High-quality thin films of the ferromagnetic insulator europium(II) sulfide (EuS) were fabricated by pulsed laser deposition on Al 2 O 3 (0001) and Si (100) substrates. A single orientation was obtained with the [100] planes parallel to the substrates, with atomic-scale smoothness indicates a near-ideal surface topography. The films exhibit uniform ferromagnetism below 15.9 K, with a substantial component of the magnetization perpendicular to the plane of the films. Optimization of the growth condition also yielded truly insulating films with immeasurably large resistance. This combination of magnetic and electric properties open the gate for novel devices that require a true ferromagnetic insulator.Over more than 50 years a wealth of new effects and properties have been discovered in binary lanthanide compounds. In particular, compounds of europium with elements of the sixth group (O,S,Se,Te) exhibit a rocksalt (NaCl)-type crystal structure with ordered magnetic states at low temperatures. As the lattice parameter increases from EuO to EuTe, a ferromagnetic ordered state of moments localized on Eu ions appear in EuO (T C ≈ 69 K) and in EuS (T C ≈ 16.7 K), 1,2 while EuSe and EuTe show collinear antiferromagnetic ordering with T N ≈ 4.2 K, T N ≈ 9.8 K respectively. 3,4 In these chalcogenide compounds, the S ground state of Eu 2+ ions and their simple face centered cubic (FCC) magnetic lattice facilitate testings of the Heisenberg model of ferromagnetism and theories of critical phenomena. 5-9 A variety of applications were proposed or implemented utilizing these magnetic semiconductors. 10-12 A class of magnetoelectric applications, such as π-Josephson junctions for quantum qubits 13-15 and recently proposed topological magnetoelectric effect associated with the surface state of topological insulators, 16-20 require fabrication of highquality insulating ferromagnet thin films with robust magnetic properties.Here we focus on EuS, which is a semiconductor with an indirect energy gap between the 4f 7 Eu states and the conduction band minimum at 300 K is 1.65 eV. [21][22][23] The lattice parameter of bulk crystals of EuS is a 0 = 5.967Å, with a ferromagnetic Curie temperature T C ≈ 16.7 K. When strained, the lattice constant change is accompanied by a change in Curie temperature, e.g. thin films of EuS grown on KCl show an increase in T C by as much as 2 K, due to compression induced by differential expana) Electronic mail: qiyang@stanford.edu sions of the film and substrate. 24 At the same time, very thin films will exhibit slightly lower T C due to dimensionality reduction. 25 However, although good electric insulation (ρ ∼ 10 4 Ω·cm) was obtained in high-quality single crystals, difficulties in material fabrication lead to disorder and unintentional doping, which may drastically reduce the resistivity to as low as ρ ∼ 10 −2 Ω·cm. 2,26 Such reduction in resistivity was found to be accompanied by increased Curie temperatures due to interactions between charge carriers and the Eu 2+ ions. 1,26-28 Particularly for t...