Structural electronic properties and enhancement of electrical polarization in er2nimno6 la2nimno6 superlattice by first principles calculations

Structural, electronic properties and enhancement of electrical polarization in Er2NiMnO6/La2NiMnO6 superlattice by first-principles calculations , Haipeng Lu, Xun Sun , Zhihua Hou, Wen Yang, Siyuan Wang, Jianliang Xie, and Longjiang Deng Citation: AIP Advances 6, 035219 (2016); doi: 10.1063/1.4945394 View online: http://dx.doi.org/10.1063/1.4945394 View Table of Contents: http://aip.scitation.org/toc/adv/6/3 Published by the American Institute of Physics AIP ADVANCES 6, 035219 (2016) Structural, electronic properties and enhancement of electrical polarization in Er2NiMnO6/ La2NiMnO6 superlattice by first-principles calculations Haipeng Lu,1,2 Xun Sun,2,a Zhihua Hou,2 Wen Yang,2 Siyuan Wang,2 Jianliang Xie,2 and Longjiang Deng1,2 National Engineering Research Center of Electromagnetic Radiation Control Materials, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054, China (Received March 2016; accepted 21 March 2016; published online 30 March 2016) Employing first-principles calculations, structural, electronic properties of new multiferroic material Er2NiMnO6/La2NiMnO6 perovskite superlattice are investigated This structure is computed as monoclinic phase with obvious distortion The average in-plane anti-phase rotation angle, average out-of-plane in-phase rotation angle and other microscopic features are reported in this paper Ni and Mn are found in this superlattice that stay high spin states These microscopic properties play important roles in multiferroic properties Based on these microscopic features, the relationship between the direction of spontaneous polarization and the order of substitution in neighboring A-O layers is explained Finally, we try to enhance the electrical polarization magnitude by 32% by altering the previous superlattice as LaEr2NiMnO7 structure Our results show that both repulsion force of A site rare earth ions and the arrangement of B site ions can exert influences on spontaneous polarization C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4945394] I INTRODUCTION Multiferroic materials with their coexisting of ferroelectric and ferromagnetic properties may be used in many technological applications such as memory devices and sensors.1–4 But these two properties are rare to coexist.5,6 Therefore, searching materials that are ferromagnetic with electrical polarization at room temperature becomes current research focus Perovskite oxides attract considerable research interest due to rich structural features and potential multiferroic properties Perovskite material BiFeO3 is a famous multiferroic system with electrical polarization near to 100µC/cm2 with magnetic feature as antiferromagnetism.7,8 Perovskite Ba0.8Sr0.2TiO3 synthesized by hydroxide co-precipitation method shows obvious ferroelectric behavior.9,10 Double perovskites R2NiMnO6 (R is a rare earth ion) have attracted increasing attention because of their near room ferromagnetic Curie temperature.11,12 For example, La2NiMnO6 has a ferromagnetic Curie temperature 287k which may be used in thermoelectric coolers.13 But these materials cannot induce electrical polarization by themselves Recent study14 has shown in theory that short-period [001] ordered R2NiMnO6/La2NiMnO6 superlattices can exhibit an electrical polarization if both R2NiMnO6 and La2NiMnO6 adopt an a-a-c+ rotation pattern It is well known that geometry structure of system especially BO6 octahedral rotation pattern plays an important role in ferroelectric feature and magnetic structure determines ferromagnetic feature Therefore, such micro properties predicated in theory will allow researchers to design new perovskites materials with multiferroic feature With this background, in this paper, we report structure, electronic properties of Er2NiMnO6/La2NiMnO6 a Corresponding author, e-mail: sunxunphy@hotmail.com 2158-3226/2016/6(3)/035219/6 