  We report the precise control of tunneling magnetoresistance (TMR) in devices
of self-assembled core/shell Fe$_3$O$_4$/Co$_{1-x}$Zn$_x$Fe$_2$O$_4$
nanoparticles ($0\leq x\leq 1$). Adjusting the magnetic anisotropy through the
content of Co$^{2+}$ in the shell, provides an accurate tool to control the
switching field between the bistable states of the TMR. In this way, different
combinations of soft/hard and hard/soft core/shell configurations can be
envisaged for optimizing devices with the required magnetotransport response.
