Nano-Hall sensors with granular Co-C

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.