Magnetic properties of single biogenic magnetite nanoparticles

Biogenic magnetite nanoparticles (MNP) extracted from the magnetotactic bacterium Magnetospirillum gryphiswaldense MSR-1 have been systematically studied by atomic force microscopy (AFM) and magnetic force microscopy (MFM). Isolated single MNP and chains of MNP were obtained from diluted MNP aqueous suspension dried on mica surfaces in a homogeneous in-plane magnetic field. The size of the MNP was determined by employing AFM tip deconvolution procedures. The obtained result has been confirmed by scanning electronic microscopy. Magnetic properties of isolated single MNP and chains of MNP in remanence and in the presence of external magnetic fields were investigated by MFM. In particular, the magnetization reversal of a two-particle chain has been revealed and the dipolar interaction between the MNP is estimated. The change in the magnetic contrast on application of an external magnetic field is consistent with the hysteresis curve obtained by cantilever magnetometry.     

Electrical resistivity due to electron scattering with magnetic domain walls and magnetic properties of Ni-Fe alloy thin films

The physical properties of magnetic domain walls and electrical conductivity of permalloy thin films under external magnetic fields were studied. Using a magnetic force microscope (MFM), we observed the variation of domain configurations with the change of applied magnetic field for different film thicknesses of 245, 320, and 415 nm. A superconducting quantum interference device (SQUID) was exploited to measure the magnetization loop for the applied magnetic field either parallel or perpendicular to the normal direction of the surface. We also found that the resistivity increases significantly as the electrical current conduction changed from parallel to perpendicular to the domain walls.     

Magnetic properties of field-annealed FeCo thin films

Fe₅₀Co₅₀ thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 °C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films.     

Competing magnetoresistance contributions in sputtered FePt thin films

Fe-Pt thin films were deposited by rf sputtering on an MgO substrate heated at different temperatures to induce the formation of the perpendicular Fe-Pt L1₀ phase with a different grain morphology on the nanometer scale. All films are characterized by a mazelike pattern of FePt nanograins with interconnected bases. MFM images and magnetization curves indicate that all samples have a strong perpendicular magnetic anisotropy arising from (0 0 1) growth. The temperature behaviour of the electrical resistance indicates that a percolating path exists for conduction electrons in the mazelike pattern. The magnetoresistance was measured as a function of magnetic field (applied longitudinally) and temperature in the ranges −70 kOe<H<+70 kOe and 4 K<T<150 K, respectively. All samples display a complex behaviour of the electrical resistance as a function of applied field. The role of the different magnetoresistance effects (both intrinsic and extrinsic) measured in these FePt thin films is elucidated.     

Versatile UHV system for combined far- and near-field magneto-optical microscopy of thin films

We have developed a UHV system for in situ studies of magnetic domains and magnetization reversal of thin films in the presence of external magnetic fields and at variable temperature. The system comprises a setup for magneto-optical Kerr effect measurements of magnetization curves, a Kerr-microscope for far-field magnetic-domain imaging, and a magneto-optical scanning near-field microscope in combination with a Sagnac interferometer (Sagnac-SNOM) for high-resolution imaging on a sub-μm scale. All components have successfully been tested, and the feasibility of studying ultrathin films has been demonstrated.     

Investigation of ferromagnetic resonance and damping properties of CoFeB

In this study, the influences of thin film thickness and post-annealing process on the magnetic properties of CoFeB thin films were investigated. The angular dependency and linewidth of the ferromagnetic resonance signal were used to explore the magnetic behavior of sputtered single-layer and trilayer thin film stacks of CoFeB. A micromagnetic simulation model was employed based on the metropolis algorithm comprising the demagnetizing field and in-plane induced uniaxial anisotropy terms with all relevant contributions. Our results reveal that the direction of magnetization changes from in-plane to out-of-plane as a result of the annealing process and induces a perpendicular magnetic anisotropy in the 1-nm thick CoFeB thin film. The ferromagnetic resonance (FMR) linewidth can be defined well by the intrinsic Gilbert damping effect and the magnetic inhomogeneity contribution in both as-grown and annealed samples. The difference between the linewidths of the single and trilayer film is mainly caused by the spin pumping effect on damping which is associated with the interface layers.     

