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$\alpha$-RuCl$_3$ has been studied extensively because of its proximity to the Kitaev quantum-spin-liquid (QSL) phase and the possibility of approaching it by tuning the competing interactions. Here we present the first polarized inelastic neutron scattering study on $\alpha$-RuCl$_3$ single crystals to explore the scattering continuum around the $\Gamma$ point at the Brillouin zone center, which was hypothesized to be resulting from the Kitaev QSL state but without concrete evidence. With polarization analyses, we find that while the spin-wave excitations around the M point vanish above the transition temperature $T_{\rm N}$, the pure magnetic continuous excitations around the $\Gamma$ point are robust against temperature. Furthermore, by calculating the dynamical spin-spin correlation function using the cluster perturbation theory, we derive magnetic dispersion spectra based on the $K$-$\Gamma$ model, which involves with a ferromagnetic Kitaev interaction of -7.2 meV and an off-diagonal interaction of 5.6 meV. We find this model can reproduce not only the spin-wave excitation spectra around the M point, but also the non-spin-wave continuous magnetic excitations around the $\Gamma$ point. These results provide evidence for the existence of fractional excitations around the $\Gamma$ point originating from the Kitaev QSL state, and further support the validity of the $K$-$\Gamma$ model as the effective minimal spin model to describe $\alpha$-RuCl$_3$., Comment: Published in Chinese Physics Letters as Express Letter, 8 pages, 4 figures

${\mathrm{Ba}}_{3}{\mathrm{NbFe}}_{3}{\mathrm{Si}}_{2}{\mathrm{O}}_{14}$ (langasite) is structurally and magnetically single-domain chiral with the magnetic helicity induced through competing symmetric exchange interactions. Using neutron scattering, we show that the spin waves in antiferromagnetic langasite display directional anisotropy. On applying a time-reversal symmetry breaking magnetic field along the $c$ axis, the spin-wave energies differ when the sign is reversed for either the momentum transfer $\ifmmode\pm\else\textpm\fi{}\stackrel{P\vec}{Q}$ or applied magnetic field $\ifmmode\pm\else\textpm\fi{}{\ensuremath{\mu}}_{0}\mathrm{H}$. When the field is applied within the crystallographic $ab$ plane, the spin-wave dispersion is directionally isotropic and symmetric in $\ifmmode\pm\else\textpm\fi{}{\ensuremath{\mu}}_{0}\mathrm{H}$. However, a directional anisotropy is observed in the spin-wave intensity. We discuss this directional anisotropy in the dispersion in langasite in terms of a field-induced precession of the dynamic unit cell staggered magnetization resulting from a broken twofold symmetry. Directional anisotropy, often referred to as nonreciprocal responses, can occur in antiferromagnetic phases in the absence of the Dzyaloshinskii-Moriya interaction or other effects resulting from spin-orbit coupling.

Hidden-order phases that occur in a number of correlated $f$-electron systems are among the most elusive states of electronic matter. Their investigations are hindered by the insensitivity of standard physical probes, such as neutron diffraction, to the order parameter that is usually associated with higher-order multipoles of the $f$ orbitals. The heavy-fermion compound ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{20}{\mathrm{Si}}_{6}$ exhibits magnetically hidden order at subkelvin temperatures, known as phase II. Additionally, for magnetic field applied along the [001] cubic axis, another phase ${\mathrm{II}}^{\ensuremath{'}}$ was detected, but the nature of the transition from phase II to phase ${\mathrm{II}}^{\ensuremath{'}}$ remained unclear. Here we use inelastic neutron scattering to argue that this transition is most likely associated with a change in the propagation vector of the antiferroquadrupolar order from (111) to (100). Despite the absence of magnetic Bragg scattering in phase ${\mathrm{II}}^{\ensuremath{'}}$, its ordering vector is revealed by the location of an intense magnetic soft mode at the (100) wave vector, that is orthogonal to the applied field. At the II-${\mathrm{II}}^{\ensuremath{'}}$ transition, this mode softens and transforms into quasielastic and nearly $\mathbf{Q}$-independent incoherent scattering, which is likely related to the non-Fermi-liquid behavior recently observed at this transition. Our experiment also reveals sharp collective excitations in the field-polarized paramagnetic phase, after phase ${\mathrm{II}}^{\ensuremath{'}}$ is suppressed in fields above 4 T.

