  Hall effects have been employed as sensitive detectors of magnetic fields and
magnetizations. In spintronics, exotic phenomena often emerge from a
non-equilibrium spin polarization or magnetization, that is very difficult to
measure directly. The challenge is due to the tiny total moment, which is out
of reach of superconducting quantum interference devices and vibrating sample
magnetometers or spectroscopic methods such as X-ray magnetic circular
dichroism. The Kerr effect is sufficiently sensitive only in direct gap
semiconductors, in which the Kerr angle can be resonantly enhanced. Here we
demonstrate that even one excess spin in a million can be detected by a Hall
effect at room temperature. The novel Hall effect is not governed by the spin
Hall conductivity but by its energy derivative thereby related to the spin
Nernst effect.
