Quantitative measurement of the magnetic moment of individual magnetic nanoparticles by magnetic force microscopy

The quantitative measurement of the magnetization of individual magnetic nanoparticles (MNPs) using magnetic force microscopy (MFM) is described. Quantitative measurement is realized by calibration of the MFM signal using an MNP reference sample with traceably determined magnetization. A resolution of the magnetic moment of the order of 10(-18) A m(2) under ambient conditions is demonstrated, which is presently limited by the tip's magnetic moment and the noise level of the instrument. The calibration scheme can be applied to practically any magnetic force microscope and tip, thus allowing a wide range of future applications, for example in nanomagnetism and biotechnology.     

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.     

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.     

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.     

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.     

磁力显微镜在磁性材料领域的应用

磁力显微镜是种新发展起来的用于研究物质磁性的重要技术。本文简要地介绍了ＭＦＭ的基本工作原理、影响分辨率的主要因素，以及ＭＦＭ在磁记录体系，磁性薄膜和铁磁学基本现象研究方面的应用情况。本文还报道了ＭＦＭ在有机铁磁体和生物分子磁性研究方面应用的重要意义及其发展趋势。     

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.     

The advantages of the magnetic structure in ferromagnetic-film-coated carbon nanotube probes

The magnetic structures of ferromagnetic-film-coated carbon nanotube (CNT) probes and conventional pyramidal probes for a magnetic force microscope (MFM) were simulated using three-dimensional micromagnetic simulation. The CNT-MFM probes with a total probe diameter less than 60?nm are almost uniformly magnetized along the longitudinal direction of the CNT, which is the ideal magnetic structure for MFM observations. On the other hand, the pyramidal probes had a vortex structure around the point tip, which suggests that they require a greater thickness of the ferromagnetic film because only part of the magnetic moment participates in the detection of the z-component of the stray field from samples. The advantages of the CNT-MFM probe are uniform magnetization along the longitudinal direction and magnetic imaging ability using a smaller coating thickness.     

Magnetic,morphological and structural investigations of CoFe/Si interfacial structures

CoFe/Si interfacial structures have been realised by electron beam evaporation of CoFe magnetic alloy on p- and n-Si substrates. These realised interfacial structures have been characterised from X-ray diffraction (XRD), atomic force microscopy (AFM), magnetic force microscopy (MFM) and magnetisation (M–H) characteristics. XRD data have shown the presence of CoFe (bcc phase) and β-FeSi₂ phases for CoFe/p-Si interfacial structures, whereas CoFe/n-Si interfacial structures have shown the presence of CoFe (bcc and fcc) phases along with Fe₃Si, ?-FeSi and β-FeSi₂ silicide phases having nanodimension crystallites. The M–H characteristics have shown the superparamagnetic type behaviour for CoFe/p-Si structure, whereas CoFe/n-Si structure shows a feature of interfacial antiferromagnetic (AF) coupling. The observed magnetic behaviour has been understood due to the presence of various magnetic phases and their nanosized grains. The MFM data of domain size have also been correlated with the observed magnetic behaviour. CoFe/n-Si structures have shown a significant feature of giant magnetoresistance (GMR) as compared to CoFe/p-Si structures. It has been found that CoFe/p-Si structures show a distinctly different behaviour than CoFe/n-Si structures for their chemical structure, morphology and magnetic behaviour.     

Susceptibility of systematic error-compensating algorithms to random noise in phase-shifting interferometry

In phase-shifting interferometry, many algorithms have been reported that suppress systematic errors caused by, e.g., nonlinear motion of the phase shifter and nonsinusoidal signal waveform. However, when a phase-shifting algorithm is designed to compensate for the systematic phase-shift errors, it becomes more susceptible to random noise and gives larger random errors in the measured phase. The susceptibility of phase-shifting algorithms to random noise is analyzed with respect to their immunity to phase-shift errors and harmonic components of the signal. It is shown that for the most common group of error-compensating algorithms for nonlinear phase shift, both random errors and the effect of high-order harmonic components of the signal cannot be minimized simultaneously. It is also shown that if an algorithm is designed to have extended immunity to nonlinear phase shift, simultaneous minimization becomes possible.     

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.     

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.     

Magnetization transitions obtained by deconvolution of measured replay pulses in perpendicular magnetic recording

Magnetization transitions in perpendicular magnetic recording have been calculated from measured replay pulses by a deconvolution algorithm using an analytical expression for the field of a probe head (PH). The transitions appear to be asymmetric whereby a pronounced magnetization peak occurs whose shape depends on the medium coercivity. For the experiments double layer media with double sided probe heads have been used. For the deconvolution only perpendicular head field and medium magnetization components are considered.     

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.     

磁场力对磁性杂质颗粒的作用的研究

磁场对处于其中的磁性颗粒有力的作用,通过对交流磁场和稳恒磁场的磁场力的比较分析,证明这2种磁场力可以统一,与磁性颗粒的作用规律相似.采用磁平衡法测量了MnFeAl36和MnFeSiAl42种金属间化合物的磁化率,证明它们是顺磁性的物质,利用磁场力可以将其分离.利用自制的交变磁场和稳恒磁场装置对铝熔体进行分离试验,结果表明磁场力可以将铝熔体中的磁性杂质颗粒分离出来.     

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.     

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.     

磁流体浸没物磁场力分析及磁浮特性

物体浸没于磁流体中表现出磁浮特性，对其受力状态分析是准确描述其悬浮状态的前提和基础。基于非线性磁流体应力张量模型和稳态Bernoulli方程，建立磁流体中浸没物受力模型。借助非磁性体受力模型的简化计算方法以及磁性体与磁流体之间的多场效应关系，分别对非磁性体、磁性体两类浸没物在磁流体中所受磁浮力进行分析，结果表明非磁性浸没物在磁流体中仅受到外加磁场贡献的一次磁浮力，而磁性浸没物除受到一次磁浮力外，还受到其自身激发磁场贡献的二次磁浮力．永磁体在磁流体中位置决定的磁浮力满足一定条件时，永磁体能够自悬浮于磁流体中。     

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.