The resolution of giant magneto-impedance (GMI)based magnetic sensor is limited by its equivalent magnetic noise level. It presents a white noise and a low-frequency 1/f noise behavior. Previous works have showed that the white noise is mainly determined by the electronic conditioning noise, nevertheless, the 1/f noise's origin is not totally clear yet. To optimize the 1/f noise, in this paper, first, we develop an optimized negative feedback GMI sensor. It contains of a low-distortion oscillator, a high-performance amplitude detector, and a GMI sensing element with adjustable static working point. And then, the sensitivity and the 1/f noise level of the GMI sensor are studied when considering the electronic conditioning parameters and the static working point. The experiment results show that the adjustment of the static working point has a significant effect on the 1/f noise at low frequencies. The optimal 1/f noise is obtained by setting the static working point at the position where the GMI element possesses with the maximum intrinsic sensitivity. Finally, we achieve the GMI sensor noise performance of 20 pT/√Hz at white noise region and 100 pT/√Hz at 1 Hz.
The thermopower and thermal conductivity of superconducting perovskite $MgCNi_3$ ($T_c \approx$ 8 K) have been studied. The thermopower is negative from room temperature to 10 K. Combining with the negative Hall coefficient reported previously, the negative thermopower definetly indicates that the carrier in $MgCNi_3$ is electron-type. The nonlinear temperature dependence of thermopower below 150 K is explained by the electron-phonon interaction renormalization effects. The thermal conductivity is of the order for intermetallics, larger than that of borocarbides and smaller than $MgB_2$. In the normal state, the electronic contribution to the total thermal conductivity is slightly larger than the lattice contribution. The transverse magnetoresistance of $MgCNi_3$ is also measured. It is found that the classical Kohler's rule is valid above 50 K. An electronic crossover occures at $T^* \sim 50 K$, resulting in the abnormal behavior of resistivity, thermopower, and magnetoresistance below 50 K.
A seismic data acquisition system based on wireless network transmission is designed to improve the low-frequency response and low sensitivity of the existing acquisition system. The system comprises of a piezoelectric transducer, a high-resolution data acquisition system, and a wireless communication module. A seismic piezoelectric transducer based on a piezoelectric simply supported beam using PMN-PT is proposed. High sensitivity is obtained by using a new piezoelectric material PMN-PT, and a simply supported beam matching with the PMN-PT wafer is designed, which can provide a good low-frequency response. The data acquisition system includes an electronic circuit for charge conversion, filtering, and amplification, an FPGA, and a 24-bit analog-to-digital converter (ADC). The wireless communication was based on the ZigBee modules and the WiFi modules. The experimental results show that the application of the piezoelectric simply supported beam based on PMN-PT can effectively improve the sensitivity of the piezoelectric accelerometer by more than 190%, compared with the traditional PZT material. At low frequencies, the fidelity of the PMN-PT piezoelectric simply supported beam is better than that of a traditional central compressed model, which is an effective expansion of the bandwidth to the low-frequency region. The charge conversion, filtering, amplification, and digitization of the output signal of the piezoelectric transducer are processed and, finally, are wirelessly transmitted to the monitoring centre, achieving the design of a seismic data acquisition system based on wireless transmission.
A silicon-based stress-coupled optical racetrack resonator with a crossbeam mass is proposed to detect acceleration for seismic prospecting. Acceleration applied on the crossbeam mass can result in optical phase changes in one racetrack cycle, which leads to a resonant wavelength shift. By systematically optimizing the resonator structure and mechanical characteristics, a very large wavelength shift of 52 pm under 1 g acceleration is demonstrated by numerical simulation. The maximum frequency of input signal can be up to 200 Hz. This silicon-based compact and high-performance racetrack resonator can have great potential for seismic prospecting.
The effects of various doping materials (amorphous phases: TiO 2 , SiO 2 , MgO, and C and crystalline phases: HfO 2 and ZrO 2 ) on the microstructure and magnetic properties of FePt thin films have been investigated. It is found that FePt films with amorphous doping materials especially MgO and C have better (001) texture and grain isolation than doping with crystalline materials. Nevertheless, doping with crystalline materials, FePt films were preferred to form columnar structures with a larger aspect ratio compared with semi-spherical grain shape for amorphous doping materials. Moreover, all the FePt-X films exhibit better perpendicular anisotropy except for FePt-ZrO 2 films due to some FePt (200) textured films epitaxial grown directly on tetragonal (002) textured ZrO 2 . For crystalline materials doping, the (001) texture and perpendicular anisotropy could be improved by tuning crystalline oxide materials with appropriate lattice structure, which may offer a method for application in FePt in heat-assisted magnetic recording media.
The normal state resistivtity, upper critical field $H_{c2}$ and Hall coefficient $R_H$ in superconducting perovskite $MgCNi_3$ ($T_c \approx 8 K$) have been studied. Above 70 K, $\rho(T)$ fits well curve predicted by Bloch-Gruneisen theory consistently with electron-phonon scattering. $H_{c2}(0)$ was estimated to be about 15.0 Tesla within the weak-coupling BCS theory, and the superconducting coherence length $\xi(0)$ is approximately 47 A. $R_H$ of $MgCNi_3$ is negative for the whole temperature range which definitely indicates that the carrier in $MgCNi_3$ is electron-type. $R_H$ is temperature independent between $T_c$ and $\sim$ 140 K. Above $\sim$ 140 K, the magnitude of $R_H$ decreases as temperature rises. At T = 100 K, the carrier density is $1.0 \times 10^{22}/cm^3$, which is comparable with that in perovskite $(Ba,K)BiO_3$, and less than that of the metallic binary $MgB_2$.
With the application of magnetic thin films becoming more and more widespread, people pay more and more attention to the performance characterization. In order to obtain a magnetic film with a specific performance, it is very important to judge the quality of the magnetic film and measure the magnetic properties of the film. However, with the increase of the film preparation process, the thickness of the prepared film is getting thinner and the magnetic moment signal contained therein is also decreased. This brings a certain degree of difficulty to the traditional measurement methods. For example, the VSM system that obtains the hysteresis loop by measuring the magnetic moment signal has become somewhat inadequate for the measurement of ultra-thin films. In order to solve this issue, a new method based on anomalous Hall effect is introduced in this paper. The test system of this system adopts the four-probe measuring method, a constant current is applied across the surface of the film sample, and the abnormal Hall voltage is measured at the other two ends. The R-H curve of the sample can be obtained through calculation. As compared to VSM measurement, this method is simpler and stable, more accurate, which can greatly reduce the anomalous Hall-effect device R-H characteristic measurement cost.
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