X-ray Magnetic Circular Dichroism Microscopy Research Articles

Analysis on bimodal complex permeability spectrum of a noise suppression sheet and single constituent flakes

In recent years, highly integrated active components inside small mobile communication devices have led to serious electromagnetic interference problems. The countermeasure to this problem is covering a noise source with a noise suppression sheet (NSS) which is composed of ferromagnetic flakes and polymers. The NSS absorbs effectively electromagnetic waves due to a large imaginary part of relative permeability μr″ at the specific frequency region. This function is attributed to the bimodal μr″ spectrum of NSS, but its physical origin has not been revealed so far. In this study, the bimodal μr″ spectrum of NSS has been analyzed and its physical mechanism has been discussed. The μr″ spectrum of a constituent single magnetic flake was measured by using a newly developed high-sensitivity permeability measurement system, which showed a bimodal spectrum quite similar to that of NSS. This fact indicates that the physical origin of the bimodal μr″ spectrum is attributed to the magnetization dynamics of each single constituent flake. After measuring the μr″ spectra of single flakes with various materials, such as Fe, Fe-Al, Fe-Co-V, and Fe-Si-Al alloys, these bimodal μr″ spectra were quantitatively explained by the magnetization dynamics of the flux closure states inside the flake. The magnetic domain structure of a single flake was observed by a micro-beam X-ray magnetic circular dichroism microscopy. The result confirmed that the segmented magnetic domains form the local flux closure states inside the flake.

Demagnetization processes of Nd-Fe-B sintered magnets and ferrite magnets as demonstrated by soft X-ray magnetic circular dichroism microscopy

The demagnetization processes of Nd-Fe-B sintered magnets and ferrite magnets are investigated using X-ray magnetic circular dichroism microscopy (XMCD). The average XMCD demagnetization curves are compared with the demagnetization curves from magnetic measurements. It is found that the magnetization reversal of the magnets proceeds in two steps. First, the grains are discretely demagnetized in a weak demagnetization field. When the strength of the demagnetization field increases, the magnetization of the grains adjacent to the discretely reversed grains is also reversed to form a cluster of reversed grains, and this cluster grows. Coercivity is achieved through the growth of such clusters. The demagnetization curves from XMCD and the demagnetization curves from the magnetic measurements agree well with each other when the self-demagnetization field and the magnetic after-effect in the XMCD measurement are taken into account. This result shows that the information obtained from the XMCD reflects the bulk information.

Post-acquisition upsampling method for scanning x-ray microscopy.

A method of post-acquisition upsampling for scanning x-ray microscopy is developed in this study to improve the spatial resolution beyond the Nyquist frequency as determined by the intervals of a raster scan grid. The proposed method is applicable only when the probe beam size is not negligibly small compared with the pixels that constitute a raster micrograph-the Voronoi cells of a scan grid. The unconvoluted spatial variation in a photoresponse is estimated by solving a stochastic inverse problem at a higher resolution than that at which the data are acquired. This is followed by a rise in the spatial cutoff frequency due to a reduction in the noise floor. The practicability of the proposed method was verified by applying it to raster micrographs of x-ray absorption in Nd-Fe-B sintered magnets. The improvement thus achieved in spatial resolution was numerically demonstrated via spectral analysis by using the discrete Fourier transform. The authors also argue for a reasonable decimation scheme for the spatial sampling interval in relation to an ill-posed inverse problem and aliasing. The computer-assisted enhancement in the viability of scanning x-ray magnetic circular dichroism microscopy was illustrated by visualizing magnetic field-induced changes in domain patterns of the Nd2Fe14B main-phase.

