Magnetic Holography Research Articles

Dispersing Magnetic Nanoparticles into Staggered, Porous Nano‐Frameworks: Weaving and Visualizing Nanoscale Magnetic Flux Lines for Enhanced Electromagnetic Absorption

AbstractBy dispersing magnetic materials within various nanoscale structural frameworks, a complex and weavable magnetic network of nanoscale magnetic fields is created. This network not only systematically reconstructs interactions with external electromagnetic fields but also inversely adjusts its magnetic properties, potentially leading to anomalous or enhanced magnetic behaviors. However, the challenges lie in finding an effective nano‐framework capable of efficiently regulating magnetic flux lines and visualizing these reconstructed fields at the nanoscale. In this study, magnetic Co nanoparticles are strategically dispersed into a meticulously layered nanoporous framework with the bottom and upper nanopores exhibiting a staggered arrangement. The staggered nanoporous structure intricately shapes the magnetic flux lines into a Chinese knot shape, significantly altering its magnetic characteristics. The innovative use of advanced in situ magnetic holography, coupled with magnetic simulation calculations, has been instrumental in visualizing these modifications to the magnetic flux lines. Such a transformation remarkably enhances the material's efficacy in absorbing electromagnetic waves, covering the Ku band with remarkable efficiency even at a minimal thickness of 1.7 mm. Looking ahead, these breakthroughs not only provide a roadmap for developing more efficient electromagnetic wave absorbers but also open up new possibilities in manipulating magnetic properties for various advanced technological applications.

Realizing optimized interfacial polarization and impedance matching with CNT-confined Co nanoparticles in hollow carbon microspheres for enhanced microwave absorption

The hollow porous structure with exceptional interfacial effect and customizable internal environment shows significant potential for application as electromagnetic shielding and absorption materials. However, designing hollow porous electromagnetic absorbers with both desirable impedance matching and high loss capability remains a challenge. Herein, 3D hollow porous electromagnetic microspheres were constructed by assembling 0D Co magnetic nanoparticles, 1D carbon nanotubes, and 2D carbon nanosheets. Due to the sufficient sites for Co2+ riveting, the high loading of magnetic carbon nanotubes (CoNC) and porous carbon spheres formed high-density interfaces, enhancing the interfacial polarization. Furthermore, high-density CoNC were grown in situ on the hollow porous carbon (HPC) microsphere, forming a highly dispersed 3D magnetic network that inhibited the aggregation of magnetic nanoparticles and enhanced magnetic coupling. Therefore, the as-prepared CoNC/HPC microspheres exhibited excellent microwave absorption (MA) performance, with a minimum reflection loss of -33.2 dB and an effective bandwidth of 5.5 GHz at a thickness of only 1.8 mm. The interfacial polarization mechanism for enhanced MA performance was demonstrated by electron holography and density functional theory calculations. Magnetic holography and micromagnetic simulations also revealed magnetic confinement and coupling mechanism. This work provides a new approach for designing electromagnetic absorbers with optimized impedance matching and loss capability.

Magnetic Holography and Its Application to Data Storage

The principle of magnetic holograms and its application to holographic memory are reviewed. A magnetic hologram was recorded through a thermomagnetic recording as a difference in magnetization direction and reconstructed with the magneto-optical effect. To achieve a bright reconstruction image, it is important to record deep magnetic fringes on the materials with large Faraday rotation coefficients. This technique was applied to the holographic memory using transparent magnetic garnets as a recording material. The first reconstruction image was dark and noisy, but improvements in the recording conditions resulted in error-free recording and reconstruction of the magnetic hologram. To form deep magnetic fringes, insertion of heat dissipation (HD) layers into recording layer was proposed. It was found that this HD multilayer medium showed diffraction efficiency higher than that of a single layer medium, and error-free recording and reconstruction were also achieved, using magnetic assisted recording. These results suggest that HD multilayer media have potential applications in recording media of magnetic holographic data storage. In future, a high recording density technique, such as multiple recording, should be developed.

