  We analyzed the performance of Hall sensors with different Co-C ratios,
deposited directly in nano-structured form, using $Co_2(CO)_8$ gas molecules,
by focused electron or ion beam induced deposition. Due to the enhanced
inter-grain scattering in these granular wires, the Extraordinary Hall Effect
can be increased by two orders of magnitude with respect to pure Co, up to a
current sensitivity of $1 \Omega/T$. We show that the best magnetic field
resolution at room temperature is obtained for Co ratios between 60% and 70%
and is better than $1 \mu T/Hz^{1/2}$. For an active area of the sensor of $200
\times 200 nm^2$, the room temperature magnetic flux resolution is $\phi_{min}
= 2\times10^{-5}\phi_0$, in the thermal noise frequency range, i.e. above 100
kHz.
