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.     

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.     

Magnetic anisotropy study of triangular-shaped Co nanostructures

Atomic force microscopy (AFM), X-ray magnetic circular dichroism (XMCD), magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) have been used to measure the magnetic and geometrical characteristics of triangular-shaped Co structures of lateral size 730 nm and thickness 32 nm, prepared by nanosphere lithography (NSL). Evidence of in-plane six-fold magnetic anisotropy induced by the symmetry of the structure has been found. By means of XMCD measurements, performed at remanence after applying a pulsed field, a structure rotation angle-dependent oscillation of about 15% with a periodicity of 60° has been observed for both the orbital and spin moments. Furthermore, the system exhibits the angular hysteresis effect. The magnetic measurements performed by MFM show a reduction of the magnetic configurations to only two states, one quasi-single domain Y state and second, a combination of vortex and Y state.     

Structural and magnetic properties of evaporated Fe thin films on Si(1 1 1), Si(1 0 0) and glass substrates

We present experimental results on the structural and magnetic properties of series of Fe thin films evaporated onto Si(1 1 1), Si(1 0 0) and glass substrates. The Fe thickness, t, ranges from 6 to110 nm. X-ray diffraction (XRD) and atomic force microscopy (AFM) have been used to study the structure and surface morphology of these films. The magnetic properties were investigated by means of the Brillouin light scattering (BLS) and magnetic force microscopy (MFM) techniques. The Fe films grow with (1 1 0) texture; as t increases, this (1 1 0) texture becomes weaker for Fe/Si, while for Fe/glass, the texture changes from (1 1 0) to (2 1 1). Grains are larger in Fe/Si than in Fe/glass. The effective magnetization, 4πM_eff, inferred from BLS was found to be lower than the 4πM_S bulk value. Stress induced anisotropy might be in part responsible for this difference. MFM images reveal stripe domain structure for the 110 nm thick Fe/Si(1 0 0) only.     

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.     

Field polarity dependent nucleation of superconductivity in quasi-one-dimensional magnetic templates

We investigate the nucleation of superconductivity in an Al/Al₂O₃/Py trilayer system by electrical transport measurements. Magnetic force microscopy images taken at room temperature show that the 0.7 μm thick Py-film form stripes of magnetic domains with alternating out-of-plane stray field. After applying a strong out of plane magnetic field H the superconductor/normal phase boundary becomes asymmetric with respect to H = 0. This lack of field polarity symmetry results from the unbalanced size distribution of domains with opposite polarity.     

Application of citrate-stabilized gold-coated ferric oxide composite nanoparticles for biological separations

Gold-coated magnetic nanoparticles were synthesized with size ranging from 15 to 40 nm using sodium citrates as the reducing agent. Oxidized magnetites (Fe₃O₄) fabricated by co-precipitation of Fe²⁺ and Fe³⁺ in strong alkaline solution were used as magnetic cores. The structures of gold (Au) shell and magnetic core (Au–Fe) were studied by transmission electron microscopy (TEM) image and energy dispersive spectroscopy (EDS) spectrum. Results from high-resolution X-ray diffraction (HR XRD) show that the Au–Fe oxide nanoparticles have a face-centered cubic shape with the crystalline faces of {1 1 1}. The Au-coated magnetic nanoparticles exhibited a surface plasmon resonance peak at 528 nm. The nanoparticles are well dispersed in distilled water. A 3000 G permanent magnet was successfully used for the separation of the functionalized nanoparticles. Magnetic properties of the nanoparticles were determined by magnetic force microscope (MFM) in nanometric resolution and vibrating sample magnetometer (VSM). Magnetic separation of biological molecules using Au-coated magnetic oxide composite nanoparticles was examined after attachment of protein immunoglobulin G (IgG) through electrostatic interactions. Using this method, separation was achieved with a maximum yield of 35% at an IgG concentration of 400 ng/ml.     