6, 035219-1 © Author(s) 2016 035219-2 Lu et al AIP Advances 6, 035219 (2016) ordered perovskite superlattice via first-principles calculation Then we try to enhance the electrical polarization by changing the previous superlattice structure II COMPUTATIONAL METHOD This work is performed by using the Vienna ab initio Simulation Package (VASP) Electronic correlation effects are considered by the generalized gradient approximation GGA+U method with the parameterization due to Perdew-Burke-Ernzerhof revised for solids (PBEsol)15 in this work The value of Hubbard U is eV for Ni and Mn.15,16 Here, the f electrons of rare earth ions are treated as core electrons for the reason that partially f states may cause convergence problems and also cannot be well described by the current density function.17 The k-space mesh (6×6×4) is carried out for sampling over Brillouin zone The energy cut-off for plane wave is 500 eV The structure is relaxed until force on each atom is smaller than 0.005eV/Å In order to test the accuracy of this method, the lattice parameters and magnetic moments of ground state of La2NiMnO6 are calculated The lattice constants a, b and c are 5.531 Å, 5.524 Å and 7.791 Å, respectively The magnetic moments of Ni2+ and Mn4+ are calculated as 1.55 µB and 3.16 µB, respectively Our results agree reasonably with experimental data,13 which are 5.506 Å, 5.456 Å and 7.732 Å for a, b and c lattice vectors, 1.9 µB for Ni2+ and 3.0 µB for Mn4+ ions III RESULTS AND DISCUSSION A Structure This structure (Fig 1(a)) which contains two formula units with 20 atoms is built by replacing the first La-O layer with Er-O in La2NiMnO6 (P21/n ground state) bulk Here Ni/Mn of B site stack along [111] direction alternatively After calculating total energy, we find the FM state (Ni ion and Mn ion are coupled ferromagnetically) is the ground state After full structural optimization is completed, the Er2NiMnO6/La2NiMnO6 superlattice shows obvious distortion The space group is P21 The lattice constants a, b and c are 5.292 Å, 5.492 Å and 7.597 Å, respectively Note that the lattice vectors a, b and c are along the pseudo-cubic [110], [-110], and [001] directions Monoclinic angle β = 90.194◦ Detail structural information is shown in TABLE I NiO6 and MnO6 octahedrons themselves are found have slight distortion Ni ion displaces slightly along [-1 -1 -1] direction from the center position Three coplanar oxygens lie below Ni along [-1 -1 -1] direction at 1.12 Å and three sit above at 1.19 Å Mn ion has similar displacement along opposite direction, with three neighboring oxygens at 1.10 Å and three others at 1.12 Å The average angles of Ni-O-Mn in ab plane and perpendicular to ab plane are 150.30◦ and 151.69◦, respectively Both NiO6 and MnO6 octahedrons mutually possess similar a-a-c+ rotation pattern18 (Fig 1(b)) The average in-plane anti-phase rotation angle ωR is 12.0◦ and the average out-of-plane in-phase rotation angle ωM is 10.1◦ FIG (a) Structure of Er2NiMnO6/La2NiMnO6 superlattice, (b) rotation pattern, and (c) displacements of Er, La ions 035219-3 Lu et al AIP Advances 6, 035219 (2016) TABLE I Energy-minimized structural parameters of Er2NiMnO6/La2NiMnO6 superlattice a (Å) b (Å) c (Å) α (◦) β (◦) γ (◦) Ω (Å3) 5.292 5.492 7.597 90.000 90.194 90.000 220.792 La Er Ni Mn O1 O2 O3 x y z 0.98939 0.01690 0.00145 0.50686 0.30847 0.28654 0.60560 0.95226 0.06917 0.50249 0.00018 0.28938 0.30351 0.96708 0.74861 0.24732 0.00667 0.00949 0.05969 0.44185 0.24649 In the system, rare earth ions from two neighboring A-O layers have obvious displacements along b axis as can be seen from Fig 1(c), the displacement of Er (δ1) is 0.27 Å and the displacement of La (δ2) is -0.11 Å The displacements along a axis (δ3) of any two first-nearest rare earth ions in the same AO layers average out to zero Also, there is no displacement along c axis Here, the displacements are calculated by choosing the paraelectric ground state of La2NiMnO6 as reference phase.13 B Electronic properties