Magnetic Force Microscopy Images Research Articles

Relating surface roughness and magnetic domain structure to giant magneto-impedance of Co-rich melt-extracted microwires

Understanding the relationship between the surface conditions and giant magneto-impedance (GMI) in Co-rich melt-extracted microwires is key to optimizing their magnetic responses for magnetic sensor applications. The surface magnetic domain structure (SMDS) parameters of ~45 μm diameter Co69.25Fe4.25Si13B13.5-xZrx (x = 0, 1, 2, 3) microwires, including the magnetic domain period (d) and surface roughness (Rq) as extracted from the magnetic force microscopy (MFM) images, have been correlated with GMI in the range 1–1000 MHz. It was found that substitution of B with 1 at. % Zr increased d of the base alloy from 729 to 740 nm while retaining Rq from ~1 nm to ~3 nm. A tremendous impact on the GMI ratio was found, increasing the ratio from ~360% to ~490% at an operating frequency of 40 MHz. Further substitution with Zr decreased the high frequency GMI ratio, which can be understood by the significant increase in surface roughness evident by force microscopy. This study demonstrates the application of the domain period and surface roughness found by force microscopy to the interpretation of the GMI in Co-rich microwires.

High coercivity Sm-Co thin films from elemental Sm/Co multilayer deposition and their microstructural aspects

Hard magnetic thin films with high coercivity were fabricated by magnetron sputtering on MgO(100) and quartz substrates. The films were grown by depositing sequentially Sm and Co layers at an elevated substrate temperature of 500°C. Subsequent post-annealing was carried out at various temperatures in range of 500–700°C to form Sm-Co hard magnetic thin films. X-ray diffraction studies revealed the formation of randomly oriented SmCo5 crystallites on quartz substrate, whereas, a textured growth of Sm2Co7 with strong (110) crystalline phases was observed on MgO substrate. Microstructural analyses were carried out using Transmission Electron Microscopy (TEM) for samples grown on MgO substrate at 650°C and inferred the presence of high density planar defects along with large grain boundaries. Further microdiffraction studies confirmed the presence of SmCo3 as an impurity phase in the films. Magnetic hysteresis measurements indicate the square hysteresis behaviors with high coercivity value of 3.1T and 2.7T for 650°C annealed samples on both MgO and quartz substrates, respectively. The origin of such high coercivity value was then correlated with pinning type of spin reversal mechanism as confirmed through the analyses of demagnetization curves. The magnetic force microscopy images for films on MgO substrate, annealed at 650°C, revealed the presence of magnetic domains with size higher than 1µm. The formed magnetic domains lacked well defined boundaries indicating an enhanced exchange coupling between the grain clusters.

Stripe magnetic domains in CeY2Fe5O12 (Ce:YIG) epitaxial films

Thin epitaxial films of CeY2Fe5O12 are deposited on (111) Gd3Ga5O12 substrates using pulsed laser ablation. The films exhibit low coercivity (3 mT), high saturation magnetization (102 mT), and excellent epitaxy. High resolution magnetic force microscopy (MFM) images of the film reveal a prominent stripe magnetic domain pattern due to a non-coplanar magnetization vector M→. Intensity analysis of the MFM images allows extraction of the orientation angle θc of M→ with respect to the plane of the film. We have recorded the changes in the width of the stripes and their disappearance as the strength of an in-plane magnetic field is increased. A simple model based on sinusoidal variation of M→ correctly predicts the change in the width of the stripe domains.

Stochastic domain wall depinning in permalloy nanowires with various types of notches

Stochastic phenomena in magnetic nanowires based on domain wall (DW) motion is scientifically important thus to understand and control such behaviors are very meaningful. Here we report on the investigation of pinning and depinning of DWs in permalloy nanowires with six types of longitudinally asymmetric notches using focused magneto-optic Kerr effect (FMOKE) magnetometer and magnetic force microscopy (MFM). The hysteresis loops obtained by FMOKE indicate the generation of one or two distinct depinning fields by creating one notch close to the edge of the nanowires, in comparison multiple depinning processes occur in the nanowires with two identical notches symmetrically placed along the transverse direction, indicating more remarkable stochastic DW depinning phenomena. The MFM images verify the existence of DW in each type of nanowires and the DW sizes in the latter kind of nanowires are generally larger than those in the former ones. These observations can be explained by considering the thermal perturbation and edge or surface roughness effects in nanowires.

