Magnetic Force Microscopy Technique Research Articles

Structure, control, and dynamics of altermagnetic textures

We present a phenomenological theory of altermagnets, that captures their unique magnetization dynamics and allows modeling magnetic textures in this new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g., RuO2, we can explain intuitively the characteristic lifted degeneracy of their magnon spectra, by the emergence of an effective sublattice-dependent anisotropic spin stiffness arising naturally from the phenomenological theory. We show that as a consequence the altermagnetic domain walls, in contrast to antiferromagnets, have a finite gradient of the magnetization, with its strength and gradient direction connected to the altermagnetic anisotropy, even for 180° domain walls. This gradient generates a ponderomotive force in the domain wall in the presence of a strongly inhomogeneous external magnetic field, which may be achieved through magnetic force microscopy techniques. The motion of these altermagentic domain walls is also characterized by an anisotropic Walker breakdown, with much higher speed limits of propagation than ferromagnets but lower than antiferromagnets.

Simultaneous magnetic field and field gradient mapping of hexagonal MnNiGa by quantitative magnetic force microscopy

Magnetic force microscopy (MFM) is a scanning microscopy technique that is commonly employed to probe the sample’s magnetostatic stray fields via their interaction with a magnetic probe tip. In this work, a quantitative, single-pass MFM technique is presented that maps one magnetic stray-field component and its spatial derivative at the same time. This technique uses a special cantilever design and a special high-aspect-ratio magnetic interaction tip that approximates a monopole-like moment. Experimental details, such as the control scheme, the sensor design, which enables simultaneous force and force gradient measurements, as well as the potential and limits of the monopole description of the tip moment are thoroughly discussed. To demonstrate the merit of this technique for studying complex magnetic samples it is applied to the examination of polycrystalline MnNiGa bulk samples. In these experiments, the focus lies on mapping and analyzing the stray-field distribution of individual bubble-like magnetization patterns in a centrosymmetric [001] MnNiGa phase. The experimental data is compared to calculated and simulated stray-field distributions of 3D magnetization textures, and, furthermore, bubble dimensions including diameters are evaluated. The results indicate that the magnetic bubbles have a significant spatial extent in depth and a buried bubble top base.

Poly(vinyl alcohol)/sodium alginate/magnetite composites: magnetic force microscopy for tracking magnetic domains.

Hydrogels of poly(vinyl alcohol) (PVA)/sodium alginate (SA), and magnetic nanoparticles (MNPs) were prepared by solvent casting in the absence and in the presence of magnets, in order to obtain MNPs distributed randomly (PVA/SA-rMNP) and magnetically oriented MNPs (PVA/SA-gMNP) in the polymer matrix. Atomic force microscopy (AFM) and magnetic force microscopy (MFM) techniques were used to evaluate the topography and to map the distribution of magnetic domains in the polymer matrix, respectively. The tip-surface distance (lift distance) of 50 nm during the MFM analyses facilitated the mapping of magnetic domains because the van der Waals forces were minimized. The magnetic signal stemming from clusters of MNPs were more easily identified than that from isolated MNPs. PVA and SA, PVA/SA, PVA/SA-rMNP, and PVA/SA-gMNP coatings with surface roughness (Ra) values of 3.8 nm, 28.7 nm, and 49.8 nm, respectively, were tested for the proliferation of mouse hippocampal HT-22 cells. While PVA/SA, PVA/SA-rMNP, and PVA/SA-gMNP coatings preserved cell viability >70% in comparison to the control (plastic plate) over 48 h, cell proliferation tended to decrease on surfaces with higher Ra values (PVA/SA-gMNP). These findings showed that the orientation of magnetic domains led to an increase of surface roughness, which decreased the viability of HT-22 cells. Thus, these results might be interesting for situations, where the control of cell proliferation is necessary.

