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.     

交变力磁力显微镜动态成像技术的研究

近年来磁力显微镜(magnetic force microscopy,MFM)对动态磁场信号的测量与分析由于其特殊的工业要求和重要用途而受到广泛关注,本文旨在利用交变磁力对磁性探针的周期性调制发展一种交变力磁力显微镜技术,为磁信息存储工业等重要领域关键技术的发展提供新型的有力的工具.与目前标准MFM采用的设计思路不同,本文的关键在于合理利用MFM频率调制机理,优化设计MFM磁性探针,并且引入动态信号处理模块,实现对交变磁场信号的MFM成像.为达到这些目的,需要从理论上研究MFM探针的频率调制机理,并由实验上设计出动态信号提取模块,二者相辅结合优化设计出具有动态信号测试和分析能力的交变力磁力显微镜技术,由此来测量和解释纳米尺度磁畴结构.     

Stability of the magnetic domain structure of nanoparticle thin films against external fields

We investigate the effect of external magnetic fields on the magnetic structure of thin films from magnetic nanoparticles (MNP) with dipolar interaction. Such fields are present, for example, if samples are scanned with magnetic probes. Numerical simulations and experimental magnetic force microscopy (MFM) studies are presented. Numerically, we have calculated the magnetization pattern of single-layer and multilayer MNP thin films. The calculations show that unperturbed single-layer MNP films have an in-plane orientation of the magnetization with a flux-closure-domain pattern. An external field generated by a point dipole above the film induces locally an out-of-plane configuration of the magnetization. In the corresponding MFM images, the domain pattern in the film is erased and a stripe-like contrast enhancement at the edges appears. Multilayer films are found to be more robust against external fields than monolayers.     

MFM probe control of magnetic vortex chirality in elliptical Co nanoparticles

Magnetic force microscopy (MFM) methods were applied to investigate the peculiarities of magnetization distribution in elliptical 400×600×27 nm Co particles. Reversible transitions between the uniform and vortex states under inhomogeneous magnetic field of MFM probe were observed. Possibility to control the chirality of a magnetic vortex in these particles by MFM probe manipulation was shown.     

Local observation of reverse-domain superconductivity in a superconductor-ferromagnet hybrid

Nanoscale magnetic and superconducting properties of the superconductor-ferromagnet Nb/PbFe12O19 hybrid were studied as a function of applied magnetic fields. Low-temperature scanning laser microscopy (LTSLM) together with transport measurements were carried out in order to reveal local variations of superconductivity induced by the magnetic field template produced by the ferromagnetic substrate. Room temperature magnetic force microscopy (MFM) was performed and magnetization curves were taken at room and low temperature to investigate the magnetic properties of the hybrid. Comparative analysis of the LTSLM and the MFM images has convincingly demonstrated the presence of the reverse-domain superconductivity.     

In situ magnetic hysteresis measurement of magnetic tips in magnetic force microscope

In situ magnetic hysteresis measurements of magnetic tips in a magnetic force microscope (MFM) are demonstrated using alternating gradient force magnetometry. The measured magnetic moments of MFM tips are estimated in the range from 10⁻⁶ to 10⁻⁵ emu by this technique and the whole MFM tips in cantilevers are considered to be measured from the value of measured magnetic moments. The relationship between the magnetic hysteresis loops of MFM tips and those of coated magnetic films is discussed.     

Magnetic force microscopic study of the magnetic field induced antiferro to ferrimagnetic transition in Mn1.85Co0.15Sb

Magnetic field induced first order antiferromagnetic (AFM) to ferrimagnetic (FRI) transition in polycrystalline Mn_1.85Co_0.15Sb has been studied using magnetic force microscopy (MFM) at 60 K and up to 8 T magnetic fields. Our MFM studies provide real space visualization of AFM to FRI transition. It shows growth (decay) of FRI phase with increasing (decreasing) magnetic field. The hysteretic behavior and co-existing FRI and AFM phases across the critical field required for FRI-AFM transition in Mn_1.85Co_0.15Sb are highlighted. This study demonstrates the potential of MFM for studying phase co-existence at high field and low temperatures.     

