Magnetic Force Microscopy Signals Research Articles

The Role of Nanoparticles on Topographic Cross‐Talk in Electric Force Microscopy and Magnetic Force Microscopy

Topographic cross‐talk is still an issue in magnetic force microscopy (MFM) as well as in electric force microscopy (EFM). Using interleave mode measurements, combining a first topographic scan with a second scan in a certain distance from the surface following the topography from the first scan, capacitive coupling effects occur while measuring nanoparticles. This article focuses on the influence of the dielectric constant of polystyrene nanoparticles in interleave mode MFM measurements. To investigate the contribution of the capacitive coupling effect to the signal, nonmagnetic polystyrene nanoparticles are investigated. The tip‐substrate distance change above the nanoparticle leads to a positive phase shift in MFM signals. Simulations and fits to the experimental data allow the investigation of the influence of the dielectric constant of the nanoparticles on topographic effects in interleave mode measurements in general.

Influence of dielectric layer thickness and roughness on topographic effects in magnetic force microscopy.

Magnetic force microscopy (MFM) has become a widely used tool for the characterization of magnetic properties. However, the magnetic signal can be overlapped by additional forces acting on the tip such as electrostatic forces. In this work the possibility to reduce capacitive coupling effects between tip and substrate is discussed in relation to the thickness of a dielectric layer introduced in the system. Single superparamagnetic iron oxide nanoparticles (SPIONs) are used as a model system, because their magnetic signal is contrariwise to the signal due to capacitive coupling so that it is possible to distinguish between magnetic and electric force contributions. Introducing a dielectric layer between substrate and nanoparticle the capacitive coupling can be tuned and minimized for thick layers. Using the theory of capacitive coupling and the magnetic point dipole–dipole model we could theoretically explain and experimentally prove the phase signal for single superparamagnetic nanoparticles as a function of the layer thickness of the dielectric layer. Tuning the capacitive coupling by variation of the dielectric layer thickness between nanoparticle and substrate allows the distinction between the electric and the magnetic contributions to the MFM signal. The theory also predicts decreasing topographic effects in MFM signals due to surface roughness of dielectric films with increasing film thickness.

Indirect magnetic force microscopy.

Magnetic force microscopy (MFM) is an atomic force microscopy (AFM)-based technique to map magnetic domains in a sample. MFM is widely used to characterize magnetic recording media, magnetic domain walls in materials, nanoparticles and more recently iron deposits in biological samples. However, conventional MFM requires multiple scans of the samples, suffers from various artifacts and is limited in its capability for multimodal imaging or imaging in a fluid environment. We propose a new modality, namely indirect magnetic force microscopy (ID-MFM), a technique that employs an ultrathin barrier between the probe and the sample. Using fluorescently conjugated superparamagnetic nanoparticles, we demonstrate how ID-MFM can be achieved using commercially available silicon nitride windows, MFM probes and AFM equipment. The MFM signals obtained using ID-MFM were comparable to those obtained using conventional MFM. Further, samples prepared for ID-MFM were compatible with multi-modal imaging via fluorescence and transmission electron microscopy. Thus ID-MFM can serve as a high-throughput, multi-modal microscopy technique which can be especially attractive for detecting magnetism in nanoparticles and biological samples.

Quantitatively probing the magnetic behavior of individual nanoparticles by an AC field-modulated magnetic force microscopy.

Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with nanometer scale spatial resolution remains an outstanding challenge. Current approaches, for example, Hall micromagnetometer and nitrogen-vacancy magnetometer, are limited by highly complex experimental apparatus and a dedicated sample preparation process. Here we present a new AC field-modulated magnetic force microscopy (MFM) and report the local and quantitative measurements of the magnetic information of individual magnetic nanoparticles (MNPs), which is one of the most iconic objects of nanomagnetism. This technique provides simultaneously a direct visualization of the magnetization process of the individual MNPs, with spatial resolution and magnetic sensitivity of about 4.8 nm and 1.85 × 10−20 A m2, respectively, enabling us to separately estimate the distributions of the dipolar fields and the local switching fields of individual MNPs. Moreover, we demonstrate that quantitative magnetization moment of individual MNPs can be routinely obtained using MFM signals. Therefore, it underscores the power of the AC field-modulated MFM for biological and biomedical applications of MNPs and opens up the possibility for directly and quantitatively probing the weak magnetic stray fields from nanoscale magnetic systems with superior spatial resolution.

Electric contributions to magnetic force microscopy response from graphene and MoS2 nanosheets

Magnetic force microscopy (MFM) signals have recently been detected from whole pieces of mechanically exfoliated graphene and molybdenum disulfide (MoS2) nanosheets, and magnetism of the two nanomaterials was claimed based on these observations. However, non-magnetic interactions or artefacts are commonly associated with MFM signals, which make the interpretation of MFM signals not straightforward. A systematic investigation has been done to examine possible sources of the MFM signals from graphene and MoS2 nanosheets and whether the MFM signals can be correlated with magnetism. It is found that the MFM signals have significant non-magnetic contributions due to capacitive and electrostatic interactions between the nanosheets and conductive cantilever tip, as demonstrated by electric force microscopy and scanning Kevin probe microscopy analyses. In addition, the MFM signals of graphene and MoS2 nanosheets are not responsive to reversed magnetic field of the magnetic cantilever tip. Therefore, the observed MFM response is mainly from electric artefacts and not compelling enough to correlate with magnetism of graphene and MoS2 nanosheets.