The Demagnetization Processes of Nd-Fe-B Sintered Magnets and Ferrite Magnets as Demonstrated by Soft X-Ray Magnetic Circular Dichroism Microscopy

The Demagnetization Processes of Nd-Fe-B Sintered Magnets and Ferrite Magnets as Demonstrated by Soft X-Ray Magnetic Circular Dichroism Microscopy

Effect of hydrogenation-disproportionation-desorption-recombination powder processing on the demagnetization process of Nd-Fe-B sintered magnets analyzed by soft X-ray magnetic circular dichroism microscopy

The demagnetization process of a variety of Nd-Fe-B-based magnets produced using different manufacturing techniques are studied by soft X-ray magnetic circular dichroism microscopy. The four magnets studied include a hot deformed magnet, sintered magnets produced from either a N2 jet-milled or a He jet-milled fine powder, and a magnet produced by hydrogenation-disproportionation-desorption-recombination (HDDR) followed by He jet-milling. The coercivity of the HDDR processed magnet is lower than the magnets produced with N2 jet-milled powder even though the size of the grains is smaller. By following the magnetic domain reversal behavior during the demagnetization process, it was found that, while the magnets produced with N2 jet-milled powder and the hot deformed magnet presents a predominance of the reversed magnetic domain propagation, in the HDDR processed magnet the nucleation of individual grains occurs before the reversed magnetic domains propagate. This behavior is associated with a decreased nucleation field of each grain.

Magnetization reversal of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets as per soft x-ray magnetic circular dichroism microscopy

We investigated magnetization reversal of (Sm, Ce)2(Co, Fe, Cu, Zr)17 magnets as per x-ray magnetic circular dichroism microscopy. Magnetization reversal initially occurs at the ferromagnetic grain boundaries or in the vicinity of nonmagnetic Sm oxides. As the demagnetization field increases after magnetization reversal, the reversal region extends into the grain from these areas by the magnetic domain wall motion. Energy-dispersive x-ray analysis using an electron probe micro-analyzer shows that, at the grain boundaries, the Fe concentration is higher and the Cu concentration is lower compared to that inside of the grains; and concentrations of Sm, Co, Fe, and Cu vary in the vicinity of Sm oxides. By measuring the Co L3 absorption intensity, we verified that local coercivities in these areas are very low compared to those inside of the grain. These results imply that the magnetization reversal that occurs in these areas is induced by the variation in the composition. The results obtained in our research will be useful for improving the magnetic properties of Sm–Co magnets.

Microwave-assisted switching in CoCrPt granular medium under continuous microwave fields

We have fabricated a device for microwave-assisted switching (MAS) experiments with perpendicularly magnetized CoCrPt-SiO2 granular film with anisotropy field Hk = 18.8 kOe. The device was carefully designed to be able to apply a microwave field of 0.95 kOe in amplitude continuously, allowing direct evaluation of microwave-assisted switching properties of the medium without any assumptions about time-dependent switching behavior. The coercivity of CoCrPt-SiO2 medium linearly decreases with increasing microwave frequency, and the maximum coercivity reduction ratio reaches 80% for hrf = 0.95 kOe at frf = 25 GHz after subtracting the effect of temperature rise due to microwave field application. The linear slope of coercivity against the frequency becomes steeper with increasing microwave field amplitude, which is a unique frequency and amplitude dependencies in granular media. Correlation length obtained by X-ray magnetic circular dichroism microscopy measurement decreases by applying a microwave field with higher frequency, suggesting that the magnetic clustering dimensions of the medium possibly depend on microwave frequency during microwave-assisted switching.

Temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on first-order reversal curve analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Temperature Dependent Magnetization Reversal Process of a Ga-Doped Nd-Fe-B Sintered Magnet Based on First-Order Reversal Curve Analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well verified using soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well explained by the temperature dependent saturation field of multidomain grains. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Observation of the magnetoelectric reversal process of the antiferromagnetic domain

We investigated the switching process of the perpendicular exchange bias, which is driven by the magnetoelectric effect, by conducting magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. Isothermal and simultaneous application of magnetic and electric fields switches the perpendicular exchange bias polarity. The switching process proceeds by the nucleation and growth of reversed domains. The correspondence among the ferromagnetic/antiferromagnetic domains and exchange bias polarity indicates that interfacial antiferromagnetic spin/domain reversal is responsible for the magnetoelectric switching of the perpendicular exchange bias polarity.