We have systematically studied physical properties of Ba(Fe_{0.97}Cr_{0.03})_{2}(As_{1-x}P_{x})_{2}, where superconductivity in BaFe_{2}(As_{1-x}P_{x})_{2} is fully suppressed by just 3% of Cr substitution of Fe. A quantum critical point is revealed at x∼0.42, where non-Fermi-liquid behaviors similar to those in BaFe_{2}(As_{1-x}P_{x})_{2} are observed. Neutron diffraction and inelastic neutron scattering measurements suggest that the quantum critical point is associated with the antiferromagnetic order, which is not of conventional spin-density-wave type as evidenced by the ω/T scaling of spin excitations. On the other hand, no divergence of low-temperature nematic susceptibility is observed when x is decreased to 0.42 from higher doping level, demonstrating that there are no nematic quantum critical fluctuations. Our results suggest that non-Fermi-liquid behaviors in iron-based superconductors can be solely resulted from the antiferromagnetic quantum critical fluctuations, which cast doubts on the role of nematic fluctuations played in the normal-state properties in iron-based superconductors.

We have systematically studied physical properties of Ba(Fe_{0.97}Cr_{0.03})_{2}(As_{1-x}P_{x})_{2}, where superconductivity in BaFe_{2}(As_{1-x}P_{x})_{2} is fully suppressed by just 3% of Cr substitution of Fe. A quantum critical point is revealed at x∼0.42, where non-Fermi-liquid behaviors similar to those in BaFe_{2}(As_{1-x}P_{x})_{2} are observed. Neutron diffraction and inelastic neutron scattering measurements suggest that the quantum critical point is associated with the antiferromagnetic order, which is not of conventional spin-density-wave type as evidenced by the ω/T scaling of spin excitations. On the other hand, no divergence of low-temperature nematic susceptibility is observed when x is decreased to 0.42 from higher doping level, demonstrating that there are no nematic quantum critical fluctuations. Our results suggest that non-Fermi-liquid behaviors in iron-based superconductors can be solely resulted from the antiferromagnetic quantum critical fluctuations, which cast doubts on the role of nematic fluctuations played in the normal-state properties in iron-based superconductors.

Orthoferrite solid solution HoFe1−xCrxO3 (x=0, 0.2,…,1.0) was synthesized via solid state reaction methods. The crystal structure, magnetism and spin reorientation properties of this system were investigated by X-ray diffraction, neutron powder diffraction and magnetic measurements. For compositions of x≤0.6, the system exhibits similar magnetic properties to HoFeO3. With increasing Cr-doping, the system adopts a Γ4(GxAyFz) magnetic configuration with a decreased Neel temperature from 640 K to 360 K. A Γ42 spin reorientation of Fe(Cr)3+ was also observed in this system with an increase in transition temperature from 56 K to about 200 K due to competition between the Fe(Cr)–Fe(Cr) and Ho–Fe(Cr) interactions. For the x≥0.8, the system behaves more like HoCrO3 which adopts a Γ2(FxCyGz) configuration with no spin reorientation below the Neel temperature TN. Throughout the whole substitution range, we found that the saturated moment of Fe(Cr) was less than the ideal value for a free ion, which implies the existence of spin fluctuation in this system. A systematic magnetic structure variation with Cr-substitution is revealed by Rietveld refinement. A phase diagram combining the results of the magnetic measurements and neutron powder diffraction results was obtained.

In this paper we present a comprehensive study of magnetic dynamics in the rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron scattering, we observed that the spectrum of magnetic excitations consists of two collective modes well separated in energy: 3D gapped magnons with a bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM) ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe subsystem changes very little through the SR transition and could be well described in the frame of semiclassical linear spin-wave theory. On the other hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$ drastically changes the excitation spectrum of the Yb subsystem, inducing the transition between two regimes with magnon and spinon-like fluctuations. At $T < T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced ferromagnetic (FM) ground state, and the spectrum consists of a sharp single-magnon mode, a two-magnon bound state, and a two-magnon continuum, whereas at $T > T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum dominates the spectrum. In this work we show that a weak quasi-1D coupling within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies, creates unusual quantum spin dynamics on the low energy scales. The results of our work may stimulate further experimental search for similar compounds with several magnetic subsystems and energy scales, where low-energy fluctuations and underlying physics could be "hidden" by a dominating interaction., Main text (12 pages, 7 figures) & Supplementary Information (5 pages, 8 figures); v.2 with the correct references