Single-pass magnetic force microscopy technique, with topography feedback based on scanning polarization force microscopy

We propose the use of a hybrid scanning probe microscopy technique, which combines magnetic force microscopy with scanning polarization force microscopy, for rapid scans of relatively large areas. Topography feedback relies on maintaining a constant electric interaction between tip and sample, and is achieved by monitoring the off-resonance tip oscillation induced by electric modulation. Magnetic sensing is based on recording changes of the amplitude and phase of the cantilever’s mechanically modulated oscillation at resonance. By using electric interaction instead of van der Waals for topography feedback, the tip is held in feedback at relatively large distances from the surface – tens to hundreds of nanometers. This reduces the cross-talk between topography and magnetic signals, thus eliminating the need for two-pass scanning. Moreover, it allows imaging at higher speeds with lower risk of tip damage, opening this field to studies of dynamic phenomena. Advantages and disadvantages of this technique are discussed.

High-resolution alternating magnetic force microscopy using an amorphous FeB-based tip driven by an inverse magnetostrictive effect: Imaging of the high-density magnetic recording media

The improved spatial resolution of the alternating magnetic force microscopy (A-MFM) technique developed in this work is demonstrated using a tip coated with a very thin magnetostrictive Fe-B-based amorphous film. Its magnetization can be periodically reversed using an applied alternating current magnetic field, which leads to alternating mechanical stress at the tip end, thus affecting localized magnetization reversal via an inverse magnetostrictive effect. Spatial resolution improvement is achieved via effective reversible magnetization volume at the sharpened tip end, which promotes a decrease in the total magnetic volume and maintains sufficient sensitivity at a lower coating thickness. A-MFM uses frequency modulation induced by an off-resonant alternating magnetic force between the tip and the sample. Therefore, a magnetic force can be selectively detected near the sample surface, independent of non-magnetic interactions. Due to their smooth surfaces without crystalline grain boundaries, large magnetization, and large magnetostriction compared with those of other magnetic materials, Fe-based amorphous films are suitable for tip coating applications. A 500-kfci perpendicular magnetic recording medium (PMR) was imaged using a 6-nm thick amorphous Fe-B coated tip to produce highly resolved images with minimal detection of the wavelength of the magnetic field of less than 10 nm. Also, 1600-kfci PMR imaging was successfully achieved, with the images showing individual ∼ 15-nm bits.

DJ-1 Acts as a Scavenger of α-Synuclein Oligomers and Restores Monomeric Glycated α-Synuclein.

Glycation of α-synuclein (αSyn), as occurs with aging, has been linked to the progression of Parkinson’s disease (PD) through the promotion of advanced glycation end-products and the formation of toxic oligomers that cannot be properly cleared from neurons. DJ-1, an antioxidative protein that plays a critical role in PD pathology, has been proposed to repair glycation in proteins, yet a mechanism has not been elucidated. In this study, we integrate solution nuclear magnetic resonance (NMR) spectroscopy and liquid atomic force microscopy (AFM) techniques to characterize glycated N-terminally acetylated-αSyn (glyc-ac-αSyn) and its interaction with DJ-1. Glycation of ac-αSyn by methylglyoxal increases oligomer formation, as visualized by AFM in solution, resulting in decreased dynamics of the monomer amide backbone around the Lys residues, as measured using NMR. Upon addition of DJ-1, this NMR signature of glyc-ac-αSyn monomers reverts to a native ac-αSyn-like character. This phenomenon is reversible upon removal of DJ-1 from the solution. Using relaxation-based NMR, we have identified the binding site on DJ-1 for glycated and native ac-αSyn as the catalytic pocket and established that the oxidation state of the catalytic cysteine is imperative for binding. Based on our results, we propose a novel mechanism by which DJ-1 scavenges glyc-ac-αSyn oligomers without chemical deglycation, suppresses glyc-ac-αSyn monomer–oligomer interactions, and releases free glyc-ac-αSyn monomers in solution. The interference of DJ-1 with ac-αSyn oligomers may promote free ac-αSyn monomer in solution and suppress the propagation of toxic oligomer and fibril species. These results expand the understanding of the role of DJ-1 in PD pathology by acting as a scavenger for aggregated αSyn.

