  A ferromagnetic needle is predicted to precess about the magnetic field axis
at a Larmor frequency $\Omega$ under conditions where its intrinsic spin
dominates over its rotational angular momentum, $N\hbar \gg I\Omega$ ($I$ is
the moment of inertia of the needle about the precession axis and $N$ is the
number of polarized spins in the needle). In this regime the needle behaves as
a gyroscope with spin $N\hbar$ maintained along the easy axis of the needle by
the crystalline and shape anisotropy. A precessing ferromagnetic needle is a
correlated system of $N$ spins which can be used to measure magnetic fields for
long times. In principle, by taking advantage of rapid averaging of quantum
uncertainty, the sensitivity of a precessing needle magnetometer can far
surpass that of magnetometers based on spin precession of atoms in the gas
phase. Under conditions where noise from coupling to the environment is
subdominant, the scaling with measurement time $t$ of the quantum- and
detection-limited magnetometric sensitivity is $t^{-3/2}$. The phenomenon of
ferromagnetic needle precession may be of particular interest for precision
measurements testing fundamental physics.
