Field Detection in Spin Valve Sensors Using CoFeB/Ru Synthetic-Antiferromagnetic Multilayers as Magnetic Flux Concentrators

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co
$_{93}$
Zr
$_{3}$
Nb
$_{4}$
(CZN) or (Co
$_{70}$
Fe
$_{30}$
)
$_{80}$
B
$_{20}$
-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (
$\chi =196$
for SAF vs.
$\chi =753$
for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from
${\sim}61$
nT/Hz
$^{0.5}$
for single spin valve sensors down to
${\sim}1.8$
nT/Hz
$^{0.5}$
when CZN concentrators with a steep-profile are used.