Visualizing ferromagnetic domain behavior of magnetic topological insulator thin films

AbstractA systematic magnetic force microscopy (MFM) study of domain behavior in thin films of the magnetic topological insulator Sb1.89V0.11Te3 reveals that in the virgin domain state, after zero-field cooling, an equal population of up and down domains occurs. Interestingly, the cooling field dependence of MFM images demonstrates that a small cooling magnetic field (approximately 5–10 Oe) is sufficient to significantly polarize the film despite the coercive field (HC) for these films being on the order of a tesla at low temperature. By visualizing the magnetization reversal process around HC of V-doped Sb2Te3, we observed a typical domain behavior of a ferromagnet, i.e., domain nucleation and domain wall propagation. Our results provide direct evidence of ferromagnetic behavior of the magnetic topological insulator, a necessary condition for a robust quantum anomalous Hall effect.

Quantitative Nanoscale Magnetic Study of Isolated Diameter-Modulated FeCoCu Nanowires.

The comprehension of the magnetic configuration in FeCoCu nanowires with a diameter-modulated cylindrical geometry will allow controlling the domain wall motion in this low-dimensional system under the application of magnetic fields and/or the injection of current pulses. Here we perform a quantitative magnetic characterization of isolated diameter-modulated FeCoCu nanowires by combining nanoscale magnetic characterization techniques such as electron holography, magnetic force microscopy, and micromagnetic simulations. Local reconstructions of the magnetic distribution show the diameter-modulated geometry of the wires induces the formation of vortex-like structures and magnetic charges in the regions where the diameter is varied. Vortex-like structures modify the axial alignment of the magnetization in large-diameter segments. Moreover, the magnetic charges control the demagnetizing field distribution, promoting a flux-closure stray field configuration around large-diameter segments and keeping the demagnetizing field parallel to the NW's magnetization around small diameter segments. The detailed description of the remanent state in diameter-modulated cylindrical FeCoCu nanowires allows us to provide a clear explanation of the origin of bright and dark contrast observed in magnetic force microscopy images, which have the same feature of magnetic domain walls. This work establishes the primary knowledge required for future magnetization reversal studies with the aim of searching efficient modulated geometries that allow an optimum and controlled domain wall propagation.

Magnetostrictive iron gallium thin films grown onto antiferromagnetic manganese nitride: Structure and magnetism

We report structural and magnetic properties of magnetostrictive Fe100−xGax (x ≈ 15) alloys when deposited onto antiferromagnetic manganese nitride and non-magnetic magnesium oxide substrates. From X-ray diffraction measurements, we find that the FeGa films are single crystalline. Scanning tunneling microscopy imaging reveals that the surface morphologies are dictated by the growth temperature, composition, and substrate. The magnetic properties can be tailored by the substrate, as found by magnetic force microscopy imaging and vibrating sample magnetometry measurements. In addition to pronounced tetragonal deformations, depositing FeGa onto manganese nitride leads to the formation of stripe-like magnetic domain patterns and to the appearance of perpendicular magnetic anisotropy.

Development and calibration of a MFM-based system for local hysteresis loops measurements

A measurement technique derived from a field-dependent magnetic force microscope (MFM) is presented for the measurement of local hysteresis loops on patterned micrometric and sub-micrometric magnetic structures. The technique exploits the synchronisation of the applied field variations with the end-of-line signal of the microscope, while keeping the slow scan axis disabled. In this way, a single MFM image contains the whole field evolution of the magnetisation processes in the sample along a user-defined profile. An analysis procedure is presented for the subsequent determination of local hysteresis loops on magnetic dots. The system has been calibrated for what concerns the applied field values. No significant artifacts induced in the measurements by the applied field have been observed up to applied fields of ≈ 1000 Oe.

