[en] An effective method for realizing ultra-low-frequency single-mode band gap in pentamode metamaterials is proposed based on constituent materials. Results show that the decreasing ratio E/ρ (stiffness/mass density) of constituent material can significantly lower the frequency range of single-mode band gap. By merely replacing the constituent material from Al to rubber, the center frequency f _c of single-mode band gap can be reduced nearly 600 times (from 3621 Hz to 6.5 Hz), while the normalized bandwidth Δf/f _c and the ratio of bulk modulus B to shear modulus G of pentamode structure keep substantially stable. The nonlinear fitting demonstrates that the relation between f _c and E/ρ satisfies the logarithmic function. The two-component pentamode structure is designed to further explore the ultra-low-frequency single-mode band gap. The effects of thick-end diameter D of double-cone, diameter D ₀ and material type of additional sphere, on single-mode band gap of two-component system are analyzed. This work is attractive for several ∼Hz acoustic/elastic wave regulations using pentamode metamaterials. (paper)

[en] Highlights: • A polymer-film microfluidic impedance cytometer integrated with inertial focusing and facing electrodes for cell sensing. • The proposed device with a thickness of 0.45 mm is made of ITO-coated polymer films and double-sided adhesive tapes. • The whole fabrication process is shortened from the traditional 3–4 days to less than 5 min by using UV laser cutting. • The device was successfully applied for standard particle, MCF-7, and PANC-1 cell sensing. We report here an easy-fabricated and disposable polymer-film microfluidic impedance cytometer (PMIC) integrated with inertial focusing and parallel facing electrodes for cell sensing. The cells are first focused in an asymmetric serpentine channel, and then their impedance signals are measured when passing through the electrode region. The proposed PMIC device is the first impedance cytometer that is fabricated into a flexible sheet (with a thickness of 0.45 mm) by using the materials of commonly-available ITO-coated polymer films and double-sided adhesive tapes, the whole fabrication process is shortened from traditional 3–4 days to less than 5 min by using UV laser cutting. To verify the feasibility of our device for cell sensing, we explore the focusing behaviors of three differently sized particles and two types of tumor cells, and analyze their impedance signals. The results show that our device is capable of obtaining impedance information on numbers, diameters, and longitudinal positions of cells. We envision that our PMIC device is promising in label-free cell sensing owning to the advantages of low cost, small footprint, and simple fabrication.

[en] Here, the magnetic interactions between rare-earth and Fe ions in hexagonal rare-earth ferrites (h–RFeO_3), may amplify the weak ferromagnetic moment on Fe, making these materials more appealing as multiferroics. To elucidate the interaction strength between the rare-earth and Fe ions as well as the magnetic moment of the rare-earth ions, element-specific magnetic characterization is needed. Using x-ray magnetic circular dichroism, we have studied the ferrimagnetism in h–YbFeO_3 by measuring the magnetization of Fe and Yb separately. The results directly show antialignment of magnetization of Yb and Fe ions in h–YbFeO_3 at low temperature, with an exchange field on Yb of about 17 kOe. The magnetic moment of Yb is about 1.6μ_B at low temperature, significantly reduced compared with the 4.5μ_B moment of a free Yb"3"+. In addition, the saturation magnetization of Fe in h–YbFeO_3 has a sizable enhancement compared with that in h–LuFeO_3. These findings directly demonstrate that ferrimagnetic order exists in h–YbFeO_3; they also account for the enhancement of magnetization and the reduction of coercivity in h–YbFeO_3 compared with those in h–LuFeO_3 at low temperature, suggesting an important role for the rare-earth ions in tuning the multiferroic properties of h–RFeO_3.

[en] Core–shell pentamode metamaterial is designed and its phonon band structure is calculated by finite element method. Influences of factors, including constituent material and dimensions of the core-double-cone, on the mechanical properties and single-mode band gap (i.e. single-mode frequency regime, in which only compression wave exists and shear waves are completely suppressed) of the core–shell pentamode metamaterials are systematically investigated based on the static continuum mechanics calculations. It’s found that compared with the traditional non-core–shell pentamode metamaterial, the core–shell one has broader mechanical response and higher sensitivity (ratio of bulk modulus B to shear modulus G: B/G ∼ 10⁴). Especially when the thin-end diameter $d_c$ of core-double-cone is smaller, the B/G ratio exceeds 6 × 10⁴, which is nearly 54 times than that of non-core–shell structure. The smaller Young’s modulus E _c of core material and the smaller dimension of core-double-cone will be conducive to obtain single-mode band gap with lower frequency and wider bandwidth. (paper)

[en] We have carried out the growth of h-RFeO₃ (0 0 1) (R  =  Lu, Yb) thin films on Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) substrates, and studied the effect of the h-RFeO₃ (0 0 1)/Fe₃O₄ (1 1 1) interfaces on the epitaxy and magnetism. The observed epitaxial relations between h-RFeO₃ and Fe₃O₄ indicate an unusual matching of Fe sub-lattices rather than a matching of O sub-lattices. The out-of-plane direction was found to be the easy magnetic axis for h-YbFeO₃ (0 0 1) but the hard axis for Fe₃O₄ (1 1 1) in the h-YbFeO₃ (0 0 1)/Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) films, suggesting a perpendicular magnetic alignment at the h-YbFeO₃ (0 0 1)/Fe₃O₄ (1 1 1) interface. These results indicate that Fe₃O₄ (1 1 1)/Al₂O₃ (0 0 1) could be a promising substrate for epitaxial growth of h-RFeO₃ films of well-defined interface and for exploiting their spintronic properties. (paper)

