Speaker
Mr.
Adolfo Otero Fumega
(Universidade de Santiago de Compostela)
Description
Ferromagnetic insulating oxides are uncommon materials and yet they are of extreme importance in the fabrication of spintronic devices [1–4]. Due to their technological relevance and rarity, it is interesting to explore and clarify the properties of one of such systems since this could help in designing other oxides with improved properties.
LaCoO3 (LCO) in its bulk form presents a diamagnetic response at low temperatures, all the Co3+ cations (surrounded by an oxygen octahedron) being in a non-magnetic low-spin state, with its all 6 electrons occupying the lower-lying t2g levels. However, when LCO is grown as a thin film on a substrate that induces epitaxial strain on it, a ferromagnetic phase arises at low temperatures, characterized by a saturation magnetization close to 0.8 μB/Co, in case the substrate is SrTiO3 (STO), and a Tc of about 80 K. Apart from that, oxygen vacancy planes are formed perpendicular to the substrate with a repetition of one vacancy plane every three unit cells [5, 6]. This situation is sketched in Fig. 1.
In this study (see Ref. [7] for further details) we have analysed the ferromagnetic-insulating behavior of LCO when it is grown on (001) STO. We have used DFT (density functional theory) calculations to analyze the electronic structure properties of LCO under strain and for various stoichiometries including different oxygen vacancy configurations and concentration. Our calculations were carried out using the WIEN2k software [8], which provides a full-potential, all-electron method to solve Kohn-Sham equations.
Based on a full structural relaxation and our total energy calculations, we have found that the ground state of LCO when it is grown on STO is given by an off-stoichiometry of the form LaCoO2.83 produced by about 6% oxygen vacancies. We have shown that the vacancies form chains perpendicular to the (001) direction and are contained in the plane perpendicular to the film/substrate interface (as shown in Fig. 1), consistent with experimental findings. Other structural features like the distance between La layers or the lattice parameter in the (001) direction agree with the experimental measurements.
The total magnetic moment is in close agreement with experimental measurements. We have found that the Co atoms that lie in the plane of vacancies are a mixture between Co2+ low-spin state (S=1/2) and Co2+ high-spin state (S=3/2). The ferromagnetic insulating behavior of the film is readily obtained through a simple ionic model, the gap opening occurring naturally in that electron count due to the appearing crystal field splittings together with the addition of a reasonable U value.
The results presented in this work could lead to a better understanding of other ferromagnetic-insulating oxides and how defects play a role in the magnetic properties of oxides in general.
This work has been funded by the MINECO of Spain through the project MAT2016-80762-R.
References
[1] A. Fert, Angewandte Chemie International Edition 47 (2008) 5956.
[2] M. Gtte, M. Joppe, and T. Dahm, Scientific Reports 6 (2016) 36070.
[3] P. Michetti and P. Recher, Phys. Rev. B 84 (2011) 125438.
[4] F. Katmis, V. Lauter, F. S. Nogueira, B. A. Assaf, M. E. Jamer, P. Wei, B. Satpati, J. W. Freeland, I. Eremin, D. Heiman, P. Jarillo-Herrero, and J. S. Moodera, Nature 533 (2016) 513.
[5] V. V. Mehta, N. Biskup, C. Jenkins, E. Arenholz, M. Varela, and Y. Suzuki, Phys. Rev. B 91 (2015) 144418.
[6] N. Biškup, J. Salafranca, V. Mehta, M. P. Oxley, Y. Suzuki, S. J. Pennycook, S. T. Pantelides, and M. Varela, Phys. Rev. Lett. 112 (2014) 087202.
[7] Adolfo O. Fumega, V. Pardo, arXiv:1704.03240 (2017).
[8] K. Schwarz and P. Blaha, Comp. Mater. Sci. 28 (2003) 259.
Primary author
Mr.
Adolfo Otero Fumega
(Universidade de Santiago de Compostela)
Co-author
Dr.
Victor Pardo
(Universidade de Santiago de Compostela)
