. The underlying microscopic origin of this reversal process was studied in detail by high-resolution magnetic force microscopy, showing that the reversal is triggered by in-plane domain walls propagating through the ferromagnetic layer. In contrast, thin ferromagnetic layers show a hysteresis-free reversal, which is nucleation-dominated due to grain-to-grain variations in magnetic anisotropy of the Co/Ni/Pt multilayer and an inhomogeneous exchange coupling with the magnetically hard TbFeCo layer, as confirmed by micromagnetic simulations.

Atomic scale insights into the segregation/partitioning behaviour in as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets

As-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets typically exhibit a higher coercivity than as-sintered single-main-phase Nd-Ce-Fe-B magnets with identical content of Ce. To clarify the microstructural features that are relevant to the coercivity, as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets are characterised by high-resolution magnetic force microscopy and atom probe tomography down to the atomic scale. Though inhomogeneous distribution of rare earth elements (Nd, Pr, and Ce) is observed inside 2:14:1 matrix grains, Ce atoms unexpectedly show similar partitioning behaviour (∼14–27% of rare earth element) at the edge of these grains in the vicinity of rare earth-rich grain boundary compared with as-sintered single-main-phase Nd-Ce-Fe-B magnets with a 20% Ce substitute of Nd and Pr. By contrast, the observation of stripe-liked magnetic domains pinned at the grain boundaries and a thin (Nd,Pr,Ce)-rich grain boundary with a composition of Nd9.9Pr2.6Ce19.0Fe65.3Co1.9Balance1.3 (at.%) jointly indicates the enrichment of Ce in grain boundaries benefit to the magnetic isolation of neighbouring matrix grains in the as-sintered state. Unusual Ce-rich clustering regions are also observed in the grain boundary, which also contribute to the coercivity enhancement. The segregation of Ce, Co, and Cu and rare-earth-Cu clusters are only observed at the interface between rare earth-Cu rich phase and matrix phase while no obvious elemental segregation and clusters are observed at the interface between the rare earth-Ga rich phase and matrix phase, suggesting the occurrence of rare earth-Cu rich phase may be one of the reasons for achieving high coercivity. These findings offer fresh insights into the as-sintered multi-main-phase Nd-Ce-Fe-B permanent magnets.

Luminescence properties and ROS generation of magnetic porous silicon nanoparticles

Magnetite‒Porous Silicon 100−150 nm size nanoparticles (MPSi) were obtained combining luminescent and magnetic properties from silicon and magnetite, respectively.MPSi hybrids were characterized by high-resolution transmission electron microscopy, atomic and magnetic force microscopy and X-ray photoelectron spectroscopy.The presence of magnetite quenches statically visible luminescence of Porous Silicon toluene suspensions. Whereas MPSi, maintain the luminescence in the 300−450 nm spectral region.Particles retained the capacity for singlet oxygen and superoxide radical ion generation (Reactive Oxygen Species, ROS). However quantum yield singlet oxygen generation is much lower than the PSi analogues and superoxide radical ion concentration dismiss when magnetite is incorporated in the PSi matrix.Silanization of Porous Silicon and MPSi yield nanoparticles with − SH terminal groups with unique luminescence properties.

Magnetization Reversal of Strongly Exchange-Coupled Double Nanolayers for Spintronic Devices

Contemporary spintronics devices, notably GMR and TMR-sensors with high linear ranges, stand to benefit from large bias fields in excess of 1T that have been observed in exchange-coupled double layers using rare earth-transition metal ferrimagnets as pinning layers. However, before exploiting the strong coupling the reversal process must be understood at the scales of the smallest devices, where it would be effective. Little is known about these processes to-date, owing to the technical difficulty of imaging magnetic structures smaller than 20nm in fields of several Tesla. Here we image the nanoscale magnetic structures of a double layer comprising a polycrystalline Co/Pt-multilayer (Co/Pt) which is strongly exchange-coupled to an amorphous and magnetically-hard ferrimagnetic TbFe alloy layer. High resolution magnetic force microscopy provides snapshots of the magnetic structures along various applied fields between 0 and 7T as the magnetization reversal takes place. In contrast to a magnetization reversa...