A quantitative analysis of magnetic vortices in Permalloy nanoparticles characterized by electron holography

The present work investigates experimentally curling magnetic configurations locally observed in almost dispersed Permalloy nanoparticles in the remanent state. Magnetic analysis is performed in a field emission TEM using off-axis electron holography. Particularly, electron holography is used to characterize the magnetic microstructure of Fe₃₀Ni₇₀ nanoparticles, whose average diameter (50 nm) is expected to be close to the critical size for a curling magnetic structure (vortex) formation. The vortex core diameter D_core and the bulk magnetic profile of the vortex are measured and compared with a “rigid vortex” micromagnetic model. The connection between vortex structure and the characteristic micromagnetic length of the system deduced from magnetization curve measurements is discussed.     

Demonstration of ultra-high-resolution MFM images using Co90Fe10-coated CNT probes

We demonstrate ultra-high-resolution magnetic force microscopy images of perpendicular magnetic storage media using carbon nanotube probes coated by ferromagnetic Co₉₀Fe₁₀ films (20, 30, 40, and 50 nm). By optimizing ferromagnetic film thickness (effective tip diameter), we obtained best magnetic domain image with an 40 nm-Co₉₀Fe₁₀-coated tip (50 nm tip diameter) about a lateral detect density of 1200 k flux per inch on perpendicular magnetic storage medium, one of the highest resolutions in MFM imaging reported for this material system and structure. The observed dependence of tip dimension on signal contrast and image resolution was successfully explained by a theoretical analysis indicating that the signal contrast, along with the physical probe-tip dimension, should be taken into account to design magnetic probes tips for high-resolution magnetic force microscopy.     

利用扫描透射X射线显微镜观测磁涡旋结构

利用上海光源软X射线谱学显微光束线站(STXM)并结合X射线的磁圆二色效应, 我们对方形、圆形和三角形的Ni₈₀Fe₂₀薄膜微结构中的磁涡旋结构进行了定量实验观测, 并利用同步辐射光源的元素分辨特性, 分别在Fe和Ni的L₃吸收边对涡旋磁结构进行了观测. 我们还对磁涡旋中磁矩的分布进行了定量分析, 发现实验结果与微磁学模拟结果完全符合.     

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.     

Hard disk magnetic domain nano-spatial resolution imaging by using a near-field scanning microwave microscope with an AFM probe tip

A near-field scanning microwave microscope (NSMM) incorporating an atomic force microscope (AFM) probe tip was used for the direct imaging of magnetic domains of a hard disk under an external magnetic field. We directly imaged the magnetic domain changes by measuring the change of reflection coefficient S₁₁ of the NSMM at an operating frequency near 4.4 GHz. Comparison was made to the magnetic force microscope (MFM) image. Using the AFM probe tip coupled to the tuning fork distance control system enabled nano-spatial resolution. The NSMM incorporating an AFM tip offers a reliable means for quantitative measurement of magnetic domains with nano-scale resolution and high sensitivity.     

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Studies of the domain structure of anisotropic sintered SmCo5 permanent magnets

In this work, investigations of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets with high coercivity have been made using the colloid-scanning electron microscopy (SEM) technique and magnetic force microscopy (MFM). The magnets were produced by powder metallurgy (sintering) process and consisted of oriented grains with an average size of about 20 μm. They were studied in the thermally demagnetized state. Owing to the application of digital image recording, enhancement and analysis, high-quality images of the magnetic microstructure were obtained and analyzed not only qualitatively but also quantitatively. Improvements over previous results were achieved. The grains show the presence of magnetic domains, as expected. At the surface perpendicular to the alignment axis, the coarse domain structure in the form of a maze pattern with surface reverse spikes is observed. The main (maze) domains had typical widths 3–5 μm. The reverse spike domains were imaged as circles typically 1–2 μm in diameter or as elongated regions up to about 6 μm in length. Interestingly, in addition to the coarse maze domains and reverse spikes near the surface, a fine surface domain structure is revealed with MFM. The fine scale domains are found to be magnetized perpendicular to the surface and their occurrence is attributed to further reduction of the magnetostatic energy at the cost of a larger domain wall energy. On the surface parallel to the alignment axis, the main domains within individual grains are imaged as stripe domains with domain walls running approximately parallel to the alignment axis, while reverse spike domains are displayed in the form of triangular domains and occur near some grain boundaries, pores or precipitations. The magnetic alignment of grains was found to be good, but certainly not perfect. In most cases the domain structures within grains were independent of their neighbors, but in some cases (not so rare) observations indicated the existence of significant magnetostatic coupling between neighboring grains. The main and surface domain widths were determined by digital means using the stereologic method of Bodenberger and Hubert. Moreover, the domain wall energy and other intrinsic parameters for the studied magnets were determined.     