Heuristic Description of Magnetoelectricity of Cu2OSeO3.

CuO2SeO3 is an insulating material that hosts topologically nontrivial spin whirls, so-called skyrmions, and exhibits magnetoelectric coupling allowing to manipulate these skyrmions by means of electric fields. We report magnetic force microscopy imaging of the real-space spin structure on the surface of a bulk single crystal of CuO2SeO3. Based on measurements of the electric polarization using Kelvin-probe force microscopy, we develop a heuristic description of the magnetoelectric properties in CuO2SeO3. The model successfully describes the dependency of the electric polarization on the magnetization in all magnetically modulated phases.

Customized MFM probes with high lateral resolution.

Magnetic force microscopy (MFM) is a widely used technique for magnetic imaging. Besides its advantages such as the high spatial resolution and the easy use in the characterization of relevant applied materials, the main handicaps of the technique are the lack of control over the tip stray field and poor lateral resolution when working under standard conditions. In this work, we present a convenient route to prepare high-performance MFM probes with sub-10 nm (sub-25 nm) topographic (magnetic) lateral resolution by following an easy and quick low-cost approach. This allows one to not only customize the tip stray field, avoiding tip-induced changes in the sample magnetization, but also to optimize MFM imaging in vacuum (or liquid media) by choosing tips mounted on hard (or soft) cantilevers, a technology that is currently not available on the market.

Magnetic domain imaging of nano-magnetic films using magnetic force microscopy with polar and longitudinally magnetized tips.

Perpendicular or parallel magnetic fields are used to magnetize the tips used in magnetic force microscopy (MFM). In this process, perpendicular or parallel magnetic dipole moments are produced on the tip plane, thus leading to the formation of polar magnetized tips (PM-tips) or longitudinally magnetized tips (LM-tips), respectively. The resolution of an MFM image of a magneto-optic disk is used for calibration of these tips, and the saturated magnetic fields of the PM- and LM-tips are found to be 2720 Oe and 680 Oe, respectively. Because both tips can simultaneously magnetize the sample during the scanning process when measuring a Co thin film, clear MFM images are captured, which enable the identification of magnetizable regions and the distribution of the magnetic domains on the sample surface. These results will be useful for improving the manufacturing processes required for soft nano-magnetic film production.

Effect of swift heavy ion irradiation on dilute Fe-doped Sb0.95Se0.05 magnetic semiconductor

ABSTRACTA thin film of dilute Fe (0.008)-doped Sb0.95Se0.05 alloy was grown on silicon substrate using the thermal evaporation technique. This film was irradiated with swift heavy ions (SHIs) Ag+15 having 200 MeV energy at ion fluences of 1 × 1012 and 5 × 1012 ions per cm2, respectively. The thickness of the thin film was ∼500 nm. We study the effect of irradiation on structural, electrical, surface morphology and magnetic properties of this film using grazing angle XRD (GAXRD), DC resistivity, atomic force microscopy (AFM) and magnetic force microscopy (MFM), respectively. GAXRD suggests that no significant change is observed in this system due to SHI irradiation. The average crystallite size increases with fluence, whereas the AFM image shows the rms roughness decreases due to irradiation with respect to the un-irradiated thin film. The MFM image shows that the magnetic interaction in irradiated film decreases due to the irradiation effect. Although the un-irradiated sample shows metal to semiconducting transition, but after irradiation with fluence of 5 × 1012 ions per cm2, the sharpness of the metal to semiconducting phase transition is observed to increase dramatically at ∼300 K. This characteristic of the thin film makes it a promising candidate for an electrical switching device after irradiation.