Antiferromagnetic domain wall creep driven by magnetoelectric effect

We observed the magnetoelectric induced domain wall propagation in a Pt/Co/Au/Cr2O3/Pt stacked thin film based on magnetic domain observations using scanning soft X-ray magnetic circular dichroism microscopy. The antiferromagnetic (Cr2O3) domain wall velocity was estimated by a quasi-static approach using a pulsed voltage. At a pulse voltage amplitude of −12 V, corresponding to an electric field of −8.0 × 102 kV/cm, the domain wall velocity was very low, at 0.3 m/s. The domain wall velocity increased with increasing voltage amplitude, reaching 22 m/s at −20 V (−1.3 × 103 kV/cm). The change in the domain wall velocity with the applied voltage amplitude indicates the creep motion of the domain wall. Using a phenomenological model, we estimated the domain wall depinning energy, and found that the bulk and interface terms of the magnetic anisotropy affect the effective magnetic field to the same degree, suggesting that the magnetic domain wall motion may be controllable by the antiferromagnetic layer thickness.

First-order reversal curve analysis of a Nd-Fe-B sintered magnet with soft X-ray magnetic circular dichroism microscopy

First-order reversal curve (FORC) diagram, which visualizes the variation of magnetic susceptibility on a field plane, has been applied to a Nd-Fe-B sintered magnet. The FORC diagram exhibits the characteristic behavior of two remarkable spots in low-field and high-field regions. The high-field FORC spot corresponds to the irreversible magnetization reversal at a coercive field, whereas the low-field FORC spot indicates the appearance of a large magnetic susceptibility state during the demagnetization process. Moreover, this low-field FORC spot becomes dominant at high temperature, accompanied by a significant reduction in coercivity. These results suggest that the low-field FORC spot has a strong correlation with the degradation of magnetic properties of a Nd-Fe-B sintered magnet. To clarify the actual magnetization reversal processes corresponding to these two FORC spots, soft X-ray magnetic circular dichroism (XMCD) microscopy observation was employed with similar field sequences of the FORC measurements. Consequently, the low-field FORC spot is mainly attributed to the domain wall motion in multi-domain grains, whereas the high-field FORC spot corresponds to the magnetization reversal of single-domain grains. These indicate that a FORC diagram is a powerful evaluation method for the magnetization reversal processes of permanent magnets.

Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures.

Magnetic skyrmions are chiral spin structures with a whirling configuration. Their topological properties, nanometre size and the fact that they can be moved by small current densities have opened a new paradigm for the manipulation of magnetization at the nanoscale. Chiral skyrmion structures have so far been experimentally demonstrated only in bulk materials and in epitaxial ultrathin films, and under an external magnetic field or at low temperature. Here, we report on the observation of stable skyrmions in sputtered ultrathin Pt/Co/MgO nanostructures at room temperature and zero external magnetic field. We use high lateral resolution X-ray magnetic circular dichroism microscopy to image their chiral Néel internal structure, which we explain as due to the large strength of the Dzyaloshinskii-Moriya interaction as revealed by spin wave spectroscopy measurements. Our results are substantiated by micromagnetic simulations and numerical models, which allow the identification of the physical mechanisms governing the size and stability of the skyrmions.

XMCD microscopy with synchronized soft X-ray and laser pulses at PETRA III for time-resolved studies

We present the setup of a new transmission X-ray microscope using zone plates at the soft X-ray beamline P04 at PETRA III, designed to utilize the X-ray magnetic circular dichroism (XMCD) contrast for imaging the magnetic orientation of nanostructured materials. We present the first results obtained with this setup at PETRA III using vacuum chambers with built-in multi-axis micropositioning stages that can be easily customized to mount a variety of components like condenser, samples, coils, zone plates etc. For further experiments the X-ray microscope will be equipped with a synchronized femtosecond laser system to implement a time-resolved detection of transient magnetic states in nanostructured media using a pump-probe scheme. The synchronization between laser and X-ray pulses has already been demonstrated at both the P04 and P11 beamlines at PETRA III with a jitter of σ < 15 ps.