Magnetic Force Microscopy Measurements Research Articles

Room-temperature skyrmion lattice in a layered magnet (Fe0.5Co0.5)5GeTe2.

Novel magnetic ground states have been stabilized in two-dimensional (2D) magnets such as skyrmions, with the potential next-generation information technology. Here, we report the experimental observation of a Néel-type skyrmion lattice at room temperature in a single-phase, layered 2D magnet, specifically a 50% Co–doped Fe5GeTe2 (FCGT) system. The thickness-dependent magnetic domain size follows Kittel’s law. The static spin textures and spin dynamics in FCGT nanoflakes were studied by Lorentz electron microscopy, variable-temperature magnetic force microscopy, micromagnetic simulations, and magnetotransport measurements. Current-induced skyrmion lattice motion was observed at room temperature, with a threshold current density, jth = 1 × 106 A/cm2. This discovery of a skyrmion lattice at room temperature in a noncentrosymmetric material opens the way for layered device applications and provides an ideal platform for studies of topological and quantum effects in 2D.

Tailoring magnetic properties by tuning the interface in a Pt/Co/Pt/IrMn system with perpendicular and double-exchange biases

We prepared Pt/Co/Pt(t Pt)/IrMn heterostructures with perpendicular exchange bias (PEB) by inserting a Pt spacer between Co and IrMn. X-ray diffraction demonstrates that the IrMn and Pt layers exhibit a (111) texture promoting PEB. Here, the samples for various Pt spacer thicknesses exhibit double-shifted hysteresis loops with the coexistence of positive and negative exchange biases (EBs). Magnetic force microscopy measurement indicates that this behavior in the as-grown state results from the formation of an antiferromagnetic bidomain state with opposite signs. Also, the magnetic loop behaviors can be tailored by tuning the EB, coercive and switching fields which are sensitive to sub-nanometer changes in the spacer layer (Pt) thickness. It is found that the optimum thickness of the Pt layer is 0.8 nm by considering a well-defined single remanence state, where H EB is about seven times as large as the coercivity. Our results indicate that large EB and rather small coercivity necessary for the single-remanence state in the double-shifted Pt/Co/Pt/IrMn systems can be achieved by tuning the interface at atomic level. Moreover, magnetic properties were analyzed in detail depending on Pt space spacer layer thickness between Co and IrMn layers. These results may be useful for potential applications in future multilevel memory devices.

Impact of ferromagnetic electrode length and thickness on Magnetic Tunnel Junction-Based Molecular Spintronic Devices (MTJMSD)

A knowledge gap exists about the impact of variation in length and thickness of ferromagnetic(FM) electrodes on molecular spintronics devices' ability to manifest molecular coupling impact. Magnetic Tunnel Junction-Based Molecular Spintronic Devices (MTJMSDs) are potential candidates for inventing highly correlated materials and devices and investigating the fundamental science of organic spintronics. This paper reports our experimental observations providing the dramatic impact of variation in ferromagnetic electrode length and thickness on paramagnetic molecule-based MTJMSD. Room temperature transport studies were performed to investigate the effect of FM electrode thickness. On the other hand, magnetic force microscopy measurements were conducted to understand the effect of FM electrode length extending beyond the molecular junction area, i.e., the site where paramagnetic molecules bridged between two FM. In the strong molecular coupling regime, transport study suggested thickness variation caused ∼1000 to million-fold differences in junction conductivity. MFM study revealed near-zero magnetic contrast for pillar-shaped MTJMSD without any extended FM electrode. However, MFM images showed a multitude of microscopic magnetic phases on cross junction shaped MTJMSD where FM electrodes extended beyond the junction area. To understand the intriguing experimental results, we conducted an in-depth theoretical study using Monte Carlo Simulation (MCS) approach. MCS study utilized a Heisenberg atomic model of cross junction shaped MTJMSD to gain insights about room temperature transport and MFM experimental observations of microscopic MTJMSD. To make this study applicable for a wide variety of MTJMSDs, we systematically studied the effect of variation in molecular coupling strength between magnetic molecules and ferromagnetic (FM) electrodes of various dimensions. Our theoretical results suggest 25 atoms thick FM electrode show very weak molecular coupling impact and of the same order of experimentally studied FM electrode dimensions showing weak effect. Also, MCS results agree with experimentally observed multiple magnetic phases on lengthy FM electrodes of an MTJMSD.

