Possible origin of ferromagnetism in undoped monoclinic HfO2 film

Abstract

Using density functional theory calculations, we have studied the electronic structures and magnetic properties of oxygen-deficient HfO2 (111) and (1¯11) surface. We find that the complex defects of oxygen vacancies (CDOV) may be responsible for the unexpected ferromagnetism in monoclinic HfO2 film without doping. The results show that each kind of oxygen vacancy (VO) alone can not introduce the magnetism on the surface, so neither (111) nor (1¯11) surface is magnetic with only one VO. When the CDOV which are composed of two oxygen vacancies were introduced on the HfO2 (111) or (1¯11) surface, the system will be magnetic. However, the CDOV have to meet two prerequisites for the existence of a high-spin defect ground state, i.e., one non-ground state of ferromagnetism exists for one VO in the CDOV, and the distance between the two oxygen vacancies in the CDOV should be less than or equal the critical distance. The spins induced by the special CDOV will form a ferromagnetic ground state, and they can produce a magnetic moment of 2.0μB. In addition, the magnetic moment mainly results from the d orbitals of low-charge-state Hafnium ions adjacent to the CDOV. The experimentally observed d0-ferromagnetism behavior in HfO2 system is in good agreement with our calculated results.