A Python software package for data reduction and visualization of continuous angle multiple energy analysis (CAMEA) type detector backend MultiFLEXX is presented. The software concept focuses on unambiguous, automated aggregation of experimental data and preservation of raw data structure in graphical representation, enabling on-the-fly analysis of experimental data from MultiFLEXX with minimal amount of user input, reducing confusion and human error in studies involving multiple parameters. The software also provides a set of interfaces for versatile tweaking of graphing parameters, facilitating generation of production-quality graphs for use in publications. The software enhances the role of MultiFLEXX as a swift mapping option available at the cold-neutron triple-axis spectrometer FLEXX. Keywords: Inelastic neutron scattering, Three-axis spectroscopy, Visualization tool

Graphical abstract: Oxygen atmosphere played an important role in inhibiting electrons hopping between Fe{sup 2+} and Fe{sup 3+} and reducing both of magnetic and dielectric losses of Ba{sub 3}Co{sub 2}Fe{sub 24}O{sub 41}, which is favorable in view of antenna substrate applications. - Highlights: • Co{sub 2}Z-type hexaferrites were sintered in different atmospheres. • The losses of the sample sintered in O{sub 2} were reduced effectively without additive. • A 3RC equivalent circuit model was put forward in the impedance analysis. - Abstract: Co{sub 2}Z-type hexaferrites with stoichiometric composition of Ba{sub 3}Co{sub 2}Fe{sub 24}O{sub 41} were fabricated by using a conventional solid–state reaction method. The influence of sintering atmosphere, namely air, O{sub 2} and N{sub 2}, on magnetic and electrical properties of the hexaferrites was systematically investigated. This work reveals that O{sub 2} played an important role in inhibiting the formation of Fe{sup 2+} and thus reducing magnetic and dielectric losses. The Co{sub 2}Z specimen sintered at 1300 °C in O{sub 2} atmosphere showed promising magneto-dielectric properties. Specifically, the magnetic and dielectric losses were less than 0.05 and 0.005 in the frequency range of 30–300 MHz, respectively. Impedance spectra were performed to reveal the underlying mechanisms for the different electrical propertiesmore » of the Co{sub 2}Z ceramics sintered in different atmospheres.« less

Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane "Equation missing" single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.

We report a systematic crystal growth and characterization of Ca$_{1-y}$La$_{y}$Fe$_{1-x}$Ni$_{x}$As$_{2}$, the newly discovered 112-type iron-based superconductor. After substituting Fe by a small amount of Ni, bulk superconductivity is successfully obtained in high quality single crystals sized up to 6 mm. Resistivity measurements indicate common features for transport properties in this 112-type iron pnictide, suggest strong scattering from chemical dopants. Together with the superconducting transition temperature $T_c$, and the Neel temperature $T_N$ determined by the elastic neutron scattering, we sketch a three-dimensional phase diagram in the combination of both Ni and La dopings., Comment: 8 pages, 8 figures

The first experimental characterization of a multiple energy analysis wide angle backend for a cold triple-axis spectrometer is reported. The multi-analyzer module MultiFLEXX employs 155 detection channels which simultaneously probe an extensive range in wavevector and energy transfer. Successful mapping of magnetic excitations in MnF2 and Ho demonstrate order of magnitude gains in data collection efficiency using this novel type backend. MultiFLEXX is competitive to standard triple-axis spectroscopy in terms of energy resolution and signal-to-noise ratio. A minority of the detector channels is affected by spurious signals inherent to this multiplexing concept. The characteristic signature of these spurious signals easily allows for their discrimination. The instrument concept focuses on detection efficiency in the horizontal scattering plane which makes it an ideal technique for fast mapping and parametric studies including extreme sample environment.

