Resolution Magnetic Force Microscopy Research Articles

Magnetization Reversal of Strongly Exchange-Coupled Double Nanolayers for Spintronic Devices

Contemporary spintronics devices, notably GMR and TMR-sensors with high linear ranges, stand to benefit from large bias fields in excess of 1T that have been observed in exchange-coupled double layers using rare earth-transition metal ferrimagnets as pinning layers. However, before exploiting the strong coupling the reversal process must be understood at the scales of the smallest devices, where it would be effective. Little is known about these processes to-date, owing to the technical difficulty of imaging magnetic structures smaller than 20nm in fields of several Tesla. Here we image the nanoscale magnetic structures of a double layer comprising a polycrystalline Co/Pt-multilayer (Co/Pt) which is strongly exchange-coupled to an amorphous and magnetically-hard ferrimagnetic TbFe alloy layer. High resolution magnetic force microscopy provides snapshots of the magnetic structures along various applied fields between 0 and 7T as the magnetization reversal takes place. In contrast to a magnetization reversa...

Magnetic domain structure imaging near sample surface with alternating magnetic force microscopy by using AC magnetic field modulated superparamagnetic tip

For magnetic domain imaging, with a very high spatial resolution magnetic force microscope, the tip-sample distance should be as small as possible. However, magnetic imaging near the sample surface is very difficult with conventional magnetic force microscopy (MFM) because the interactive forces between the tip and sample include van der Waals and electrostatic forces along with a magnetic force. In this study, we proposed alternating MFM which only extracts a magnetic force near the sample surface without any topographic and electrical crosstalk. In the present method, the magnetization of an FeCo–GdOx superparamagnetic tip is modulated by an external AC magnetic field in order to measure the magnetic domain structure without any perturbation from the other forces near the sample surface. Moreover, it is demonstrated that the proposed method can also measure the strength and identify the polarities of the second derivative of the perpendicular stray field from a thin film permanent magnet with a DC demagnetized state and remanent state.

Magnetic Domain Patterns in Bilayered Ribbons Studied by Magnetic Force Microscopy and Magneto-Optical Kerr Microscopy.

The magnetic domain patterns of amorphous bilayered FeSiB/FeNbSiB and FeNbCuSiB/CoSiB ribbons are observed and analysed using the magneto-optical Kerr microscopy (MOKM) and magnetic force microscopy (MFM). Both microscopic techniques are highly sensitive to the sample surface; possibility of Kerr microscopy to visualize the domains separately in both layers is achieved by focusing the laser spot on the ribbon cross section. Wide curved domains as well as fine fingerprint domains were detected at the surface of ribbons due to presence of local stresses coming from the preparation process. With respect to high lateral resolution of MFM and its out-of-plane magnetization sensitivity, the perpendicularly magnetized crossed stripe domain patterns can be selected as well. Coiling of the ribbons on the half-round-end sample holder is often used to induce and control the magnetic anisotropy of these alloys. Changes in the magnetic domain structure at the outer-coiled surface and its dependence on the sign of magnetostriction coefficient are discussed in detail. Finally, the MFM images in the presence of external in-plane magnetic field up to ±40 kA/m are shown.

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.

Specialized probes with nanowhisker structures for high resolution magnetic force microscopy

Creation and study of specialized nanowhisker probes with magnetic coating are performed for high-precision imaging of various objects by means of atomic force and magnetic force microscopy. Thin layers of Ni and Co are deposited on the surface of nanowhisker structures to perform visualization of magnetic fields on the sample surface, in particular, structure of pits on a hard disk drive (HDD). It is revealed that probes with nanowhisker structures covered with magnetic coating due to their high aspect ratio demonstrate a higher spatial resolution and contrast of magnetic fields visualization in comparison with standard magnetic probes.

