  Searches for the axion and axionlike particles may hold the key to unlocking
some of the deepest puzzles about our universe, such as dark matter and dark
energy. Here we use the recently demonstrated spin-based amplifier to constrain
such hypothetical particles within the well-motivated ``axion window'' (1
$\mu$eV-1 meV) through searching for an exotic spin-spin interaction between
polarized electron and neutron spins. The key ingredient is the use of
hyperpolarized long-lived $^{129}$Xe nuclear spins as an amplifier for the
pseudomagnetic field generated by the exotic interaction. Using such a spin
sensor, we obtain a direct upper bound on the product of coupling constants
$g_p^e g_p^n$. The spin-based amplifier technique can be extended to searches
for a wide variety of hypothetical particles beyond the Standard Model.