Highly ordered arrays of Co nanowires with different diameters have been fabricated by electrodeposition in nanoporous anodic aluminum oxide membranes. The prepared Co nanowires have hexagonal close-packed structure and are [002] preferred oriented crystalline with the c-axis parallel to the long axis of the nanowires. The high anisotropy of Co nanowires offers the possibility to control their magnetic properties and microwave properties, which are experimentally characterized by a vibrating sample magnetometer and frequency-swept ferromagnetic resonant flip-chip based techniques, respectively. The nanowire arrays exhibit strong magnetic anisotropies. As for the ferromagnetic resonance frequency, it increases with increasing value of the applied field, and zero-field ferromagnetic resonance frequencies up to 28–34 GHz. Also, the magnetic anisotropies and high ferromagnetic resonance frequency of Co nanowire arrays, are shown to be determined by the combined effect of shape anisotropy, crystalline anisotropy, and dipolar interactions between the nanowires.

M-type barium ferrite (BaM) film is a promising candidate for magnetic recording media and the next-generation high-frequency microwave material. BaM films were fabricated using a chemical solution spin-coating deposition method on (001) plane sapphire. The effect of ethylene glycol (EG) on precursor solution and annealing temperature on microstructure and magnetic properties were investigated. X-ray diffraction showed that strong out-of-plane (001) texture in BaM films can be achieved with adequate EG addition. Films fabricated without EG addition or annealed at lower temperatures showed partial hematite secondary phase. The epitaxial formation of hematite secondary phase on sapphire substrate was confirmed by the X-ray diffraction φ-scan technique. Magnetic measurements showed out-of-plane anisotropy in BaM films. The crystallographic texture and magnetic anisotropy of the films are related to the film microstructure.

Quasi-epitaxial barium hexaferrite thin films (BaM) with crystallographic c -axis parallel to film normal were prepared through a topotactic reactive diffusion process using two-step solution deposition on c -plane sapphire. The two-step spin coating process involves preparing an epitaxial hematite film, coating the film with barium precursor solution and thermal annealing. The crystal orientation and magnetic anisotropy of BaM thin films were investigated by X-ray diffraction analysis, SEM observation and magnetic measurements. Hysteresis loops showed good magnetic anisotropy and high remanence ratio (RR) Mr/Ms = 0.97. The films fabricated by two-step spin coating process displayed wider rocking curve width but better magnetic anisotropy than one-step spin coating. The possible mechanism of this discrepancy is discussed in this paper.

High–energy-storage density capacitors with thin films of 0.5Bi(Ni1/2Ti1/2)O3–0.5PbTiO3 (BNT–PT) were fabricated by chemical solution deposition technique on Pt/Ti/SiO2/Si substrates. The dense thin films with pure-phase perovskite structure could be obtained by annealing at 750°C. High capacitance density (~1925 nF/cm2 at 1 kHz) and extremely high-energy density (~45.1 J/cm3) under an electric field of 2250 kV/cm were achieved at room temperature. The energy-storage density and efficiency varied little in a wide temperature range from −190°C to 250°C. The high–energy-storage density and good temperature stability make BNT–PT films promising candidates for high power electric applications.

Epitaxial spinel cobalt ferrite thin films have been prepared by spin-coating method. -axis in-plane and -axis out-of-plane oriented films are prepared on a-plane and c-plane sapphire substrates, respectively. The key process is to fabricate a layer of α-Fe2O3 precursor film first. The final cobalt ferrite film is formed by coating the precursor film with Cobalt nitrate solution and subsequently activating the topotactic reactive diffusion by thermal annealing. In-plane magnetic anisotropy has been confirmed in the in-plane oriented film. This process enables the fabrication of epitaxial cobalt ferrite film with desired orientations without utilizing expensive physical vapor deposition systems.

A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states, most of which are characterized by exotic spin excitations with fractional quantum numbers (termed `spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO4. The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO4 that has a perfect spin-1/2 triangular lattice as in the original proposal of quantum spin liquids., Comment: 23 pages, 10 figures

In-plane c-axis oriented M-type barium hexaferrite (BaM) films were grown on a-plane sapphire (1120) substrates by radio frequency magnetron sputtering with ex-situ annealing. Oxygen in sputtering atmosphere played a critical role in the c-axis orientation growth of BaM films. With incorporation of 10% O2 during sputtering, in-plane c-axis highly oriented BaM films with high hysteresis loop squareness (Mr/Ms∼0.94) along in-plane easy axis and low Mr/Ms∼0.08 along in-plane hard axis was achieved. In contrast, films sputtered without O2 incorporation showed poor magnetic anisotropy and low hysteresis loop squareness along both directions. The oxygen induced formation of oriented hematite intermediate phase was found, which was supposed to be responsible for the in-plane orientation growth of BaM grains.

