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A multilayer structure with a high-quality (Bi,La)(Fe,Co)O3 multiferroic thin film/[Co/Pd] perpendicular magnetic thin film dots was fabricated for demonstrating magnetization reversal of [Co/Pd] dots under an applied electric field. Although the magnetization direction of the multiferroic thin film was reversed under the electric field, the magnetic properties of the multiferroic thin films were generally low. If the multiferroic thin film in this structure can control the magnetization direction of the highly functional magnetic thin film under an electric field, high-performance magnetic devices with low power consumption are easily obtained. The magnetic domain structure of the [Co/Pd] dots fabricated on the (Bi,La)(Fe,Co)O3 thin film was analyzed by magnetic force microscopy (MFM). The structure was de-magnetized before the local electric-field application and magnetized after applying the field, showing reduced magnetic contrast of the dot. The line profile of the MFM image revealed a downward magnetic moment of 75%, which reversed to upward under the local electric field. Magnetic interaction between the (Bi,La)(Fe,Co)O3 and [Co/Pd] layers was also observed in magnetization hysteresis measurements. These results indicate that the magnetization direction of the [Co/Pd] dots was transferred through the magnetization reversal of the (Bi,La)(Fe,Co)O3 layer under a local electric field. That is, the magnetization of [Co/Pd] dots were reversed by applying a local electric field to the multilayer structure. This demonstration can potentially realize high-performance magnetic devices such as large capacity memory with low power consumption. [ABSTRACT FROM AUTHOR]

Magnetic force microscopy (MFM) has become one of the most important instruments for characterizing magnetic materials with nanoscale spatial resolution. When analyzing magnetic particles by MFM, calibration of the magnetic tips using reference magnetic nanoparticles is a prerequisite due to similar orientation and dimension of the yielded magnetic fields. However, in such a calibration process, errors caused by extra electrostatic interactions will significantly affect the output results. In this work, we evaluate the magnetic moment and dipole radius of the MFM tip on Fe3O4 nanoparticles by considering the associated electrostatic force. The coupling of electrostatic contribution on the measured MFM phase is eliminated by combining MFM and Kelvin probe force microscopy together with theoretical modeling. Numerical simulations and experiments on nickel nanoparticles demonstrate the effectiveness of decoupling. Results show that the calibrated MFM tip can enable a more accurate analysis of micro-and-nano magnetism. In addition, a fast and easy calibration method by using bimodal MFM is discussed, in which the acquisition of multiple phase shifts at different lift heights is not required. [ABSTRACT FROM AUTHOR]

Magnetic force microscopy (MFM) is utilized to characterize the magnetic moment in nanostructured plant leaf-derived graphene quantum dot clusters (GQDCs). The MFM signal reveals that the magnetic response of the GQDCs depends on the height and width of the GQDCs. However, individual GQDs, and smaller clusters with widths of less than 20 nm, have not shown any observable magnetic signal. Importantly, experimental analyses suggest that the magnetic signal of GQDCs distributed in a plane can be effectively detected at room temperature. These results could pave the way for future graphene-based magnetic storage media and spin manipulation quantum devices. [ABSTRACT FROM AUTHOR]

We demonstrate a method to dope monolayer chemical vapor deposited (CVD) graphene with nitrogen and make it ferromagnetic. CVD graphene was first functionalized with hydroxyl groups by treating with H2O2 in the presence of UV light and then annealed in ammonia gas to dope it with nitrogen. Magnetization measurements showed a ferromagnetic hysteresis loop at low temperatures with a coercivity of 222 Oe at 2 K. We also investigated the effect of a change in the angle of the applied magnetic field on the anisotropic magnetoresistance effect (AMR) in the doped CVD graphene devices. Graphene shows positive AMR for temperatures from 2 K to 50 K, negative AMR at 100 K and 150 K, and no AMR for temperatures higher than 150 K. A maximum AMR of 0.92% was observed at 2 K for an in-plane magnetic field of 30 kOe. Magnetic force microscopy also confirms the introduction of magnetism in CVD graphene after doping, and electron spin resonance spectroscopy shows resonance when scanned in a magnetic field, which confirms the presence of unpaired electrons in doped graphene. The process introduced in this paper for nitrogen doping of graphene with attendant magnetism could pave the way for the applications of graphene in spintronics and other devices. [ABSTRACT FROM AUTHOR]

The X/Co 3 nm/Y (where X, Y = Au, Pt) trilayers with as deposited in-plane magnetization alignment were irradiated with 30 keV Ga+ ions in the wide range of ion fluence. The samples were investigated by means of complementary techniques: magneto-optical magnetometry and spectroscopy (in the photon energy range from 1.2 eV to 4.5 eV), magnetic force microscopy, positron annihilation spectroscopy, x-ray diffraction and reflectivity. Difference in miscibility of interface atoms is clearly manifested in various intermixing extent at Co/Pt and Co/Au interfaces and consequently in magnetic properties of the irradiated trilayers. Low irradiation fluence (∼1014 ions cm−2) leads to ∼1 nm interfaces broadening without visible surface etching for all samples, which is related with a distinct drop of magnetic anisotropy. However, the high irradiation fluence (∼5 × 1015 ions cm−2) results in enhanced interface broadening and significant surface etching (∼5 nm) partially removing also Co atoms. Tensile strains (up to 0.5%) were developed in the cover layers. The tensile strain, layers intermixing and the creation of Co–Pt(Au) alloys with different composition formed by irradiation are correlated with the increase of magnetic anisotropy. Moreover it was observed that substitution of Au instead of Pt (as a cap or buffer layer) results in substantial increase of perpendicular magnetic anisotropy. Maximal increase of magnetooptical parameters was observed for Pt/Co/Pt layer. Irradiation induced changes of concentration profiles are revealed using magnetooptical spectra, x-ray reflectivity spectra and simulations with use of binary collision approximation. [ABSTRACT FROM AUTHOR]