Phase diagram of magnetic states in nickel submicron disks

We present a detailed study of the magnetic states in sub-micron nickel disks in the diameter range of 300–900 nm and thickness of 15–55 nm using magnetic force microscopy and micromagnetic simulations. In this dimensional range, we find three main magnetic states, namely: the single domain state, the vortex state, and the stripe domain state. We identify the crossover dimensions between these magnetic states, demonstrating that the disk thickness is the dominant parameter, which is further supported by the fact that the magnetocrystalline anisotropy of the Ni films is thickness dependant and plays a dominant role at large thicknesses. Experimental results are in a very good agreement with micromagnetic modelling using the out-of-plane effective anisotropy constant. Using in-situ high resolution Magnetic Force Microscopy techniques, we investigate in detail the motion of the vortex core under applied in-plane magnetic field and find that vortex nucleates and annihilates at ∼100 Oe.

Visualizing magnetocrystalline anisotropy field distribution of patterned triangular L10 FePt nanodots

Measuring magnetocrystalline anisotropy field distribution of patterned magnetic nanodots using a microscopically visualizing way is important for understanding some important magnetic behaviors such as switching field distribution (SFD) of patterned recording media. We present a detailed analysis of the remanent domain structures of L10-FePt triangular nanodots as revealed by high resolution magnetic force microscopy (MFM) with the help of micromagnetic simulation, showing that the domain structure diversity can effectively account for a dot-to-dot variation of the magnetocrystalline anisotropy field. Our method could shed light not only on understanding the fundamental causes of a wider SFD but also on designing future nanostructured magnetic devices.

Micromagnetic Studies of Ultrahigh Resolution Magnetic Force Microscope Tip Coated by Soft Magnetic Materials

Magnetic force microscope (MFM) tips coated by soft magnetic materials can achieve a spatial resolution above 10 nm. It is interesting to analyze why tips coated with soft magnetic materials can achieve such a high resolution. In experiment, an MFM tip coated by amorphous FeB can achieve a resolution of 8 nm; therefore, we chose an FeB tip as an example and establish a micromagnetic model to understand the measurement mechanism of the soft magnetic MFM tip. In the FeB film simulation, the random crystalline anisotropy results in a soft magnetic loop with an in-plane coercivity of 0.2 Oe, and the film surface roughness will raise the coercivity to the order of 1 Oe. In the tip simulation, it is found that the FeB-coated tip can be switched in a uniform field of the order of 100 Oe, but can remain near a remanent state in a stray field resulting from media. A simple model is set up to analyze the MFM images of bits in hard disk drivers using the simulated magnetic properties of the tip and resolution ~10 nm is confirmed.

A study on dynamic heat assisted magnetization reversal mechanisms under insufficient reversal field conditions

We report an experimental study on the dynamic thermomagnetic (TM) reversal mechanisms at around Curie temperature (Tc) for isolated 60 nm pitch single-domain [Co/Pd] islands heated by a 1.5 μm spot size laser pulse under an applied magnetic reversal field (Hr). Magnetic force microscopy (MFM) observations with high resolution MFM tips clearly showed randomly trapped non-switched islands within the laser irradiated spot after dynamic TM reversal process with insufficient Hr strength. This observation provides direct experimental evidence by MFM of a large magnetization switching variation due to increased thermal fluctuation/agitation over magnetization energy at the elevated temperature of around Tc. The average percentage of non-switched islands/magnetization was further found to be inversely proportional to the applied reversal field Hr for incomplete magnetization reversal when Hr is less than 13% of the island coercivity (Hc), showing an increased switching field distribution (SFD) at elevated temperature of around Tc (where main contributions to SFD broadening are from Tc distribution and stronger thermal fluctuations). Our experimental study and results provide better understanding and insight on practical heat assisted magnetic recording (HAMR) process and recording performance, including HAMR writing magnetization dynamics induced SFD as well as associated DC saturation noise that limits areal density, as were previously observed and investigated by theoretical simulations.

Magnetization behaviors of Fe3O4 nanoparticles studied by frequency-modulated magnetic force microscopy

Fe3O4 nanoparticles were synthesized through a sol–gel method. The structure and morphology features of the nanoparticles were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution magnetic force microscopy (HR-MFM). The magnetization behaviors of Fe3O4 nanoparticles were investigated by using the frequency-modulated magnetic force microscopy (FM-MFM). It was observed that the prepared individual nanoparticles were within 20nm. The high resolution amplitude and phase images of the nanoparticles were obtained separately by using the FM-MFM technique with an AC magnetic field. The magnetic distribution of the individual nanoparticles (~20nm) was demonstrated by this technique.