Structures and microwave dielectric properties of Ba2−2xSr2xSmSbO6 (x = 0–1) ceramics were studied in this article. The phase transitions were analyzed using X-ray powder diffraction, transmission electron microscopy, and Raman spectroscopy. All structures are proved to be ordered double perovskites, and a series of composition-induced phase transitions are confirmed in the Ba2−2xSr2xSmSbO6 system via the sequence: Fm-3m → I2/m → P21/n. Dielectric properties of the ceramics were measured at microwave frequencies (9–11 GHz). With the increase of x, the dielectric constant (er) varies gradually from 20.7 to 16.0, the quality factor (Q × f) decreases first from 86 500 GHz to a minimum value at x = 0.4 and then increases slightly to 93 000 GHz, and the temperature coefficient of resonant frequency (τf) decreases from +25 to −50 ppm/°C. The relationships between composition, structure, and microwave dielectric properties in Ba2−2xSr2xSmSbO6 were discussed from the phase transition consideration.

Perovskite ceramics of Ba[Ti1−x(Co0.5W0.5)x]O3 (x = 0.40–0.90) were synthesized by a conventional solid-state reaction method. Crystal phase, microstructure, and microwave dielectric property of the ceramics were investigated. A phase transition from simple cubic (x = 0.40–0.55) to hexagonal and then to cubic double pervoskite (x = 0.75–0.90) was detected. A single hexagonal perovskite structure (P63/mmc) was obtained at 1425°C and stabilized at room temperature only at x = 0.61. For ceramics x = 0.55–0.60 and x = 0.62–0.75, they are mixtures that consisted of cubic and hexagonal perovskites. With increasing x, Qxf increased from 2800 to 20 700 GHz, whereas dielectric constant (er) decreased from 65 to 18, and temperature coefficient of resonant frequency (τƒ) decreased from 44 to −17 ppm/°C. Hexagonal perovskite (er = 18, Qxf = 7700 GHz, τƒ = −14 ppm/°C) had decreased er, τƒ and increased Qxf values, compared with the cubic perovskite phases. The phase formation and the relationship between structures and microwave dielectric properties were discussed.

Structures and microwave dielectric properties of (Ba, Sr)2 LnSbO6 (Ln = La, Pr, Nd, Sm, Gd, Dy) ceramics were studied. Ba2 LnSbO6 undergoes a cubic-rhombohedral structural transition, and Sr2 LnSbO6 adopts the monoclinic P21/n structure. The microwave dielectric properties, especially the ϵr and τf values, change with composition obviously. The Ba2 LnSbO6 ceramics have ϵr in the range of 19.2 ∼ 20.7, Q×f = 6 700 ∼ 86 500 GHz, and τf = −37 ∼ 38 ppm/°C; the Sr2 LnSbO6 ceramics have ϵr in the range of 15.9 ∼ 16.8, Q×f = 5 700 ∼ 93 000 GHz, and τf = −46 ∼ −55 ppm/°C.

Ba2Ca1−xSrxWO6 (x=0–1) microwave dielectric ceramics have been prepared using the two-step solid-state reaction method. The substitution of Sr2+ for Sr2+ can effectively lower the densification temperature of the ceramics from 1500° to 1200°C. Single-phase ceramics with double perovskite structures were obtained, and structural transitions occurred in Ba2Ca1−xSrxWO6 via the phase sequence: cubic→rhombohedral→monoclinic with an increase of x. The microwave dielectric properties of the Ba2Ca1−xSrxWO6 ceramics can be effectively tuned by tailoring x. The dielectric constant (ɛr) remains almost unvaried, the quality factor (Q × f) decreases monotonically, and the temperature coefficient of resonant frequency (τf) changes in sign with the increase of x. A good combination of the microwave dielectric properties was obtained for Ba2Ca0.975Sr0.025WO6 sintered at 1250°C: ɛr=23.9, Q × f=80 200 GHz, and τf=+18 ppm/°C.

