Nowadays, the most popular method to increase ferromagnetic resonance (FMR) frequency ( f) in self-bias soft magnetic films is to improve the anisotropy field H. However, to push f to higher frequencies only via raising H becomes increasingly challenging because f is already higher than 10 GHz by now. In this study, we fabricated a series of magnetically anisotropic FeCoB/Ru/FeCoB sandwich films possessing antiferromagnetic-like coupling and gradually increased uniaxial stress in the FeCoB sublayers from 52 to 110 MPa. It is quite remarkable that the acoustic mode of FMR gradually disappears, whereas the optical mode is enhanced in these structures. We observed simultaneous enhancement of H and interlayer coupling field ( J) with the uniaxial stress, which leads to a very pronounced optical-mode frequency increase from 8.67 to 11.62 GHz with a very sensitive stress response of 51 Hz/Pa. In contrast, the f in a FeCoB single layer (acoustic mode) only varies from 3.47 to 5.05 GHz under similar stress. We believe that the strain-induced electron density variation of the Ru spacer's Fermi surface in the out-of-plane direction is responsible for the enhancement of J. This study demonstrates that the antiferromagnetic coupling is a new route to achieve higher f and provides the possibility of engineering and manipulating optical-mode resonance simply by controlling the interlayer coupling strength via stress.
A series of FeGa, FeGaN and FeGaB films with varied oblique angles were deposited by sputtering method on silicon substrates, respectively. The microstructure, soft magnetism, microwave properties, and damping factor for the films were investigated. The FeGa films showed a poor high frequency magnetic property due to the large stress itself. The grain size of FeGa films was reduced by the additional N element, while the structure of FeGa films was changed from the polycrystalline to amorphous phase by the involved B element. As a result, N content can effectively improve the magnetic softness of FeGa film, but their high frequency magnetic properties were still poor both when the N2/Ar flow rate ratio is 2% and 5% during the deposition. The additional B content significantly led to the excellent magnetic softness and the self-biased ferromagnetic resonance frequency of 1.83 GHz for FeGaB film. The dampings of FeGa films were adjusted by the additional N and B contents from 0.218 to 0.139 and 0.023, respectively. The combination of these properties for FeGa films are helpful for the development of magnetostrictive microwave devices.
In this study, a dual-mode Metglas/Pb(Zr,Ti)O3 magnetoelectric (ME) sensor was prepared for measuring weak magnetic fields. It is interesting to note that this ME sensor can work at alternating current (AC) and direct current (DC) dual-modes with high field resolution. In AC mode, a very accurate AC magnetic field resolution of 0.8 nT was achieved at a mechanical resonance frequency of 72.2 kHz; moreover, the operating frequency band for resolution better than 1 nT is as wide as 3.4 kHz. We proposed a DC bias field perturbation (DBFP) method to detect the DC magnetic field using lock-in amplifier technology. As a result, an ultra-accurate DC field resolution of 0.9 nT with noise power spectral density as low as 100 pT/Hz was obtained in the studied ME sensor via the DBFP method. The dual-mode ME sensor enables simultaneous measurement for DC and AC magnetic fields with wideband and accurate field resolution, which greatly enhances the measurement flexibility and application scope.
Recently, rapidly increased demands of integration and miniaturization continuously challenge energy densities of dielectric capacitors. New materials with high recoverable energy storage densities become highly desirable. Here, by structure evolution between fluorite HfO2 and perovskite hafnate, we create an amorphous hafnium-based oxide that exhibits the energy density of ~155 J/cm3 with an efficiency of 87%, which is state-of-the-art in emergingly capacitive energy-storage materials. The amorphous structure is owing to oxygen instability in between the two energetically-favorable crystalline forms, in which not only the long-range periodicities of fluorite and perovskite are collapsed but also more than one symmetry, i.e., the monoclinic and orthorhombic, coexist in short range, giving rise to a strong structure disordering. As a result, the carrier avalanche is impeded and an ultrahigh breakdown strength up to 12 MV/cm is achieved, which, accompanying with a large permittivity, remarkably enhances the energy storage density. Our study provides a new and widely applicable platform for designing high-performance dielectric energy storage with the strategy exploring the boundary among different categories of materials.
Dielectric capacitors are fundamental for electric power systems due to the fast charging/discharging rate and high-power density.[1,2] Recently, rapidly increased demands of miniaturization and integration continuously challenge energy storage density of dielectric capacitors, especially for that could be compatible with the complementary metal-oxide-semiconductor (CMOS) technology, for developing energy-autonomous systems and implantable/wearable electronics, where high-κ capacitors become increasingly desirable in the next-generation applications.[3-5] However, their recoverable energy storage densities (Urec) are low in emerging capacitive energy storage materials. Here, by structure evolution between fluorite HfO2 and perovskite hafnate who have similar metal sublattices, we create an amorphous hafnium-based oxide that exhibits a giant Urec of ~155 J/cm3 with an efficiency (η) of 87%, which is record-high in high-κ materials and state-of-the-art in dielectric energy storage. The improved energy density is owing to the strong structure disordering in both short and long ranges induced by oxygen instability in between the two energetically-favorable crystalline forms. As a result, the carrier avalanche is impeded and an ultrahigh breakdown strength (Eb) up to 12 MV/cm is achieved, which, accompanying with a large permittivity (εr), remarkably enhances the dielectric energy storage. Our study provides a new and widely applicable playground for designing high-performance dielectric energy storage with the strategy exploring the boundary among different categories of materials.
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