[en] From the Cr 2p_3/2 x-ray magnetic circular dichroism signal, there is clear evidence of interface polarization with overlayers of both Pd and Pt on chromia (Cr₂O₃). The residual boundary polarization of chomia is stronger for a Pt overlayer than in the case of a Pd overlayer. The reduction of chromia boundary magnetization with a paramagnetic metal overlayer, compared to the free surface, is interpreted as a response to the induced spin polarization in Pt and Pd. Magnetization induced in a Pt overlayer, via proximity to the chromia boundary magnetization, is evident in the polar magneto-optical Kerr measurements. These results are essential to explainations why Pt and Pd are excellent spacer layers for voltage controlled exchange bias, in the [Pd/Co]_n/Pd/Cr₂O₃ and [Pt/Co]_n/Pt/Cr₂O₃ perpendicular magneto-electric exchange bias systems. The findings pave the way to realize ultra-fast reversal of induced magnetization in a free moment paramagnetic layer, with possible application in voltage-controlled magnetic random access memory. (letter)

[en] We studied the crystal structure and magnetic properties of the rare-earth-free intermetallic compound Fe₃Co₃Nb₂, which has recently been demonstrated to have potentially high magnetic anisotropy, using temperature-dependent neutron powder diffraction. The temperature dependence of the diffraction spectra reveals a magnetic transition between 300 and 400 K, in agreement with the magnetometry measurements. According to the structural refinement of the paramagnetic state and the substantial magnetic contribution to the diffuse scattering in the ferromagnetic state, the Fe/Co anti-site mixing is so strong that the site occupation for Fe and Co is almost random. The projection of the magnetic moments turned out to be non-zero along the c axis and in the a–b plane of Fe₃Co₃Nb₂, most likely because of the exchange interactions between the randomly orientated nanograins in the samples. These findings suggest that future studies on the magnetism of Fe₃Co₃Nb₂ need to take the Fe/Co anti-site mixing into account, and the exchange interactions need to be suppressed to obtain large remanence and coercivity. (paper)

[en] Despite low resistivity (∼1 mΩ cm), metallic electrical transport has not been commonly observed in inverse spinel NiCo₂O₄, except in certain epitaxial thin films. Previous studies have stressed the effect of valence mixing and the degree of spinel inversion on the electrical conduction of NiCo₂O₄ films. In this work, we studied the effect of nanostructural disorder by comparing the NiCo₂O₄ epitaxial films grown on MgAl₂O₄ (1 1 1) and on Al₂O₃ (0 0 1) substrates. Although the optimal growth conditions are similar for the NiCo₂O₄ (1 1 1)/MgAl₂O₄ (1 1 1) and the NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, they show metallic and semiconducting electrical transport, respectively. Post-growth annealing decreases the resistivity of NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films, but the annealed films are still semiconducting. While the semiconductivity and the large magnetoresistance in NiCo₂O₄ (1 1 1)/Al₂O₃ (0 0 1) films cannot be accounted for in terms of non-optimal valence mixing and spinel inversion, the presence of anti-phase boundaries between nano-sized crystallites, generated by the structural mismatch between NiCo₂O₄ and Al₂O₃, may explain all the experimental observations in this work. These results reveal nanostructural disorder as being another key factor for controlling the electrical transport of NiCo₂O₄, with potentially large magnetoresistance for spintronics applications. (paper)

[en] The structural transition at about 1000 °C, from the hexagonal to the orthorhombic phase of LuFeO_3, has been investigated in thin films of LuFeO_3. Separation of the two structural phases of LuFeO_3 occurs on a length scale of micrometer, as visualized in real space using X-ray photoemission electron microscopy. The results are consistent with X-ray diffraction and atomic force microscopy obtained from LuFeO_3 thin films undergoing the irreversible structural transition from the hexagonal to the orthorhombic phase of LuFeO_3, at elevated temperatures. The sharp phase boundaries between the structural phases are observed to align with the crystal planes of the hexagonal LuFeO_3 phase. The coexistence of different structural domains indicates that the irreversible structural transition, from the hexagonal to the orthorhombic phase in LuFeO_3, is a first order transition, for epitaxial hexagonal LuFeO_3 films grown on Al_2O_3.

[en] The electronic structure for the conduction bands of both hexagonal and orthorhombic LuFeO_3 thin films have been measured using x-ray absorption spectroscopy at oxygen K (O K) edge. Dramatic differences in both the spectral features and the linear dichroism are observed. These differences in the spectra can be explained using the differences in crystal field splitting of the metal (Fe and Lu) electronic states and the differences in O 2p-Fe 3d and O 2p-Lu 5d hybridizations. While the oxidation states have not changed, the spectra are sensitive to the changes in the local environments of the Fe"3"+ and Lu"3"+ sites in the hexagonal and orthorhombic structures. Using the crystal-field splitting and the hybridizations that are extracted from the measured electronic structures and the structural distortion information, we derived the occupancies of the spin minority states in Fe"3"+, which are non-zero and uneven. The single ion anisotropy on Fe"3"+ sites is found to originate from these uneven occupancies of the spin minority states via spin–orbit coupling in LuFeO_3. (paper)