Optimal ferromagnetically-coated carbon nanotube tips for ultra-high resolution magnetic force microscopy

Using single-walled carbon nanotubes homogeneously coated with ferromagnetic metal as ultra-high resolution magnetic force microscopy probes, we investigate the key image formation parameters and their dependence on coating thickness. The crucial step of introducing molecular beam epitaxy for deposition of the magnetic coating allows highly controlled fabrication of tips with small magnetic volume, while retaining high magnetic anisotropy and prolonged lifetime characteristics. Calculating the interaction between the tips and a magnetic sample, including hitherto neglected thermal noise effects, we show that optimal imaging is achieved for a finite, intermediate-thickness magnetic coating, in excellent agreement with experimental observations. With such optimal tips, we demonstrate outstanding resolution, revealing sub-10 nm domains in hard magnetic samples, and non-perturbative imaging of nanoscale spin structures in soft magnetic materials, all at ambient conditions with no special vacuum, temperature or humidity controls.

Micromagnetic study of effect of tip-coating microstructure on the resolution of magnetic force microscopy

The properties of a magnetic force microscopy (MFM) tip are very important for high-resolution magnetic imaging. In this work, micromagnetic models of tips are set up to study the effect of tip-coating microstructure, especially the randomness of anisotropy on tip edge and tip end, on the resolution of MFM. The effective coating height and the resolution potential of tips with various microstructures and magnetic properties have been characterized by investigating the obtained signals from high-density continuous granular thin film disk media with a bit size of 8×16 nm2 and bit-patterned media with a pattern period p of 50 nm. The magnetic moment distribution at the tip end should be perpendicular to the sample to realize a ‘magnetically sharp’ tip, which explains further the improved resolution in the recent experimental reports. Tips with well-controlled grain structure and magnetic anisotropy of coating materials can be applied to both high-density thin film disk media and bit-patterned media.

Advanced Magnetic Force Microscopy for High Resolution Magnetic Imaging

Magnetic force microscopy (MFM) is one of the primary imaging tools for studying magnetic nanostructures. The resolution of MFM has been considered as a main issue in characterizing magnetic nanostructures smaller than 30 nm; especially for high density recording media beyond 1 Tbit/in 2 . In this letter, we investigated three different kinds of MFM tips such as those with a perpendicular magnetic anisotropy (PMA) derived from a crystallographic texture, antiferromagnetic coupled perpendicular (AFC) tips and tips with no crystallographic textures, resulting in no PMA. Their resolution, as measured using a recording media with written information, was compared with that of commercial MFM tips. The tip in the PMA configuration was found to provide the best resolution among all of them.

Characteristics of magnetic force microscopy magnetics on high moment perpendicular magnetic recording writers with high coercivity probes

High resolution magnetic force microscopy (MFM) imaging with high coercivity probes on perpendicular magnetic recording (PMR) writers directly characterizes magnetic field contour for the writer main pole as well as its shields’ magnetic state. Evolution of write bubble and return field was analyzed by MFM imaging in dynamic phase detection scheme. Different write field components and their out of plane second order derivatives were calculated via finite element modeling. The MFM imaged write field distribution correlates well with the PMR writer out of plane write field component. Magnetic responses of the PMR writer main pole, trailing and side shields are quantified. The trailing and side shields exhibit complicated magnetic saturation behaviors comparing with the PMR writer pole. The side shield’s magnetic response is dependent upon its initial equilibrium state.

