Field-dependent Magnetic Force Microscopy Research Articles

Imaging Large Iron-Oxide Nanoparticle Clusters by Field-Dependent Magnetic Force Microscopy

Iron-oxide nanoparticles are intensively considered for high-performance biomedical applications, where simultaneous functionalities, such as magnetic state, large surface area for maximal protein/enzyme binding, high magnetization values to provide large signals, and good dispersion in liquid media, are usually required. In this context, the association of individual nanoparticles into large clusters is of particular interest. Here, we present a magnetic force microscopy (MFM) approach capable to image individual nanoparticulate clusters as large as 350 nm at room temperature and under variable magnetic fields. It is shown that an in situ removal of electrostatic interactions─particularly important for large particle sizes supported by dielectric substrates─in MFM experiments based on phase detection allows us to image the magnetic state of individual clusters. After taking into account the magnetization behavior of the microscope tip, the phase signal reveals a gradual and uniform rotation of the magnetization with the magnetic field and the absence of a hysteretic behavior for all investigated clusters.

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

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

Rotatable magnetic anisotropy in Fe78Si9B13 thin films displaying stripe domains

Fe78Si9B13 thin films having a controlled weak perpendicular anisotropy resulting in dense stripe domain configuration have been prepared to investigate their rotatable anisotropy properties. Vector vibrating sample magnetometry, an innovative field-dependent magnetic force microscopy, and ferromagnetic resonance techniques have been jointly exploited to correlate the perpendicular anisotropy to the threshold field value that must be overcome to induce the stripes realignment. A linear relationship between these two quantities is found. The presence of the threshold field is attributed to the portions of the samples whose magnetisation must flip its perpendicular component during a rotation process, therefore encountering the energy barrier of the perpendicular anisotropy.

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.

Enhancement of the magneto-optical properties in 2-dimensional bilayered magnetic anti-dot lattice

The magneto-optical (MO) properties of two-dimensional bilayered magnetic anti-dot lattice (BMAL) structure, which consists of upper perforated “Co” layer over uniform “Ni” underlayer in square-lattice arrangement, were investigated. Enhancement in the MO activity was found in such structures. Nearly 4 times larger MO-Kerr activity was observed for the first-order diffracted beam as compared with that of the zeroth-order one. To understand the origin of the enhanced MO rotation, the magnetization-domain configuration was also investigated by field-dependent magnetic-force microscopy. Additionally, comparison between MO response and magnetization-domain configuration for the rhomboid BMAL structure was done, which allowed us to understand more clearly the enhancement of MO properties.

Magnetization reversal mechanism of bilayered magnetic anti-dot lattices

Micropatterned ordered arrays of cobalt (Co) anti-dots on a uniform thin nickel (Ni) underlayer, known as bilayered magnetic anti-dot lattices (BMALs), were fabricated using photolithography and a controlled wet-etching process. The magnetization reversals in a rhomboid BMAL under the application of a field along 0° (i.e., easy axis) and 90° (i.e., hard axis) were investigated using field-dependent magnetic-force microscopy, and the angular dependence of squareness (Mr/Ms) and coercivity (Hc) were studied using magneto-optical Kerr effects. Although the magnetic reversals were dominated by domain-wall motions or domain rotations, reflecting the easy and hard axis characteristics, various other complex reversal processes were also found that depends on the history of the field application. The angular dependence of Mr/Ms and Hc in a BMAL system was completely different from that in a single-layer, Co magnetic anti-dot lattice. Interestingly, a unidirectional and uniaxial component representing the overall magnetic anisotropy was revealed in the BMAL system. The details of these complicated magnetization behaviors were investigated and elaborated.

The magnetic domain configuration in Co/Ni/Co nanoscale antidot arrays

We performed superconducting quantum interference devices and magnetic force microscopy (MFM) measurements on magnetic multilayer Co(60 Å)/Ni(90 Å)/Co(60 Å) nanoscale antidot samples. The antidot samples were fabricated on nanochannel glass substrates with different antidot diameters and the antidots ordered as a two dimensional hexagonal lattice structure. The results indicate that a self-organized domain structure is formed due to the pinning effect of the antidots. The strong uniaxial anisotropy of Co suppresses the shape anisotropy of the antidots and results in an uncommon domain structure. The field dependent MFM data reveal a reversal of magnetization.

Half-metallic chromium dioxide (CrO2) nanostructures and field-dependent magnetic domain evolution

Both polycrystalline and epitaxial chromium dioxide structures in nanoscale have been fabricated using selective-area growth technique. Multiple grains were formed in the polycrystalline CrO2 dots larger than 100nm, however, below which only a single grain was allowed to exist. The lateral growth rate of epitaxial CrO2 nanocrystals was found to be highly anisotropic, which is affected by both the orientation and the deposited thickness. Field-dependent magnetic force microscopy was used to study the domain structures of single crystal CrO2 nanowires. The magnetization states were found to be closely related to the wire width. A stripelike domain structure with alternating magnetization parallel to the magnetic easy axis was observed within the CrO2 nanowires aligned along the [010] directions, indicating the existence of a strong uniaxial magnetocrystalline anisotropy.

Thermal stability and switching field distribution of CoNi/Pt patterned media

The thermal dependence and distribution of the switching fields of arrays of magnetic Co50Ni50/Pt nanodots has been studied. These dots, with a diameter of 90 nm, are arranged in a hexagonal pattern with a periodicity of 300 nm. Field-dependent magnetic force microscopy was used to measure the switching field distribution of the array, which was found to range from 80 to 192 kA/m, a value which is confirmed by vibrating sample magnetometer measurements. Additionally, the temperature dependence on the collective behaviour of the switching fields of the array has been investigated. The energy barrier at zero field was estimated to have a value in between 1.8×10−19 and 2.1×10−19 J. Combining this value with the effective anisotropy determined by torque measurements, the switching volume can be estimated to lie in between 1.2×103 and 1.4×103 nm3.