Development of Heat Dissipation Multilayer Media for Volumetric Magnetic Hologram Memory

Holographic memory is a strong candidate for next-generation optical storage, featuring high recording densities and data transfer rates, and magnetic hologram memory using a magnetic garnet, as the recording material is expected to be used as a rewritable and stable storage technology. However, the diffraction efficiency of magnetic holography depending on the Faraday rotation angle is insufficiently high for actual storage devices. To increase the diffraction efficiency, it is important to record deep magnetic fringes, whereas it is necessary to suppress the merging of fringes owing to heat diffusion near the medium surface. In this work, we investigated the recording process of magnetic holograms in detail with experiments and numerical simulations, and developed a multilayer media with transparent heat dissipation layers to record deep and clear magnetic holograms by controlling the heat diffusion generated during the thermomagnetic recording process. To suppress lateral heat diffusion near the medium surface, we designed and fabricated a multilayer magnetic medium in which the recording magnetic layers are discrete in a film, approximately 12-µm thick. This medium exhibited diffraction efficiency higher than that of the single-layer medium, and error-free recording and reconstruction were achieved using the magnetic assist technique.

Reconstruction of non-error magnetic hologram data by magnetic assist recording

Hologram memory is expected to be the next-generation of optical data storage technology. Bismuth-substituted yttrium iron garnet is typically used for rewritable magnetic hologram media. The diffraction efficiency of magnetic holography depends on the Faraday rotation angle, but the experimental diffraction efficiency is not as high as that expected from calculations. This difference could be caused by incomplete magnetization reversal at the recorded region. In this study, we investigated the effects of magnetic assist (MA) recording through numerical simulation and experiment to improve the diffraction efficiency and the resulting reconstructed images. The improvement of diffraction efficiency was more effective in garnet films thinner than the width of a fringe, and a suitable value of the assist magnetic field was identified for the improvement. In addition, MA recording improved the intensity of reconstructed images and broadened the non-error recording conditions to the low energy region. This technique shows promise in improving the reconstructed quality of magnetic hologram data.

Three-dimensional supercritical resolved light-induced magnetic holography.

In the era of big data, there exists a growing gap between data generated and storage capacity using two-dimensional (2D) magnetic storage technologies (for example, hard disk drives), because they have reached their performance saturation. 3D volumetric all-optical magnetic holography is emerging rapidly as a promising road map to realizing high-density capacity for its fast magnetization control and subwavelength magnetization volume. However, most of the reported light-induced magnetization confronts the problems of impurely longitudinal magnetization, diffraction-limited spot, and uncontrollable magnetization reversal. To overcome these challenges, we propose a novel 3D light-induced magnetic holography based on the conceptual supercritical design with multibeam combination in the 4π microscopic system. We theoretically demonstrate a 3D deep super-resolved [Formula: see text] purely longitudinal magnetization spot by focusing six coherent circularly polarized beams with two opposing high numerical aperture objectives, which allows 3D magnetic holography with a volumetric storage density of up to 1872 terabit per cubic inches. The number and locations of the super-resolved magnetization spots are controllable, and thus, desired magnetization arrays in 3D volume can be produced with properly designed phase filters. Moreover, flexible magnetization reversals are also demonstrated in multifocal arrays by using different illuminations with opposite light helicity. In addition to data storage, this magnetic holography may find applications in information security, such as identity verification for a credit card with magnetic stripe.

Achieving a high cutting-off frequency in the oriented CoFe2O4 nanocubes

An effective way for the improvement of resonance frequency was demonstrated by the orientation of CoFe2O4 nanocubes (NCs) after applying a magnetic field. The magnetic holography image of the oriented CoFe2O4 NC arrays was obtained, and the vortex domain structure was found in single CoFe2O4 NCs, which greatly depends on dipolar interactions. Moreover, the cutting-off frequency of oriented CoFe2O4 NCs of 8.8 GHz is achieved with a slight increase in permeability at an extremely low mass fraction (about 10%). This work provides an effective way for optimizing the high-frequency properties of magnetic nanomaterials above GHz.

Magnetooptical and crystalline properties of sputtered garnet ferrite film on spinel ferrite buffer layer

The purpose of this study is to provide garnet films for volumetric magnetic holography. Volumetric magnetic holography usually employs an easily obtainable short-wavelength laser (visible light, not infrared light) with a large diffraction intensity. Bi-substituted garnet ferrite with a large Faraday rotation is promising for volumetric magnetic holography applications in the visible light region. However, a garnet film without a deteriorated layer must be obtained because a deteriorated layer (minute polycrystalline grains containing an amorphous phase) is formed during the initial deposition on a glass substrate. In particular, the required magnetooptical properties have not been obtained in a thin garnet film (100 nm or less) after annealing (1 h, 700 °C, oxygen atmosphere). Therefore, there is a need for excellent garnet films with the required magnetooptical (MO) properties even if the films are thin. By using a spinel ferrite buffer layer for garnet film deposition, we could obtain a thin garnet film with excellent MO properties. We determined the effect of the initial buffer layer on the crystallinity of the deposited garnet films by observing the film cross section. In addition, we undertook a qualitative estimation of the influence of the crystallinity and optical properties of the garnet film on a glass substrate with a spinel ferrite buffer layer.

