Magnetic Force Microscopy Data Research Articles

Quantitative Magnetic Force Microscopy: Transfer-Function Method Revisited

Magnetic force microscopy (MFM) is a characterization method used to obtain images of the stray field emanating from the surface of magnetic samples with high spatial resolution. The MFM signal arises from a convolution of the magnetic moment distribution of the MFM tip with the stray field of the sample. Therefore, deconvolution of the stray field of a magnetic sample requires a well-defined micromagnetic structure of a tip. While the magnetic moment distribution of the tip remains challenging to assess, it is convenient to calibrate the tip-equivalent magnetic charge in a plane parallel to the sample surface running through the tip apex using a suitable tip-calibration method. Once the tip-equivalent magnetic charge is calibrated, MFM data can be deconvolved to obtain the stray field of the sample or the equivalent magnetic surface charge, for example, at the sample surface. Here we use low-noise MFM data obtained with high-quality-factor cantilevers and a modified L-curve method for the optimization of a Tikhonov deconvolution procedure used for the calibration of the MFM tip, and---once the tip is calibrated---for obtaining the stray field or magnetic surface charge of the sample.

Visualizing Atomically Layered Magnetism in CrSBr.

2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual atomic layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extracted from MFM data and force-distance curves, revealing a characteristic onset of both intra- and interlayer magnetic correlations as a function of temperature and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temperature antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temperatures close to the bulk Néel temperature (TN ). Furthermore, the AFM phase can be reliably suppressed using modest fields (≈16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the critical role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).

Domain Wall Patterning and Giant Response Functions in Ferrimagnetic Spinels.

The manipulation of mesoscale domain wall phenomena has emerged as a powerful strategy for designing ferroelectric responses in functional devices, but its full potential is not yet realized in the field of magnetism. This work shows a direct connection between magnetic response functions in mechanically strained samples of Mn3O4 and MnV2O4 and stripe‐like patternings of the bulk magnetization which appear below known magnetostructural transitions. Building off previous magnetic force microscopy data, a small‐angle neutron scattering is used to show that these patterns represent distinctive magnetic phenomena which extend throughout the bulk of two separate materials, and further are controllable via applied magnetic field and mechanical stress. These results are unambiguously connected to the anomalously large magnetoelastic and magnetodielectric response functions reported for these materials, by performing susceptibility measurements on the same crystals and directly correlating local and macroscopic data.

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.

Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H2−20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.

Correlating the Nanoscale Structural, Magnetic, and Magneto-Transport Properties in SrRuO3-Based Perovskite Thin Films: Implications for Oxide Skyrmion Devices

We investigated the structural and magnetic properties of bare SrRuO3 (SRO) ultrathin films and SrRuO3/SrIrO3/SrZrO3 (SRO/SIO/SZO: RIZ) trilayer heterostructures between 10 and 80 K, by comparing macroscopic data using the magneto-optical Kerr effect (MOKE) and magneto-transport (anomalous Hall effect: AHE), with nanoscale fingerprints when applying noncontact scanning force microscopy (nc-SFM) and magnetic force microscopy (MFM). SRO and RIZ ultrathin films were epitaxially grown at 650 °C onto vicinal SrTiO3 (100) single-crystalline substrates to a nominal thickness of 4 and 4/2/2 unit cells (uc), respectively. Our correlated analysis allows associating topographic sample features of overgrown individual layers to their residual magnetization, as is shown here to be relevant for interpreting the macroscopic AHE data. Although the hump-like features in the AHE suggest a magnetically textured skyrmion phase to exist around 55 K associated with the topological Hall effect (THE), both our MOKE and MFM data ...

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.

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.

Heat Treatment Effect on Magnetic Microstructure of Fe73.9Cu1Nb3Si13.2B8.9 Thin Films

Fe73.9Cu1Nb3Si13.2B8.9 (Finemet) thin films were deposited on the glass substrates by means of radio frequency sputtering. The films thickness was varied from 10 to 200 nm. Heat treatment at temperatures of 350, 400 and 450 °C were performed for 30 minutes in order to control thin film structural state. The X-ray powder diffractometry revealed that the crystallization of α-FeSi nanograins took place only at 450 °C whilst the other samples stayed in the amorphous state. Relation between the structure and magnetic properties of the films was discussed in the framework of random magnetic anisotropy model and the concept of stochastic magnetic domains. The latter was investigated using magnetic force microscopy (MFM). MFM data showed formation of such magnetic domains only in samples thermally treated at 450 °C. There was a tendency of the magnetic domain size reduction with the thickness decrease.

