Calibration of Coercive and Stray Fields of Commercial Magnetic Force Microscope Probes

Variable-field magnetic force microscope (MFM) is introduced to characterize the magnetic behavior of commercially available MFM probes that is relevant to interpret MFM imaging. A Nanotec Electronica S.L. microscope has been conveniently modified to apply magnetic fields in axial direction (up to 1.5 kOe) and in-plane direction (up to 2.0 kOe). Axial and transeverse hysteresis loops of the probes have been generated by measuring the changes in the MFM contrast observed when the magnetic field is applied. The variation of the MFM signal is ascribed to the modification of the magnetic state of the tips. This is enabled by the large coercitivity (~1.7 kOe) of the checked longitudinal recording media. The properties of the probes depend on the coating material, the macroscopic tip shape, and tip radius. In only a few cases, the magnetization of the probe can be oriented along in-plane orientation. In addition, the stray field of the tips has been deduced by measuring the influence of the probe in the magnetic state of the checked samples.     

Magnetic Force Microscopy of Low-Coercivity Ferromagnetic Nanodiscs

We report the results of magnetic force microscopy (MFM) investigations of low-coercivity Co nanodiscs, with 50 nm lateral size and 20 nm height, fabricated by e-beam lithography and ion etching. We observed two types of MFM contrast in the form of Gaussian and ring distributions caused by strong probe–particle interaction. We compared experimentally the transformation of the MFM contrast from these low-coercivity nanodiscs caused by an external magnetic field applied in situ, and compared the experimental results with theoretical simulations.      

Comparison and Validation of Different Magnetic Force Microscopy Calibration Schemes

The future of consumer electronics depends on the capability to reliably fabricate nanostructures with given physical properties. Therefore, techniques to characterize materials and devices with nanoscale resolution are crucial. Among these is magnetic force microscopy (MFM), which transduces the magnetic force between the sample and a magnetic oscillating probe into a phase shift, enabling the locally resolved study of magnetic field patterns down to 10 nm. Here, the progress done toward making quantitative MFM a common tool in nanocharacterization laboratories is shown. The reliability and ease of use of the calibration method based on a magnetic reference sample, with a calculable stray field, and a deconvolution algorithm is demonstrated. This is achieved by comparing two calibration approaches combined with numerical modeling as a quantitative link: measuring the probe's effect on the voltage signal when scanning above a nanosized graphene Hall sensor, and recording the MFM phase shift signal when the probe scans across magnetic fields produced by metallic microcoils. Furthermore, in the case of the deconvolution algorithm, it is shown how it can be applied using the open‐source software package Gwyddion. The estimated magnetic dipole approximation for the most common probes currently in the market is also reported.     

Field induced vortex dynamics in magnetic Ni nanotriangles

The magnetization states in Ni triangular dots under an applied magnetic field have been studied using variable-field magnetic force microscopy (VF-MFM) imaging. In order to understand their dynamics we performed micromagnetic simulations which are in remarkable agreement with the experimental MFM results. The nanostructures present magnetic vortices as ground states which move under an external magnetic field. The combination of micromagnetic simulations and MFM imaging allows us to identify correctly the vortex chiralities and polarizations. The triangular geometry produces an improved contrast of the vortex core. Additionally, the vortices of different chiralities present clearly different MFM images under an?applied field.     

Sintering Effect of Annealed FePt Nanocrystal Films Observed by Magnetic Force Microscopy

Chemically synthesized FePt nanocrystals can exhibit room temperature ferromagnetism after being annealed at temperatures above 500degC. In thick films composed of FePt nanocrystals, the coercivity can be quite large. However, the coercivity of thin films has been found to decrease significantly with decreasing thickness, to the point that ferromagnetism at room temperature is lost. We studied 12 to 55 nm thick films by using magnetic force microscopy (MFM) under external applied fields. We made smooth films by spin casting 4-nm-diameter FePt nanocrystals and annealing them at 605degC-630degC. Thin FePt films showed lower coercivity than thick films. To help interpret the MFM images, we obtained complementary magnetic and structural data by superconducting quantum interference device (SQUID) magnetometry, transmission electron microscopy (TEM), and X-ray diffraction. We conclude that the magnetic properties of these films are strongly affected by nanocrystal aggregation that occurs during annealing     

Nanoscale Magnetic Behavior of C60 Thin Films in Earth Magnetic Field under Polarization Light Influences

Magnetic behavior of C₆₀ thin films in the Earth's magnetic field under polarization light influence is presented. Transformation of magnetic field for two fullerene thin films of different thickness is investigated. Two proton magnetometers were used for these measurements. Samples of 30 nm and 250 nm thickness illustrate a significant change of magnetic field intensity under the influence of polarization light, in range from 3.4 to 12.9 nT, for 200 measurement data per sample.     

