Antiferromagnetic Order in the Rare Earth Halide Perovskites CsEuBr$_3$ and CsEuCl$_3$

Bulk CsEuBr$_3$ and CsEuCl$_3$ are experimentally shown to be magnetic semiconductors that order antiferromagnetically at Neel temperatures of 2.0 K and 1.0 K respectively. Given that nanoparticles and thin films of CsEuCl$_3$ have been reported to order ferromagnetically at a similar temperature, our observation of antiferromagnetic ordering in CsEuBr$_3$ and CsEuCl$_3$ expands the possible applications of halide perovskites to now include spintronic devices where both ferromagnetic and antiferromagnetic devices can be fabricated from a single material. The conclusion that CsEuCl$_3$ can be used as a switchable magnetic material is also supported by our density-functional theory calculations.