High temperature magnetic order in Zn1−xMnxSnSb2+MnSb nanocomposite ferromagnetic semiconductors

We present studies of structural, magnetic, and electrical properties of Zn1−xMnxSnSb2+MnSb nanocomposite ferromagnetic semiconductors with the average Mn-content, , changing from 0.027 up to 0.138. The magnetic force microscope imaging done at room temperature shows the presence of a strong signal coming from MnSb clusters. Magnetic properties show the paramagnet-ferromagnet transition with the Curie temperature, TC, equal to about 522 K and the cluster-glass behavior with the transition temperature, TCG, equal to about 465 K, both related to MnSb clusters. The magnetotransport studies show that all investigated samples are p-type semiconductors with high hole concentration, p, changing from 1021 to 1022 cm−3. A large increase in the resistivity as a function of the magnetic field is observed at T < 10 K and small magnetic fields, mT, for all the studied samples with a maximum amplitude of the magnetoresistance about 460% at T = 1.4 K. The large increase in the resistivity is most probably caused by the appearance of the superconducting state in the samples at T < 4.3 K.

Weak dipolar interaction between CoPd multilayer nanodots for bit-patterned media application

The morphology and magnetic performance of CoPd multilayer nanodots have been investigated by a combination of DC sputtering, anodic aluminum oxide(AAO) template method and Ar ion etching. The nanodots exhibit a normal switching field distribution(SFD) of 17%, which is comparative to Bit-patterned media (BPM) made by e-Beam. It is found that the first-order reverse curve (FORC) data of CoPd nanodots have a good match with the mean-field model, which shows dipolar interaction contributes as small as 8.4% to the total SFD. Magnetic force microscope(MFM) imaging at a certain location confirms the FORC results by observing the effect of dipolar interaction directly. Our CoPd multilayer nanodots can be a good candidate for BPM application.

MFM Study of a Sintered Nd–Fe–B Magnet: Analyzing Domain Structure and Measuring Defect Size in 3-D View

This paper describes the use of magnetic force microscopy (MFM) to investigate the domain structure of sintered Nd–Fe–B magnet in a thermally demagnetized state. Observations are made at both perpendicular and parallel surfaces to the alignment axis (easy axis). Clear magnetic contrast images in 3-D view with defined domain structures are presented. Mazelike magnetic domains and stripe magnetic domains are observed in the perpendicular and parallel surfaces, respectively. Superficial spike domains and reverse spikes are observed within the main domains and analyzed in 3-D view. MFM images illustrate the grains of Nd–Fe–B magnets, and indicate the multi-domain structures within individual grains. The effect of interfacial defects upon the domain structure is studied in 3-D, and evident reverse magnetic domains (nucleation sites) are observed on the imperfection region. The height and width of the defect region are determined using the XEI imaging software. In addition, the volume and surface area of nucleation region are evaluated and validated theoretically by developing an approach involving coercivity versus the nucleus volume. Defect regions have been identified by using Raman spectroscopy as an oxide in character rather than metallic. The average domain width and domain wall energy have been determined for both directions.

Annealing influence on the exchange stiffness constant of Permalloy films with stripe domains

An investigation of the annealing influence on the stripe domains structure in Permalloy films has been performed by comparing the static and dynamic magnetic properties. An increasing exchange stiffness constant Aex is found when the annealing temperature rises. The magnetic force microscopy images show that the width of stripe domain increases with the increase of annealing temperature, and the increasing exchange stiffness constant is considered to be the most prominent reason. A perpendicular standing spin-wave model is used to deduce the Aex from the evolution of resonance frequencies, and the Aex increases nearly twice from 7.2 × 10−7 ergs cm−1 for the as-deposited film to 12.9 × 10−7 ergs cm−1 for the film annealing at 400 °C. Micromagnetic calculation is also used to compute the magnetization configuration, and the results agree well with the experimental conclusions.

Removal of electrostatic artifacts in magnetic force microscopy by controlled magnetization of the tip: application to superparamagnetic nanoparticles.

