Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

Abstract

Ferromagnetic insulating behaviour has recently been demonstrated in cobalt-substituted SrTiO 3 at room temperature [1]. Experimentally, it was found that a well-defined hysteresis loop only occurs at high Co concentrations of 30-40%. X-ray photoelectron spectroscopy also indicated that Co substitutes for Ti with Co being in high-spin +2 oxidation state. In this work, we employ density-functional theory to explain the experimentally observed properties of cobalt-substituted SrTiO 3 . We examine in detail the role of oxygen vacancy (OV) defects and their formation of defect complexes with the Co ions as the origin of the ferromagnetic insulating behaviour. Our first-principles thermodynamic calculations indicate that OV defects are much more likely to occur next to Co atoms where their formation energies could be reduced by as much as 1.28 eV compared to that in bulk SrTiO 3 . We also find that Co in these Co-OV complexes occurs in the high-spin state in agreement with core level spectroscopy, and identify a linear arrangement of the Co-OV defect complexes to be the most energetically favourable structure. These defect complexes are also shown to interact ferromagnetically and that their magnetic interaction is found to be short-ranged, consistent with the relatively high Co concentrations needed experimentally for ferromagnetism to be observed in cobalt-substituted SrTiO 3 . I.Introduction The discovery of room-temperature ferromagnetism in doped transition metal oxides by Matsumoto et al. [2] has initiated many theoretical and experimental studies, owing to potential applications of these materials in spintronics. For example, , SnO 2 [5-6], In 2 O 3 [7][8] and TiO 2 [9][10], have all been reported to show room-temperature ferromagnetism when doped with magnetic ions. Increasing the Curie temperature and being able to control the spin degrees of freedom remain major challenges in this field. In this respect, defects in these materials, both intrinsic and extrinsic, play an important role. Defects provide additional carriers to the system (electrons or holes) which could significantly alter magnetic interactions and the Curie temperature in these materials.

Defects-Driven Ferromagnetism in Undoped Dilute Magnetic Oxides: A Review

“…The report of RTFM in Co doped TiO2 films has obviously encouraged experimental studies [14] . Since then, intense research efforts have followed on these to realize robust RTFM in doped oxide materials and many wide-band-gap oxides, such as ZnO [13,[15][16][17][18][19][20] , TiO2 [14,[21][22][23][24] , SnO2 [25][26][27][28][29] , In2O3 [30][31][32][33][34] , SrTiO3 [35][36][37][38] , etc. People understood that the RTFM resulted from the interaction between the dopant moments and carrier spins with different theoretical models such as RKKY [39,40] , mean field theory [13,41,42] , or double exchange [43,44] etc., and considered that doping played a key role in the RTFM of DMOs [45] .…”

Multiferroic Phases and Transitions in Ferroelectric Lead Titanate Nanodots

“…The projector-augmented wave (PAW) potentials was employed for electron-ion interaction31, which explicitly treats the Pb 5 d , 6 s , and 6 p , the Ti 3 s , 3 p , 3 d , and 4 s , and the O 2s and 2 p electrons as valence states. The DFT+ U scheme provides accurate atomic structures and electronic properties in wide bandgap oxides (i.e., PbTiO 3 ) with defects or nonstoichiometry323334. The validity of the method presented here has been further verified by repeating the calculations for small nanowires (Fig.…”

Electronic and magnetic properties of structural defects in SrTiO3(Co)

“…An important role of the oxygen vacancies forming the Co 2+ B -nearest vacancy complexes whose magnetic mo-ment is 1µ B [15] or even 3µ B [37] (for the same structure) was demonstrated in LDA+U studies. [15], the magnetic interaction between two nearest-neighbor Co 4+ B ions (S = 1/2) is ferromagnetic (2J = 60 meV/cell) and becomes very weak for second and third neighbors.…”

“…[36] in which the nearest Co atoms are ordered antiferromagnetically and the largest ferromagnetic interaction was obtained for the next-nearest-neighboring Co 4+ B ions. The ferromagnetic interaction of two Co 2+ B -V O complexes forming a linear chain is significantly weaker (2J = 8-15 meV/cell) [37].…”

“…In addition to the experimental studies of the roomtemperature ferromagnetism, the theoretical studies that discuss possible mechanisms of this phenomenon were performed using the HSE06 hybrid functional [36] and LDA+U approach [15,37]. In Ref.…”