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.     

Magnetic interactions in ferromagnetic manganite nanotubes of different diameters

In this work we present a magnetic study of La_0.67Sr_0.33MnO₃ (LSMO) and La_0.67Ca_0.33MnO₃ (LCMO) nanotubes with nominal external diameters (?) of 100, 200, 600 and 800 nm. The 800 nm diameter nanotubes have walls of around 50 nm thickness in all the cases. The walls are constituted by an assembly of nanoparticles with a non-Gaussian size distribution presenting a maximum at 24 ± 6 nm (LSMO) and 25 ± 8 nm (LCMO). We carried out isothermal remanent magnetization (IRM) and dc demagnetization (DCD) experiments. We determined that the crystallites are single magnetic domains with a magnetic dead layer on the surface which avoids exchange interactions among grains. We conclude that the dominating interactions are of dipolar type of the same magnitude for all the samples.     

Direct observation of room temperature magnetism in (In,Mn)As thin films by magnetic force microscopy

Variable-temperature magnetic force microscopy (MFM) has been performed over the temperature range of 298-348 K on ferromagnetic (In,Mn)As thin films deposited by metal-organic vapor phase epitaxy (MOVPE). Ferromagnetic domains were observed with submicron resolution in both single and two phase (In,Mn)As films, persisting up to 328 K. Isolated cylindrical domains ranging from 100 to 350 nm in diameter with densities of 2-5 × 10⁸ cm⁻² were observed in phase pure films. Longer range magnetic order, in the form of ribbon-like domains up to 1 μm in length, are present in the regions between the cylindrical domains. Two phase (In,Mn)As films produced a well-resolved complex domain structure consisting of 180° parallel and antiparallel domains. Excellent agreement between the temperature dependence of the relative magnetization obtained by MFM and superconducting quantum interference device measurements was observed.     

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.     

Magnetic and magneto-optical studies on Zn1−xCrxTe (x=0.05) films grown on glass substrate

Zn_1−xCr_xTe (x=0.05) films were prepared by thermal evaporation onto glass substrates. X-ray diffraction (XRD) was used to determine the crystalline quality of the ZnTe:Cr film. Magnetic force microscopy (MFM) investigation has shown a non-uniform distribution of magnetic domains with an average size of 4 nm at room temperature. SQUID measurements have further shown that the non-uniform distribution of domains does not affect the room temperature ferromagnetism of this material. Electron spin resonance spectroscopy (ESR) was done to determine the Cr valence state in the ZnTe lattice. Magnetic circular dichroism (MCD) analysis was used to confirm the ZnCrTe phase contributing to the ferromagnetic behavior.     

Ferromagnetism in MnxGe1−x films prepared by magnetron sputtering

Mn_xGe_1−x thin films were prepared by magnetron sputtering with a substrate temperature of 673 K and subsequently annealed at 873 K. The X-ray diffraction (XRD) measurements showed that all samples had a single Ge cubic structure. No films showed clear magnetic domain structure under a magnetic force microscope (MFM). Atom force microscope (AFM) measurements showed that the films had an uniform particle size distribution, and a columnar growth pattern. X-ray photoelectron spectroscopy (XPS) measurements indicated that the valences of both Mn and Ge atoms increase with the Mn concentration. The resistance decreased with increasing temperature, suggesting that the films were typical semiconductors. Magnetic measurements carried out using a Physical Property Measurement System (PPMS) showed that all samples exhibited ferromagnetism at room temperature. There was a small concentration of Mn₁₁Ge₈ in the films, but the ferromagnetism was mainly induced by Mn substitution for Ge site.