PHYSICAL REVIEW B 101, 054441 (2020) Electronic structure and x-ray magnetic circular dichroism in ferroelectric CaMnTi 2 O 6 V. N. Antonov, 1, 2 D. A. Kukusta, 1, 3 S. V. Moklyak, 1 D. V. Mazur , 1 and L. V. Bekenov 1 1 G. V. Kurdyumov Institute for Metal Physics of the N.A.S. of Ukraine, 36 Academician Vernadsky Boulevard, UA-03142 Kyiv, Ukraine 2 University of Bialystok, K. Ciolkowskiego 1M, PL-15-245 Bialystok, Poland 3 Max-Planck-Institut für Festkörperforschung, Heisenberg Strasse 1, D-70569 Stuttgart, Germany (Received 10 October 2019; revised manuscript received 24 January 2020; accepted 18 February 2020; published 27 February 2020) We study the electronic and magnetic properties of ferroelectric CaMnTi 2 O 6 within density functional theory using the generalized gradient approximation (GGA) with the consideration of strong Coulomb correlations (GGA + U ) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band structure method. The x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) at the Mn, Ti L 2,3 , and O K edges have been investigated theoretically. The calculated results are in good agreement with experimental data. The core-hole effect in the final state as well as the effect of the electric quadrupole E 2 and magnetic dipole M 1 transitions have been investigated. The core-hole effect has improved the agreement with the experimental XAS and XMCD spectra at the Ti and Mn L 2,3 edges. DOI: 10.1103/PhysRevB.101.054441 I. INTRODUCTION Multiferroics (MFs) are compounds where long-range magnetic and electric dipolar orders coexist [1]. There is plenty of fascinating physics in these materials, owing to the strong entanglement of spin-charge-orbital degrees of freedom [2,3], and they have great potential for technological applications in energy-efficient information processing and storage [4–6]. For widespread implementation of new tech- nology, the coexistence of long-range magnetic and electric orders at room temperature will be required; at present, there is only one material, BiFeO 3 , known to exhibit ferroelectric and antiferromagnetic (AFM) orders above room temperature [7]. Recently, several manganese and iron oxides have been shown to be strong-coupling materials promising for realizing room-temperature multiferroics. However, ferroelectricity in these materials is rather weak [8]. So far, room-temperature ferroelectric materials are gener- ally associated with single perovskites based on second-order Jahn-Teller (SOJT) active cations with d 0 (Ti 4+ , Zr 4+ , Nb 5+ ) or s 2 (Pb 2+ , Bi 3+ ) valence band electron configurations. These SOJT active cations are prone to anisotropic covalent bonding with ligands inducing structural distortions that may eventu- ally lead to the appearance of a collective polar mode and a macroscopic electric dipolar moment P, whenever the relevant distortions do not array generating antiparallel effects [9]. Recently, Aimi et al. [10] synthesized a new tetragonal double perovskite ferroelectric CaMnTi 2 O 6 with less common A-site order and proved that A-site ordering and SOJT distortions can couple to enable ferroelectricity. Gou et al. [11] iden- tified the origin of the ferroelectricity in CaMnTi 2 O 6 using first-principles calculations combined with detailed symmetry analyses. They explored the properties of CaMnTi 2 O 6 includ- ing its ferroelectric polarization, dielectric and piezoelectric responses, magnetic order, electronic structure, and optical absorption coefficient. It was found that CaMnTi 2 O 6 exhibits room-temperature-stable ferroelectricity and moderate piezo- electric responses. Soft x-ray absorption and magnetic circular dichroism in CaMnTi 2 O 6 were measured recently by Herrero- Martin et al. [9]. CaMnTi 2 O 6 presents a new class of ferro- electric perovskites for potential applications in ferroelectric photovoltaic solar cells. The aim of this paper is the theoretical study from first principles of the electronic and magnetic structures and x-ray magnetic circular dichroism in the ferroelectric CaMnTi 2 O 6 compound. The energy band structure of ferro- electric CaMnTi 2 O 6 is calculated within the ab initio approach taking into account strong electron correlations by applying a local spin-density approximation (LSDA) to density func- tional theory supplemented by a Hubbard U term [generalized gradient approximation (GGA) + U ][12]. The paper is organized as follows. The computational de- tails are presented in Sec. II. Section III presents the electronic structure, x-ray absorption spectra (XAS), and x-ray magnetic circular dichroism (XMCD) of ferroelectric CaMnTi 2 O 6 at the Mn, Ti L 2,3 , and O K edges calculated in the GGA + U approximation. Theoretical results are compared to the exper- imental measurements. Finally, the results are summarized in Sec. IV. II. CRYSTAL STRUCTURE AND COMPUTATIONAL DETAILS A. X-ray magnetic circular dichroism Magneto-optical (MO) effects refer to various changes in the polarization state of light upon interaction with materials possessing a net magnetic moment, including rotation of the plane of linearly polarized light (Faraday, Kerr rotation) and the complementary differential absorption of left and right circularly polarized light (circular dichroism). In the near- visible spectral range these effects result from excitation of electrons in the conduction band. Near x-ray absorption edges, 2469-9950/2020/101(5)/054441(10) 054441-1 ©2020 American Physical Society