Observation of interacting Josephson vortex chains by magnetic force microscopy

The ability to control Josephson vortices is instrumental for development of superconducting cryoelectronics. However, direct visualization of multivortex states in Josephson junctions is a challenging task. Here, we employ a magnetic force microscopy (MFM) for the analysis of planar Josephson junctions. We observe a specific MFM response, seen as a chain of small rings. By changing the applied field, we show that the number of rings is equal to the number of flux quanta in the junction. Therefore, each ring represents an individual vortex in a one-dimensional vortex chain within the junction. Our observation demonstrates that the MFM technique can be used for visualization of Josephson vortices and for probing their spatial configurations and mutual interaction.

Microwave assisted gating of spin wave propagation

Magnonics is an emerging research area where magnons or spin waves are used as a medium of information processing. Efficient manipulation/gating of magnons on-chip is crucial for realization of logic circuitry and device integration. Here, we show a simple method for gating of the magnons in a magnetic wire based on the dipolar coupled chain of nanomagnets. Spin wave propagation has been directly measured using the micro-Brillouin light scattering technique. We observed a significant reduction of spin wave amplitude by switching the nanomagnets using microwave current through a coplanar waveguide, which was also used for spin wave generation. Microwave assisted magnetization switching has been probed using the magnetic force microscopy technique. The results have potential implications in the area of wave based devices for next generation high frequency communication technologies.

Reconfigurable magnetic and microwave properties of a ferrimagnetic-type artificial crystal

We have demonstrated a ferrimagnetic-type anti-parallel remanent state of rhomboid shaped nanomagnets of two different widths alternated in a dipolar coupled nanomagnetic array. The anti-parallel state is reconfigured to a parallel remanent state by using a simple field initialization process. Magnetic states were directly visualized using the magnetic force microscopy technique. Due to a variation of the stray field distributions for the two different remanent states, distinct dynamic responses are found using the broadband ferromagnetic resonance spectroscopy technique without applying any external bias magnetic field. Experimental observations were supported by the micromagnetic simulations. The results are important in the context of bias-field-free and reprogrammable microwave magnetic devices and creation of artificial ferrimagnetic crystals.

AFM and MFM techniques for enzyme activity imaging and quantification

ABSTRACTIn this work, a new approach for enzyme activity monitoring is suggested. It is based on the real-time imaging, by atomic force microscopy (AFM) and magnetic force microscopy (MFM), of the degradation of a nanoparticles-loaded enzyme responsive layer, namely Fe3O4-nanoparticles-loaded gelatin. The roughness analysis allowed the quantification of the trypsin-induced ferrogel degradation and the correlation of the AFM and MFM data obtained.

Fabrication and Characterization of Permanent Magnetic SmCo<sub>5 </sub>Thin Films by SQUID Magnetometer

Fabrication of hard magnetic thin films is a key issue on the development of new micro electro mechanical systems. As the magnetically hard SmCo thin-films offer excellent intrinsic magnetic properties, such as moderate saturation magnetization, large magnetic anisotropy, and high Curie temperature, they are considered as a promising candidate to be used for novel MEMS applications. In this work, SmCo5 thin films with Cu underlayer were grown onto Si (100) substrate at room temperature by RF magnetron sputtering technique. The samples were annealed at 400 ̊C and 500 ̊C under Ar atmosphere condition. Microstructural and magnetic properties of sputtered SmCo5 thin films were investigated by a number of advanced characterization tools and techniques. Phase composition of SmCo5 thin films was analyzed by grazing incident X-ray diffraction (GIXRD) with Cu-Kα radiation. Surface morphology was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Magnetic force microscopy (MFM) technique was used to take stray-field images of SmCo5 thin films, and finally magnetic properties were investigated to explain the coercivity of SmCo5 thin films using superconducting quantum interference device (SQUID) as a magnetometer.