The Zn 2 SiO 4 based ceramics with the compositions of Zn 2 SiO 4 , Zn 1.8 SiO 3.8 , Zn 2.2 SiO 4.2 , and Zn 2 SiO 4 -11wt%TiO 2 were prepared by a sol-gel process. The formation process of the crystal phases in the precursor powders was characterized by XRD techniques. The results showed that the willemite phase was formed from the reaction between ZnO and SiO 2 , and the presence of TiO 2 precursor could reduce the temperature of phase formation. The excellent microwave properties of ϵ r ∼ 6.55, Q× f ∼ 198,400 GHz, τ f ∼ -41.6ppm/°C and ϵ r ∼ 9.13, Q× f∼ 150,800GHz, τ f ∼ 1.0ppm/°C were achieved for the pure and TiO 2 -doped Zn 2 SiO 4 ceramics, respectively. The phase characterization revealed that the pure phases formed from sol-gel process might be responsible for the excellent properties.

Microwave dielectric composite ceramics with compositions of (1-x)Ba(Zn 1/3 Nb 2/3 )O 3 -BaWO 4 (x = 0.1∼ 0.4) were prepared. The increasing BaWO 4 content can effectively lower the sintering temperature of the composite ceramics from 1400°C to 1250°C. XRD indicated that Ba(Zn 1/3 Nb 2/3 )O 3 and BaWO 4 coexisted. SEM observation revealed that the grain size increased with the sintering temperature rising up. The microwave dielectric properties of the sintered ceramics varied with the BaWO 4 content. The samples with x = 0.4 sintered at 1225°C exhibited the best microwave dielectric property: a near zero temperature coefficient of resonant frequency of τ f = −2.1 ppm/°C, ϵ r = 24.0, Q×f = 65300 GHz was obtained.

(1−x)Zn2SiO4–xTiO2 (x=0, 5, 8, 11, and 15 wt%) ceramics have been successfully prepared through a sol–gel process. The thermal gravimetry-differential thermal analysis and X-ray diffraction analysis showed that the addition of TiO2 can lower the nucleation temperature of Zn2SiO4 and enhance the sinterability of the powders synthesized. Dense ceramics with a pure phase could be achieved at a lower temperature. The pure Zn2SiO4 ceramics sintered at 1325°C exhibited microwave dielectric properties: a dielectric constant (ɛr) of 6.6, a quality factor Q×f of 198 400 GHz, and a temperature coefficient of resonant frequency (τf) of −41.6 ppm/°C. The τf value can be adjusted to near zero by adding an appropriate amount of TiO2. The Zn2SiO4 ceramics containing 11 wt% of TiO2 sintered at 1200°C showed excellent microwave dielectric properties: an ɛr value of 9.1, a Q×f value of 150 800 GHz, and a τf value of −1.0 ppm/°C. Our results show that the sol–gel process can achieve pure phase as well as ultrafine powders, which is beneficial to optimize the performance of Zn2SiO4 ceramics. Zn2SiO4 ceramics containing 11 wt% of TiO2 are a promising candidate for microwave and millimeter-wave applications.

New low-temperature co-fired microwave dielectric composites with compositions of (1–x)Ba3(VO4)2–xBaWO4 (x=0–1) were prepared by firing mixtures of Ba3(VO4)2 and BaWO4. Thermal mechanical analysis indicated the ceramic composites had relatively low densification temperatures and could be sintered in the temperature range 925°∼950°C. X-ray diffraction patterns showed that Ba3(VO4)2 and BaWO4 coexisted and no secondary phase was detected in the sintered bodies, implying good chemical compatibility between the two phases. Near-zero temperature coefficients of the resonant frequency (τf) could be achieved by changing the relative content of the two phases, due to their positive and negative τf values, respectively. With increasing BaWO4 (x from 0.50 to 0.65), the τf value of the composites decreased from +10.9 to −1.9 ppm/°C, and the quality factor (Q×f value) increased from 66 700 to 79 100 GHz. The best microwave dielectric properties were obtained for x=0.65 samples sintered at 925°C with a dielectric constant of 11.1, a Q×f value of 79 100 GHz, and a τf value of −1.9 ppm/°C. Chemical compatibility experiments showed the ceramics are compatible with silver during cofiring process.