Assembled Comb-Drive XYZ-Microstage With Large Displacements and Low Crosstalk for Scanning Force Microscopy

This paper proposes and demonstrates a chip-level-microassembly comb-drive XYZ-microstage for providing large displacements and low crosstalk in scanning force microscopy applications at low temperatures. The three-dimensional structure of the comb-drive XYZ-microstage, consisting of an in-plane XY-microstage, two out-of-plane Z-actuators, and a base substrate, was accurately and orderly constructed using microassembly technology. This configuration can overcome the out-of-plane stroke-space limitation of conventional monolithic-wafer-based XYZ-microstages, and the crosstalk movements resulting from the coupling connection between in-plane and out-of-plane actuation units can be avoided. The in-plane actuation unit of the XY-microstage can provide low-crosstalk movements in the X- and Y-directions, due to the design of the decoupling-motion structure and constraint of the capacitance-coupling crosstalk of the actuation voltages. Folded-flexure springs with high stiffness were adopted in the XYZ-microstage to enhance the lateral stability of movable combs and improve the range of achievable strokes. The assembled comb-drive XYZ-microstage could provide quite large displacements of 49.2 <inline-formula> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$27.9~\mu \text{m}$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$50.5~\mu \text{m}$ </tex-math></inline-formula> in the X-, Y-, and Z-directions, respectively. Furthermore, to demonstrate the feasibility of the fabricated XYZ-microstage, a magnetic resonance force microscopy measurement system was constructed using the scanning XYZ-microstage with a specimen of 1, 1-Diphenyl-2-picrylhydrazyl radical. The magnetic resonance force from the electron spin resonance excited in the specimen could be detected by scanning the specimen in a specific resonance region. The results demonstrate that the proposed microassembly with the optimized actuation-unit structure is a promising means of establishing a comb-drive XYZ-microstage with large displacements, low crosstalk, and high adaptability. [2021-0112]

Tunable magnetic skyrmions in ferrimagnetic Mn4N

Thin films of ferrimagnetic Mn4N are candidate materials to host magnetic skyrmions that have demonstrated thermal stability up to 450 °C. However, there are no experimental reports observing skyrmions in this system. Here, we discuss the results of sputter grown 15–17 nm Mn4N thin films on the MgO substrate capped with Pt1−xCux layers. Vibrating sample magnetometry measurement of out-of-plane hysteresis loops confirmed that magnetic properties are insensitive to the cap layer composition. Imaging based on magnetic force microscopy measurements observed 300–50 nm sized skyrmions, as the Cu concentration was increased from x = 0–0.9. We performed density functional theory calculations and found that the interfacial Dzyaloshinskii–Moriya interactions (iDMI) follow a trend: Mn4N/MgO(001) &amp;lt; Cu/Mn4N(001) &amp;lt; Pt/Mn4N(001). We infer from these calculations that x in the Pt1−xCux capping layer can serve as a robust tuning knob to tailor the iDMI and control the skyrmion size. This work provides guidance to achieve smaller Néel-type skyrmions in Mn4N thin films, which is an important step forward for building thermally stable skyrmionic devices.

Magnetic Force Microscopy on Nanofibers—Limits and Possible Approaches for Randomly Oriented Nanofiber Mats

Magnetic force microscopy (MFM) belongs to the methods that enable spatially resolved magnetization measurements on common thin-film samples or magnetic nanostructures. The lateral resolution can be much higher than in Kerr microscopy, another spatially resolved magnetization imaging technique, but since MFM commonly necessitates positioning a cantilever tip typically within a few nanometers from the surface, it is often more complicated than other techniques. Here, we investigate the progresses in MFM on magnetic nanofibers that can be found in the literature during the last years. While MFM measurements on magnetic nanodots or thin-film samples can often be found in the scientific literature, reports on magnetic force microscopy on single nanofibers or chaotic nanofiber mats are scarce. The aim of this review is to show which MFM investigations can be conducted on magnetic nanofibers, where the recent borders are, and which ideas can be transferred from MFM on other rough surfaces towards nanofiber mats.