Quantitative Analysis of Magnetic Nanoparticles by Means of Magnetic Force Microscopy

Magnetic force microscopy (MFM) is a powerful and simple tool to analyze magnetic materials and structures down to the nano-meter scale. However, there is still a demand for satisfactory models describing measured MFM data quantitatively, thereby allowing us to quantitatively analyze the magnetic properties of the sample. Here, we present an application of the recently proposed pseudo-pole model, which assumes that the tip can be represented by a cone covered homogeneously with dipoles pointing to the tip of the cone. Nano-composites of ferromagnetic Co nanoparticles (NPs) made by inert gas condensation and subsequently deposited non-magnetic SiO x cover layers with different layer thicknesses on single-crystalline Si substrates have been investigated in this paper. We extract quantitative information about the magnetization of individual NPs, as well as their depth below the surface of the SiO x films. In addition, we present an alternative method to extract information about the NP magnetic moment from the width of the NP signal in lift images, which does not require knowledge about the van der Waals interaction. We find good agreement between the magnetic moments determined by both approaches.

Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation

The effect of graded anisotropy on static and dynamic magnetic properties of ${\mathrm{Ar}}^{+}$-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by ${\mathrm{Ar}}^{+}$ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing $\ensuremath{\alpha}$, a decreasing thickness of the hard $\mathrm{L}{1}_{0}$ phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (${\ensuremath{\nu}}_{0}\ensuremath{\approx}6$ GHz), which could be reproduced assuming the presence of a nonzero ``rotatable'' anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.

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.

G-mode magnetic force microscopy: Separating magnetic and electrostatic interactions using big data analytics

In this work, we develop a full information capture approach for Magnetic Force Microscopy (MFM), referred to as generalized mode (G-Mode) MFM. G-Mode MFM acquires and stores the full data stream from the photodetector, captured at sampling rates approaching the intrinsic photodiode limit. The data can be subsequently compressed, denoised, and analyzed, without information loss. Here, G-Mode MFM is implemented and compared to the traditional heterodyne-based MFM on model systems, including domain structures in ferromagnetic Yttrium Iron Garnet and the electronically and magnetically inhomogeneous high entropy alloy, CoFeMnNiSn. We investigate the use of information theory to mine the G-Mode MFM data and demonstrate its usefulness for extracting information which may be hidden in traditional MFM modes, including signatures of nonlinearities and mode-coupling phenomena. Finally, we demonstrate detection and separation of magnetic and electrostatic tip-sample interactions from a single G-Mode image, by analyzing the entire frequency response of the cantilever. G-Mode MFM is immediately implementable on any atomic force microscopy platform and as such is expected to be a useful technique for probing spatiotemporal cantilever dynamics and mapping material properties, as well as their mutual interactions.

Interrelation Between the Structural, Magnetic and Magnetoresistive Properties of Double-Perovskite Sr2FeMoO6−δ Thin Films

Investigations of the formation conditions of Sr2FeMoO6−δ compound films, depending on the substrate temperature and sputtering rate with their subsequent thermal treatment, have made it possible to obtain the single-phase homogeneous films. The analysis of x-ray diffractometry and magnetic force microscopy data has made it possible to reveal the superstructural ordering of Fe and Mo cations, determine the spin polarization degree of conduction electrons and magnetic structure of the films. Studies of temperature and magnetic field dependences of magnetization in the zero-field cooling and field-cooling modes have shown the existence of magnetic regions with low coercitive force, which bears witness of the domination of the ferrimagnetic state at temperatures above the blocking temperature. Lower than this temperature, the nanosized grains are present, where homogeneous magnetization promotes the realization of a superparamagnetic state, which is “obscured” at the higher temperatures. The negative magnetoresistive effect, reaching 14% at low temperatures in a magnetic field of 8 T, is by its nature the giant magnetoresistance with monodirectional spin-polarized electrons.

Magnetic fluctuations sorted by magnetic field in MnSb clusters embedded in GaMnSb thin films

Dynamics of magnetization relaxation of the MnSb clusters under applied magnetic field has been studied in the 8–300 K temperature range. The lognormal distribution of ferromagnetic MnSb cluster sizes f(D) has been extracted from the field and temperature dependences of magnetic viscosity S(T, H) in GaMnSb thin films. An average cluster diameter D = 55 nm is in agreement with the magnetic force microscopy data. Magnetic anisotropy constant ∼3.2 × 104 erg/cm3 has been determined. The fluctuation field HF = 7 Oe and the activation volume VA = 1.7 × 10−16 cm3 have been calculated from the magnetic viscosity data. Stepped sweeping of the magnetic field expands the windows of experimentally detectable fluctuations. The change in the reversal magnetic field provides the scanning of the MnSb clusters sorting them by fluctuation time (or frequency).

Magnetic, structural and transport properties across the Heusler alloy (Co2FeAl)/n-Si interfacial structure

Electronic and magneto-transport studies across the Co2FeAl (CFA)/n-Si interfacial structure have been carried out. Magnetic properties of the structure have also been studied from the M–H characteristics and magnetic force microscopy (MFM) features. Surface morphology of the CFA Heusler alloy thin films studied from atomic force microscopy. X-ray diffraction and X-ray photoelectron spectroscopy data of the structure reveals the formation of CFA alloy phase with L21 structure along with few silicide phases. MFM data reveals the formation of fine domain structure with average domain size of ~17 nm with average magnetic signal strength of 0.19°. Electronic transport (I–V) characteristics shows the large reverse current as compared to forward current which indicates the spin based transport from CFA Heusler alloy side to semiconductor side. I–V characteristics performed in presence of magnetic field shows the magnetic field sensitivity for reverse bias.

