  Understanding the magnetic response of the normal state of the cuprates is
considered a key piece in solving the puzzle of their high-temperature
superconductivity. The essential physics of these materials is believed to be
captured by the Fermi-Hubbard model, a minimal model that has been realized
with cold atoms in optical lattices. Here we report on site-resolved
measurements of the Fermi-Hubbard model in a spin-imbalanced atomic gas,
allowing us to explore the response of the system to large effective magnetic
fields. We observe short-range canted antiferromagnetism at half-filling with
stronger spin correlations in the direction orthogonal to the magnetization, in
contrast with the spin-balanced case where identical correlations are measured
for any projection of the pseudospin. The rotational anisotropy of the spin
correlators is found to increase with polarization and with distance between
the spins. Away from half-filling, the polarization of the gas exhibits
non-monotonic behavior with doping for strong interactions, resembling the
behavior of the magnetic susceptibility in the cuprates. We compare our
measurements to predictions from Determinantal Quantum Monte Carlo (DQMC) and
Numerical Linked Cluster Expansion (NLCE) algorithms and find good agreement.
Calculations on the doped system are near the limits of these techniques,
illustrating the value of cold atom quantum simulations for studying
strongly-correlated materials.
