G. H. Wu

Half-metallic properties for the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys: A first-principles study

The electronic structure and magnetism of the Mn2FeZ (Z=Al, Ga, Si, Ge, Sb) Heusler alloys have been studied by density functional calculations. Two half-metallic ferromagnets, namely, Mn2FeAl and Mn2FeSb, are predicted. It is found that a small expansion of the crystal lattice can restore the half-metallicity in Mn2FeSi. The calculated total magnetic moments Mtot are 1μB/f.u. for Mn2FeAl and Mn2FeGa, 2μB/f.u. for Mn2FeSi and Mn2FeGe, and 3μB/f.u. for Mn2FeSb, which agree with the Slater–Pauling curve quite well. The moments of Mn (A) and Mn (B) are large and antiparallel to each other, which is indicative of ferrimagnetism in Mn2FeZ alloys. Fe shows only a small moment and its moment is parallel to that of Mn (B). By investigating the effect of lattice distortion on the half-metallicity and magnetic moments of Mn2FeZ, it is found that the half-metallic properties of Mn2FeSb are insensitive to the lattice distortion and a 100% spin polarization can be obtained within the wide range of 5.4–6.05 A. This is ...

Giant exchange bias based on magnetic transition in gamma-Fe2MnGa melt-spun ribbons

Giant exchange bias with a shift up to 3.86 kOe has been observed in gamma-Fe2MnGa alloy in which a ferromagnetic to antiferromagnetic transition takes place at 250 K. The transition can be suppressed to lower temperatures by higher magnetic fields at a shift rate of 10.3 K/kOe. Exchange bias and enhanced coercivity occur simultaneously, revealing an exchange coupling between the coexisting antiferromagnetic and ferromagnetic phases. Meanwhile, the internal exchange coupling inside the antiferromagnetic clusters dynamically ensures their unidirectional anisotropy during their size changing following the external magnetic field

Effect of Mo content on the structure stability of R3(Fe, Co, Mo)29

Formation of Ce3Fe29−x−yCoxMoy(x=0,3,6,10) compounds has been investigated by means of x-ray diffraction and magnetic measurements. It is found that the required Mo content to stabilize the 3:29 compounds decreases monotonically as Co content increases, totally different from the discovery in the Gd3(Fe1−xCox)29−yCry compounds. Meanwhile, we adopted a lattice inversion method in acquiring the interatomic potentials to rare-earth-transition metal intermetallic compounds, and investigated the substitution behaviors of Mo, Cr, V, and Mn in Fe-based and Co-based 3:29 compounds, respectively, by combining these potentials with computer simulation. The calculated results show that the required Mo content in R3Co29−xMox compounds is lower than that in R3Fe29−xMox compounds, whereas the Cr content in R3Co29−xCrx compounds higher than that in R3Fe29−xCrx compounds, coinciding well with the previous discovery and the experimental results in this work.

A high-performance bulk mode single crystal silicon microresonator based on a cavity-SOI wafer

A cavity-silicon-on-insulator (SOI)-based single crystal silicon (SCS) micromechanical resonator has been demonstrated in this paper. The most distinguishing feature of this method is that it solves the restrictions of being released from the sacrificial layer. The resonator structures can be fabricated and released in one step using dry anisotropic etching. The differential drive, single-ended sense configuration is implemented to measure the electrical characterization of the fabricated resonator. The fabricated square plate resonator has been excited in the Lame´ mode at a resonant frequency of 4.126 MHz and exhibits a quality factor (Q) as high as 5.49× 106 at a pressure of 0.05 mbar. This result corresponds to a frequency–Q product of 2.27× 1013, which is the highest value demonstrated to date for silicon-based resonators as far as we know. The dependence of Q and resonant frequency on the operating pressure is measured and characterized. The temperature stability of the device is also demonstrated, with the temperature coefficient of resonant frequency less than −20.8 ppm °C−1 in the temperature range from −10 to 60 °C. The high performance of the resonator not only benefits from the superior performance of SCS as a mechanical material, but also the merit of the cavity-SOI structure.

Wafer-Level Vacuum Packaging for MEMS Resonators Using Glass Frit Bonding

A wafer-level vacuum package with silicon bumps and electrical feedthroughs on the cap wafer is developed for a microelectromechanical systems (MEMS) resonator device. A MEMS resonator wafer and a cap wafer are bonded together in a vacuum chamber using glass frit bonding. The cap wafer not only provides a vacuum chamber to protect the movable resonator structure and improve the resonant performance but also realizes the redistribution of the electrical feedthroughs by using the silicon bumps. The silicon bumps provide vertical interconnections between the cap wafer and the resonator wafer, which realizes the bonding pads “transferring” from the resonator wafer to the cap wafer. A gold-aluminum eutectic is used to ensure electrical contacts between the cap wafer and the device wafer. The device fabrication and glass frit hermetic bonding process as well as the packaged MEMS resonator characterization are presented in this paper. Experimental results show that the wafer-level vacuum-packaged MEMS resonator results in over 100× higher quality factor (Q) than the resonator vibrating in atmosphere pressure, which confirms the transmission performance improvement due to vacuum packaging. Vacuum inside the package is measured indirectly by measuring the Q of the MEMS resonator inside the package. The experimental results indicate that vacuum about 1 mbar can be sealed in this approach.

