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Register a new userDual-band metacomposites containing hybrid Fe and Co-based ferromagnetic microwires
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We investigated the microwave properties of polymer based metacomposites containing hybridized parallel Fe- and Co-based microwire arrays. A dual-band left-handed feature was observed in the frequency bands of 1.5 to 5.5 GHz and 9 to 17 GHz, indicated by two transmission windows associated with ferromagnetic resonance of Fe-based microwires and long range dipolar resonance between the wire arrays. The plasma frequency after hybridization is significantly increased due to the enhanced effective diameter through the wire-wire interactions between the Fe- and Co- microwire couples. These results offer essential perspectives in designing the multi-band metamaterial for microwave applications such as sensors and cloaking devices.
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We have investigated the microwave properties of epoxy-based composites containing melt-extracted Co69.25Fe4. 25B13.5-xSi13Nbx (x=0, 1, 3) microwires of various length annealed using a so-called combined current-modulation annealing (CCMA) technique. The observation of a double-peak feature in the permittivity spectra is believed due to the coexistence of the amorphous phase and a small amount of nanocrystallites on the wires with a high Nb content. CCMA was found to be favorable for a better-defined circular anisotropy of microwires and had suppressed the highfrequency peak due to residual stress relief for the composite with 25 mm long wires. Neither the shift of resonance peak nor the characteristic double peak feature was detected for composites containing as-cast 15 or 35 mm long microwires.
We have employed Gd55Co30NixAl15-x (x = 10, 5 and 0) amorphous microwires as a model system to unravel the impact of multiple magnetic interactions on the magnetism and the magnetocaloric behavior in Gd-alloy microwire systems. Our study shows that in addition to the RKKY ferromagnetic (FM) interaction (Gd-Gd), antiferromagnetic (AFM) interactions (Gd-Co, Gd-Ni) coexist and contribute to the magnetic and magnetocaloric response of the system. The dilution effect of Al element on the FM Gd-Gd interaction is responsible for the decrease of the Curie temperature (TC), whereas the increase of the saturation magnetization (MS) is originated from the reduced AFM Gd-Ni interaction. A thorough analysis of critical exponents suggests that the presence of the AFM interactions hinders the system to exhibit a long-range FM order below the TC. Adjusting these interactions is shown to preserve the large refrigerant capacity (RC) while tuning the TC over a wide temperature range, which is desirable for active magnetic refrigeration.
A study of magnetic hysteresis and Giant magnetoimpedance (GMI) in amorphous glass covered Co-Si-B and Co-Mn-Si-B wires is presented. The wires, about 10 microns in diameter, were obtained by glass-coated melt spinning technique. Samples with positive magnetostriction (MS) have a rectangular bistable hysteresis loop. A smooth hysteresis loop is observed for wires with nearly zero MS. When MS is negative, almost no hysteresis is observed. The GMI was measured in the frequency range between 20 Hz and 30 MHz. The shapes of the impedance versus field curves are qualitatively similar to each other for both positive and zero MS samples. Impedance is maximum at zero field, and decreases sharply in the range 10-20 Oe. For the negative MS wires, when the driving current is small, the impedance is maximum at a finite external field. The position of the maximum approaches zero with increasing current. The contributions of the moment rotation and domain wall motion in the three cases are discussed.
(Ga,Fe)Sb is a promising ferromagnetic semiconductor for practical spintronic device applications because its Curie temperature ($T_{rm C}$) is above room temperature. However, the origin of ferromagnetism with high $T_{rm C}$ remains to be elucidated. Here, we use soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to investigate the valence-band (VB) structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb including the Fe-3$d$ impurity band (IB), to unveil the mechanism of ferromagnetism in (Ga,Fe)Sb. We find that the VB dispersion in (Ga$_{0.95}$,Fe$_{0.05}$)Sb observed by SX-ARPES is similar to that of GaSb, indicating that the doped Fe atoms hardly affect the band dispersion. The Fe-3$d$ resonant ARPES spectra demonstrate that the Fe-3$d$ IB crosses the Fermi level ($E_{rm F}$) and hybridizes with the VB of GaSb. These observations indicate that the VB structure of (Ga$_{0.95}$,Fe$_{0.05}$)Sb is consistent with that of the IB model which is based on double-exchange interaction between the localized 3$d$ electrons of the magnetic impurities. The results indicate that the ferromagnetism in (Ga,Fe)Sb is formed by the hybridization of the Fe-3$d$ IB with the ligand $p$ band of GaSb.
Fully understanding the properties of n-type ferromagnetic semiconductors (FMSs), complementary to the mainstream p-type ones, is a challenging goal in semiconductor spintronics because ferromagnetism in n-type FMSs is theoretically non-trivial. Soft-x-ray angle-resolved photoemission spectroscopy (SX-ARPES) is a powerful approach to examine the mechanism of carrier-induced ferromagnetism in FMSs. Here our SX-ARPES study on the prototypical n-type FMS (In,Fe)As reveals the entire band structure including the Fe-3d impurity bands (IBs) and the host InAs ones, and provides direct evidence for electron occupation of the InAs-derived conduction band (CB). A minority-spin Fe-3d IB is found to be located just below the conduction-band minimum (CBM). The IB is formed by the hybridization of the unoccupied Fe-3d states with the occupied CBM of InAs in a spin-dependent way, resulting in the large spin polarization of CB. The band structure with the IB is varied with band filling, which cannot be explained by the rigid-band picture, suggesting a unified picture for realization of carrier-induced ferromagnetism in FMS materials.
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