Induced Ferromagnetism in Epitaxial Uranium Dioxide Thin Films

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nuclear Engineering

Background:

• Actinide materials are crucial for nuclear energy and quantum computing.
• Research has primarily focused on bulk materials, with limited exploration of strain engineering in thin films.
• Tuning functional properties of actinide thin films via strain engineering remains an underdeveloped area.

Purpose of the Study:

• To explore strain engineering in actinide epitaxial thin films using uranium dioxide (UO2) as a model system.
• To investigate the origin of induced ferromagnetism in the antiferromagnetic UO2.
• To understand the correlation between strain, point defects, and ferromagnetism in UO2 thin films.

Main Methods:

• Fabrication and characterization of uranium dioxide (UO2) epitaxial thin films.
• Application of in-plane tensile and compressive strain.
• Analysis of stoichiometry (UO2+x), point defects (vacancies, interstitials), and magnetic properties.

Main Results:

• UO2+x thin films exhibited hypostoichiometry (x<0) under tensile strain and hyperstoichiometry (x>0) under compressive strain.
• Epitaxial strain in UO2 was accommodated by point defects due to low formation energy, unlike in non-actinide oxides.
• Both strain and strain relaxation-induced point defects (oxygen/uranium vacancies/interstitials) distorted the magnetic structure, inducing magnetic moments.

Conclusions:

• Strain engineering in UO2 thin films directly influences stoichiometry and defect formation.
• Point defects play a critical role in mediating the relationship between epitaxial strain and induced ferromagnetism.
• This study provides insights into coupled order parameters in actinide thin films, enabling new opportunities for property tuning.