Direct and Indirect Magnetic Force Microscopy in Histology

Iron (Fe) is an essential metal involved in a wide spectrum of physiological functions. Sub-cellular characterization of the size, composition, and distribution of ferritin(iron) can provide valuable information on iron storage and transport in health and disease. While histochemical stains (e.g. Perls stain) can provide a rapid evaluation of iron, their spatial resolution is limited. Ultra-structural characterization of the distribution and composition of iron deposits can be accomplished using transmission electron microscopy (TEM) based techniques. However, TEM involves stringent sample preparation protocols, which could alter the state of iron. Magnetism-based microscopy can serve as an attractive approach to characterize iron deposits in situ in biological tissue sections and serve as a bridge between histochemical staining, TEM studies and MRI analysis. In this study we employ the scanning probe based technique, namely magnetic force microscopy (MFM) to map sub-micron iron deposits in tissue sections. We demonstrate how MFM can identify ferritin(iron) rich lysosomes in tissue sections compatible with histochemical staining. Quantitative analysis of MFM data can reveal differences in lysosomal ferritin(iron) content. We further present a novel indirect MFM technique, which preserves the high spatial resolution and sensitivity of the conventional or direct MFM but enables high-throughput detection of magnetic domains. In indirect MFM an ultrathin membrane separates the sample and the MFM probe, thus minimizing the effect of sample topography in MFM imaging. Samples prepared for indirect MFM are compatible with multimodal imaging using light and electron microscopy. We thus elucidate the potential of direct and indirect MFM in analyzing iron content in histology.

Observation and manipulation of magnetic domains in sol gel derived thin films of spinel ferrites

Thin films of spinel ferrites, namely zinc substituted nickel, cobalt ferrite, and manganese substituted cobalt ferrite, were synthesized using sol-gel derived spin-coating techniques. The films were characterized using x-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectroscopy and Raman spectroscopy techniques for the analysis of structural, morphological and vibrational band transition properties, which confirm the spinel phase formation of the films. The magnetic force microscopy (MFM) technique was used to observe the magnetic domain structure present in the synthesized films. Further, the films were subjected to an external DC magnetic field of 2 kG to orient the magnetic domains and analyzed using an ex situ MFM technique.

Relationship between microstructural and magnetic properties of PrCo-based films prepared by the vacuum evaporation method

In this work, Ce2Ni7 type structural PrCo-based films were deposited on Si(100) substrate by ultra-high (UHV) vacuum evaporation process. The structural and magnetic properties of these films have been performed using X-ray diffraction (XRD), atomic force microscopy (AFM), vibrating sample magnetometer (VSM) and magnetic force microscopy (MFM) techniques. Two effects on structural and magnetic properties of PrCo films have been investigated: the effect of the annealing temperature (Ta) and the effect of the variation of the magnetic X-layer thickness. The as deposited PrCo films have a magnetic coercivity (Hc) of about 40–100 Oe. But after annealing at 600 °C, Hc has increased hight about 9.5 kOe for PrCo(X = 20 nm) and 10.2 kOe for PrCo(X = 50 nm) were observed. The magnetic properties were affected by the thickness due to the morphology, also the relationship between the intergrain exchange coupling (IEC), the size and quantity of the PrCo grains. The hight extrinsic properties of Hc = 10.2 kOe, maximum energy product (BH)max of 5.12 MGOe and remanence ratio Mr/Ms = 0.53 are reported for the PrCo(X = 50 nm) films. These properties are highly desirable for extremely high-density magnetic recording media applications.

Tunable room-temperature magnetic skyrmions in Ir/Fe/Co/Pt multilayers.

Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

Anisotropy Factors of Co-Based Thin Films Computed from FMR and AGFM Studies

We deposited Co x Cr100−x thin films, under vacuum, onto Si (100) substrates; x is the cobalt proportion. The hysteresis loops in the longitudinal and the polar configurations are obtained by means of an alternating gradient field magnetometer (AGFM) at room temperature. The magnetic force microscopy technique is used to observe domain configurations, especially for the thickest films. The magnetic anisotropy of the films is investigated using ferromagnetic resonance (FMR) techniques. The thinnest films do not present any domain configuration whereas the thickest films exhibit clear stripe domains with a period sensibly equal to the film thickness. The magnetic anisotropy constants are computed using the static and the dynamic magnetic studies. The K i values, issued from these techniques, are compared. These results and others are presented and discussed.