Collinear volumetric magnetic holography with magnetophotonic microcavities

Hologram memory is a candidate for high-capacity data storage. Magnetic holograms formed as magnetization directions have been studied to realize rewritable hologram media. Recently, we reported that the magnetophotonic microcavity (MPM) can improve diffraction efficiency because of enhanced Faraday rotation angle and deep hologram writing. In this study, we demonstrated a clear reconstructed image from magnetic holograms in an MPM medium. The structural condition of MPMs for high diffraction efficiency was investigated, and the MPM medium was actually fabricated. The image reconstructed from the MPM medium had approximately twice the brightness of that reconstructed using a monolayer film.

Extreme Ultraviolet Stokesmeter for Pulsed Magneto-Optics

Several applications in material science and magnetic holography using extreme ultraviolet (EUV) radiation require the measurement of the degree and state of polarization. In this work, an instrument to measure simultaneously both parameters from EUV pulses is presented. The instrument determines the Stokes parameters after a reflection on an array of multilayer mirrors at the Brewster angle. The Stokesmeter was tested at Swiss Light Source at different EUV wavelengths. The experimental Stokes patterns of the source were compared with the simulated pattern.

Effect of recording condition on the diffraction efficiency of magnetic hologram with magnetic garnet films

A holographic memory has been attracting attention as recording media with high recording density and high data transfer rate. We have studied the magnetic garnets as a rewritable and long life media for magnetic holography. However, since the signal intensity of reconstructed image was relatively low, the effects of recording conditions on the diffraction efficiency of magnetic hologram were investigated with experiments and the numerical simulation using COMSOL multi-physics. The diffraction efficiency tends to decrease as increasing the spatial frequency, and the use of short pulse laser with the pulse width of 50 ps was found to be effective to achieve high diffraction efficiency. This suggests that the formation of clear magnetic fringe similar to interference pattern can be obtained by the use of short pulse laser since undesirable heat diffusion during radiation does not occur. On the other hand, the diffraction efficiency increased as increasing the film thickness up to 3.1 μm but was saturated in the garnet film thicker than 3.1 μm in the case of spatial frequency of 1500 line pair/mm. The numerical simulation showed that the effective depth of magnetic fringe was limited about 1.8 μm irrespective of the garnet film thickness because the fringes were connected by thermal diffusion near the surface of the film, and the effective depth is limited due to this connection of the magnetic fringe. Avoiding this fringe connection, much higher diffraction efficiency will be achieved.

Exploring the limits of soft x-ray magnetic holography: Imaging magnetization reversal of buried interfaces (invited)

Only a very few experimental techniques can address the microscopic magnetization reversal behavior of the different magnetic layers in a multilayered system with element selectivity. We present an element-selective study of ferromagnetic (FM) [Co/Pt]n multilayers with perpendicular anisotropy exchange-coupled to antiferromagnetic (AFM) FeMn and IrMn films performed with a new experimental set-up developed for both soft x-ray spectroscopy and holography imaging purposes. The spectroscopy analysis allows the quantification of the unpinned (pinned) uncompensated AFM moments, providing direct evidence of its parallel (antiparallel) alignment with respect to the FM moments. The holography experiments give a direct view of both FM and uncompensated AFM magnetic structures, showing that they replicate to each other during magnetization reversal. Remarkably, we show magnetic images for effective thicknesses as small as one monolayer. Our results provide new microscopic insights into the exchange coupling phenomena and explore the sensitivity limits of these techniques. Future trends are also discussed.

Importance of Neutron Scattering for Materials Science: Expectations on Magnetic Holography

The neutron scattering technique is an indispensable probe for structural investigations of materials, particularly, for observations of light atoms and magnetic moments. Using the IMR neutron powder diffractometer HERMES, which can observe scattering patterns in wide Q rage at once, we have succeeded in performing neutron holography experiments in a Pd-H compound. Holography will become a quite important technique for local structure investigations of materials. Moreover, as the next stage, magnetic holography with polarized neutrons must be promising. [DOI: 10.1380/ejssnt.2011.422]

High-density Information Storage by Means of Magnetic Holography on Amorphous TbFe Thin Films (II)

The recording characteristics and hologram patterns of data storage by magnetic holography are described in detail. When the reconstruction was like a spatially filtered image of the original pattern, it was observed that the middle part of the contour of each character of the original pattern was saturated by the excess intensity of the object beam. It was found that the recording margin in magnetic holography is very small, and is strongly affected by the original data pattern. Adjustment of intensity of the object and reference beams at the recording stage is thus very important.