Specific features of observing magnetization inhomogeneities on the surface of permalloy thin films by means of highly sensitive magnetic-force-microscopy probes

The results of studying regions of inhomogeneous magnetization on the surface of permalloy thin films with the use of fabricated highly sensitive probes of magnetic force microscopy (MFM) are presented. The technological features of manufacturing MFM probes with a high sensitivity to magnetic-field gradient are analyzed. Regions of ordering of the vertical component of the magnetic field are revealed, and domain walls are visualized in the thin films under study. Nanoscale measurements of the domain-wall thicknesses are performed.     

Field-variable magnetic domain characterization of individual 10 nm Fe_3O_4 nanoparticles

The local detection of magnetic domains of isolated 10 nm Fe_3O_4 magnetic nanoparticles(MNPs) has been achieved by field-variable magnetic force microscopy(MFM) with high spatial resolution.The domain configuration of an individual MNP shows a typical dipolar response.The magnetization reversal of MNP domains is governed by a coherent rotation mechanism, which is consistent with the theoretical results given by micromagnetic calculations.Present results suggest that the field-variable MFM has great potential in providing nanoscale magnetic information on magnetic nanostructures,such as nanoparticles, nanodots, skyrmions, and vortices, with high spatial resolution.This is crucial for the development and application of magnetic nanostructures and devices.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Ultrawide frequency linewidth of the permeability spectra of FeCoN thin film sputtered on the flexible substrate

Analysis shows that it is possible to make use of dispersed magnetic ripple fields to obtain a wide frequency linewidth of permeability spectra of soft magnetic thin films. As-sputtered FeCoN thin film sputtered on flexible Kapton substrate is studied as an example. It has ultrawide frequency linewidths of its resonance peaks in the permeability spectra, compared to its counterpart deposited on Si substrate. The frequency linewidth of FeCoN on Kapton substrate decreases with external magnetic field, showing a different field dependence from that of FeCoN on Si substrate. The ultrawide frequency linewidth and its decrease with external magnetic field are ascribed to the dispersed magnetic ripple fields caused by the flexible substrate. This work shows that the flexible substrate is effective in obtaining a wide frequency linewidth of the permeability spectra of soft magnetic thin films.     

Recent results in magnetic force microscopy

We present domain wall images obtained by using Magnetic Force Microscope (MFM) on magnetic samples like: double layer of permalloy alloy, magnetic hard disk, BaFe₁₂O₁₉ single crystal and YGdTmGa/YSmTmGa magnetic garnet. We have imaged topography and magnetic forces of the same area. The Fe double- and single-layer thin film tips have been prepared to achieve high sensitivity (10^–12N) and high resolution of MFM.     

Study of interactions in Co–SiO2 granular films by means of MFM and magnetization measurements

A study of Co[SiO₂] granular films using magnetic force microscopy (MFM), transmission electron microscopy and DC magnetization is presented. MFM shows large strip magnetic domains in the percolated system and allows a more direct investigation of the magnetic interaction among grains in the film with the lowest metal volume fraction.     

Bifurcation magnetic resonance in films magnetized along hard magnetization axis

We study low-frequency ferromagnetic resonance in a thin film magnetized along the hard magnetization axis performing an analysis of magnetization precession dynamics equations and numerical simulation. Two types of films are considered: polycrystalline uniaxial films and single-crystal films with cubic magnetic anisotropy. An additional (bifurcation) resonance initiated by the bistability, i.e. appearance of two closely spaced equilibrium magnetization states is registered. The modification of dynamic modes provoked by variation of the frequency, amplitude, and magnetic bias value of the ac field is studied. Both steady and chaotic magnetization precession modes are registered in the bifurcation resonance range.     

Rosensweig instability of ferrogel thin films or membranes

We derive the dispersion relation of surface waves for magnetic gel membranes or thin films at the interface between two fluids in the presence of an external magnetic field normal to the free surface. Above a critical field strength surface waves become linearly unstable with respect to a stationary pattern of surface protuberances. This linear stability criterion generalizes that of the Rosensweig instability for ferrofluid and ferrogel free surfaces to take into account bending elasticity and intrinsic elastic and magnetic surface properties of the film or membrane, additionally. The latter is of interest for uniaxial ferrogel film or membranes, which show a locked-in permanent magnetization.     