Density of states are shown in Fig 2, The Er2NiMnO6/La2NiMnO6 superlattice shows insulative behavior and the majority spin bands exhibit a gap as 1.42 eV Contributions of the states at the top of valence band are almost come from O 2p electrons and Ni 3d electrons Electrons of the conduction band bottom are mainly due to O 2p and Mn 3d states O 2p electrons distribute widely near the Fermi level Under the action of O octahedral crystal field, d states of transition metal ions Ni and Mn should separate into t 2g and eg states with two distinguished peaks However, our t 2g and eg states split into several small peaks as can be seen from Fig 2(c), 2(d) This situation is made possible because of the structural distortion aforementioned The 2p electron cloud of O ions and the d x2−y2, d z2 electron cloud of Ni, Mn ions stagger slightly in the overlapping zone As can be seen from Fig 2, Ni t 2g, eg states and Mn t 2g states are under the Fermi level, while Mn eg states are above the Fermi level The eg states of Ni and Mn are quite expanded with high mobility FIG Density of states of Er2NiMnO6/La2NiMnO6 superlattice: (a) total, (b) O-2p, (c) Ni-3d, (d) Mn-3d 035219-4 Lu et al AIP Advances 6, 035219 (2016) FIG Spin density of Er2NiMnO6/La2NiMnO6 superlattice with isovalues ± 0.02 while t 2g states are localized The d electron occupation numbers and magnetic moments of Ni, Mn d d d d are nNi =8.25, µNi =1.54µB, and nMn =4.85, µMn =3.07µB, respectively Here, d electron occupation numbers and magnetic moments of both Ni and Mn ions differ from the values calculated by ionic model The reason is that d electron of Ni, Mn ions and p electron of O ions have strong covalent interaction Note that total magnetic moments are µNi =1.55µB and µMn =3.13µB, respectively To visualize the distribution of spin electron of transition metal ions, the spin density (Fig 3) is very useful The shape of spin electrons distribute around Ni ions looks like a star with sharp corners19 but that around Mn ions still keeps symmetry All of these explain that Ni stay the spin state as t 2g 6eg with the valence Ni2+, Mn stay the spin state as t 2g with the valence Mn4+ C Enhancement of electrical polarization Recent study14 has shown that Er2NiMnO6/La2NiMnO6 superlattice can exhibit an electrical polarization as 9.12µC/cm2 This electrical polarization originates from the opposite antipolar motions with different magnitude along b direction Note that, the direction of polarization in this superlattice will switch to –b direction when the second La-O layer is replaced rather than the first one For this phenomenon, it can give a satisfactory explanation by analyzing from the different of A site ions In the Er2NiMnO6/La2NiMnO6 superlattice, the radius of Er ion is significantly smaller than that of La ion Under the same driving effect (rotations ω Rω M ),20,21 Er ion will have a larger displacement along +b direction than La This can be proved from the average force on Er and La ions of unrelaxed Er2NiMnO6/La2NiMnO6 superlattice The average force on Er is 0.33 eV/Å while the average force on La is 0.01 eV/Å At the same time the magnitude of positive charges diaplacement around Er ion (along +b direction) is also larger than that of La (along –b direction) Hence, the macroscopical electrical polarization in this superlattice will along +b direction One can conclude that the direction of electrical polarization depends on the direction where the smaller A site ions moves along A T Mulder et al.22 have reported that Ruddlesden–Popper phase structure can exhibit an electrical polarization if upper and lower parts adopt a-a-c+ rotation pattern It is therefore legitimate to wonder whether we can take a similar approach to control the electrical polarization for our superlattice Here we build a new structure (LaEr2NiMnO7) shown in Fig by removing two layer of Ni/Mn-O octahedrons and one layer of La-O So neighboring Er-O layers can contact directly After full structural optimization is