Magnetic behavior in an ordered Co nanorod array

The magnetization reversal process of an ordered Co nanorod array is shown using the images obtained from successive in-field magnetic force microscope (MFM) measurements. The magnetization reversal model is discussed according to local and whole magnetization reversal properties measured by the polar magneto-optical Kerr effect (PMOKE) and an alternating gradient magnetometer (AGM), respectively. Additionally, the dipolar field was probed using in-field MFM measurements. By removing the effect of the dipolar field, an intrinsic switching field distribution (SFD) is shown in a map with a hexagonal array. A detailed study of the dipolar field in ordered nanorod arrays with various diameters and pitches was carried out by numerical calculations.     

Magnetic Force Microscopy in Liquids

In this work, the use of magnetic force microscopy (MFM) to acquire images of magnetic nanostructures in liquid environments is presented. Optimization of the MFM signal acquisition in liquid media is performed and it is applied to characterize the magnetic signal of magnetite nanoparticles. The ability for detecting magnetic nanostructures along with the well‐known capabilities of atomic force microscopy in liquids suggests potential applications in fields such as nanomedicine, nanobiotechnology, or nanocatalysis.     

Magnetic Force Microscope Contrast Simulation for Low-Coercive Ferromagnetic and Superparamagnetic Nanoparticles in an External Magnetic Field

We report the results of simulations of magnetic force microscope (MFM) contrast for low-coercive ferromagnetic and superparamagnetic nanoparticles. We show that two types of MFM contrast in the form of gaussian and ring distributions can be observed because of probe-particle interaction. We discuss stabilization of the magnetic moment of nanoparticles by an external magnetic field. We have calculated the values of stabilizing magnetic fields and their dependence on probe parameters and scanning heights.     

Ordering of free-standing Co nanoparticles

Colloidal Co particles of 11 nm diameter were deposited on Si substrate by spin coating and/or casting in magnetic field. A perpendicular magnetic field varying along the diagonal of the substrate was also applied. The samples were analyzed by transmission electron microscopy (TEM), field emission gun scanning electron microscopy (SEM-FEG), atomic and magnetic force microscopy (AFM/MFM). TEM micrographs show local order when a Co nanoparticle monolayer is deposited on Si. Drying the colloidal solution in a magnetic field leads to the formation of quite large clusters (0.3 μm) of Co nanoparticles. A stripe structure was then observed when the particles were deposited by casting in the varying magnetic field. AFM/MFM measurements show isolated Co clusters on the stripes. Magnetic features corresponding to the single Co cluster have been observed pointing out that all magnetic moments in the cluster are oriented along the field direction.     

Magnetic and screening properties of amorphous ferromagnetic ribbons

We have studied the magnetic and screening properties of cylindrical shields made of amorphous ferromagnetic ribbons. It is established that the relative magnetic permeability of this material can exceed 106. The action of an alternating decaying magnetic field (demagnetization) converts amorphous ferromagnetic ribbons into the state of anhysteretic magnetization, which is characterized by extremely high (above 2 × 10⁷) values of the relative magnetic permeability in magnetic fields at a level of 10 nT. The results of measurements were used to estimate the coefficient of attenuation of the Earth’s magnetic field inside cylindrical shields with open ends, depending on the diameter and the number of layers of an amorphous ferromagnetic ribbon.     

AC Magnetic Technique to Measure Specific Absorption Rate of Magnetic Nanoparticles

The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm³ in the frequency range f=10–150 kHz and for magnetic field amplitudes up to H ₀=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter D=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10^?5 g, dispersed in a solid matrix. The maximal SAR ～?80 W/g has been obtained for the magnetite nanoparticle assembly investigated.     

Intrinsic Switching Field Distribution of Arrays of Ni80Fe20 Nanowires Probed by In Situ Magnetic Force Microscopy

The progress of magnetization reversal of weakly packed ferromagnetic Ni₈₀Fe₂₀ nanowire arrays of different diameters (40, 50, 70, and 100 nm) electrodeposited in polycarbonate membranes was studied by magnetic force microscopy (MFM). For such a low packing density of nanomagnets, the dipolar interactions between neighboring wires can be neglected. The intrinsic switching field distribution has been extracted from in situ MFM images and its width was found to be considerably smaller than for densely packed nanowire arrays.     

Magnetic force microscopy of superparamagnetic nanoparticles

The use of magnetic force microscopy (MFM) to detect probe-sample interactions from superparamagnetic nanoparticles in vitro in ambient atmospheric conditions is reported here. By using both magnetic and nonmagnetic probes in dynamic lift-mode imaging and by controlling the direction and magnitude of the external magnetic field applied to the samples, it is possible to detect and identify the presence of superparamagnetic nanoparticles. The experimental results shown here are in agreement with the estimated sensitivity of the MFM technique. The potential and challenges for localizing nanoscale magnetic domains in biological samples is discussed.     

Fabrication and use of a nanoscale Hall probe for measurements of the magnetic field induced by MFM tips

Extraordinary Hall effect probes with 160?nm × 160?nm working area were fabricated using photo-?and electron-beam lithographic procedures with the aim of direct measurements of MFM cantilever tip magnetic properties. The magnetic field sensitivity of the probes was 35?Ω?T(-1). Magnetic induction of the MFM cantilever tips coated by Co and SmCo films was measured with the probes. It was shown that the resolution of the probes was of the order of 10?nm.     

