  Detection of vector magnetic fields at nanoscale dimensions is critical in
applications ranging from basic material science, to medical diagnostic.
Meanwhile, an all-electric operation is of great significance for achieving a
simple and compact sensing system. Here, we propose and experimentally
demonstrate a simple approach to sensing a vector magnetic field at nanoscale
dimensions, by monitoring a probabilistic nanomagnet's transition probability
from a metastable state, excited by a driving current due to SOT, to a settled
state. We achieve sensitivities for Hx, Hy, and Hz of 1.02%/Oe, 1.09%/Oe and
3.43%/Oe, respectively, with a 200 x 200 nm^2 nanomagnet. The minimum
detectable field is dependent on the driving pulse events N, and is expected to
be as low as 1 uT if N = 3 x 10^6.
