Exchange Splitting and Charge Carrier Spin Polarization in EuO

Abstract

High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.EuO is a semiconductor with a band gap of about 1.2 eV and is one of the very rare ferromagnetic oxides [1,2]. Its Curie temperature (T c ) is around 69 K and the crystal structure is rocksalt (fcc) with a lattice constant of 5.144Å. Eu-rich EuO becomes metallic below T c and the metal-insulator transition (MIT) is spectacular: the resistivity drops by as much as 8 orders of magnitude [3,4]. Moreover, an applied magnetic field shifts the MIT temperature considerably, resulting in a colossal magnetoresistance (CMR) with changes in resistivity of up to 6 orders of magnitude [4]. This CMR behavior in EuO is in fact more extreme than in the now much investigated La 1−x Sr x MnO 3 materials [5,6]. Much what is known about the basic electronic structure of EuO dates back to about 30 years ago and is based mainly on optical measurements [7][8][9] and band structure calculations [10]. With the properties being so spectacular, it is surprising that very little has been done so far to determine the electronic structure of EuO using more modern and direct methods like electron spectroscopies.Here we introduce spin-resolved x-ray absorption spectroscopy, a new type of spin-resolved electron spectroscopy technique to study directly the conduction band of EuO where most of the effects related to the MIT and CMR are expected to show up. Spin-resolved measurements of the conduction band could previously only be obtained by spin-polarized inverse photoemission spectroscopy. Spin-resolved x-ray absorption spectroscopy is an alternative technique which is especially well suited to study ferromagnetic oxides, a currently interesting broad class of materials. Using this technique we observed large changes in the conduction band across T c and we were able to show experimentally that these are caused by an exchange splitting of the conduction band below T c . Moreover, we found that this splitting is as large as 0.6 eV and show that the states close to the bottom of the conduction band are almost fully spin-polarized, which is very interesting for basic research in the field of spintronics.The experiments were performed using the helical undulator [11] based beamline ID12B [12] at the European Synchrotron Radiation Facility (ESRF) in Grenoble. Photoemission and Auger spectra were recorded using a 140 mm mean radius hemispherical analyzer coupled to a mini-Mott 25 kV spin polarimeter [13]. The spin detector had an efficiency (Sherman function) of 17%, and t...

Electronic transport through EuO spin-filter tunnel junctions

“…Among these, EuO presents the largest conduction-band exchange splitting, ∼0.54 eV, below the material's Curie temperature of 69 K. [14][15][16] In EuO, the divalent Eu ions possess a large local moment originating from the half-filled 4f band (μ Eu = 7μ B ). An energy gap of 1.0 eV separates the half-filled majority Eu-4f band from the Eu-5d conduction band.…”

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.…”

Microscopic theory for a ferromagnetic nanowire/superconductor heterostructure: Transport, fluctuations, and topological superconductivity

“…[1][2][3][4] A particularly attractive candidate ferromagnetic semiconductor is europium oxide (EuO), which is known to possess nearly spin-polarized bands. 85,86 EuO becomes ferromagnetic below 70K under ambient pressure and the Curie temperature is known to increase with pressure reaching 200K under 1.5 × 10 5 atmospheres. 87 Since its integration with with Si and GaN, 86 EuO has garnered much attention for its potential use in spintronic applications.…”