Magnetic vortex chirality determination via local hysteresis loops measurements with magnetic force microscopy.

Magnetic vortex chirality in patterned square dots has been investigated by means of a field-dependent magnetic force microscopy technique that allows to measure local hysteresis loops. The chirality affects the two loop branches independently, giving rise to curves that have different shapes and symmetries as a function of the details of the magnetisation reversal process in the square dot, that is studied both experimentally and through micromagnetic simulations. The tip-sample interaction is taken into account numerically, and exploited experimentally, to influence the side of the square where nucleation of the vortex preferably occurs, therefore providing a way to both measure and drive chirality with the present technique.

Nanoscopic Investigation of Magnetic Thin Films by Means of Magnetic Force Microscopy Technique

In the article the results of the research aimed at the recognition of the correlation between the Ni‐Fe film thickness and its magnetic domain structures are described. Magnetic thin films were prepared by pulse magnetron sputtering. Obtained thin film thicknesses were in the range of 47.6‐326.0 nm. Magnetic domain structures were imaged using magnetic force microscopy. In order to obtain quantitative description of magnetic domains images, the algorithms designed for topography parameters determination were applied, enabling the comparison of specific factors related to the magnetic properties of the samples. Utilized approach provided the analysis of the impact of sputtering parameters on the morphological and magnetic properties of obtained films. DOI: 10.12693/APhysPolA.129.1226 PACS/topics: 81.15.Cd, 68.37.Rt, 75.70.Kw

Removal of electrostatic artifacts in magnetic force microscopy by controlled magnetization of the tip: application to superparamagnetic nanoparticles.

Magnetic force microscopy (MFM) has been demonstrated as valuable technique for the characterization of magnetic nanomaterials. To be analyzed by MFM techniques, nanomaterials are generally deposited on flat substrates, resulting in an additional contrast in MFM images due to unavoidable heterogeneous electrostatic tip-sample interactions, which cannot be easily distinguished from the magnetic one. In order to correctly interpret MFM data, a method to remove the electrostatic contributions from MFM images is needed. In this work, we propose a new MFM technique, called controlled magnetization MFM (CM-MFM), based on the in situ control of the probe magnetization state, which allows the evaluation and the elimination of electrostatic contribution in MFM images. The effectiveness of the technique is demonstrated through a challenging case study, i.e., the analysis of superparamagnetic nanoparticles in absence of applied external magnetic field. Our CM-MFM technique allowed us to acquire magnetic images depurated of the electrostatic contributions, which revealed that the magnetic field generated by the tip is sufficient to completely orient the superparamagnetic nanoparticles and that the magnetic tip-sample interaction is describable through simple models once the electrostatic artifacts are removed.

Changes in magnetic domain structure during twin boundary motion in single crystal Ni-Mn-Ga exhibiting magnetic shape memory effect

The complexity of Ni-Mn-Ga single crystal originates from the interplay between ferromagnetic domain structure and ferroelastic twinned microstructure. Magnetic domain structure in the vicinity of single twin boundary was studied using magneto-optical indicator film and magnetic force microscopy technique. The single twin boundary of Type I was formed mechanically and an initial magnetization state in both variants were restored by local application of magnetic field (≈40 kA/m). The differently oriented variants exhibited either stripe or labyrinth magnetic domain pattern in agreement with the uniaxial magnetocrystalline anisotropy of the martensite. The twin boundary was then moved by compressive or tensile stress. The passage of the boundary resulted in the formation of granular or rake domains, respectively. Additionally, the specific magnetic domains pattern projected by twin boundary gradually vanished during twin boundary motion.