Magnetic skyrmions are non-trivial spin textures that resist external disturbances and are promising candidates for use in next generation spintronic devices. However, a major challenge in the realization of devices based on skyrmions is the stabilization of ordered arrangements of these spin textures under ambient temperature and zero applied field conditions. Multilayers of ferromagnetic materials (Co) interspersed with heavy metals with strong spin–orbit coupling (Pt and Ta), exhibit interesting properties, as the Dzyaloshinskii–Moriya interaction, which is an anti-symmetric exchange interaction that tilts the spins of neighboring layers, helps to stabilize skyrmions. In this work we study the formation and stabilization of magnetic skyrmions in Pt/Co/Ta films using a first order reversal curve (FORC) analysis, obtained from Hall effect measurements. The analysis of the FORC Hall diagrams was used to determine the intensity of the magnetic fields that should be applied to nucleate and stabilize skyrmions at remanence. Magnetic force microscopy images and micromagnetic simulations were compared to determine the correlation of the domain textures with the applied field and magnetic parameters of the multilayers. [ABSTRACT FROM AUTHOR]

Magnetic properties, domain morphology and magnetization reversal processes were investigated for two sets of [Pt/Co(x)]N multilayers with various (Co/Pt) bilayer repeat numbers (N) and Co layer thicknesses (x) using hysteresis loops, magnetic force microscopy (MFM), and the first-order reversal curve method. When N ranges from 7 to 20, the effective anisotropy, Keff, of the multilayers increases from 0.86 × 106 erg cc−1 to 1.48 × 106 erg cc−1 and the coercivity (Hc) is maintained at approximately 290 Oe. Moreover, when x increases, Keff drops from 0.52 × 106 erg cc−1 to 0.09 × 106 erg cc−1 and Hc decreases from 285 Oe to 75 Oe. According to the switching field distribution, evidence has been found that the magnetization switching is gradually dominated by the reversible process and the portion of the irreversible switching process declines from 86% to 40% and 58% to 10% when N and x are increased, respectively. [ABSTRACT FROM AUTHOR]

Scanning Kelvin probe force microscopy (SKPFM) measured local Volta potentials in microstructure of 22Cr-5Ni duplex stainless steel have been correlated to microstructure development with aging treatments at 475°C. Magnetic force microscopy (MFM) was employed to differentiate crystallographic phases to provide complementary information. The absolute Volta potentials of both ferrite and austenite increased after 5 hours of aging, indicating electrochemical ennoblement of the entire microstructure. Longer aging resulted in a gradual decrease of measured Volta potentials in both phases. The microstructure showed after 255 hours aging up to 2.5-times larger potential differences than in the as-received condition, indicating impaired electrochemical nobility. In the as-received microstructure, the ferrite phase was less noble than the austenite, whereas after 5 hours aging both phases had similar, balanced Volta potentials which indicated a balanced nobility of ferrite and austenite. Longer aging treatment caused severe loss of nobility for the entire microstructure, with ferrite showing larger changes in Volta potential than the austenite. Spinodal microstructure decomposition and associated phase reactions of the ferrite, with elemental redistribution in the austenite, are the reason for the observed changes in microstructure nobility. [ABSTRACT FROM AUTHOR]

Domain configuration has been studied by magnetic force microscopy and micromagnetic simulations in NdCo honeycomb lattices in comparison with similar patterned structures made of polycrystalline Co. The change in material anisotropy from in-plane to weak perpendicular magnetic anisotropy (wPMA) modifies the basic domain structure and relevant topological defects in the magnetization in each case: from in-plane domains, vortices, antivortices and half vortices in Co lattices to parallel stripe patterns with dislocations in NdCo samples with large enough thickness. A characteristic feature of wPMA materials is the possibility to drive the system from in-plane to stripe pattern configuration playing with sample thickness (during growth) or with magnetic anisotropy (as a function of temperature). It has allowed us to observe complex magnetic textures within the stripe pattern of NdCo samples imprinted from previous magnetic vortex and antivortex states that nucleate within the honeycomb lattice during the early stages of deposition and, then, become frozen by local rotatable anisotropy as sample thickness increases.