A Near-Infrared Spectroscopy in Magnetic Fields Above 100 T

Development of the first measurement system for near-infrared absorption spectra between 0.9 and 1.7 μm under ultra-high magnetic fields is reported. Spectral signals were integrated for 1 μs with an InGaAs photodiode array around the top of a very short pulsed megagauss field. The magnetic fields were generated using a single-turn coil system. The measurement system was demonstrated in the study of exciton states in single-walled carbon nanotubes up to megagauss fields. A nearly noiseless absorption spectrum with well-resolved absorption peaks was obtained at 105.9 T in the Voigt configuration where the magnetic field was applied parallel to the alignment of the nanotubes.     

GISAXS studies of self-assembling of colloidal Co nanoparticles

In this work we report on the formation of ordered monolayers (2-D) and arrays of rods (3-D) of magnetic Co nanoparticles in magnetic field perpendicular to the substrate surface. Samples were prepared by drying a droplet of colloidal solution of Co nanoparticles (10 nm diameter) on Si/Si₃N₄ substrates in magnetic field between 0.2 and 0.9 T. The samples were characterized by high resolution scanning electron microscopy (SEM), atomic and magnetic force microscopy (AFM/MFM) and grazing incidence small angle X-ray scattering (GISAXS). SEM studies of monolayers show well-ordered 2-D arrays with hexagonal symmetry of 200 nm × 500 nm in size forming a mosaic structure. Rods, about 500 nm in diameter, aligned with the field direction and forming a hexagonal pattern were obtained when higher concentration of colloid and low evaporation rate of the solvent were used. The ordering of nanoparticles in the monolayer analyzed by GISAXS is described by the local order with hexagonal symmetry. The model of close packing of hard spheres is used for ordering of particles inside the rods. Magnetic features corresponding to the 3-D arrays have been observed by MFM pointing out that all magnetic moments in the rod are oriented along the field direction.     

Enhancement and Correlation of MFM Images: Effect of the Tip on the Magnetic Configuration of Patterned Co Thin Films

A technique of numerical treatment of magnetic force microscopy (MFM) data matrices has been exploited to enhance the quality of raw MFM images of patterned Co thin films obtained by Electron Beam Lithography on RF sputtered 30-nm-thick Co samples. The pattern consists of chains of elliptical cylinders whose major axis is around 2.5 $mu$ m and whose minor axis is around 0.5 $mu$m (aspect ratio 5:1). In this work, a new differential approach is proposed. Two or more MFM images of the same surface area of a soft ferromagnetic material submitted to different magnetic fields $H$ are examined, and the different arrangements of the local magnetization, as emerging from contrast differences in MFM images, are analyzed as functions of $H$. It is shown that this differential approach is able to account for the effect of the MFM tip on the magnetization of the investigated soft magnetic material. The patterned Co samples used to demonstrate this method have been demagnetized before each MFM scan in the plane of the film by applying an alternate field of progressively small absolute value.      

Thermometers in Low Magnetic Fields

In this article the effect of low amplitude DC magnetic fields on different types of thermometers is discussed. By means of a precision water-cooled electromagnet, the effect of a magnetic field on platinum resistance thermometers, thermistors, and type T, J, and K thermocouples was investigated, while thermometers were thermally stabilized in thermostatic baths. Four different baths were used for temperatures from 77 K (?196 °C) to 353 K (80 °C): liquid nitrogen bath (nitrogen boiling point at atmospheric pressure), ice-point bath, room-temperature air bath, and hot-water bath. The generated DC magnetic field of high relative precision (2 × 10^?4 at 1 T, 4 × 10^?5 short-term stability) and high relative uniformity (2 × 10^?5 over 1 cm², 10 mm gap) had a magnetic flux density of 1 T in the center of the gap between the magnet pole caps. The results indicate a magnetic effect of up to 100 mK due to a 1 T magnetic field for the types of thermocouples composed of ferromagnetic materials (Fe, Cr, Ni). For platinum resistance thermometers, thermistors, and non-magnetic type T thermocouples, the detected magnetic effect was weaker, i.e., under 10 mK.     

Electrical Force Microscopy measurements on nanoconductors deposited by electropulsed SPM

The reduced dimensions of novel integrated devices and systems stresses the need for new nanopatterning techniques. We showed that metallic pixels of about 30 nm can be deposited by electropulsed SPM. Deposition is obtained by applying −12 to −17 V pulses to a tip coated with a CoCr metallic film while scanning in tapping mode AFM. By assembling arrays of these pixels it is possible to obtain custom-shaped conductors. We deposited rectangular conductors with dimensions from 50×100 nm up to 2000×400 nm and thickness between 2 and 3 nm.A typical conductor line of 2000×400 nm deposited at 40 Hz pulse frequency, 800 nm/s tip velocity and at a line density of 0.64 nm⁻¹ shows a thickness of approximately 3 nm. For electrical measurements this line is deposited perpendicularly and welded from the edge of a conventional Al pad used for microelectronics. The opposite end of the nanoconductor line is located by AFM and the electrical conductivity is then asserted using the conductive AFM tip in Electrical Force Microscopy (EFM) mode.