  Material structures containing tetrahedral FeAs bonds, depending on their
density and geometrical distribution, can host several competing quantum ground
states ranging from superconductivity to ferromagnetism. Here we examine
structures of quasi two-dimensional (2D) layers of tetrahedral Fe-As bonds
embedded with a regular interval in a semiconductor InAs matrix, which
resembles the crystal structure of Fe-based superconductors. Contrary to the
case of Fe-based pnictides, these FeAs/InAs superlattices (SLs) exhibit
ferromagnetism, whose Curie temperature (Tc) increases rapidly with decreasing
the InAs interval thickness t_InAs (Tc ~ t_InAs^-3), and an extremely large
magnetoresistance up to 500% that is tunable by a gate voltage. Our first
principles calculations reveal the important role of disordered positions of Fe
atoms in the establishment of ferromagnetism in these quasi-2D FeAs-based SLs.
These unique features mark the FeAs/InAs SLs as promising structures for
spintronic applications.