MFM study of magnetic vortex cores in circular permalloy dots: behavior in external field

In a circular dot of permalloy with an appropriate size, a vortex structure with perpendicular (turned-up) magnetization at the core is realized. The existence of the perpendicular magnetization spot has been confirmed and the direction of the magnetization, up or down, has been determined by magnetic force microscopy (MFM) for permalloy dots with the diameter of 0.1–1 μm. The switching field of turned-up magnetization is determined by applying external fields perpendicularly and in tilted directions to the plane. By comparing the MFM results and the magnetization curves measured by a SQUID magnetometer, the switching process of turned-up magnetization is argued.     

Is MFM really useful?

The ill-posed linear inverse problems, characterised by Fredholm integral equations of the first kind, are encountered in many areas of science and technology. This type of problems present some loss of information under the inversion process. The loss of information often makes the inversion process very difficult. Magnetic force microscopy (MFM) is a technique where problems related to loss of information occur. Work is presented here to understand what can be measured by the magnetic force microscope. A simple model is constructed, where the magnetic tip is approximated by a point dipole. Given the force F() acting on the dipole tip, we attempt to determine the magnetization distributlon in a thin ferromagnetic film, M(). This calculation should be interesting due to the rapidiy growing interest in magnetic thin films and magnetic multilayers. Received 3 December 2001 and Received in final form 11 March 2002     

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.     

Quantitative analysis of magnetization reversal in patterned strip wire by magnetic force microscopy

Magnetic force microscopy (MFM) was used to investigate the magnetization reversal process in a patterned strip wire of permalloy thin film. The magnitude of the phase-shift of tapping mode MFM changed with the varying interactive magnetic force between the magnetic tip and the sample. By analyzing the change in values of the phase-shift, the behaviors of magnetization reversal of different local regions in a patterned strip wire can be quantitatively analyzed. The intensity of the phase-shift in the wider end is stronger than that in the narrower one. In contrast, due to a strong anisotropic effect, the coercive force in the narrower end (9 Oe) is larger than that in the wider one (8 Oe). Therefore, the H_c in the neck section could become strongly affected by the competition of the head-to-tail magnetic configurations in the two parts of the strip wire, and this results in a small H_c in the neck section. In addition, in a simple neck shape connection in a strip NiFe wire, a single domain configuration can be easily changed to a two single domain magnetic configuration.     

Magnetic force microscopy of GaAs:Mn ferromagnetic semiconductors

The dependence of magnetic properties of GaAs:Mn and MnAs epitaxial films grown on GaAs (001) by laser ablation of Mn and undoped GaAs in a hydrogen atmosphere under the growth conditions has been studied by magnetic force microscopy (MFM). Magnetic probe calibration for quantitative MFM measurements was performed by scanning across the slit of the magnetic-head of a tape recorder through which controlled direct current was passed. The dipole approximation was used to describe the magnetic properties of the MFM probe. Nonuniformity of the magnetization of GaAs:Mn films related to the formation of MnAs nanoinclusions, which are ferromagnetic at 300 K, has been observed. The typical scales of the spatial nonuniformity of the magnetization of GaAs:Mn films were varied from 270 to 550 nm depending on the film-growth conditions. The MnAs phase was identified by MFM measurements at an elevated temperature (up to 80°N).     

Magnetization reversal observed by the deflection of magnetic actuator

A magnetic actuator consisting of a silicon oxide microcantilever and a silicon oxide plate deposited on ferromagnetic multilayer thin films is fabricated using electron beam lithography and electron beam evaporation, and placed in various magnetic fields to observe its flexure. The magnetic actuator is bent by magnetic torque produced by ferromagnetic multilayer thin films under an external magnetic field owing to the fabrication of a highly sensitive microcantilever and the design of elliptic ferromagnetic thin films with high magnetic shape anisotropy. The magnetic actuator is placed in four kinds of magnetic field directions to investigate the diversity of deflections; the angles between the easy axis of the ferromagnetic multilayer thin films and the direction of the external magnetic field are 90°, 70°, 45° and 20°.