Pulsed laser deposition of high-quality thin films of the insulating ferromagnet EuS

Abstract

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...

Creation of equal-spin triplet superconductivity at the Al/EuS interface

“…At room temperature, EuS is semiconducting with an indirect energy gap, the conduction band minimum at 300 K is 1.64 eV [44]. At cryogenic temperatures, EuS is insulating with resistivity of around ρ = 10 4 Ω cm for high-quality single crystals [45]. Disorder at the atomic level and unintentional doping can lower this resistivity by several orders of magnitude and at the same time increase the Curie temperature (as observed by SQUID magnetometry measurements in Supplementary Fig.…”

Charged Particle Oscillations in Transient Plasmas Generated by Nanosecond Laser Ablation on Mg Target

“…Pulsed-laser deposition has gained a lot of attention in the past 10 years as one of the best techniques to produce complex films with relatively complicated stoichiometry [1][2][3]. The technique has a proven flexibility in terms of the deposition geometry [4] and target or background gas nature [5,6]. Great advancements have been made toward understanding the fundamental aspects of laser ablation for a better control of the thin-film deposition technique.…”

Luminescence properties of magnetic polarons in EuTe: Theoretical description and experiments in magnetic fields up to 28 T

“…However, technological interest waned when it was discovered that EuTe is an antiferromagnet with a very low Néel temperature (T N = 9.6 K), which is impractical for device applications. Nevertheless, EuTe and other Eu chalcogenides remain ideal magnetic systems that can be used to test the principles of operation of spin-related devices [8], and to investigate many-body physical phenomena, e.g., spin waves [9] and magnetic polarons [2,4,[10][11][12][13][14].…”