Influence of magnetic material composition of Fe100−xBx coated tip on the spatial resolution of magnetic force microscopy

Magnetic force microscope (MFM) tips are prepared by coating Si tips of 3 nm radius with 20-nm-thick Fe100−xBx (x = 0, 8, 18, and 34 at. %) alloys. As the B composition increases, the wettability of Fe-B alloy material to Si surface increases and a sharper and smoother MFM tip is obtained. The remanent magnetization of Fe-B alloy material decreases with increasing the B composition, which degrades the MFM signal detection sensitivity. The MFM resolution is influenced by both the tip sharpness and the remanent magnetization of coated Fe-B alloy material. Resolutions of 10.2, 9.2, 7.3, and 9.3 nm are obtained with tips coated with Fe, Fe92B8, Fe82B18, and Fe66B34 materials, respectively. Magnetic bits of 14.9 nm in length of a perpendicular magnetic medium recorded at 1700 kfci are distinguishable in the MFM image observed by using an Fe82B18-alloy coated tip. Fe-B coated tips are suitable to observe the magnetization structures of future high-density recording media.

Characterization of high‐density bit‐patterned media using ultra‐high resolution magnetic force microscopy

AbstractBit‐patterned media at one terabit‐per‐square‐inch (Tb/in2) recording density require a feature size of about 12 nm. The fabrication and characterization of such magnetic nanostructures is still a challenge. In this Letter, we show that magnetic dots can be resolved at 10 nm spacing using magnetic force microscopy (MFM) tips coated with a magnetic film possessing a perpendicular magnetic anisotropy (PMA). Compared to MFM tips with no special magnetic anisotropy, MFM tips with PMA can resolve the bits clearly, because of a smaller magnetic interaction volume, enabling a simple technique for characterizing fine magnetic nanostructures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical Considerations on Resolution of Magnetic Force Microscopy Using Carbon Nanotube Probe

Theoretical Considerations on Resolution of Magnetic Force Microscopy Using Carbon Nanotube Probe

Domain size and structure in exchange coupled [Co/Pt]/NiO/[Co/Pt] multilayers

We investigate the competing effects of interlayer exchange coupling and magnetostatic coupling in the magnetic heterostructure ([Co/Pt]/NiO/[Co/Pt]) with perpendicular magnetic anisotropy (PMA). This particular heterostructure is unique among coupled materials with PMA in directly exhibiting both ferromagnetic and antiferromagnetic coupling, oscillating between the two as a function of spacer layer thickness. By systematically tuning the coupling interactions via a wedge-shaped NiO spacer layer, we explore the energetics that dictate magnetic domain formation using high resolution magnetic force microscopy coupled with the magneto-optical Kerr effect. This technique probes the microscopic and macroscopic magnetic behavior as a continuous function of thickness and the interlayer exchange coupling, including the regions where interlayer coupling goes through zero. We see significant changes in domain structure based on the sign of coupling, and also show that magnetic domain size is directly related to the magnitude of the interlayer exchange coupling energy, which generally dominates over the magnetostatic interactions. When magnetostatic interactions become comparable to the interlayer exchange coupling, a delicate interplay between the differing energy contributions is apparent and energy scales are extracted. The results are of intense interest to the magnetic recording industry and also illustrate a relatively new avenue of undiscovered physics, primarily dealing with the delicate balance of energies in the formation of magnetic domains for coupled systems with PMA, defining limits on domain size as well as the interplay between roughness, domains and magnetic coupling.

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.

Self-Assembled Magnetic Nanohead-FeSi Nanowire Epitaxial Heterojunctions by Chemical Vapor Deposition

Self-assembled magnetic nanohead-FeSi nanowire (NW) epitaxial heterojunctions by chemical vapor deposition (CVD) are reported. Free-standing FeSi NWs, ∼30 nm in diameter and several micrometers long, were grown by reduction of Fe2O3 in a H2 atmosphere on a heated silicon substrate at 750 °C. These NWs shows very uniform dimensions, which is thought to be due to a higher diffusion activation energy for the nanostructure. A magnetic Fe3O4 head with a diameter of ∼30 nm was observed on each FeSi NW. The epitaxial relationship of these junctions is Si[001]//Fe3O4[2̅01], Si(001)//Fe3O4(102). Ferromagnetic behavior of the nanoheads was identified, and the coercivity was characterized as ∼220 Oe at 10 K. This one-step CVD approach for self-assembling epitaxially magnetic heads could enable device integration for spintronics and sensors. Particularly, such FeSi nanowires with highly localized magnetic nanoheads are ideal for high resolution magnetic force microscopy (MFM).