Various applications of scanning force microscopy in nanomechanics demand that the mechanical performance of cantilevers in use, especially their normal stiffness, be quantitatively calibrated. In this paper, a dynamic reference spring (DRS) method based on the micro-opto-electro-mechanical systems nano-force transducer developed in the PTB is proposed, in which the cantilever bending stiffness is now indirectly determined by inspection of the resonance frequency shift of the nano-force transducer before and after contacting the cantilever to be calibrated. In general, only small cantilever deflections will be generated during calibration, and thus the contact force between the cantilever tip of the atom force microscope and the transducer should be small enough and purely nondestructive to the tip. Compared with other quasi-static cantilever calibration methods, the DRS approach allows relatively fast measurements, is insensitive to the long-term noise observed within ordinary measurement environments and therefore is very suitable for in-situ cantilever spring constant calibrations under poor working conditions. A proof-of-principle experiment has verified the feasibility of this method. [ABSTRACT FROM AUTHOR]

Dynamic operation modes of scanning force microscopy based on probe resonance frequency detection are very successful methods to study force-related properties of surfaces with high spatial resolution. There are well-recognized approaches to measure vertical force components as well as setups sensitive to lateral force components. Here, we report on a concept of bidirectional force gradient microscopy that enables a direct, fast, and quantitative real space mapping of force component derivatives in both the perpendicular and a lateral direction. It relies solely on multiple-mode flexural cantilever oscillations related to vertical probe excitation and vertical deflection sensing. Exploring this concept we present a cantilever-based sensor setup and corresponding quantitative measurements employing magnetostatic interactions with emphasis on the calculation of mode-dependent spring constants that are the foundation of quantitative force gradient studies.

We study the reversal mechanisms in a self-assembled, hexagonally ordered Fe antidot array with a period of 200 nm and an antidot diameter of 100 nm which was prepared by polystyrene nanosphere lithography. Direction-dependent information in such a self-assembled sample is obtained by measuring the anisotropic magnetoresistance (AMR) through constrictions processed by focused ion beam milling in nearest neighbor and next nearest neighbor directions. We show that such an originally integral method can be used to investigate the strong in-plane anisotropy introduced by the antidot lattice. The easy and hard axis reversal mechanisms and corresponding AMR signals are modeled by micromagnetic simulations. Additional in-field magnetic force microscopy studies allow the correlation of microscopic switching to features in the integral AMR. We find that the easy axis of magnetization is connected to a distinct periodic magnetic domain pattern, which can be observed during the whole magnetization reversal. While this process is driven by nucleation and propagation of reversed domains, the hard axis reversal is characterized by a (stepwise) rotation of the magnetization via the antidot lattice’ easy axes. [ABSTRACT FROM AUTHOR]

Amorphous Nd–Co films with perpendicular magnetic anisotropy have been nanostructured in a lateral magnetic multilayer geometry in order to analyse and modify in a controlled way the configuration of its characteristic stripe domains as well as their rotation processes. Magnetic force microscopy measurements reveal that, actually, the artificial thickness modulation results in size effects on the magnetic stripes, so that the stripe domains configuration can be tuned with the lateral multilayer periodicity, and, due to the consequent reduction of the rotatable magnetic anisotropy, it is possible to modify the stripe rotation processes for in-plane magnetization reversal. [ABSTRACT FROM AUTHOR]

We present a methodology to explore experimentally the formation of thermally induced long-range ground-state ordering in artificial spin-ice systems. Our novel approach is based on the thermalization from a square artificial spin-ice array of elongated ferromagnetic nanoislands made of a FeNi alloy characterized by a Curie temperature about 100K lower than that of Permalloy (Ni81Fe19), which is commonly used for this kind of investigation. The decrease in M(T) when the sample is heated close to its Curie temperature reduces the shape anisotropy barrier of each island and allows us to bring the artificial spinice pattern above the blocking temperature of the islands, thus 'melting' the spin-ice system, without damaging the sample. The magnetization configuration resulting from the thermal excitation of the islands and the frustrated dipolar interactions among them can be then imaged by magnetic force microscopy or any other kind of magnetic microscopy imaging after cooling down the sample back to room temperature. This thermally induced melting-freezing protocol can be repeated as many times as desired on the same sample and the heating and cooling parameters (max T, heating and cooling rates, number of cycles, application of external fields) varied at will. Thereby, the approach proposed here opens up a pathway to the systematic experimental study of thermally induced frozen states in artificial spin-ice systems, which have been the subject of many recent theoretical studies due to their interesting physical properties but, because of the difficulties in obtaining them in real samples and in a controlled manner, remain experimentally an almost completely unexplored terrain. [ABSTRACT FROM AUTHOR]

We present experimental work that reveals the benefits of performing magnetic force microscopy measurements employing the torsional resonance mode of cantilever oscillation. This approach provides two clear advantages: the ability of performing magnetic imaging without topography-related interference and the significant lateral resolution improvement (approximately 15%). We believe that this work demonstrates a significant improvement to a versatile magnetic imaging technique widely used in academia and in industry. [ABSTRACT FROM AUTHOR]

Using single-walled carbon nanotubes homogeneously coated with ferromagnetic metal as ultra-high resolution magnetic force microscopy probes, we investigate the key image formation parameters and their dependence on coating thickness. The crucial step of introducing molecular beam epitaxy for deposition of the magnetic coating allows highly controlled fabrication of tips with small magnetic volume, while retaining high magnetic anisotropy and prolonged lifetime characteristics. Calculating the interaction between the tips and a magnetic sample, including hitherto neglected thermal noise effects, we show that optimal imaging is achieved for a finite, intermediate-thickness magnetic coating, in excellent agreement with experimental observations. With such optimal tips, we demonstrate outstanding resolution, revealing sub-10 nm domains in hard magnetic samples, and non-perturbative imaging of nanoscale spin structures in soft magnetic materials, all at ambient conditions with no special vacuum, temperature or humidity controls. [ABSTRACT FROM AUTHOR]

