Epitaxy, stoichiometry, and magnetic properties of Gd-doped EuO films on YSZ (001)

Abstract

We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eudistillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttriastabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu 3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.

Heteroepitaxy and ferromagnetism of EuO/MgO (001): A route towards combined spin- and symmetry-filter tunneling

“…A few works already report heteroepitaxy and bulklike magnetization of EuO on MgO, but with EuO film thicknesses above several tens of nanometers. 18,21 A recent study by Swartz et al 20 concludes that ultrathin EuO heteroepitaxy on MgO (001) cannot be obtained with film thicknesses below 2 nm. That study also reports the formation of polycrystalline EuO in the initial monolayer regime without any additional oxygen supply.…”

“…The major experimental difficulty relies in synthesizing the metastable oxide EuO in its stoichiometric ferromagnetic phase since off-stoichiometry precludes an epitaxial growth and also reduces ferromagnetic exchange in the ultrathin-film limit, which is, however, essential for efficient spin-filter tunneling. [13][14][15][16][17] In recent years, several growth studies on EuO thin films have been presented, in particular on stoichiometric EuO thin films grown coherently on perfectly lattice-matched yttriastabilized zirconia (YSZ) 14,18 and on different cubic oxides with only a few percent tensile lattice strain. 19,20 The system EuO/MgO (001), however, provides a large compressive lattice mismatch of m = (a MgO − a EuO )/a EuO = −18%.…”

Antiferromagnetic coupling in EuO1−x

“…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.…”

Lutetium-doped EuO films grown by molecular-beam epitaxy

“…For example, in films doped with gadolinium, the maximum reported T C varies between 120 K (Ref. 15) and 170 K. 14 Films doped with lanthanum have a maximum reported T C between 118 K (Ref. 1) and 200 K. 10 Likewise, for films doped with iron, the reported T C varies between 88 K and 200 K. 6,7 For Gd-doped films grown in an adsorption-controlled regime, however, the Curie temperatures are consistent and similar, 15,16 conceivably due to a minimized amount of oxygen vacancies realized in this particular growth regime.…”

“…The interaction between the Eu f-electrons and the dopant electrons enhances the ferromagnetic exchange energy 4,5 and results in an increased T C . To date, this has been accomplished through the use of trivalent cations including iron, [6][7][8] lanthanum, 1,9,10 gadolinium, [11][12][13][14][15][16][17] and holmium. 9 Alternatively, the T C can be increased by deliberately making oxygen-deficient EuO such that the resulting oxygen vacancies donate an electron.…”