Fabrication and Characteristics of High- Superconducting/Magnetoresistive Mixed Sensors

Abstract

Bicrystal grain boundary La0.7Ca0.3MnO3 (LCMO) tunneling magnetoresistance (TMR) mixed sensors with high- $T_{c}$ superconducting YBa2Cu3Oy (YBCO) transformers have been designed and fabricated. The mixed sensors show an increase in low-field magnetoresistance (MR) ratios at low temperatures and an increase in the slope of low-field MR curve at temperatures below superconducting critical temperature $T_{c}$ , indicating that the flux related to the field indeed can be captured by the superconducting YBCO loop with a constriction associated to the LCMO TMR junctions. Furthermore, the spin polarization parameter $P$ of 0.40 for LCMO at 10 K is obtained, and the temperature-dependent $P$ can be fitted by the curves of $P(T)\propto(T_{C}-T)^{0.97\pm 0.01}$ with $T_{C}=296 \text{K}$ . The temperature dependence of surface magnetization $M_{S}(T)$ deduced from $P(T)$ agrees with the theoretical models for $M_{S}(T)$ and leads to the conclusion that the charge carriers at the grain–interface boundary govern the tunneling conductivity for our bicrystal grain boundary junctions. In addition, the gain of mixed sensor as a function of temperature gets weaker as $T$ approaching $T_{c}$ . According to the voltage noise level of 6.8 $\mu\text{V/Hz}^{1/2}$ and the low-field slope of resistance variation $dR/dH\approx 781 \text{k}\Omega/\text{T}$ for the mixed sensor at 77 K, we obtain the field sensitivity ${S_{H}}^{1/2}$ of ∼0.58 $\mu{\text{T}/\text{Hz}}^{1/2}$ .