Magneto-electric coupled ordered PMN-PT/NiFe2O4 composite nanostructures

In this work, a well-ordered array of multiferroic magnetoelectric (ME) dot-like nanostructures of Pb(Mg1/3Nb2/3)O3]0.65–[PbTiO3]0.35 (PMN-PT)/NiFe2O4 is explored for high density and low power consuming memory devices. Ordered arrays of ferromagnetic NiFe2O4 nanodots underneath a ferroelectric PMN-PT layer were fabricated using silicon nitride based stencil masks and pulsed laser deposition techniques. The piezo-response and magnetic force microscopy (PFM) measurements reveal coexistence of magnetic and ferroelectric domains in PMN-PT/NiFe2O4 films at room temperature. The ferroelectric polarization can be switched with the electrically biased PFM tip. The ME coupling is evident in the PMN-PT/NiFe2O4 films, which is attributed to the transfer of the elastic strain from PMN-PT to NiFe2O4. The PMN-PT/NiFe2O4 nanodot films exhibit enhanced ME coupling coefficient (α) as compared to continuous bilayer PMN-PT/NiFe2O4 films, owing to the superior strain transfer efficiency in nanodot heterostructures. The nanodot films demonstrate electric-field controlled nonvolatile switching of α, which can be used to store binary information in memory devices, holding all the advantages of ferroelectric random access memory but overcoming the major disadvantage of destructive reading of polarization. The results reveal a versatile approach for fabrication of well-ordered nanodot arrays for low power consuming, high-density ME device applications.

Domain state modulation by interfacial diffusion in FePt/FeCo thin films: experimental approach with micromagnetic modelling

We report the complex implications of inter-diffusion between polycrystalline FePt/FeCo layers as an impact of the FeCo underlayer on the structural and magnetic properties of the system. The crystalline growth of FePt strongly reduces in an entirely diffused system compared to the one with lesser diffusion, while the crystalline structure of FeCo is apparently less affected. Charge redistribution occurs between Fe, Co and Pt ensuring increased Co–Pt and Fe–Pt interactions with higher diffusion. Thereafter, we combine hysteresis and magnetic force microscopy measurements to show that the interfacial deformations result in the distinct out-plane magnetic behaviour of the system. FeCo@FePt nano-composite like structure, originating due to interfacial diffusion, shows interactions between two magnetic phases with in-plane low anisotropy exhibiting wasp-shaped out-plane hysteresis loop. Whereas the layered structure of FePt/FeCo films shows random anisotropy with a significant out-plane contribution even in the polycrystalline films. Micromagnetic modelling demonstrates coercivity deterioration and reduction of switching field due to the formation of a slightly diffused interface. Contrarily, the experimental observations for complete diffusion between the two layers are explained by simulating the inhomogeneous distribution of anisotropies along the film plane. These studies provide deep perceptions of the magnetic properties of FePt/FeCo system governed by diffusion kinetics which are valuable to achieve desired magnetic characteristics using this system.

Emergence of exchange bias field in FeS superconductor with cobalt-doping

Among all the iron-based superconductors, the 11 series has the simplest layered structure but exhibits rich physical phenomenon. In this work, we have synthesized Fe1−x Co x S single crystals with tetragonal structure and studied their structure and magnetic properties. Magnetic susceptibility measurements indicate that the cobalt doping would suppress superconductivity and even introduce weak ferromagnetism besides antiferromagnetism. Scanning electron microscopy study reveals that the Co-doped samples exhibit intrinsic phase separation. Moreover, magnetic force microscopy measurement shows no magnetic domain in Fe1−x Co x S, indicating that neither phase is pure ferromagnetic. The coexistence of ferromagnetism and antiferromagnetism leads to the relatively large exchange bias field. Since the exchange bias effect has been widely used in the field of information storage, spin-valves, and magnetic tunnel junctions, our study provides another option for further application.