We use non-close packed colloidal lithography to prepare hexagonal magnetic antidot arrays with varying diameters at a period of 205 nm. Smaller antidots are attractive for applications as spin waveguides in magnonics. Larger antidots form a magnetically frustrated system, i.e. Kagome spin-ice, as we prove by magnetic force microscopy. The simple but effective approach successfully extends the limits of top-down lithography previously used to study spin-ice configurations and emergent magnetic monopoles towards smaller structures. [ABSTRACT FROM AUTHOR]

We visualized the evolution of the magnetic domain structure in CoFe2O4 films on different structural phases of BaTiO3 substrates with {0 0 1} orientation, using variable temperature magnetic force microscopy. When BaTiO3 underwent transitions from rhombohedral to orthorhombic to and from orthorhombic to tetragonal structures with increasing temperature, only local variations to the magnetic domains were observed. At the BaTiO3 tetragonal-cubic transition however, the magnetic domain size increased by 75% with an overall decrease in stray field contrast (33% decrease) because of reorientation of the magnetization to the in-plane directions. The reorientation of magnetization during the tetragonal to cubic phase transition of BaTiO3 was induced by release of asymmetric interfacial strain between the CoFe2O4 film and the BaTiO3 substrate, as non-uniformly distributed a and c surface ferroelectric domains in the BaTiO3 tetragonal phase disappeared at the paraelectric BaTiO3 cubic phase transition. Strain analysis based on macroscopic magnetization measurements was correlated with the microscopic magnetic domain structure to help understand the coupling in two-phase multiferroicheterostructure mediated by interfacial strain. [ABSTRACT FROM AUTHOR]

Artificial spin ices are nanoscale geometrically engineered systems that mimic the behavior of bulk spin ices at room temperature. We describe the nanoscale magnetic interactions in a square spin ice lattice by an experimentally verified model that accounts for the correct shape of the magnetic islands. Magnetic force microscopy measurements on lithographically fabricated lattices are compared to Monte Carlo simulations of the reversal process of two lattices with different lattice spacings. Lattice node statistics and correlations show significant differences in the reversal mechanism for lattices with different spacings. The effect of structural variations is also compared for the two lattice reversals. [ABSTRACT FROM AUTHOR]

The magnetization reversal mechanism in perpendicular soft/hard Fe/FePt exchange-coupled bilayers has been investigated as a function of the soft layer thickness (tFe = 2, 3.5, 5 nm) combining magnetization loops at variable angle, magnetic domain analysis by magnetic force microscopy and numerical micromagnetic simulations. The analytical model proposed in the literature can properly account for some features of the reversal mechanism, such as positive nucleation fields and the reduction of the perpendicular coercive field and remanence by increasing the soft layer thickness, but cannot satisfactorily describe the magnetization process of real systems. We showed that for a thickness of the soft layer exceeding the FePt exchange length (∼2 nm), numerical micromagnetic calculations are needed to reproduce experimental observations. Indeed, just above the coercive field, the magnetization reversal does not proceed in single step switching, as predicted by the analytical model, but according to a more complex process: evolution of nucleated magnetic domains whose magnetization is approximately along the surface normal in the hard layer and slightly out of the film plane in the soft layer, followed by rotation of Fe moments along the field direction. [ABSTRACT FROM AUTHOR]

We present observations of martensite variants and ferromagnetic domain structures of Ni53Mn24Ga23 ferromagnetic shape memory alloys with a pure tetragonal martensitic phase by using scanning electron acoustic microscopy (SEAM) and scanning thermal microscopy (SThM). Electron acoustic images show a polycrystalline morphology with martensite variants. Direct coincidence between crystallographic martensitic twin variants and magnetic domains is found. A domain-like structure, obtained by SThM, is firstly reported, and then confirmed by magnetic force microscopy (MFM). The experimental results will be helpful for investigating the local thermal properties of ferromagnets and understanding the relationship between martensite variants and magnetic domains. [ABSTRACT FROM AUTHOR]

We introduce a simple and effective model of a commercial magnetic thin-film sensor for magnetic force microscopy (MFM), and we test the model employing buried magnetic dipoles. The model can be solved analytically in the half-space in front of the sensor tip, leading to a simple 1/R dependence of the magnetic stray field projected to the symmetry axis. The model resolves the earlier issue as to why the magnetic sensors cannot be described reasonably by a restricted multipole expansion as in the point pole approximation: the point pole model must be extended to incorporate a 'lower-order' pole, which we term 'pseudo-pole'. The near-field dependence (α R-1) turns into the well-known and frequently used dipole behavior (α R-3) if the separation, R, exceeds the height of the sensor. Using magnetic nanoparticles (average diameter 18 nm) embedded in a SiO cover as dipolar point probes, we show that the force gradient-distance curves and magnetic images fit almost perfectly to the proposed model. The easy axis of magnetization of single nanoparticles is successfully deduced from these magnetic images. Our model paves the way for quantitative MFM, at least if the sensor and the sample are independent. [ABSTRACT FROM AUTHOR]