Effect of nanostructuration on the magnetic properties of CoPt films

We have nanostructured CoPt alloy layers prepared by molecular beam epitaxy, deposited directly on a MgO(0 0 1) substrate. The initial layer had the L1 0 tetragonal structure, ordered in the growth direction with an easy magnetization direction perpendicular to the layer plane. We realized by electron beam lithography and ion etching a network of dots spaced of 1 μm and lateral sizes of 1 μm, 500 nm and 300 nm, respectively. Whereas the continuous layers had a labyrinthine magnetic structure after perpendicular demagnetization, all the dots are monodomains with randomly distributed up and down magnetization. This is due to the fact that during the demagnetization the magnetic field is no longer sufficient to reach the nucleation field. Indeed, a single-dot hysteresis loop measured by field-dependent magnetic force microscopy shows a weak nucleation field distribution. However, the 3D micromagnetic calculations show a multidomain state for these dot sizes. This magnetic structure is similar to that observed on patterned samples, heated a short time (60 s) above the Curie temperature.

Nanoscale hysteresis loop of individual Co dots by field-dependent magnetic force microscopy

We present an approach in which field-dependent magnetic force microscopy (MFM) is used in order to determine the magnetic properties of individual magnetic nanodots. In this work, the integral value of the cantilever phase shift obtained in vibrating MFM experiment is used as a measure of the field dependent magnetization of single objects. The method accounts for details that are resolved at the 10 nm scale, including fine structures during magnetization reversal. Measurements have been done on a model system of embedded dots since the flatness of the free surface reduces strongly the topographic contaminations. It is shown that the method can easily be applied to investigate both out-of-plane and in-plane magnetized dots.

Fabrication of patterned magnetic nanodots by laser interference lithography

A method of fabrication of patterned magnetic nanodots by means of laser interference lithography is presented. This method includes the use of a diluted positive photoresist, and modifications in the etching angle and acceleration voltage of the ion beam etching process. Vertical standing waves were suppressed by using a high exposure dose (supra-exposure) instead of an antireflective coating. Field dependent magnetic force microscopy was used to measure the switching field distribution, which was found to range from 80 to 192 kA/m.

Magnetization reversal of individual Fe nanowires in alumites studied by magnetic force microscopy

We have studied the magnetization reversal of two-dimensional (2D) arrays of parallel ferromagnetic Fe nanowires in nanoporous alumina templates. Combining bulk magnetization measurements using a superconducting quantum interference device with field-dependent magnetic force microscopy (MFM), we decomposed the macroscopic hysteresis loop in terms of irreversible magnetic responses of individual nanowires. The field-dependent MFM provides a microscopic method by which to obtain the hysteresis curve by registering the fraction of upward and downward magnetized wires for each field. The nanowire system proves to be an excellent example of the 2D classical Preisach model, well-known from the field of hysteresis modeling and micromagnetism.

Magnetization reversal of ferromagnetic nanowires studied by magnetic force microscopy

The magnetization reversal of two-dimensional arrays of parallel ferromagnetic Fe nanowires embedded in nanoporous alumina templates has been studied. By combining bulk magnetization measurements (superconducting quantum interference device magnetometry) with field-dependent magnetic force microscopy (MFM), we have been able to decompose the macroscopic hysteresis loop in terms of the irreversible magnetic responses of individual nanowires. The latter are found to behave as monodomain ferromagnetic needles, with hysteresis loops displaced (asymmetric) as a consequence of the strong dipolar interactions between them. The application of field-dependent MFM provides a microscopic method to obtain the hysteresis curve of the array, by simply registering the fraction of up and down magnetized wires as a function of applied field. The observed deviations from the rectangular shape of the macroscopic hysteresis loop of the array can be ascribed to the spatial variation of the dipolar field through the inhomogeneously filled membrane. The system studied proves to be an excellent example of the two-dimensional classical Preisach model, well known from the field of hysteresis modeling and micromagnetism.

Magnetic force microscopy observations of the magnetic behavior in Co–C nanodot arrays

The nanomagnetic behavior of Co–C nanodot arrays was investigated by magnetic force microscopy (MFM) and an alternative gradient force magnetometer. The direction of the easy axis can be observed directly with MFM by comparing the saturated magnetization state and the remanent magnetization state. Interaction of the domain wall with local defects was observed by field dependent MFM measurements. Some types of defects that can pin domain wall movement were identified.

Magnetization reversal of a thin polycrystalline cobalt film measured by the magneto-optic Kerr effect (MOKE) technique and field-dependent magnetic force microscopy

The magnetic properties of a 100-nm-thick cobalt film, deposited thermally on a glass substrate under high-vacuum conditions, were measured ex situ through the longitudinal magneto-optic Kerr effect (MOKE) technique and through field-dependent magnetic force microscopy (MFM). The MOKE measurements showed that the film had a uniaxial anisotropy in the plane of the sample. Two series of MFM images were measured under the presence of an external magnetic field, applied separately along the easy and hard axes, with magnitudes encompassing positive and negative saturation of the sample magnetization. Each series of images recorded the magnetization reversal clearly, revealing and explaining distinctions between the two reversal processes that correlate well with the macroscopic reversals recorded by the MOKE technique. The reversal of the gray-scale polarity with the change of the applied field from negative saturation to positive saturation suggests the presence of a component to the magnetic force that was related directly to the magnetization of the individual grains. A calculation of the easy-axis MFM gray-scale image differences showed a hysteresis-like behavior that agreed qualitatively with the corresponding measured MOKE hysteresis loops.