Magnetic dynamic behavior of nanomagnets studied by Magnetic Force Microscopy with external field

The micro/nanomagnetic behavior of magnetic systems is a key issue as the size of magnetic devices is reduced to or under the micrometer range. We study the magnetic behavior of nanomagnets under different applied magnetic field conditions by Magnetic Force Microscopy (MFM). MFM is sensitive mainly to magnetization distributions that generate magnetic fields. CoCr Magnets were deposited by electropulsed SPM onto a Si substrate with sizes ranging from 400×100 to 800×400 nm and thickness between 2 and 3 nm. MFM measurements were performed using a Digital Instruments (DI) Dimension 3100 SPM upgraded for measurements with an external magnetic field applied to the sample. The home-designed modification consists in an electromagnet with field guides towards the scanning region while measuring. Different magnetic fields up to 400 Oe were applied to the samples in-plane during the MFM measurements. The magnetic configuration for the different applied fields was then imaged by MFM.     

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.     

Structural and magnetic properties of MnAs/GaAs ferromagnetic semiconductor nanocomposite material

Self-organized (Ga,Mn)As nanoclusters, embedded in GaAs and formed during thermal annealing of Ga_1-xMn_xAs layer at 500 °C or 600 °C, were studied using Transmission Electron Microscopy (TEM) and Magnetic Force Microscopy (MFM). We found that 10–20 nm large NiAs-type hexagonal MnAs nanocrystals gave magnetic contrast in MFM images, whereas smaller zinc-blende nanoinclusions were not visible by means of this technique. Theoretical simulations showed that MFM contrasts reflect interaction between magnetic tip and many randomly distributed MnAs nanocrystals.     

Shear-mode magnetic force microscopy with a quartz tuning fork in ambient conditions

We have demonstrated high-resolution shear-mode magnetic force microscopy (MFM) using a quartz tuning fork in ambient conditions. A commercial magnetic cantilever tip was attached to one prong of the tuning fork to realize shear-mode MFM operation. We have obtained MFM images with a spatial resolution of less than 100?nm and demonstrated a frequency resolution of ～1?mHz, values which are achieved by phase shift detection methods.     

Numerical simulation of MFM image of a magnetic nano-contact

A MFM image of a magnetic nano-contact is studied numerically. We show that the ring- or arc-shaped MFM images with diameter of the order of 100 nm appear while the size of a nano-contact is of the order of 1 nm. We also show that the origin of the ring or arc-shaped MFM image is the oscillation of the magnetization of a nano-contact induced by the interaction between the MFM-tip and the nano-contact.     

Low-noise magnetic force microscopy with high resolution by tip cooling

Low-noise magnetic force microscopy (MFM) was realized by using a conventional high-vacuum MFM with homemade tip-cooling equipment. The noise level of the MFM at a tip temperature of 130 K was estimated at /spl mu/N/m order. High spatial resolution of 10 nm was obtained for observing high-density recording media with recording density of 1000 kfci. The improvement of resolution by tip cooling was a result of the reduction of thermodynamic noise of a cantilever and the effective reduction of tip-sample distance due to the magnetic hardening of a tip.     

Iron-platinum-coated carbon nanocone probes on tipless cantilevers for high resolution magnetic force imaging

High coercivity iron-platinum-coated carbon nanocones (CNCs) have been fabricated for magnetic force microscopy (MFM) by direct-current plasma-enhanced chemical vapor deposition growth of nanocones on tipless cantilevers followed by sputtering and annealing of the FePt film. The FePt-coated CNC probe has many localized magnetic stray fields due to the high-aspect-ratio geometry and small radius of the tip. The MFM imaging on magnetic recording media was performed using CNC probes and compared with the imaging by FePt-coated silicon probes. An image with 20?nm lateral resolution has been demonstrated.     