Magnetic, morphological and structural investigations of CoFe/Si interfacial structures

CoFe/Si interfacial structures have been realised by electron beam evaporation of CoFe magnetic alloy on p- and n-Si substrates. These realised interfacial structures have been characterised from X-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM) and magnetisation (M–H) characteristics. XRD data have shown the presence of CoFe (bcc phase) and β-FeSi2 phases for CoFe/p-Si interfacial structures, whereas CoFe/n-Si interfacial structures have shown the presence of CoFe (bcc and fcc) phases along with Fe3Si, ϵ-FeSi and β-FeSi2 silicide phases having nanodimension crystallites. The M–H characteristics have shown the superparamagnetic type behaviour for CoFe/p-Si structure, whereas CoFe/n-Si structure shows a feature of interfacial antiferromagnetic (AF) coupling. The observed magnetic behaviour has been understood due to the presence of various magnetic phases and their nanosized grains. The MFM data of domain size have also been correlated with the observed magnetic behaviour. CoFe/n-Si structures have shown a significant feature of giant magnetoresistance (GMR) as compared to CoFe/p-Si structures. It has been found that CoFe/p-Si structures show a distinctly different behaviour than CoFe/n-Si structures for their chemical structure, morphology and magnetic behaviour.

Magnetic force microscopy: quantitative issues in biomaterials.

Magnetic force microscopy (MFM) is an atomic force microscopy (AFM) based technique in which an AFM tip with a magnetic coating is used to probe local magnetic fields with the typical AFM spatial resolution, thus allowing one to acquire images reflecting the local magnetic properties of the samples at the nanoscale. Being a well established tool for the characterization of magnetic recording media, superconductors and magnetic nanomaterials, MFM is finding constantly increasing application in the study of magnetic properties of materials and systems of biological and biomedical interest. After reviewing these latter applications, three case studies are presented in which MFM is used to characterize: (i) magnetoferritin synthesized using apoferritin as molecular reactor; (ii) magnetic nanoparticles loaded niosomes to be used as nanocarriers for drug delivery; (iii) leukemic cells labeled using folic acid-coated core-shell superparamagnetic nanoparticles in order to exploit the presence of folate receptors on the cell membrane surface. In these examples, MFM data are quantitatively analyzed evidencing the limits of the simple analytical models currently used. Provided that suitable models are used to simulate the MFM response, MFM can be used to evaluate the magnetic momentum of the core of magnetoferritin, the iron entrapment efficiency in single vesicles, or the uptake of magnetic nanoparticles into cells.

Field- and time-dependent, local and global magnetization behaviour of out-of-plane textured SmCo5 thin films

This work presents an investigation of the magnetization processes in out-of-plane textured SmCo5 thin films by means of domain observations with magnetic force microscopy (MFM). Starting from the thermally demagnetized state, an applied field was increased stepwise until saturation, and the magnetic domain pattern was recorded at the respective field. Additionally, the change in the domain structure was investigated for the demagnetization branch to observe the full reversal process from positive to negative saturation. In a second experiment, various opposite fields were applied to the film after previous saturation and the time-dependent domain evolution was measured. The directly observed domain behaviour specifies the influence of pinning and nucleation on the magnetizing and demagnetizing process.From these time-dependent domain images, magnetization curves were constructed and the resulting viscosity is compared with data from global magnetization measurements. The viscosity as determined from MFM data is measurably larger than that deduced from global relaxation measurements, which demonstrates the influence of the tip's magnetization on the reversal mechanism.

Towards a quantitative analysis of magnetic force microscopy data matrices

Fast and efficient software tools previously developed in image processing were adapted to the analysis of raw datasets consisting of multiple stacks of images taken on a sample interacting with a measuring instrument and submitted to the effect of an external parameter. Magnetic force microscopy (MFM), a follow-up of atomic force microscopy (AFM), was selected as a first testbed example. In MFM, a specifically developed ferromagnetic scanning tip probes the stray magnetic field generated from a ferromagnetic specimen. Raw scanning probe images taken on soft patterned magnetic materials and continuous thin films were used, together with synthetic patterns exploited to assess the absolute performance ability of the proposed texture analysis tools. In this case, the parameter affecting the sample-instrument interaction is the applied magnetic field. The application discussed here is just one among the many possible, including, e.g., real-time microscopy images (both optical and electronic) taken during heat treatments, phase transformations and so on. Basically any image exhibiting a texture with a characteristic spatial or angular dependence could be processed by the proposed method. Standard imaging tools such as texture mapping and novel data representation schemes such as texture analysis, feature extraction and classification are discussed. A magnetic texture stability diagram will be presented as an original output of the entropic analysis on MFM datasets.