Rapid Faraday Rotation on ε-Iron Oxide Magnetic Nanoparticles by Visible and Terahertz Pulsed Light

Nanoceramics of metastable ε-Fe2O3: effect of sintering on the magnetic properties and sub-terahertz electron resonance

In this study, we demonstrate the sintering of metastable ε-Fe2O3 nanoparticles into nanoceramics containing 98 wt% of the epsilon iron oxide phase and with a specific density of 60%. At room temperature, the ceramics retain a giant coercivity of 20 kOe and a sub-terahertz absorption at 190 GHz inherent in the initial nanoparticles. The sintering leads to an increase in the frequencies of the natural ferromagnetic resonance at 200-300 K and larger coercivities at temperatures below 150 K. We propose a simple but working explanation of the low-temperature dynamics of the macroscopic magnetic parameters of the ε-Fe2O3 materials via the transition of the smallest nanoparticles into a superparamagnetic state. The results are confirmed by the temperature dependence of the magnetocrystalline anisotropy constant and micromagnetic modeling. In addition, based on the Landau-Lifshitz formalism, we discuss features of the spin dynamics in ε-Fe2O3 and the possibility of using nanoceramics as sub-terahertz spin-pumping media. Our observations will expand the applicability of ε-Fe2O3 materials and promote their integration into telecommunication devices of the next generation.

Lignin-based covalent organic polymers with improved crystallinity for non-targeted analysis of chemical hazards in food samples

Lignin, the most abundant source of renewable aromatic compounds derived from natural lignocellulosic biomass, has great potential for various applications as green materials due to its abundant active groups. However, it is still challenging to quickly construct green polymers with a certain crystallinity by utilizing lignin as a building block. Herein, new green lignin-based covalent organic polymers (LIGOPD-COPs) were one-pot fabricated with water as the reaction solvent and natural lignin as the raw material. Furthermore, by using paraformaldehyde as a protector and modulator, the LIGOPD-COPs prepared under optimized conditions displayed better crystallinity than reported lignin-based polymers, demonstrating the feasibility of preparing lignin-based polymers with improved crystallinity. The improved crystallinity confers LIGOPD-COPs with enhanced application performance, which was demonstrated by their excellent performances in sample treatment of non-targeted food safety analysis. Under optimized conditions, phytochromes, the main interfering matrices, were almost completely removed from different phytochromes-rich vegetables by LIGOPD-COPs, accompanied by "full recovery" of 90 chemical hazards. Green, low-cost, and reusable properties, together with improved crystallinity, will accelerate the industrialization and marketization of lignin-based COPs, and promote their applications in many fields.

Low-Frequency Sub-Terahertz Absorption in HgII -XCN-FeII (X=S, Se) Coordination Polymers

Self-assembly Fe^II complexes of phenazine (Phen), quinoxaline (Qxn), and 4,4'-trimethylenedipyridine (Tmp) with tetrahedral building blocks of [Hg^II (XCN)₄ ]^2- (X=S or Se) formed six new high-dimensional frameworks with the general formula of [Fe(L)_m ][Hg(XCN)₄ ]⋅solvents (L=Phen, m/X=2/S, 1; L=Qxn, m/X=2/S, 2; L=Qxn, m/X=1/S, 3; L=Qxn, m/X=1/Se, 3-Se; L=Tmp, m/X=1/S, 4; and L=Tmp, m/X=1/Se, 4-Se). 1, 3, and 3-Se show an intense sub-terahertz (sub-THz) absorbance of around 0.60 THz due to vibrations of the solvent molecules coordinated to the Fe^II ions and crystallization organic molecules. In addition, crystals of 1, 4, and 4-Se display low-frequency Raman scattering with exceptionally low values of 0.44, 0.51, and 0.53 THz, respectively. These results indicate that heavy metal Fe^II -Hg^II systems are promising platforms to construct sub-THz absorbers.

Particle size effect on millimeter-wave absorption, rotation, and ellipticity of gallium-substituted epsilon iron oxide

Various applications employ millimeter waves. For example, the carrier frequencies of vehicle radar in advanced driver assistance systems are 76-81 GHz millimeter waves. Here, we investigate the particle size effect on millimeter-wave absorption of gallium-substituted epsilon iron oxide ε-Ga x Fe2-x O3 with x = 0.44 ± 0.01. Samples were composed of nanoparticles with sizes of 16.9(1) nm, 28.8(2) nm, and 41.4(1) nm. Millimeter wave absorption, Faraday rotation, and Faraday ellipticity were measured by terahertz time-domain spectroscopy. This series exhibits millimeter-wave absorption at 78.7, 78.2, and 77.7 GHz without an external magnetic field. The millimeter-wave absorption increases from 4.6 dB to 9.4 dB as the particle size increases. In the magnetized sample, the Faraday rotation angle increases from 9.1° to 18.4°, while the Faraday ellipticity increases from 0.27 to 0.52. The particle size effect can be explained by the change in the ratio of the surface and core of the nanoparticles. The present study should contribute to the realization of high-performance millimeter-wave absorbers.

