Ce-doped EuO: Magnetic properties and the indirect band gap

Abstract

We have prepared and investigated thin films of EuO doped with the rare-earth element cerium. X-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy were used to determine the quality of these films prepared by pulsed laser deposition. Ce doping leads to an enhanced Curie temperature near 150 K, close to that seen for oxygen-deficient EuO1−x. However, the magnetization of Ce-doped EuO exhibits differences from that observed for Gd-doped and oxygen-deficient samples. The high-resolution angular-resolved photoemission from Ce-doped EuO reveals filling of conduction-band states near the X point. This indicates that the band gap in EuO is indirect, and that at 2% doping Ce-doped EuO1−x is at least semimetallic.

Antiferromagnetic coupling in EuO1−x

“…We have successfully developed a method to prepare EuO films based on pulsed laser deposition as referenced in [23,24]. XRD patterns show the stacking planes of the films are mostly aligned with the (200) lattice planes, same as reported from prior works.…”

“…More details about the sample preparation can be found elsewhere. 23,24 To prevent the degradation of the EuO 1-x films when exposed to air, some films were protected by a Pt capping layer deposited in situ. The magnetic properties of both oxygen-deficient and stoichiometric EuO were examined using a physical properties measurement system (PPMS) from Quantum Design.…”

“…35 According to Arnold and Kroha, 29 the existence of preformed local moments at the impurity levels inside the semiconducting gap is essential to explain the distinct double-dome shape of the magnetization M(T), although it has become apparent from numerous experiments that oxygen deficiency alone, without doping localized moments, can be responsible for the unique double-dome feature. 15,16,[23][24][25] It should be pointed out that the double-dome feature is observed in single-phase clean samples and is an intrinsic property of electron doped EuO. A recent report found that the "dome" above 69 K is only observed for measurements performed at high magnetic fields and attributed it to the presence of paramagnetic Eu metal clusters.…”

“…Charge carrier doping by either oxygen vacancies 15,[23][24][25] or substituting Eu 2+ with trivalent rare earth elements such as Gd 3+ , 15-17, 19, 21-23 Ce 3+ , 24 and La 3+ 18,20 has been used to increase the Tc of EuO, often resulting in a unique double-dome feature in the M(T) curves. Upon electron doping, the extra electrons are bound in defect levels situated in the semiconductor band gap, and the transition to a ferromagnetic metal occurs when the majority states of the spin-split conduction band shift downward to overlap with the defect levels.…”

“…9 Recent studies of spin-resolved x-ray absorption spectroscopy have shown a spin-split conduction band of about 0.6 eV in its ferromagnetic state, creating a nearly 100% spin polarized electrons close to the conduction band edge. 10 Although the stoichiometric EuO has a Curie temperature (Tc) of 69 K, Tc can be enhanced significantly by pressure, [11][12][13] interfacial strain 14 or electron doping via rare-earth atoms [15][16][17][18][19][20][21][22][23][24] or oxygen vacancies. 15,16,[23][24][25] The integrations of EuO with Si, 18,[23][24][25][26][27] GaAs, 28 and GaN 18 have been successfully demonstrated.…”