  We report on the deterministic preparation of antiferromagnetic Heisenberg
spin chains consisting of up to four fermionic atoms in a one-dimensional trap.
These chains are stabilized by strong repulsive interactions between the two
spin components without the need for an external periodic potential. We
independently characterize the spin configuration of the chains by measuring
the spin orientation of the outermost particle in the trap and by projecting
the spatial wave function of one spin component on single-particle trap levels.
Our results are in good agreement with a spin-chain model for fermionized
particles and with numerically exact diagonalizations of the full few-fermion
system.