Polarized Raman spectroscopy was applied to characterize domains reorientation in the multilayer piezoelectric ceramic actuators during the poling process and in the near crack tip zone. It is found that the relative intensity of the vibration modes of E(2TO) and E(3TO + 2LO) + B1 (IE(2TO)/Isilent) is affected by electric field and mechanical stress. During the poling process, IE(2TO)/Isilent increases when the applied DC electric field exceeds the coercive electric field. In the near crack tip zone, the IE(2TO)/Isilent mapping indicates that near the crack tip zone indicated that the domains were reoriented and the mapping of Raman shift of E(3TO + 2LO) + B1 mode reveals the stress release caused by crack development. This study also confirms that the relative intensity of IE(2TO)/Isilent can be used for characterization of domains reorientation in PZT-based ceramics under electric and mechanical stresses.

Novel Ba3(VO4)2–Mg2SiO4 microwave dielectric composite ceramics with compositions of (1−x)Ba3(VO4)2–xMg2SiO4 (x=0.50–0.65) have been prepared by firing mixtures of Ba3(VO4)2 and Mg2SiO4. X-ray diffraction and scanning electron microscopy analysis revealed that they coexist without forming secondary phases. Because their resonant frequency temperature coefficients (τf) have opposite signs, the τf of the composite ceramic can be adjusted passing through zero by changing the relative content of the two phases. Dielectric property measurements were performed using the resonator method at around 11 GHz. Ceramics with Mg2SiO4 55 wt% sintered at 1175°C exhibited microwave dielectric properties of dielectric constant ɛr=9.03, Q×f=52 500 GHz, and τf=0.6 ppm/°C at 11.3 GHz. In this composite ceramic, we observed that the formation of the MgSiO3 secondary phase was suppressed.

In-plane c-axis oriented Ba-hexaferrite (BaM) thin films were prepared on a-plane (112¯0) sapphire (Al2O3) substrates by DC magnetron sputtering followed by ex-situ annealing. The DC magnetron sputtering was demonstrated to have obvious advantages over the traditionally used RF magnetron sputtering in sputtering rate and operation simplicity. The sputtering power had a remarkable influence on the Ba/Fe ratio, the hematite secondary phase, and the grain morphology of the as-prepared BaM films. Under 80 W of sputtering power, in-plane c-axis highly oriented BaM films were obtained. These films had strong magnetic anisotropy with high hysteresis loop squareness (Mr/Ms of 0.96) along the in-plane easy axis and low Mr/Ms of 0.03 along the in-plane hard axis. X-ray diffraction patterns and pole figures revealed that the oriented BaM films grew via an epitaxy-like growth process with the crystallographic relationship BaM (101¯0)//α-Fe2O3 (112¯0)//Al2O3 (112¯0).

In this letter, we present a dual band microwave absorption phenomenon in 60 nm diameter Co ferromagnetic nanowire arrays annealed at 500 °C in nitrogen atmosphere. The frequency dependent microwave response of the wires is obtained for applied field parallel to the nanowire axis, below 4 kOe and above saturation magnetization up to 8 kOe, in steps of 1 kOe. For applied magnetic fields above saturation magnetization, two sets of absorption peaks are observed, while below saturation, only one single peak is obtained. Combining structural characterization and ferromagnetic resonance measurements, it is proven that the two ferromagnetic resonance peaks are associated with a coexistence of two kinds of nanopillars with different crystal structure in annealed nanowires, one for nanopillars with hexagonal close packed structure at higher ferromagnetic resonance frequency, and the other with face center cubic structure.

A novel and facile method was successfully developed to fabricate nanotube (NT) and nanowire (NW) arrays through an anodic aluminum oxide (AAO) template-based Ti(OC4H9)4 hydrolysis process. The wall thickness of the TiO2 NTs can be controlled by adjusting the concentration of Ti(OC4H9)4 solutions. A continuous series from thin-walled NTs to completely filled NWs were synthesized using Ti(OC4H9)4 solutions of different concentrations from 0.1 to 2.0 M. The TiO2 NTs and NWs in the AAO membrane transformed from amorphous to nanocrystalline anatase completely after annealing at 650 °C, which is higher than the phase transformation temperature of TiO2 powder. Optical spectra of the synthesized TiO2 NT and NW arrays/AAO composite nanostructures were characterized and showed a blue shift of the optical absorption edge with the decrease of NT wall thickness, due to the nanoscale effect and shape dependent transformation.