Conductance of individual channels in a carbon nanotube field-effect transistor studied by magnetic force microscopy

We observed current-induced magnetic fields using magnetic force microscopy (MFM) to investigate the channel properties of carbon nanotube field-effect transistors (CNT-FETs). We first modified the shape of a MFM cantilever to enhance its response to magnetic force and then observed the MFM signals around individual CNT channels. We demonstrated that the MFM observations are quite appropriate for studying the CNT channel properties. We also found differences in the threshold gate bias and transconductance among different CNT channels and in the asymmetric conductance of a single CNT channel.

Magnetic Force Microscopic Study on Domain-Wall Molecules in NiFe Nano Rings

A pair of magnetic domain walls (DWs) in ferromagnetic NiFe rings has been investigated in terms of the magnetic force microscopy (MFM). When the distance between the rings d is greater than a threshold value dth, MFM signals indicate that a DW in the ring is dragged due to a stray magnetic field from an MFM probe tip. When d < dth, this drag signals disappears; DWs are bound to each other by the DW-DW interaction. This transition can be argued in terms of the competition between the DW-DW magnetostatic interaction and the DW-drag potential. From the d-dependent MFM data, the DW-drag potential was estimated.

Magnetic force microscopy of alternating magnetic field gradient by frequency modulation of tip oscillation

A new magnetic force microscopy (MFM) technique for measuring alternating magnetic field (ac magnetic field) was proposed by using frequency modulation (FM) phenomenon of tip oscillation. We detected a narrowband FM phenomenon in the tip oscillation of a high-coercivity MFM tip by applying an ac magnetic field to the tip by using a metal-in-gap (MIG) type ring head. In the experiment, the MFM tip was driven at a constant frequency fc near the resonant frequency of the cantilever by a piezoelectric element, and the ac magnetic field with a frequency fm up to 10 kHz was applied to the MFM tip. Two sideband spectra with a frequency of fc±fm were observed by applying ac magnetic field. The intensity of sideband spectra increased linearly with increasing applied current value of the MIG head. It was observed that FM occurred from the analysis of frequency spectra of the MFM signals. This FM phenomenon is caused by the force applied on the cantilever by the field gradient of the head. The FM phenomenon is understood by solving the differential equation for the harmonic oscillator of which the effective spring constant is changed periodically by ac magnetic field. The FM phenomenon can be used for imaging ac magnetic fields.

Theoretical analysis of magnetic force microscopy contrast in multidomain states of magnetic superlattices with perpendicular anisotropy

Recently synthesized magnetic multilayers with strong perpendicular anisotropy exhibit unique magnetic properties including the formation of specific multidomain states. In particular, antiferromagnetically coupled multilayers own rich phase diagrams that include various multidomain ground states. Analytical equations have been derived for the stray field components of these multidomain states in perpendicular multilayer systems. In particular, closed expressions for stray fields in the case of ferromagnetic and antiferromagnetic stripes are presented. The theoretical approach provides a basis for the analysis of magnetic force microscopy (MFM) images from this class of nanomagnetic systems. Peculiarities of the MFM contrast have been calculated for realistic tip models. These characteristic features in the MFM signals can be employed for the investigations of the different multidomain modes. The obtained results are applied for the analysis of multidomain modes that have been reported earlier in the literature from experiments on [Co/Cr]Ru superlattices.

Compensation walls in gallium and aluminum substituted gadolinium–bismuth–iron garnet films created by laser annealing: Measurements and simulations

Magnetic garnet films of composition (Gd2 Bi1) (Fe4.3 Ga0.2 Al0.5) O12 are grown by liquid phase epitaxy on (111) oriented substrates of gadolininium–gallium–garnet close to magnetic compensation. Local compensation walls (CW) are fabricated by laser annealing. Images of parallel and antiparallel compensation walls and of Bloch walls in annealed regions are obtained by magnetic force microscopy (MFM). The MFM signals associated with the CWs turned out to be smaller in amplitude but broader in space than the signals created by Bloch walls. To simulate the experimental results a theoretical model is developed taking into account the broadening of the CW by the preparation technique. Very good agreement between measurements and numerical simulations is obtained.

Test of response linearity for magnetic force microscopy data

The utility of vertical propagation by the Green’s function to test response linearity has been explored for magnetic force microscopy (MFM) data from current-carrying wires, by comparing the measured signal at various tip heights to the corresponding propagated MFM signals. Application of a one-dimensional Green’s function was found to be sufficient to predict signal height variation for sample regions of high to moderate field symmetry. For regions of high field asymmetry, the two-dimensional Green’s function was required to obtain good prediction of the height variation. Agreement between the measured and propagated signals was generally within 5%, except at the tails where the signal is not well behaved. The quality of agreement deteriorates gradually with the size of the height propagation. The good agreement spanning a decade of tip and sample separation suggests that the MFM signal is not significantly affected by nonlinearities and can thus be interpreted in terms of classical electromagnetic relations governing current flow.

A study of magnetization fluctuations in transition region using MFM image analysis

This paper proposed a method for analysis of MFM (magnetic force microscopy) images. By using the proposed method, the influence of the magnetization fluctuations in transition regions can be analyzed. In the proposed method, the MFM images are processed as stochastic signals, and their stochastic parameters are obtained. The variance profile (the variance of MFM signals across the track direction), one of the parameters, is closely related to the magnetization fluctuations in the transition region. Therefore, by observation of the variance profile, the fluctuations can be quantitatively analyzed. An example of the analysis, which is based on the variance profile, is presented. From the analysis, it is found that the parameter L/W, which is introduced to characterize the variance profile, has the same trend as the S/N/sub m/ (N/sub m/:medium noise) does.