Fabrication and MFM study of 60 nm track-pitch discrete track media

Discrete track magnetic recording media with a 60 nm track pitch and prewritten servo patterns were fabricated and tested for read/write performance, and a feasibility analysis of the embedded servo was performed. The fabrication process consisted of ultraviolet nanoimprint lithography (UV-NIL) and sequential ion beam etching on a conventional perpendicular magnetic recording medium. Magnetic patterns were written to the fabricated tracks at 700 kilo flux changes per inch (kFCI) using a spin stand and were read using magnetic force microscopy (MFM), with a resulting signal-to-noise ratio (SNR) of 12.15 dB. The servo pattern was also visualized with MFM. These results demonstrated the feasibility of writing to a 30 nm wide discrete data track and the workability of the embedded servo pattern.     

磁力显微镜的发展历史,原理和应用

磁力显微镜（ＭＦＭ）的分辨率高达２０～５０ｎｍ，是纳米尺度磁性材料表面磁结构研究新的有力的工具，本文简单介绍了ＭＦＭ的历史，原理，运作和应用，并介绍了中科院物理所一年来的ＭＦＭ研究，举了两个实例，最后，以ＭＦＭ研究的重要问题作了评论。     

High resolution magnetic force microscopy of patterned L1(0)-FePt dot arrays by nanosphere lithography

High resolution magnetic force microscopy (MFM) has been carried out on L1(0)-FePt dot arrays patterned by plasma modified nanosphere lithography. An ex situ tip magnetization reversal experiment is carried out to determine the magnetic domains and verify the imaging stability of MFM and the mutual perturbations between the magnetic tip and the sample. We have identified that the critical size for the single domain region is about 90?nm across. Comparison with MFM image simulation also suggests that the magnetizations of the triangular dots in both single and double domain states are parallel to one edge of the dots, indicating the large uniaxial magnetocrystalline anisotropy of the L1(0)-FePt phase and the need for decreasing the magnetostatic energy.     

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.     

Resolving magnetic nanostructures in the 10-nm range using MFM at ambient conditions

Following the demand of the magnetic data storage industry, the magnetic structures in hard disk heads are continuously shrinking. This requires a powerful tool to investigate the magnetic properties of these elements in the range of about 10 nm. To achieve this goal, we prepared MFM tips using the electron-beam deposition (EBD) contamination technique, where carbon caps and needles are grown onto the micromachined Si cantilevers. For batch production of MFM tips, however, this technique is not suited well, so we employ the focussed ion-beam (FIB) technique to produce MFM tips with a high aspect ratio similar to those tips with carbon needles. We show that the use of these tips not only improves the lateral resolution, but also considerably reduces the disturbation effects of the weak magnetic structures due to the magnetic tips.     

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.     

Effect of the tip-contact interaction on the MFM image of magnetic nanocontact

MFM images of magnetic nanocontacts fabricated in nano-oxide layers are evaluated with special attention to the tip-contact interaction. We show that a sharp ring or arc signal with a diameter on the order of 100 nm appears while the diameter of the contact is on the order of 1 nm. We also show that the position of the sharp signal is determined by the competition between the forces due to the magnetic anisotropy in the contact and due to the tip-contact interaction.     

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     

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.      

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.     

Comparison of Micromagnetic and Point Probe Model in Magnetic Force Microscope Simulation

We developed a micromagnetic model of magnetic force microscopy (MFM) tip to compare it with the simple point probe model. We simulated the MFM signal to provide an understanding of the measurement of the field generated by the write head in perpendicular recording hard disk drives. When the magnetic pole density at the air-bearing surface of the head's main pole is increased from 0.2 T to 1 T, the MFM tip with vertical anisotropy shows a flower-state magnetization, while the tip with horizontal anisotropy has more complicated switching modes. It is found that the signal ratio of the two MFM tips with vertical/horizontal anisotropy does have a one-to-one correspondence to the average magnetic field in the tip; however, the signal ratio may change sign because of the magnetic moments' switching in the tip with vertical anisotropy. The result of micromagnetic simulation is quite similar to that of the point probe model, and has good agreement with experiments.     

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.     

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.