A Ti/Pt/Co Multilayer Stack for Transfer Function Based Magnetic Force Microscopy Calibrations

Magnetic force microscopy (MFM) is a widespread technique for imaging magnetic structures with a resolution of some 10 nanometers. MFM can be calibrated to obtain quantitative (qMFM) spatially resolved magnetization data in units of A/m by determining the calibrated point spread function of the instrument, its instrument calibration function (ICF), from a measurement of a well-known reference sample. Beyond quantifying the MFM data, a deconvolution of the MFM image data with the ICF also corrects the smearing caused by the finite width of the MFM tip stray field distribution. However, the quality of the calibration depends critically on the calculability of the magnetization distribution of the reference sample. Here, we discuss a Ti/Pt/Co multilayer stack that shows a stripe domain pattern as a suitable reference material. A precise control of the fabrication process, combined with a characterization of the sample micromagnetic parameters, allows reliable calculation of the sample’s magnetic stray field, proven by a very good agreement between micromagnetic simulations and qMFM measurements. A calibrated qMFM measurement using the Ti/Pt/Co stack as a reference sample is shown and validated, and the application area for quantitative MFM measurements calibrated with the Ti/Pt/Co stack is discussed.

Investigation of structural and optical properties of Pb1-xCoxS nanocrystals embedded in chalcogenide glass

Diluted magnetic semiconductor Pb1-xCoxS nanocrystals with nominal concentrations of Co (x = 0.00, 0.10 and 0.16) embedded in a chalcogenide glass matrix were synthesized by the low-cost fusion-nucleation method that allows for the manipulation, control, and tuning of the structural, optical, and electronic properties. Transmission electron microscopy images showed the formation and growth of the Pb1-xCoxS nanocrystals to be dependent on the thermal annealing time at 500 °C. Energy dispersive X-ray spectroscopy analysis confirmed the chemical elements Pb, S and Co in the nanocrystal. X-ray diffraction measurements showed characteristic peak patterns of the PbS rock-salt crystal structure with displacement to greater angles of the plane (111) with the xCo doping. Magnetic force microscopy measurements showed the magnetic phase-contrast patterns, providing additional evidence of incorporation of Co into the PbS structure. The energy parameters of the crystal field strength (Δ = 3898 cm−1), Racah (B = 794 cm−1) and spin-orbit coupling (λ = - 555 cm−1) that are part of the sp-d exchange interactions of Co2+ ions in tetrahedral sites [CoS4]6- in Pb1-xCoxS nanocrystals were determined by UV-VIS-NIR optical absorption spectra and the crystalline field theory from the Tanabe-Sugano diagram for d7 ion in a ligand tetrahedral.

Investigation of self-nucleated skyrmion states in the ferromagnetic/nonmagnetic multilayer dot

Understanding the stability of magnetic textures in multilayer patterned dots would constitute a significant step toward skyrmion-based applications. Here, we report the observation of skyrmions in patterned nanodots composed of multilayers. We examine the stabilization of various magnetic states such as single-domain states, skyrmion states, horseshoe-like domain structures, and worm-like domain structures in submicrometer dots (diameters 150–525 nm). Dots are fabricated from Pt/Co/Au multilayer structures that exhibit the interfacial Dzyaloshinskii–Moriya interaction and perpendicular magnetic anisotropy. In particular, we show that a stack of six repetitions of Pt/Co/Au layers suffices to stabilize the skyrmion state inside a dot at room temperature. A micromagnetic simulation determines the regime of skyrmion stability. The results reveal a correlation between the magnetic-force microscopy measurements and the micromagnetic simulation. Furthermore, we explain the development of the magnetic state with increasing dot diameter. We envision that nanopatterning of multilayer magnetic films could serve as a versatile way of creating magnetic skyrmion states.