completed, the new structure shows more obvious distortion Octahedrons from upper and lower parts also adopt similar a-a-c+ rotation pattern Fig shows the displacements of rare earth ions directly (the displacements of Er and La along b axis in upper part are δ1′ and δ2′, the displacements of Er and La along b axis in lower part are δ1′′ and δ2′′, δ3′ and δ3′′ represent the displacements of any two first-nearest rare earth ions in the same A-O layers along a axis) The rotation angles are larger than that in previous Er2NiMnO6/La2NiMnO6 superlattice 035219-5 Lu et al AIP Advances 6, 035219 (2016) FIG (a) Displacements of Er, La ions in Er2NiMnO6/La2NiMnO6 superlattice, and (b) in LaEr2NiMnO7 superlattice Note that there are two interesting phenomena First, almost all displacements of Er and La ions in LaEr2NiMnO7 superlattice are improved Second, the magnitudes of δ1′ and δ1′′ (also δ2′ and δ2′′) are different Fig shows these phenomena directly For the improvement of δ1′ and δ1′′, we can analyze the average force on these two Er ions In LaEr2NiMnO7 superlattice, neighboring Er ions can meet each other directly There will be a strong repulsion force between them We calculate the average force on these two Er ions The average force on Er ions is 0.47 eV/Å before relaxing, at the same time the distance of neighboring Er ions is 5.5 Å after relaxing Such a powerful force will drive them to move significantly For the difference of magnitudes between δ1′ and δ1′′, we can explain from the view of surroundings around these two Er ions We find the arrangement of B site ions in upper and lower parts are not symmetrical about mid-plane Compared with the arrangement of B site ions in upper part, the arrangement in lower part rotate 90 degrees along [001] direction (Ni/Mn of B site stack along [111] direction in upper and lower parts) That is, the impacts on these two Er ions caused by Ni/Mn octahedral rotation are also different We also calculate the spontaneous polarization of this new structure by the Berry phase approach.23,24 The spontaneous polarization is computed to be 12.04µC/cm2 along b direction, a more than 32% increase over previous Er2NiMnO6/La2NiMnO6 superlattice One can conclude that the repulsion force of A site rare earth ions and the arrangement of B site ions can exert influences on spontaneous polarization IV CONCLUSION In summary, our results show that Er2NiMnO6/La2NiMnO6 superlattice has obvious distortion Ni and Mn are coupled ferromagnetically with spin states t 2g 6eg and t 2g The direction of electrical polarization depends on the direction where the smaller A site ions moves along The LaEr2NiMnO7 superlattice has more obvious distortion with spontaneous polarization as 12.04µC/cm2 The polarization is 32 percent larger than previous Er2NiMnO6/La2NiMnO6 superlattice due to the influences of the repulsion force of A site rare earth ions and the arrangement of B site ions These findings will allow researchers to design new perovskites materials with multiferroic features in the future ACKNOWLEDGMENT This work was supported by National Science Foundation of China under Grant No 51301031 Z Zanolli, J C Wojde, J Íđiguez, and P Ghosez, Phys Rev B 88, 060102 (2013) M Ležai´c and N A Spaldin, Phys Rev B 83, 024410 (2011) 035219-6 Lu et al AIP Advances 6, 035219 (2016) S Mukherjee, A Roy, S Auluck, R Prasad, R Gupta, and A Garg, Phys Rev Lett 111, 087601 (2013) I C Infante, J Juraszek, S Fusil, B Dupé, P Gemeiner, O Diéguez, F Pailloux, S Jouen, E Jacquet, G Geneste, J Pacaud, J Íđiguez, L Bellaiche, A Barthélémy, B Dkhil, and M Bibes, Phys Rev Lett 107, 237601 (2011) R Ramesh and N A Spaldin, Nat Mater 6, 21–29 (2007) W Eerenstein, N D Mathur, and J.F Scott, Nature 442, 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Displacements of Er, La ions in Er2NiMnO6/ La2NiMnO6 superlattice, and (b) in LaEr2NiMnO7 superlattice Note that there are two interesting phenomena First, almost all displacements of Er and La ions in LaEr2NiMnO7