Arrays of Permalloy antidot thin films have been prepared by sputtering onto hexagonally ordered anodic alumina templates. Geometry parameters of antidot arrays have been tailored so that, final diameter ranges between 35 and 66 nm, and film thickness is tuned between 18 and 138 nm while the hexagonal symmetry lattice is kept constant to 105 nm. Magnetic characteristics have been investigated by vibrating sample magnetometry, magneto-optic Kerr effect and magnetic force microscopy imaging. The magnetization process has been analysed as a function of the mentioned geometry parameters that determine the main magnetic energy contribution that is the stray-field energy coming from local distribution of magnetic charges around the antidots. The magnetization reversal mechanism is concluded to be due to displacements of domain walls for thick films while its motion is damped by the presence of antidots. In the case of thinner films and wider nanohole diameter a significant contribution from magnetization rotation mechanism is observed. [ABSTRACT FROM AUTHOR]

Magnetic reversal of an artificial square ice pattern subject to a sequence of magnetic fields applied slightly off the diagonal axis is investigated via magnetic force microscopy of the remanent states that result. Sublattice independent reversal is observed via correlated incrementally pinned flip cascades along parallel dipolar chains, as evident from analysis of vertex populations and dipolar correlation functions. Weak dipolar interactions between adjacent chains favour antialignment and give rise to weak charge ordering of 'monopole' vertices during the reversal process. [ABSTRACT FROM AUTHOR]

The sub-microdoughnut-like copper periodic structure is fabricated by an electroplating method on a patterned Ni template. The diameter of the cover Cu doughnut hole can be controlled by plating duration. This structure can be applied in a biosensor due to surface plasmonic resonance, and for a novel optical device with a variable hole diameter and period of that array. The magnetic force microscope was used for evaluating the biosensing response under the addition of magnetic proteins (metallothionein, Mn2Cd5 metallothionein, and ferritin) on that sample before and after magnetization. In addition, the optical properties before and after magnetization were measured and scanned by Fourier transform JR spectroscopy. Some interesting behaviors were observed in the spectra and transmitting ratio under different polarization incident conditions. [ABSTRACT FROM AUTHOR]

The temperature-dependent properties of a triangular flux-line lattice (FLL) in the low-flux density regime were investigated by evaluating the imaged flux-line (FL) size and the lattice regularity observed in real space utilizing magnetic force microscopy (MFM). At low temperatures, pinning by randomly distributed point defects in the anisotropic type-II superconductor Bi2Sr2CaCu2O8+δ results in curved FLs and lateral disorder within the FLL (Bragg glass). Above 30 K, depinning of pancake vortices (PVs) leads to straightening of FLs and a better-ordered lattice. Evaluation of the temperature-dependent imaged FL size allows us to determine the stiffness of the potential, in which FLs in the lattice are caged due to mutual repulsion between them. At 54.1 K, far below melting temperatures reported so far, thermal fluctuations plus the lateral force exerted by the scanning tip facilitate decoupling of PVs near the surface and the image contrast exhibit a liquid-like behavior. Our analysis demonstrates the ability of MFM to obtain three-dimensional information on the arrangement of PVs. [ABSTRACT FROM AUTHOR]

The typical magnetic domains of Sm(CobalFe0.25Cu0.07Zr0.02)7.4 magnets quenched through various heat-treatment steps have been revealed by using magnetic force microscopy (MFM). For the specimens in which the nominal c-axis is perpendicular to the imaging plane, the domain configurations change from plate-like for the as-sintered magnet to corrugation and spike-like for the homogenized one, and then to a coarse and finally to a finer domain structure when isothermally aged at 830°C and then annealed at 400°C. However, only plate-like domains can be detected on the surfaces with the nominal c-axis parallel to the imaging plane. The finer domain (so-called interaction domain) is a characteristic magnetic domain pattern of the Sm-Co 2:17-type magnets with high coercivities. Domain walls in a zigzag shape are revealed by means of MFM in final bulk Sm-Co 2:17-type sintered magnets. [ABSTRACT FROM AUTHOR]

Arrays of self-assembled polystyrene spheres with various particle sizes have been used as a substrate to study the exchange bias effect along the out-of-plane direction of Pt/Co multilayers capped with IrMn layers. The evolution of the reversal process of the resulting magnetic nanocaps was investigated by magnetic force microscopy (MFM) and magnetic transmission x-ray microscopy (M-TXM). Tip-sample interaction-induced irreversible and reversible switching events have been observed during multiple scanning cycles in MFM imaging which are ascribed to the so-called training effect. During M-TXM imaging a drastic change in morphology has been found due to the x-ray exposure, leading to the formation of much larger spherical particles. Interestingly, these merged particles reveal again an exchange coupled single-domain magnetic cap with magnetic behaviour similar to magnetic films deposited directly on spheres of similar size. [ABSTRACT FROM AUTHOR]

We report on a systematic electrical transport study of the effect of an exchange tab on the magnetization switching process of magnetic nanowires. The exchange tab consists of a small segment of a ferromagnetic layer coupled antiferromagnetically to a small portion of the magnetic wire via a thin Ru spacer. The magnetic force microscopy image shows that the exchange tab has a single domain structure. The magnetoresistance of exchange tab stabilized nanowires is characterized by a reversible switching process in both the longitudinal and the transverse field directions. The former is in sharp contrast to the magnetoresistance curve of nanowires with the same geometry but without an exchange tab. The dependence of the exchange field on the tab size has also been investigated, which revealed that a shorter tab generally leads to a stronger coupling between the tab and the nanowire. Micromagnetic modelling has been carried out to interpret the experimental results. [ABSTRACT FROM AUTHOR]