Magnetic force microscopy (MFM) has been demonstrated as valuable technique for the characterization of magnetic nanomaterials. To be analyzed by MFM techniques, nanomaterials are generally deposited on flat substrates, resulting in an additional contrast in MFM images due to unavoidable heterogeneous electrostatic tip-sample interactions, which cannot be easily distinguished from the magnetic one. In order to correctly interpret MFM data, a method to remove the electrostatic contributions from MFM images is needed. In this work, we propose a new MFM technique, called controlled magnetization MFM (CM-MFM), based on the in situ control of the probe magnetization state, which allows the evaluation and the elimination of electrostatic contribution in MFM images. The effectiveness of the technique is demonstrated through a challenging case study, i.e., the analysis of superparamagnetic nanoparticles in absence of applied external magnetic field. Our CM-MFM technique allowed us to acquire magnetic images depurated of the electrostatic contributions, which revealed that the magnetic field generated by the tip is sufficient to completely orient the superparamagnetic nanoparticles and that the magnetic tip-sample interaction is describable through simple models once the electrostatic artifacts are removed.

Enhanced high-frequency magneto-impedance response of melt-extracted Co69.25Fe4.25Si13B13.5 microwires subject to Joule annealing

We report an enhancement of low-field, high-frequency magneto-impedance (MI) response and circumferential domain structures of melt-extracted amorphous Co69.25Fe4.25Si13B13.5 microwires by dc Joule annealing. The current amplitudes of 5 mA, 10 mA, and 20 mA were applied to amorphous microwires for different time treatments. The magnetic field dependence of impedance and circumferential domain structure of the microwires were explored using an impedance analyzer over a high frequency range (20 MHz–1000 MHz) and magnetic force microscopy (MFM), respectively. Consequently, the optimum value of 5 mA current for 20 min markedly improved the MI ratio up to 610% (1.7 times of 380% from the as-cast state) at a working frequency of 20 MHz. Moreover, the magnetic field sensitivity has been achieved up to 500%/Oe (more than two times of 225%/Oe from the as-cast one) at a frequency of 40 MHz. MFM images clearly show the development of a well-defined circumferential domain structure of the annealed microwire comparing with the as-cast one. The improvement of the high-frequency MI response in Co-rich amorphous microwires is promising for sensitive magnetic microsensors for low field detection.

Reconfigurable logic via gate controlled domain wall trajectory in magnetic network structure.

An all-magnetic logic scheme has the advantages of being non-volatile and energy efficient over the conventional transistor based logic devices. In this work, we present a reconfigurable magnetic logic device which is capable of performing all basic logic operations in a single device. The device exploits the deterministic trajectory of domain wall (DW) in ferromagnetic asymmetric branch structure for obtaining different output combinations. The programmability of the device is achieved by using a current-controlled magnetic gate, which generates a local Oersted field. The field generated at the magnetic gate influences the trajectory of the DW within the structure by exploiting its inherent transverse charge distribution. DW transformation from vortex to transverse configuration close to the output branch plays a pivotal role in governing the DW chirality and hence the output. By simply switching the current direction through the magnetic gate, two universal logic gate functionalities can be obtained in this device. Using magnetic force microscopy imaging and magnetoresistance measurements, all basic logic functionalities are demonstrated.

Nanoscale patterning of CrPt3 magnetic thin films by using ion beam irradiation

We have successfully fabricated planar patterned CrPt3 ordered L12 alloy films by Kr+ ion irradiation. Planar-patterned CrPt3 nanodots with various bit sizes from 200nm to 50nm were successfully fabricated by 30keV Kr+ ion irradiation at a dose of 2×1014ions/cm2, where e-beam lithography was used for creating the resist mask. We have confirmed that the nanofabrication process didn’t change the magnetic properties of CrPt3 ordered L12 alloy films. As-prepared film exhibited perpendicular hysteresis loop with the coercivity of 5.5kOe. The typical perpendicular maze domain structure with the stripe structure was clearly seen in as-prepared CrPt3 film. Magnetic force microscopy (MFM) images of patterned CrPt3 nanodots indicated that each un-irradiated bit consists of localized perpendicular magnetic domain structures, which corresponds to perpendicular magnetization direction. Nanodots with bit size ⩽80nm show either dark or bright contrast, suggesting single domain structure. No magnetic contrast in irradiated space is due to the suppressing of the magnetization by Kr+ ion irradiation.