Fabrication and Characteristics of High-${T}_{{c}}$ Superconducting/Magnetoresistive Mixed Sensors

Bicrystal grain boundary La
_0.7
Ca
_0.3
MnO
₃
(LCMO) tunneling magnetoresistance (TMR) mixed sensors with high-T
_c
superconducting YBa
₂
Cu
₃
O
_y
(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) ∝ (T
_C
- T)
^0.97±0.01
with T
_C
= 296 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 μV/Hz
^1/2
and the low-field slope of resistance variation dR/dH ≈ 781 kΩ/T for the mixed sensor at 77 K, we obtain the field sensitivity S
_H
^1/2
of ~0.58 μT/Hz
^1/2
.