Magnetic force microscopy (MFM) has been used to analyse the behaviour of domain walls in uniaxial amorphous CoxSi1[?]x films patterned with diluted arrays of antidots by electron beam lithography. The walls are found to be pinned by the antidot array when the antidot density is high enough along the easy axis. The expansion of reversed nuclei under the influence of the tip stray field has been observed in several consecutive MFM images of the same area, showing how the competition between line tension effects and pinning by the patterned holes governs the creep motion of the 180deg walls across the array of antidots. [ABSTRACT FROM AUTHOR]

We present a magnetic force microscopy study of alternate sputtered (001) oriented L10phase FePt films. It is found that the root-mean-square value of phase shift of magnetic force images, (??)rms, can be used to characterize the perpendicular anisotropy for a series of specimens. Therefore, the considerable improvement of the perpendicular anisotropy after post-annealing can be characterized. In addition, the magnetic properties, magnetic and crystalline microstructures before and after post-annealing are compared for the typical [Fe5 nmPt5 nm]10film with substrate temperature Ts= 500°C, single layer thickness d= 5nm and total layer thickness D= 100nm to confirm the effect of post-annealing on improving the perpendicular anisotropy for Fe-Pt films. [ABSTRACT FROM AUTHOR]

The requirements for imaging magnetic vortices properly by magnetic force microscopy are discussed. Special attention is devoted to the influence of the tip on the sample magnetic configuration. It is shown that one can obtain a quantitative understanding of the experiments using micromagnetic simulations within a general framework for reversible tip-induced perturbations. Finally, preliminary results obtained with specially prepared ‘nanowire tips’ are presented. [ABSTRACT FROM AUTHOR]

In this paper, we propose an approach that combines in situ mechanical-stress measurement and magnetic force microscopy (MFM) imaging and enable manipulation of the spin configuration of flexible films by tailoring the stress-induced anisotropy. We demonstrate that the stress-induced anisotropy can effectively rotate stripe domains. We see that, without the application of the magnetic field, the stripe domains tend to align parallel (perpendicular) to the direction of tensile (compressive) stress. The expansion (shrinkage) of flux-closure cap domains in response to applied tensile (compressive) stress, changes the out-of-plane stray field and the profile in the MFM image. The experimental results have been reproduced very well by micromagnetic simulations assuming a stress-induced anisotropy. Furthermore, we studied the evolution of stripe domains in the film with compressive stress against a decreasing magnetic field. It is indicated that in-plane mechanical stress can effectively convert stripe domains with Bloch walls to those with Néel walls. The observed stress-modulation of magnetic stripe domains may find application in microwave absorbers and flexible electronics. [ABSTRACT FROM AUTHOR]

In this paper, the in situ control of radial vortex polarity and chirality at room temperature mediated by perpendicular magnetic field pulse has been reported via micromagnetic simulations. The correlative results show that the polarity and chirality are switched simultaneously without resulting in a large core movement, the simulation built-in magnetic force microscopy provides a deeply insight into the evolution of magnetic configuration during the reversal process. The reversal mechanism reported in this work differs from the reported vortex-antivortex pair creation and annihilation mechanisms, which is useful for bit reading and probing in magnetic data storage devices. Meanwhile, the thermal perturbations at nonzero temperatures will weaken the effect of magnetic field, so the threshold field strength need increase with the elevated temperature to guarantee chirality and polarity reversed completely. This approach presents an alternative efficient means to achieve an ultrafast radial vortex chirality and polarity switching, which paves the way for the practical application of magnetoresistive memory based on radial vortex. [ABSTRACT FROM AUTHOR]

We reported a study of tunnel magnetoresistance (TMR) effect in single manganite nanowire via the combination of magnetotransport and magnetic force microscopy imaging. TMR value up to 290% has been observed in single (La1 − yPry)1 − xCaxMnO3 nanowires with varying width. We find that the TMR effect can be explained in the scenario of opening and blockade of conducting channels from inherent magnetic domain evolutions. Our findings provide a new route to fabricate TMR junctions and point towards future improvements in complex oxide-based TMR spintronics. [ABSTRACT FROM AUTHOR]

We present a new procedure that takes advantage of the magnetic flux quantization of superconducting vortices to calibrate the magnetic properties of tips for magnetic force microscopy (MFM). Indeed, a superconducting vortex, whose quantized flux is dependent upon Plank constant, speed of light and electron charge, behaves as a very well defined magnetic reference object. The proposed calibration procedure has been tested on new and worn tips and shows that the monopole point-like approximation of the probe is a reliable model. This procedure has been then applied to perform quantitative MFM experiments on a soft ferromagnetic thin film of permalloy, leading to the determination of the local out-of-plane component of the canted magnetization, together with its spatial variations across a few μm2 scan area. [ABSTRACT FROM AUTHOR]

