  The tuning of sensitivity and dynamic range in linear magnetic sensors is
required in various applications. We demonstrate the control and design of the
sensitivity in magnetic tunnel junction (MTJ) sensors with a vortex-type
sensing layer. In this work, we develop sensor MTJs with NiFe sensing layers
having a vortex magnetic configuration. We demonstrate that by varying the
pinned layer size, the sensitivity to magnetic field is tuned linearly. We
obtain a high magnetoresistance ratio of 140 %, and we demonstrate a
controllable sensitivity from 0.85 to 4.43 %/Oe, while keeping the vortex layer
fixed in size. We compare our experimental results with micromagnetic
simulations. We find that the linear displacement of vortex core by an applied
field makes the design of vortex sensors simple. The control of the pinned
layer geometry is an effective method to increase the sensitivity, without
affecting the vortex state of the sensing layer. Furthermore, we propose that
the location of the pinned layer can be used to realize more sensing
functionalities from a single sensor.