Resonant Magnetic Field Sensor With Capacitive Driving and Electromagnetic Induction Sensing

This letter presents a novel magnetic field sensor, which consists of a square extensional mode resonator with a planar induction coil. This sensor exploits the principle of electromagnetic induction to detect external magnetic field through the electromotive force in the induction coil, which is placed on top of the resonant plate. The proposed sensor employs capacitive driving and electromagnetic induction sensing approach to detect the external magnetic field. The capacitive driving method reduces the power dissipation, and the electromagnetic induction sensing approach makes it easy to measure the output signal with high precision. The operation principle and fabrication process as well as the characterization of the magnetic field sensor are presented. Experimental result shows that the device offers a sensitivity of 3 μV/mT at its resonant frequency f0 = 4.33 MHz in air. In this letter, the vibrating characterization of the sensor is largely reduced due to air damping. Therefore, vacuum packaging is needed to enhance the performance of the magnetic field sensor.

Phononic crystal strip based anchors for reducing anchor loss of micromechanical resonators

Micromechanical resonators must be clamped to the substrate via anchors to support the suspended microstructure. However, these anchors will introduce anchor loss, and decrease quality factors (Qs) of the micromechanical resonators. To reduce the anchor loss, one dimensional phononic crystal based strips are employed as anchors of the microresonators in this paper. The dispersion relations and eigenmodes of the phononic crystal strips are presented. Flexural mode ring resonator and Lame mode square plate resonator are designed to verify the effect of phononic crystal strips. The calculated results and finite-element simulations indicate that the leaky energy could be effectively reduced by the phononic crystal strip anchor design. Resonators with different anchor designs are also fabricated and characterized. The measured Qs of the microresonators show that the phononic crystal strips could reduce the energy dissipated through anchors, and with increasing the number of phononic strip periods, Qs of the re...

Uncooled Thermoelectric Infrared Sensor With Advanced Micromachining

A simple mass producible uncooled thermoelectric infrared microsensor has been designed and fabricated. To improve the cost-efficiency, an advanced micromachining process, which combines wet anisotropic pre-etching and XeF2 dry isotropic post-etching, is adopted for the sensor fabrication. The wet anisotropic pre-etching removes bulk silicon from back-side and forms a thin silicon membrane for device fabrication, the XeF2 dry isotropic post-etching undercuts silicon membrane and releases the microstructure. Experimental results show that the sensor with advanced micromachining exhibits a two times higher responsivity and detectivity than the sensor with only XeF2 front-side etching. In air at room temperature, the sensor with advanced micromachining has a responsivity of 71.57 V W-1, noise equivalent power of 0.64 nW Hz-1/2, detectivity of 6.21×107 cm Hz1/2 W-1 and a time constant of 13.2 ms. The effect of back-side etch window size on sensor performance is also characterized by finite-element method simulation.

Large magnetization change and magnetoresistance associated with martensitic transformation in Mn2Ni1.36Sn0.32Co0.32 alloy

In this paper we report on the realization of magnetic field-induced martensitic phase transformation in Mn-rich Heusler alloy Mn2Ni1.36Sn0.32Co0.32. The saturation magnetization of the austenite reached 111 emu/g at 70 kOe, which decreased rapidly to 8 emu/g upon transforming to martensite. This is attributed to the crystallographic distortion from cubic structure to tetragonal structure with c/a >u20091, turning the Mn moments at B sites and D sites from parallel alignment to antiparallel alignment. A large magnetoresistance of 40% was observed through the field-induced transformation. The increase of conduction electrons accompanying this field-induced martensitic transformation is estimated to be 67%. These intriguing properties render the alloy a good candidate for applications in smart devices.

Synthesis, thermal expansion, and magnetic properties of Gd3(Fe,Co,Cr)29 compounds

We have obtained single-phase compounds Gd3Fe29−x−yCoxCry with the Nd3 (Fe,Ti)29-type. Cr is a stabilizing element and also increases the solid-solution limit of Co in the Gd3Fe29−x−yCoxCry compounds. Substitution of Co by Fe results in a significant change of the magnetocrystalline anisotropy and changes the easy magnetization direction from basal-plane to easy-axis when x>10 and y=4. The intensity of the magnetic interactions in Co-rich Gd3Fe29−x−yCoxCry compounds has been calculated from the high-field magnetization measurements. The Co substitution suppresses the Invar anomaly found in the Gd3(FeCr)29 compounds.