For magnetic domain imaging, with a very high spatial resolution magnetic force microscope, the tip-sample distance should be as small as possible. However, magnetic imaging near the sample surface is very difficult with conventional magnetic force microscopy (MFM) because the interactive forces between the tip and sample include van der Waals and electrostatic forces along with a magnetic force. In this study, we proposed alternating MFM which only extracts a magnetic force near the sample surface without any topographic and electrical crosstalk. In the present method, the magnetization of an FeCo–GdOx superparamagnetic tip is modulated by an external AC magnetic field in order to measure the magnetic domain structure without any perturbation from the other forces near the sample surface. Moreover, it is demonstrated that the proposed method can also measure the strength and identify the polarities of the second derivative of the perpendicular stray field from a thin film permanent magnet with a DC demagnetized state and remanent state. [ABSTRACT FROM AUTHOR]

Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing the surface magnetic state of barium titante–hexagonal barium ferrite (BaTiO3–BaFe12O19) ceramic composites using magnetic force microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe12O19, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO3 grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetoelastic anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An average change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetoelastic anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K1 magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio. [ABSTRACT FROM AUTHOR]

The 3D stray field landscape above an exchange bias layer system with engineered domain walls has been fully characterized by quantitative magnetic force microscopy (qMFM) measurements. This method is based on a complete quantification of the MFM tip’s imaging properties and the subtraction of its contribution from the measured MFM data by deconvolution in Fourier space. The magnetically patterned Ir17Mn83/Co70Fe30-exchange-bias-multilayers have been designed to contain asymmetric head-to-head (hh)/tail-to-tail (tt) Néel walls between domains of different magnetic anisotropies for potential use in guided particle transport. In the current application, qMFM reveals the effective magnetic charge profile on the surface of the sample—with high spatial resolution and in an absolute quantitative manner. These data enable to calculate the magnetostatic potential and the full stray field landscape above the sample surface. It has been successfully tested against: (i) micromagnetic simulations of the magnetization structure of a comparable exchange-bias layer system, (ii) measurements of the magnetization profile across the domain boundary with x-ray photoemission electron microscopy, and (iii) direct stray field measurements obtained by scanning Hall probe microscopy at elevated scan heights. This approach results in a quantitative determination of the stray field landscape at close distances to the sample surface, which will be of importance for remote magnetic particle transport applications in lab-on-a-chip devices. Furthermore, the highly resolving and quantitative MFM approach reveals details of the domain transition across the artificially structured phase boundary, which have to be attributed to a continuous change in the materials parameters across this boundary, rather than an abrupt one. [ABSTRACT FROM AUTHOR]

In this work we have used focused electron beam induced deposition of cobalt to functionalize atomic force microscopy Akiyama tips for application in magnetic force microscopy. The grown tips have a content of  ≈ Co after exposure to ambient air. The magnetic tips were characterized using energy dispersive x-ray spectroscopy and scanning electron microscopy. In order to investigate the magnetic properties, current loops were prepared by electron beam lithography. Measurements at room temperature as well as were carried out and the coercive field of  ≈ mT of the Co tip was estimated by applying several external fields in the opposite direction of the tip magnetization. Magnetic Akiyama tips open new possibilities for wide-range temperature magnetic force microscopy measurements. [ABSTRACT FROM AUTHOR]

Superparamagnetic iron oxide nanoparticles (SPIONPs) incorporated into the base material of implants are used as contrast agents in magnetic resonance imaging for the delineation of the implants from the surrounding tissue. However, the delineation quality is strongly related to the structural characteristics of the incorporated SPIONPs and their interparticle interaction as well as their interaction with the polymer matrix of the implant. Consequently, a profound knowledge of the formation of aggregates inside the polymer matrix, which are responsible for strong interparticle interactions, and of their structural characteristics, is required for controlling the magnetic resonance image quality of the implants. In this work, transmission electron microscopy methods such as electron tomography and nano-electron diffraction were used to depict SPIONP aggregates inside the melt-spin polyvinylidene fluoride fibers used for the assembly of implants and to determine the crystal structure of individual nanocrystals inside these aggregates, respectively. Using these techniques it was possible for the first time to characterize the aggregates inside the fibers of implants and to validate the magnetization measurements that have been previously used to assess the interaction phenomena inside the fibers of implants. With electron tomography, inhomogeneously sized distributed aggregates were delineated and 3D models of these aggregates were constructed. Furthermore, the distribution of the aggregates inside the fibers was verified by means of magnetic force microscopy. With nano-diffraction measurements, the SPIONP crystal structure inside the fibers of the implant could not be clearly assigned to that of magnetite (Fe3O4) or maghemite (γ-Fe2O3). Therefore, additional electron energy loss spectroscopy measurements were performed, which revealed the presence of both phases of Fe3O4 and γ-Fe2O3, probably caused by oxidation processes during the manufacture of the fibers by melt-spinning. [ABSTRACT FROM AUTHOR]

The formation conditions of single magnetic bubbles through in-plane field demagnetization are investigated in an array of Co/Ni circular dots by magnetic force microscopy and compared to micromagnetic calculations. We demonstrate high success rates in nucleating stable bubbles. The efficiency of single bubble formation is found to depend not only on the dot size, material thickness and intrinsic material parameters but also on the bubble nucleation path. Experimental phase diagrams and micromagnetic calculations highlight the influence of the starting in-plane field amplitude and dipolar interactions in stabilizing the bubble. The identification of a systematic procedure for controlling nucleation of single bubbles, multidomain states or a uniform state is important from a technological point of view, opening a path toward the realization of reprogrammable magnonic crystals for the control of spin-wave propagation. [ABSTRACT FROM AUTHOR]