Quantifying the nucleation effect of correlated matrix grains in sintered Nd-Fe-B permanent magnets

The long-standing issue-the ferromagnetic coupling of adjacent Nd2Fe14B matrix grains across grain boundaries severely impedes further coercivity enhancement of sintered Nd-Fe-B permanent magnets. However, the quantitative role of the ferromagnetic coupling across grain boundaries on the coercivity degradation of sintered Nd-Fe-B magnets is still not well understood. Here, we quantified the nucleation effect of correlated (ferromagnetically coupled) matrix grains in sintered Nd-Fe-B magnets by integrating electron backscatter diffraction, energy dispersive spectroscopy, atom probe tomography, and high-resolution magnetic force microscopy in conjunction with micromagnetic simulations. We revealed that (1) the de-nucleation effect of correlated matrix grains (αψ) across intergranular grain boundaries is linearly enhanced with the increasing thickness (d) of grain boundaries (αψ = kd + l) and k is reduced with increasing exchange stiffness of these grain boundaries when the d is comparable to the domain wall width of the Nd2Fe14B matrix grains in sintered Nd-Fe-B magnets; (2) The thickness variation of the ferromagnetic grain boundary at the nanoscale is harmful to achieving high intrinsic coercivity of sintered Nd-Fe-B magnets; (3) The nucleation field is reduced if there is a little misalignment between correlated (ferromagnetically coupled) matrix grains. These findings shed light on grain boundary engineering at the nanoscale for enhancing the coercivity of Nd-Fe-B permanent magnets.

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.

Stripe magnetic domains in CeY2Fe5O12 (Ce:YIG) epitaxial films

Thin epitaxial films of CeY2Fe5O12 are deposited on (111) Gd3Ga5O12 substrates using pulsed laser ablation. The films exhibit low coercivity (3 mT), high saturation magnetization (102 mT), and excellent epitaxy. High resolution magnetic force microscopy (MFM) images of the film reveal a prominent stripe magnetic domain pattern due to a non-coplanar magnetization vector M→. Intensity analysis of the MFM images allows extraction of the orientation angle θc of M→ with respect to the plane of the film. We have recorded the changes in the width of the stripes and their disappearance as the strength of an in-plane magnetic field is increased. A simple model based on sinusoidal variation of M→ correctly predicts the change in the width of the stripe domains.

Formation of Magnetic Nanoparticles by Annealing Continuous CoFeB/Cu Bilayer Thin Films

This paper reports a novel yet simple maskless method for fabricating monodisperse submicrometer magnetic islands by annealing continuous CoFeB/Cu bilayer films. Annealing drives the diffusion of Co and Fe into the Cu underlayer, causing them to aggregate into submicrometer islands at the Cu grain boundaries. This was established by in situ Auger electron spectroscopy (AES), along with scanning electron microscopy and high-resolution atomic and magnetic force microscopy, to determine the surface composition at various stages of preparation. The composition of the magnetic clusters was established by energy dispersive X-ray spectroscopy. Our results indicate that Co and Fe are coalesced, with their relative ratio remaining similar to the starting composition of the sputtering target, while boron is dissociated from the magnetic species. Also, the areal density of the islands was found to be strongly dependent on the Cu thickness and annealing temperature, while the size of magnetic particles was only weakly dependent. This feature was utilized to control the distribution of monosized magnetic islands.

Specialized probes with nanowhisker structures for high resolution magnetic force microscopy

Creation and study of specialized nanowhisker probes with magnetic coating are performed for high-precision imaging of various objects by means of atomic force and magnetic force microscopy. Thin layers of Ni and Co are deposited on the surface of nanowhisker structures to perform visualization of magnetic fields on the sample surface, in particular, structure of pits on a hard disk drive (HDD). It is revealed that probes with nanowhisker structures covered with magnetic coating due to their high aspect ratio demonstrate a higher spatial resolution and contrast of magnetic fields visualization in comparison with standard magnetic probes.