Real-space observation of ferroelectrically induced magnetic spin crystal in SrRuO3

Unusual features in the Hall Resistivity of thin film systems are frequently associated with whirling spin textures such as Skyrmions. A host of recent investigations of Hall Hysteresis loops in SrRuO3 heterostructures have provided conflicting evidence for different causes for such features. We have constructed an SrRuO3-PbTiO3 (Ferromagnetic – Ferroelectric) bilayer that exhibits features in the Hall Hysteresis previously attributed to a Topological Hall Effect, and Skyrmions. Here we show field dependent Magnetic Force Microscopy measurements throughout the key fields where the ‘THE’ presents, revealing the emergence to two periodic, chiral spin textures. The zero-field cycloidal phase, which then transforms into a ‘double-q’ incommensurate spin crystal appears over the appearance of the ‘Topological-like’ Hall effect region, and develop into a ferromagnetic switching regime as the sample reaches saturation, and the ‘Topological-like’ response diminishes. Scanning Tunnelling Electron Microscopy and Density Functional Theory is used to observe and analyse surface inversion symmetry breaking and confirm the role of an interfacial Dzyaloshinskii–Moriya interaction at the heart of the system.

Preliminary report on MFM measurements on magnetic nanofiber mats

Nanofiber mats can be produced unambiguously by electrospinning. Besides pure polymers or polymer blends, such nanofibers can also contain metals, ceramics, etc., often introduced in the form of nanoparticles embedded in the spinning solution. Especially in case of magnetic nanoparticles, the physical properties of the whole nanofiber mats will strongly depend on the dispersion of the nanoparticles in the fibers – while small single nanoparticles may show superparamagnetic behavior, larger agglomerations will rather tend to showing ferromagnetic properties. Investigations of the magnetic properties of a sample with high spatial resolution are mostly performed by magnetic force microscopy (MFM). This technique, however, is usually applied on very flat surfaces of thin-film or nanostructured samples. Here, we report for the first time on MFM measurements on magnetic nanofiber mats, proving in principle that this technique can be used to investigate magnetic nanofiber mats, while the highly uneven nanofiber structure still causes large problems which have to be solved in the future.

Room-temperature ferromagnetism in oxidized-graphenic nanoplatelets induced by topographic defects

This work reports on room-temperature ferromagnetism of pyrolytic oxidized-graphene nanoplatelets obtained from bamboo pyroligneous acid by varying the density of extended defects. Topographic defects, created during the fabrication process, arise from a natural formation of clusters; such clusters drastically distort the graphitic basal plane, giving rise to abrupt surface curvatures. Topographic defects were found to be sources of the magnetic signal, as evidenced by bulk magnetization and magnetic force microscopy measurements. Increased defect density, tuned by carbonization temperature, results in enhanced magnetization.

Magnetic Imaging of Encapsulated Superparamagnetic Nanoparticles by Data Fusion of Magnetic Force Microscopy and Atomic Force Microscopy Signals for Correction of Topographic Crosstalk.

Encapsulated magnetic nanoparticles are of increasing interest for biomedical applications. However, up to now, it is still not possible to characterize their localized magnetic properties within the capsules. Magnetic Force Microscopy (MFM) has proved to be a suitable technique to image magnetic nanoparticles at ambient conditions revealing information about the spatial distribution and the magnetic properties of the nanoparticles simultaneously. However, MFM measurements on magnetic nanoparticles lead to falsifications of the magnetic MFM signal due to the topographic crosstalk. The origin of the topographic crosstalk in MFM has been proven to be capacitive coupling effects due to distance change between the substrate and tip measuring above the nanoparticle. In this paper, we present data fusion of the topography measurements of Atomic Force Microscopy (AFM) and the phase image of MFM measurements in combination with the theory of capacitive coupling in order to eliminate the topographic crosstalk in the phase image. This method offers a novel approach for the magnetic visualization of encapsulated magnetic nanoparticles.

Domain Structure and Reversal Mechanisms through Diffracted Magneto-optics in Fe80B20 Microsquare Arrays

In this paper, the predictive power of diffracxtive magneto-optics concerning domain structure and reversal mechanisms in ordered arrays of magnetic elements is demonstrated. A simple theoretical model based on Fraunhoffer diffraction theory is used to predict the magnetisation reversal mechanisms in an array of magnetic elements. Different domain structures and simplified models (or educated guesses) of the associated reversal mechanisms produce marked differences in the spatial distributions of the magnetisation. These differences and the associated magnetisation distribution moments are experimentally accessible through conventional and diffractive magneto-optical Kerr effect measurements. The domain and magnetisation reversal predictions are corroborated with Magnetic Force Microscopy (MFM) measurements.