The millimeter-sized Nd9.5Fe61.5Co10Ti2.5Nb0.5B16−xCx (x  =  0–1.25) alloy rods with various compositions were fabricated by direct casting. Nano-sized hard phase Nd2(FeCo)14B, soft phase ɑ-FeCo, and amorphous phase were observed in all alloys. An optimized amount of carbon additions improved the magnetic properties by enhancing the glass forming ability and forming near single domain-sized Nd2(FeCo)14B grains around the rod surface. Various intergranular structures were observed in the alloys with x  =  0.25–1. Micromagnetic simulation using the images obtained from the magnetic force microscope and transition electron microscope indicates that the distribution and magnetism of the intergranular phase have an important influence on the magnetic properties and demagnetization process of the alloys. A uniformly distributed nonmagnetic intergranular amorphous phase may enhance the magnetic properties, but the coercivity decreases when the amorphous phase is magnetic. It is important to modify the structure and distribution of the inter-grain amorphous phase in order to achieve high hard magnetic properties in these alloys. [ABSTRACT FROM AUTHOR]

We apply an in-plane external magnetic field during growth of amorphous TbCo thin films and examine the effects on the magnetic anisotropy and domain structure. A combinatorial approach is employed throughout the deposition and analysis to study a continuous range of compositions between 7–95 at.% Tb. Magnetometry measurements show that all samples have a strong out-of-plane anisotropy, much larger than any in-plane components, regardless of the presence of a growth field. However, magnetic force microscopy demonstrates that the growth field does indeed have a large effect on the magnetic domain structure, resulting in elongated domains aligned along the imprinting field direction. The results show that the anisotropy can be tuned in intricate ways in amorphous TbCo films giving rise to unusual domain structures. Furthermore the results reveal that a combinatorial approach is highly effective for mapping out these material properties. [ABSTRACT FROM AUTHOR]

Permalloy films with different thickness were prepared for comparison, and the dynamic properties of the stripe domains were investigated by using a vector network analyzer ferromagnetic resonance (VNA-FMR) technique. A top-down approach was used to control the thickness of the films. In the measurement of permeability spectra, an applied magnetic field Happ was employed parallel and perpendicular to the stripe domains, respectively. Dynamic pseudo-uniaxial anisotropy fields are derived. In order to explain the anisotropy variation of permalloy films, a theoretical model has been proposed, and the result shows that the pseudo-uniaxial anisotropy can be attributed to the contributions of saturation magnetization, exchange interaction, perpendicular anisotropy, and film thickness. In addition, with the increase of Happ, the stripe domains gradually rotate towards the direction of the applied field, which can be clearly observed by magnetic force microscopy. Moreover, the dynamic permeability spectra exhibit a rhythmic variation dependent on the rotation of the stripe domains, i.e. the relationship between the rotational stripe domains and resonance frequency. [ABSTRACT FROM AUTHOR]

The use of nondestructive magnetic signatures for continuous monitoring of the degradation of structural materials in nuclear reactors is a promising yet challenging application for advanced functional materials behavior modeling and measurement. In this work, a numerical model, which is based on the Landau–Lifshitz–Gilbert equation of magnetization dynamics and the phase field approach, was developed to study the impact of defects such as nonmagnetic precipitates and/or voids, free surfaces and crystal orientation on magnetic domain structures and magnetic responses in magnetic materials, with the goal of exploring the correlation between microstructures and magnetic signatures. To validate the model, single crystal iron thin films (~240 nm thickness) were grown on MgO substrates and a focused ion beam was used to pattern micrometer-scale specimens with different geometries. Magnetic force microscopy (MFM) was used to measure magnetic domain structure and its field-dependence. Numerical simulations were constructed with the same geometry as the patterned specimens and under similar applied magnetic field conditions as tested by MFM. The results from simulations and experiments show that 1) magnetic domain structures strongly depend on the film geometry and the external applied field and 2) the predicted magnetic domain structures from the simulations agree quantitatively with those measured by MFM. The results demonstrate the capability of the developed model, used together with key experiments, for improving the understanding of the signal physics in magnetic sensing, thereby providing guidance to the development of advanced nondestructive magnetic techniques. [ABSTRACT FROM AUTHOR]

A low temperature dynamic scanning force microscope has been constructed using commercially available piezoresistive cantilevers that can be coated with a ferromagnetic material for MFM application. The setup is able to work in a temperature range from room temperature down to 1.5 K. The performance of the piezoresistive cantilevers has been investigated under different working conditions. Topographic as well as magnetic images of a magnetite thin film sample have been taken at 50 and 4.2 K confirming the proper operation of the microscope at cryogenic temperatures. Furthermore, force-distance-curves taken on thin lead films at 4.2 K demonstrate the levitation forces between the magnetized cantilever tip and the superconducting films. Flux lines were generated by the magnetized cantilever tip itself when approaching the sample. It has also been shown that the microscope is sensitive to the detection of single magnetic flux lines penetrating the lead films. [ABSTRACT FROM AUTHOR]