Enhanced imaging of magnetic structures in micropatterned arrays of Co dots and antidots

A specific technique of numerical treatment of atomic force microscopy (AFM) and magnetic force microscopy (MFM) signal has been developed to enhance the quality of raw images, in order both to improve their contrast and to gain better insight on the sample topography and on the local arrangement of the magnetisation vector. Basically, the technique consists in computing the optimum conformal transformation that allows one to superimpose two AFM images of the same area, acquired performing subsequent scans whose fast scan axis were mutually perpendicular, and applying the inverse transform to the second image. After MFM image superposition, the two datasets were either summed or subtracted, in order to improve the magnetic contrast. Computations have been done in a Matlab^® workspace with the help of Image Processing Toolbox 4.2. Improved MFM images obtained on both dots and antidots thin evaporated Co arrays in the demagnetised state (after performing alternate field demagnetisation parallel and perpendicular to the array plane) have been interpreted. Samples consisting of large-size patterns (1×1 mm) of circular dots/antidots with square/hexagonal lattices and minimum diameters of 1 μm were prepared by optical lithography. The magnetic film thickness was chosen depending on resist thickness, and varied between 25 and 150 nm, with a fixed ratio 1:4 between metal/resist film thickness. MFM was exploited to obtain images of either intra-dot or inter-antidot magnetic structures.     

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states have been revealed by using magnetic force microscopy (MFM) with high coercivity tips. MFM results indicate that specimens in a remanent state are single domain and their magnetizations align with the direction of the magnetizing field. The evolution of the magnetic domains with annealing temperatures shows that the thermal demagnetization process consists of four stages. Nd-Fe-B should be heat-treated at about 120-170 °C to make its magnetic state stable before practical applications.     

Magnetization reversal of ferromagnetic nanoparticles under inhomogeneous magnetic field

We investigated remagnetization processes in ferromagnetic nanoparticles under inhomogeneous magnetic field induced by the tip of magnetic force microscope (MFM) in both theoretical and empirical ways. Systematic MFM observations were carried out on arrays of submicron-sized elliptical ferromagnetic particles of Co and FeCr with different sizes and periods. It clearly reveals the distribution of remanent magnetization and processes of local remagnetization of individual ferromagnetic particles. Modeling of remagnetization processes in ferromagnetic nanoparticles under magnetic field induced by MFM probe was performed on the base of Landau–Lifshitz–Gilbert equation for magnetization. MFM-induced inhomogeneous magnetic field is very effective to control the magnetic state of individual ferromagnetic nanoparticles as well as to create different distribution of magnetic field in array of ferromagnetic nanoparticles.     

In-plane magnetic pattern separation in NiFe/NiO and Co/NiO exchange biased bilayers investigated by magnetic force microscopy

Ion bombardment induced magnetic patterning (IBMP) was used to write in-plane magnetized micro and submicron patterns in exchange biased magnetic bilayers, where the magnetization directions of the adjacent patterns are antiparallel to each other in remanence. These magnetic patterns were investigated by non-contact magnetic force microscopy (MFM). It is shown that the recorded MFM images of the IBMP patterns in two exemplarily chosen standard layer systems (NiFe (4.8 nm)/NiO (68 nm) and Co (4.8 nm)/NiO (68 nm)) can be well described by a model within the point-dipole approximation for the tip magnetization. For 5 and 0.9 μm wide bar patterns the domain wall widths between adjacent magnetically patterned areas were determined to a≈1 μm. The minimum magnetically stable pattern width was estimated to be 0.7 μm in the standard system Co (4.8 nm)/NiO (68 nm).     

Individual bit island reversal and switching field distribution in perpendicular magnetic bit patterned media

The switching of single bit magnetic islands in bit patterned media (BPM) for two cases with 10 times difference in coercivity, as well as the switching field distribution (SFD) of the islands, has been studied using magnetic force microscopy (MFM) measurements. The intrinsic SFD is measured to be ∼9-11% of the remanence coercivity (H_cr), which contributes only ∼20-50% of the total SFD broadening (∼23-41% of H_cr). High resolution MFM observations clearly showed the influence of surrounding islands on the switching behaviour and switching fields of individual bit islands, resulting in significant contributions in SFD broadening due to non-intrinsic dipolar interactions. It was further observed that single magnetic islands could be switched within a very narrow switching field range as small as 4 Oe, which indicates very sharp and uniform switching for each individual island of BPM.     

