Investigation of room-temperature ferromagnetism in $\hbox {SrTiO}_3$ perovskite structure via substitutional doping

Abstract

The C and N atomic doping and co-doping at all possible lattice sites in $\hbox {SrTiO}_3$ (STO) perovskite crystal structure have been investigated using density functional theory (DFT) calculations. Calculated formation energies indicate that C and N doping at O site in STO is more stable than at Sr site and Ti site. C incorporation at anion site significantly affects the electronic bandgap of STO by inducing spin-polarized defect states in bandgap region. Consequently transforming the non-magnetic STO to a magnetic with a magnetic moment of 2.0 $\mu _{\mathrm{{B}}}$ per C atom and reduced the direct (indirect) bandgap 0.86 eV (0.18 eV) for spin-up (spin-down) channel, respectively. Further, with an increase of C atom concentration the magnetic moment of 2.0 $\mu _{\mathrm{{B}}}$ per C atom remains constant, and also, half-metallicity is observed due to spin-down channel. Similarly, N atom substitution at anion site in STO crystal structure induces magnetism of 1.0 $\mu _{\mathrm{{B}}}$. N causes defect states due to which the bandgap of the spin-up channel decreased to 1.68 eV and shows half-metallicity due to the spin-down channel. We found that further increasing the N atom concentration enhanced the half-metallicity by increasing the number of states of the spin-down channel at Fermi energy. Finally, we studied the FM and AFM alignment of the magnetic moments at the dopants C, N and their co-doping in STO with different distances. We found that the C atom prefers AFM states, while the N atom prefers FM states. The calculated Curie temperature $\hbox {T}_c$ for two N atoms at near and far dopants is 722.16 K and 860.91 K, respectively. The results of tailored electronic and magnetic properties above room temperature are interesting from a theoretical perspective and may open opportunities for STO in electronic and spintronic devices.

Graphic Abstract

(a) Side view of $2 \times 2 \times 2$ supercell of SrTiO3 (STO) perovskite crystal structure. (b) Top view of the translationally asymmetric STO conventional cell of a $2 \times 2 \times 2$ supercell indicating the possible doping sites. (c) For FM and AFM calculations, we considered two positions: near interacting positions denoted as (1, 2) and far interacting positions denoted as (1, 3), to study the dopants interactions. The red circles denote host O sites. In FM interaction, both atoms have positive magnetization (spin-up), while in AFM calculations, one of the two dopants has negative magnetization (spin-down)