High-Density Information Storage by Means of Magnetic Holography of Amorphous TbFe Thin Films (II)

Previously we reported the possibility of recording information by means of magnetic holography (using amorphous TbFe thin films as a hologram medium) onto a spot of diameter less than 1 mm, reconstructing data using the Faraday effect. However, we had not studied the basic characteristics of information storage in detail, and therefore were not able to make good-quality holograms with good reproducibility. Because reproducibility is the characteristic of greatest importance for recording, we studied the recording margin for information storage by magnetic holography. In addition, we considered the relationship between the power density of the two beams (object beam and reference beam) and the domain pattern (hologram). It was confirmed that the domain pattern is closely related to the beam density, and the recording margin depends on the information pattern. From these results, we were able to create good-quality holograms with satisfactory reproducibility.

非晶質ＴｂＦｅ薄膜を用いた磁気ホログラフィによる高密度記録 ＩＩ

Previously we reported that it is possible to record information by means of magnetic holography (using amorphous TbFe thin films as a hologram medium) onto a spot whose diameter is less than 1 mm and to reconstruct it by using the Faraday effect. However, we had not studied the basic characteristics for information storage in detail, and therefore did not then have a proper way of making good holograms reproducibly. Because reproducibility is the most important characteristic for recording, we studied the recording margin for information storage by magnetic holography. In addition, we discussed the relationship between the power density of two beams (object beam and reference beam) and the domain pattern (hologram). It was confirmed that the domain pattern has a close relation to the beam density, and the recording margin depends on the information pattern. From these results, we can make a good hologram reproducibly.

High-Density Information Storage by Means of Magnetic Holography on Amorphous TbFe Thin Films

High-density information storage by means of magnetic holography is demonstrated by using amorphous TbFe thin films as a magnetic hologram medium. The object beam, which emerges from a two-dimensional information pattern (in most cases 35 mm photographic films), is focused on TbFe film by a lens using the spatial Fourier-transform pattern. A magnetic hologram, that is, a magnetic domain pattern corresponding to the interference pattern of the object beam and the reference beam, is established on the TbFe film by thermo-magnetic writing. A Q-switched ruby laser (of peak light power=0.6MW and light pulse durations30 ns) was used as the writing light source. An illuminated spot, from 0.2 to 1 mm in diameter, was chosen depending on the size of information. Image reconstruction was done by means of either the Faraday effect (transmitted light) or the polar Kerr effect (reflected light) using a first-order diffracted beam. A He-Ne laser (1 mW or 30 mW) was used for the reconstruction light source. It was confirmed that satisfactory images of not only documents but also picture images can be reconstructed by magnetic holography. The storage density estimated from the preliminary experiment is about 4×103 characters/mm2. Some considerations for improving the reconstructed image quality are also described.

Magnetic Holography

The conditions for magnetic hologram formation are reviewed and an analysis is given to explain some of the unique polarization effects seen on reconstruction, with both the Faraday and polar Kerr effect reconstructions treated. Experiments are presented which illustrate the features and storage potentials of magnetic holography.

Reconstruction effects in magnetic holography

Magnetic holography, a storage method based on the combination of techniques of magneto-optics and holography, is shown to have features which are unique to either of these techniques. In contrast to the usual situation in magneto-optics, reconstruction may be obtained with either the Faraday or polar Kerr effect, using polarized or unpolarized light. Furthermore the reconstruction will not be ellipticaly polarized if linearly polarized light is used for readout. In contrast to the usual case in holography, the first-order diffracted wave is polarized at 90° to both the incident and zeroth-order waves. This feature allows a considerable increase in the attainable signal-to-noise in the reconstruction. These and other features are described analytically and demonstrated experimentally.

Polarization and efficiency in magnetic holography

Reconstruction effects in a magnetic hologram can easily be interpreted if one assumes a linearly polarized plane wave readout beam. The field transmitted through or reflected off the hologram is rotated via the Faraday or Kerr effect. By decomposing the emerging field into two orthogonal components, one can define separate transmission functions for each polarization. The diffracted field and the efficiency of the magnetic grating are readily computed from these transmission functions. For a binary magnetic grating all diffraction orders, with the exception of zeroth order, are polarized orthogonally to the incoming polarization. For a sinusoidal magnetic grating, the various diffraction orders alternate in polarization with the zeroth order polarized in the same direction as the incoming polarization.