Phase diagram of magnetic states in nickel submicron disks

We present a detailed study of the magnetic states in sub-micron nickel disks in the diameter range of 300–900 nm and thickness of 15–55 nm using magnetic force microscopy and micromagnetic simulations. In this dimensional range, we find three main magnetic states, namely: the single domain state, the vortex state, and the stripe domain state. We identify the crossover dimensions between these magnetic states, demonstrating that the disk thickness is the dominant parameter, which is further supported by the fact that the magnetocrystalline anisotropy of the Ni films is thickness dependant and plays a dominant role at large thicknesses. Experimental results are in a very good agreement with micromagnetic modelling using the out-of-plane effective anisotropy constant. Using in-situ high resolution Magnetic Force Microscopy techniques, we investigate in detail the motion of the vortex core under applied in-plane magnetic field and find that vortex nucleates and annihilates at ∼100 Oe.

Visualizing magnetocrystalline anisotropy field distribution of patterned triangular L10 FePt nanodots

Measuring magnetocrystalline anisotropy field distribution of patterned magnetic nanodots using a microscopically visualizing way is important for understanding some important magnetic behaviors such as switching field distribution (SFD) of patterned recording media. We present a detailed analysis of the remanent domain structures of L10-FePt triangular nanodots as revealed by high resolution magnetic force microscopy (MFM) with the help of micromagnetic simulation, showing that the domain structure diversity can effectively account for a dot-to-dot variation of the magnetocrystalline anisotropy field. Our method could shed light not only on understanding the fundamental causes of a wider SFD but also on designing future nanostructured magnetic devices.

Direct Measurement of the Magnetic Cluster Size Using High-Resolution Magnetic Force Microscopy

High-resolution magnetic force microscopy was used to image high linear density tracks and measure the magnetic cluster size of the ac-erased state of perpendicular magnetic recording media. The full-width at half-maximum (FWHM) of the auto-correlation function corresponding to an ac-erased region determines the correlation length, i.e., the magnetic cluster size. This is demonstrated by comparing the calculated FWHM of auto-correlation function obtained on simulated Voronoi granular structures with the expected value for magnetically decoupled grains. The cluster size we obtained using the auto-correlation procedure is about four grain sizes. The impact of the finite cluster size on the recording performance was assessed by following the trend of tracks' auto-correlation function with increasing the linear density.

A study on dynamic heat assisted magnetization reversal mechanisms under insufficient reversal field conditions

We report an experimental study on the dynamic thermomagnetic (TM) reversal mechanisms at around Curie temperature (Tc) for isolated 60 nm pitch single-domain [Co/Pd] islands heated by a 1.5 μm spot size laser pulse under an applied magnetic reversal field (Hr). Magnetic force microscopy (MFM) observations with high resolution MFM tips clearly showed randomly trapped non-switched islands within the laser irradiated spot after dynamic TM reversal process with insufficient Hr strength. This observation provides direct experimental evidence by MFM of a large magnetization switching variation due to increased thermal fluctuation/agitation over magnetization energy at the elevated temperature of around Tc. The average percentage of non-switched islands/magnetization was further found to be inversely proportional to the applied reversal field Hr for incomplete magnetization reversal when Hr is less than 13% of the island coercivity (Hc), showing an increased switching field distribution (SFD) at elevated temperature of around Tc (where main contributions to SFD broadening are from Tc distribution and stronger thermal fluctuations). Our experimental study and results provide better understanding and insight on practical heat assisted magnetic recording (HAMR) process and recording performance, including HAMR writing magnetization dynamics induced SFD as well as associated DC saturation noise that limits areal density, as were previously observed and investigated by theoretical simulations.

Magnetization behaviors of Fe3O4 nanoparticles studied by frequency-modulated magnetic force microscopy

Fe3O4 nanoparticles were synthesized through a sol–gel method. The structure and morphology features of the nanoparticles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution magnetic force microscopy (HR-MFM). The magnetization behaviors of Fe3O4 nanoparticles were investigated by using the frequency-modulated magnetic force microscopy (FM-MFM). It was observed that the prepared individual nanoparticles were within 20nm. The high resolution amplitude and phase images of the nanoparticles were obtained separately by using the FM-MFM technique with an AC magnetic field. The magnetic distribution of the individual nanoparticles (~20nm) was demonstrated by this technique.