Induced Ferromagnetism in Epitaxial Uranium Dioxide Thin Films

Abstract

Actinide materials have various applications that range from nuclear energy to quantum computing. Most current efforts have focused on bulk actinide materials. Tuning functional properties by using strain engineering in epitaxial thin films is largely lacking. Using uranium dioxide (UO 2 ) as a model system, in this work, the authors explore strain engineering in actinide epitaxial thin films and investigate the origin of induced ferromagnetism in an antiferromagnet UO 2 . It is found that UO 2+x thin films are hypostoichiometric (x<0) with in-plane tensile strain, while they are hyperstoichiometric (x>0) with in-plane compressive strain. Different from strain engineering in non-actinide oxide thin films, the epitaxial strain in UO 2 is accommodated by point defects such as vacancies and interstitials due to the low formation energy. Both epitaxial strain and strain relaxation induced point defects such as oxygen/uranium vacancies and oxygen/uranium interstitials can distort magnetic structure and result in magnetic moments. This work reveals the correlation among strain, point defects and ferromagnetism in strain engineered UO 2+x thin films and the results offer new opportunities to understand the influence of coupled order parameters on the emergent properties of many other actinide thin films.

Strain‐driven Switching Between Antiferromagnetic States in Frustrated Antiferromagnet UO2 Probed by Exchange Bias Effect

“…Another factor that could potentially explain the difference lies in the presence of distinct variations in the stoichiometry of the UO 2 layer and/or the morphology of the UO 2 /Fe 3 O 4 interface. The former was shown [ 26 ] to be responsible for ferromagnetism in the UO 2+x thin films by incorporating point defects. Hence, the stoichiometry of the UO 2 layers was closely monitored after depositing each layer.…”

“…(As a point of comparison: in the case of hyperstoichiometric UO 2+x thin film samples investigated in Ref. [26], the associated photoemission study documented in Ref. [35] indicated that the satellite peaks are scarcely visible).…”

“…The former is split between the dipole‐dipole (DD) and quadrupole‐quadrupole (QQ) terms acting within the ground‐state (GS) Γ 5 triplets of U 4+ ion as defined in Ref. [26]. The tetragonal crystal field (CF) splits the GS triplet inducing the SIA.…”

“…In UO 2 thin films strain can induce ferromagnetism by incorporating point defects and generating hypo‐ ( x < 0) or hyper‐ ( x > 0) stoichiometric UO 2+ x . [ 26 ] In this study, we apply epitaxial strain through substrate selection and switch between AFM structures in stoichiometric UO 2 films. Determining the exact type of AFM ordering in UO 2 films is challenging, requiring a large amount of material [ 22 ] and a sophisticated neutron experiment.…”

Analysis of Coronado State Historic Site artifacts using X‐rays

“…XRD, SEM–EDS, and FT‐IR were effectively utilized together to study uncommon copper alloy artifacts dating back to the eighth‐century BC and their complex degradation phenomena to develop strategies for long‐term conservation 7 . Finally, Raman is widely used to study paintings, especially pigments, dyes, and paints and deep UV Raman is especially valuable to characterize geologic, 8 oxide, 9 and paleontological specimens 10 …”