Terahertz time-domain ellipsometry with high precision for the evaluation of GaN crystals with carrier densities up to 1020 cm-3

Gallium nitride (GaN) is one of the most technologically important semiconductors and a fundamental component in many optoelectronic and power devices. Low-resistivity GaN wafers are in demand and actively being developed to improve the performance of vertical GaN power devices necessary for high-voltage and high-frequency applications. For the development of GaN devices, nondestructive characterization of electrical properties particularly for carrier densities in the order of 1019 cm-3 or higher is highly favorable. In this study, we investigated GaN single crystals with different carrier densities of up to 1020 cm-3 using THz time-domain ellipsometry in reflection configuration. The p- and s-polarized THz waves reflected off the GaN samples are measured and then corrected based on the analysis of multiple waveforms measured with a rotating analyzer. We show that performing such analysis leads to a ten times higher precision than by merely measuring the polarization components. As a result, the carrier density and mobility parameters can be unambiguously determined even at high conductivities.

Advances in magnetic films of epsilon-iron oxide toward next-generation high-density recording media

Iron oxide magnets, which are composed of the common elements iron and oxygen, are called ferrite magnets. They have diverse applications because they are chemically stable and inexpensive. Epsilon-iron oxide (ε-Fe2O3) is a polymorph that shows an extremely large coercive field as a magnetic oxide. It maintains its ferromagnetic ordering even when downsized to a single nano-sized scale (i.e.,<10 nm). Due to these characteristics, ε-Fe2O3 is highly expected to be used for high-density magnetic recording media in the big data era. Here, we describe the recent developments of magnetic films composed of metal-substituted ε-iron oxide, ε-MxFe2-xO3 (M: substitution metal), toward the next-generation of magnetic media.

3D porous graphene-assisted capsulized cholesteric liquid crystals for terahertz power visualization

We demonstrate a high-efficiency visualized terahertz (THz) power meter based on the THz-photothermochromism of capsulized cholesteric liquid crystals (CCLCs) embedded in three-dimensional porous graphene (3DPG). The graphene is a broadband perfect absorber for THz radiation and transfers heat efficiently, and its black background is beneficial for color measurement. Quantitative visualization of THz intensity up to 2.8×10²mW/cm² is presented. The minimal detectable THz power is 0.009 mW. With multi-microcapsule analysis, the relationship between THz power and the average hue value of CCLCs achieves linearity. The device can convert THz radiation to visible light and is lightweight, cheap, and easy to use.

Magnetic hyperthermia with ε-Fe2O3 nanoparticles

Biocompatibility restrictions have limited the use of magnetic nanoparticles for magnetic hyperthermia therapy to iron oxides, namely magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃). However, there is yet another magnetic iron oxide phase that has not been considered so far, in spite of its unique magnetic properties: ε-Fe₂O₃. Indeed, whereas Fe₃O₄ and γ-Fe₂O₃ have a relatively low magnetic coercivity, ε-Fe₂O₃ exhibits a giant coercivity. In this report, the heating power of ε-Fe₂O₃ nanoparticles in comparison with γ-Fe₂O₃ nanoparticles of similar size (∼20 nm) was measured in a wide range of field frequencies and amplitudes, in uncoated and polymer-coated samples. It was found that ε-Fe₂O₃ nanoparticles primarily heat in the low-frequency regime (20-100 kHz) in media whose viscosity is similar to that of cell cytoplasm. In contrast, γ-Fe₂O₃ nanoparticles heat more effectively in the high frequency range (400-900 kHz). Cell culture experiments exhibited no toxicity in a wide range of nanoparticle concentrations and a high internalization rate. In conclusion, the performance of ε-Fe₂O₃ nanoparticles is slightly inferior to that of γ-Fe₂O₃ nanoparticles in human magnetic hyperthermia applications. However, these ε-Fe₂O₃ nanoparticles open the way for switchable magnetic heating owing to their distinct response to frequency.

Reconfiguration of magnetic domain structures of ErFeO3 by intense terahertz free electron laser pulses

Understanding the interaction between intense terahertz (THz) electromagnetic fields and spin systems has been gaining importance in modern spintronics research as a unique pathway to realize ultrafast macroscopic magnetization control. In this work, we used intense THz pulses with pulse energies in the order of 10 mJ/pulse generated from the terahertz free electron laser (THz-FEL) to irradiate the ferromagnetic domains of ErFeO₃ single crystal. It was found that the domain shape can be locally reconfigured by irradiating the THz − FEL pulses near the domain boundary. Observed domain reconfiguration mechanism can be phenomenologically understood by the combination of depinning effect and the entropic force due to local thermal gradient exerted by terahertz irradiation. Our finding opens up a new possibility of realizing thermal-spin effects at THz frequency ranges by using THz-FEL pulses.