  Electrons in materials with linear dispersion behave as massless Weyl- or
Dirac-quasiparticles, and continue to intrigue physicists due to their close
resemblance to elusive ultra-relativistic particles as well as their potential
for future electronics. Yet the experimental signatures of Weyl-fermions are
often subtle and indirect, in particular if they coexist with conventional,
massive quasiparticles. Here we report a large anomaly in the magnetic torque
of the Weyl semi-metal NbAs upon entering the "quantum limit" state in high
magnetic fields, where topological corrections to the energy spectrum become
dominant. The quantum limit torque displays a striking change in sign,
signaling a reversal of the magnetic anisotropy that can be directly attributed
to the topological properties of the Weyl semi-metal. Our results establish
that anomalous quantum limit torque measurements provide a simple experimental
method to identify Weyl- and Dirac- semi-metals.