Quantification of the lift height for magnetic force microscopy using 3D surface parameters

In this work, the quantitative conditions for the lift height for imaging of the magnetic field using magnetic force microscopy (MFM) were optimized. A thin cobalt film deposited on a monocrystalline silicon (1 0 0) substrate with a thickness of 55 nm and a thin nickel film deposited on a glass with a thickness of 600 nm were used as samples. The topography of the surface was acquired by tapping mode atomic force microscopy (AFM), while MFM imaging was performed in the lift mode for various lift heights. It was determined that the sensitivity of the measurements was about 10% higher for images obtained at a scan angle of 90° compared to a scan angle of 0°. Therefore, the three-dimensional surface texture parameters, i.e., average roughness, skewness, kurtosis and the bearing ratio, were determined in dependence on the lift height for a scan angle of 90°. The results of the analyses of the surface parameters showed that the influence of the substrate and its texture on the magnetic force image could be neglected for lift heights above 40 nm and that the upper lift height limit is 100 nm. It was determined that the optimal values of the lift heights were in the range from 60 to 80 nm, depending on the nature of the sample and on the type of the tip used.     

MFM imaging of micron-sized permalloy ellipses

The magnetic domain structure of micron-sized elliptic permalloy elements has been studied by magnetic force microscope (MFM) measurements, and has been compared to results from micromagnetic simulations. The elements all have the same aspect ratio, whereas the inter-elemental distance has been varied. Both the zero-field states and in field domain configurations have been studied. In zero-applied field, two different stable flux-closure states were found in both the MFM measurements and in the simulations. The different stable domain states occur as a result of minute differences in the local magnetic environment occurring during the demagnetization process.     

Enhanced pinning of superconducting vortices at circular magnetic dots in the magnetic-vortex state

Low-temperature magnetic force microscopy (LT-MFM) was used to study the distribution of superconducting vortices in Nb above a square array of 1 μm-sized circular ferromagnetic dots in a magnetic-vortex state. The force that the MFM tip exerts on the individual vortex in the depinning process was used to estimate the spatial modulation of the pinning potential. It was found, that the superconducting vortices which are preferably located on top of the Py dots experience a pinning force, about 15 times stronger as compared to the pinning force in the pure Nb film. This strong pinning exceeds the repulsive interaction between the superconducting vortices and allows vortex clusters to be located above the dots.     

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.     

Advantages of CNT-MFM probes in observation of domain walls of soft magnetic materials

The advantage of the CoFe-coated carbon nanotube (CNT) probes in a magnetic force microscope (MFM) is verified on in-plane magnetized soft magnetic materials. CoFe-coated CNT, standard and low-moment MFM probes were used to observe closure domains in square and rectangular Permalloy elements. The perturbative effect of the CNT-MFM probe was far less than that of a standard MFM probe. Domain walls were clearly observed as a pair of dark and bright lines which was in agreement with the micromagnetic simulations. The vortex core was also clearly observed using the CNT-MFM probes.     

磁力显微镜对相分离锰氧化物薄膜中反铁磁绝缘相融化的成像

用自制磁力显微镜研究了一个受各向异性应变的锰氧化物薄膜中的相分离以及由磁场导致的从反铁磁绝缘相到铁磁金属相的转变．磁力显微镜图片显示,在0 T这两种竞争的相就已经共存,且两种相 的畴呈非常明显的各向异性的条状分布,这可以定性解释输运上的各向异性．在2.1 T 以上,反铁磁绝缘相逐渐转变为铁磁金属相,并在3.2 T时结束．当去掉磁场时铁磁金属相能够保持．     

MFM studies of magnetic domain patterns in bulk barium ferrite (BaFe12O19) single crystals

Magnetic domain patterns in bulk barium ferrite (BaFe₁₂O₁₉; BaM) single crystals on the basal plane and the prism plane were measured and studied by magnetic force microscopy (MFM). The surface domain pattern is in the form of flowers or star on the basal plane and long elongated spikes or stripe domains on the prism plane. The change in domain structure with applied field (H_app) and the thickness (T) dependence on domain width (δ) was observed. The domain width decreased from 32 to 9 μm for the crystals of 800-100 μm thicknesses, respectively.     

Computer simulation of magnetic force microscopy images with a static model of magnetization distribution and dipole-dipole interaction

An algorithm for computer simulation of images obtained by magnetic force microscopy (MFM) is suggested. It is based on the Brown formalism and takes into account the shapes and the magnetic properties of the MFM tip and sample studied. The robustness and efficiency of the algorithm are tested by simulating the MFM image of a point magnetic dipole for the case where the tip is approximated by a nonmagnetic truncated cone covered by a thin uniformly magnetized layer. From the computer simulation of the MFM images of the dipole, the optimum parameters of the MFM probe are obtained.