Growth and characterization of ferromagnetic Fe-doped GaSb quantum dots with high Curie temperature

We report the structural and magnetic properties of the Fe-doped GaSb quantum dots (QDs) (nominal Fe concentration x = 4.7%–16.6%) grown on GaAs (001) substrates by molecular beam epitaxy. The QDs with nanometer-scale dimensions consist of two areas with different crystal structures, a zinc-blende GaAsSb wetting layer and a new phase of FeGaSb alloy that has a simple cubic lattice. The size and distribution of the QDs depend on the Fe concentration, as revealed by atomic force microscopy. Magnetic force microscopy measurements at zero applied magnetic field show the presence of ferromagnetism in the QDs at room temperature with an easy axis in the 1¯10 direction, which is consistent with magnetometry measurements. The Curie temperature in these QDs is very high (&amp;gt;400 K), which is promising for spintronic applications at room temperature.

Plasma-Assisted Chemical Vapor Deposition of F-Doped MnO2 Nanostructures on Single Crystal Substrates.

MnO2 nanostructures were fabricated by plasma assisted-chemical vapor deposition (PA-CVD) using a fluorinated diketonate diamine manganese complex, acting as single-source precursor for both Mn and F. The syntheses were performed from Ar/O2 plasmas on MgAl2O4(100), YAlO3(010), and Y3Al5O12(100) single crystals at a growth temperature of 300 °C, in order to investigate the substrate influence on material chemico-physical properties. A detailed characterization through complementary analytical techniques highlighted the formation of highly pure and oriented F-doped systems, comprising the sole β-MnO2 polymorph and exhibiting an inherent oxygen deficiency. Optical absorption spectroscopy revealed the presence of an appreciable Vis-light harvesting, of interest in view of possible photocatalytic applications in pollutant degradation and hydrogen production. The used substrates directly affected the system structural features, as well as the resulting magnetic characteristics. In particular, magnetic force microscopy (MFM) measurements, sensitive to the out-of-plane magnetization component, highlighted the formation of spin domains and long-range magnetic ordering in the developed materials, with features dependent on the system morphology. These results open the door to future engineering of the present nanostructures as possible magnetic media for integration in data storage devices.

Influence of thermal treatment on the magnetic properties and morphology of electrodeposited Fe-Co films

Fe-Co films were prepared by electrodeposition from an electrolyte containing citric acid and annealed afterwards at different temperatures up to 700 °C. The grain sizes of the deposits increased from 11 nm to 30 nm with increasing annealing temperature, which lead to changes in magnetic properties of deposits. SQUID measurements indicate that there is a difference between the coercive force, Hc, of the as deposited samples (17 Oe) and the heat-treated samples (25–40 Oe) with a measurement error of 1–2 Oe. The remnant magnetization, Mr, decreased from 190 ± 12 emu/cm3 for the as deposited to 75 ± 5 emu/cm3 for the annealed samples, respectively. The saturation magnetization, Ms, seems not to be influenced strongly by the thermal treatment, with the only exception for the samples annealed at 500 °C. Thus, Ms has a slightly decreasing tendency from 1880 ± 90 emu/cm3 for the as-deposited samples to 1780 ± 85 emu/cm3 for samples annealed at 700 °C. The biggest value for Ms (2100 ± 105 emu/cm3) was obtained if the samples were annealed at 500 °C. The thermal treatment generated cracks in the deposits. Interestingly, these cracks had a regular rectangular shape only if the deposits were annealed at 600 °C. The coercivity of the layers annealed at 600 °C was lower compared to layers annealed at the other temperatures. Magnetic force microscopy measurements indicated the magnetic domain distribution and the topography of the annealed deposits. The deposits showed the best soft magnetic properties if annealed between 500 and 600 °C.