ABSTRACT
Recently, giant coercivities (20-42 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) at 100-300 GHz were observed for single-domain M-type hexaferrite particles with high aluminum substitution. Herein, we fabricated dense ceramics of Sr0.67Ca0.33Fe8Al4O19 and, for the first time, investigated their magnetostatic and magnetodynamic properties in the temperature range of 5-300 K. It was shown that dense ceramics maintain their high magnetic hardness (a coercivity of 10-20 kOe) and NFMR frequencies of 140-200 GHz durably in the entire temperature range. Magnetizing the initially non-magnetized ceramics leads to a considerable decrease in the resonance absorption and to almost complete vanishing of the resonance line at 5 kOe. At the same time, an efficient linear frequency tuning by the external magnetic field was observed for the remanent sample. These findings open new horizons for developing industrial terahertz electronics based on dielectric ferrimagnets.
ABSTRACT
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.
ABSTRACT
The temperature behavior of the magnetic properties is crucial for the application of magnetic materials. Recently, giant room temperature coercivities (20-36 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) frequencies (160-250 GHz) were observed for single-domain M-type hexaferrites with high aluminum substitution. Herein, the temperature dependences of the magnetic properties and natural ferromagnetic resonance are studied at 5-300 K for single-domain Sr1-x/12Cax/12Fe12-xAlxO19 (x = 1.5-5.5) particles. It is shown that the samples maintain their magnetic hardness over the whole temperature range. The coercivity and NFMR frequencies have a maximum shifting to the low-temperature region with a rise in aluminum concentration. The highest coercivity of 42 kOe and the maximum NFMR frequency of 297 GHz are observed for x = 5.5 at 180 K.
ABSTRACT
Correction for 'High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance' by Evgeny A. Gorbachev et al., Mater. Horiz., 2022, 9, 1264-1272, DOI: https://doi.org/10.1039/D1MH01797G.
ABSTRACT
Following the game-changing high-pressure CO (HiPco) process that established the first facile route toward large-scale production of single-walled carbon nanotubes, CO synthesis of cm-sized graphene crystals of ultra-high purity grown during tens of minutes is proposed. The Boudouard reaction serves for the first time to produce individual monolayer structures on the surface of a metal catalyst, thereby providing a chemical vapor deposition technique free from molecular and atomic hydrogen as well as vacuum conditions. This approach facilitates inhibition of the graphene nucleation from the CO/CO2 mixture and maintains a high growth rate of graphene seeds reaching large-scale monocrystals. Unique features of the Boudouard reaction coupled with CO-driven catalyst engineering ensure not only suppression of the second layer growth but also provide a simple and reliable technique for surface cleaning. Aside from being a novel carbon source, carbon monoxide ensures peculiar modification of catalyst and in general opens avenues for breakthrough graphene-catalyst composite production.
ABSTRACT
Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.
ABSTRACT
In this paper, fabrication of a new material is reported, the so-called Aero-Ga2O3 or Aerogallox, which represents an ultra-porous and ultra-lightweight three-dimensional architecture made from interconnected microtubes of gallium oxide with nanometer thin walls. The material is fabricated using epitaxial growth of an ultrathin layer of gallium nitride on zinc oxide microtetrapods followed by decomposition of sacrificial ZnO and oxidation of GaN which according to the results of X-ray diffraction (XRD) and X-ray photoemission spectroscopy (XPS) characterizations, is transformed gradually in ß-Ga2O3 with almost stoichiometric composition. The investigations show that the developed ultra-porous Aerogallox exhibits extremely low reflectivity and high transmissivity in an ultrabroadband electromagnetic spectrum ranging from X-band (8-12 GHz) to several terahertz which opens possibilities for quite new applications of gallium oxide, previously not anticipated.
