Strong coupling of Jahn-Teller distortion to oxygen-octahedron rotation and functional properties in epitaxially strained orthorhombic LaMnO3

Abstract

First-principles calculations reveal a large cooperative coupling of Jahn-Teller (JT) distortion to oxygenoctahedron rotations in perovskite LaMnO 3 . The combination of the two distortions is responsible for stabilizing the strongly orthorhombic A-AFM insulating (I ) eP bnm ground state relative to a metallic ferromagnetic (FM-M) phase. However, epitaxial strain due to coherent matching to a crystalline substrate can change the relative stability of the two states. In particular, coherent matching to a square-lattice substrate favors the less orthorhombic FM-M phase, with the A-AFM phase stabilized at higher values of tensile epitaxial strain due to its larger volume per formula unit, resulting in a coupled magnetic and metal-insulator transition at a critical strain close to 1%. At the phase boundary, a very large magnetoresistance is expected. Tensile epitaxial strain enhances the JT distortion and opens the band gap in the A-AFM-I c-eP bnm phase, offering the opportunity for band-gap engineering. Compressive epitaxial strain induces a transition within the FM-M phase from the c-eP bnm orientation to the ab-eP bnm orientation with a change in the direction of the magnetic easy axis relative to the substrate, yielding strain-controlled magnetization at the phase boundary. Similar behavior is expected in other JT active P bnm perovskites. (La,M)MnO 3 (M = Ca, Pr, Sr, Ba) has been of great interest due to the couplings among structure, magnetic and orbital orderings, and the resulting functional properties, such as colossal magneto-resistance (CMR). 1-4 As a result, there have been many experimental 5-8 and theoretical 9-25 studies of the end-member compound LaMnO 3 (LaMnO 3 ). The observed sequence of phases with decreasing temperature is as follows: 8,26 At very high temperature (above 1010 K), LaMnO 3 has a rhombohedral R3c structure, produced by rotation of the oxygen octahedron network of the ideal perovskite structure. From 1010 K down to 750 K, it has an orthorhombic P bnm structure produced by a different pattern of octahedral rotations. At 750 K, an orbital-order transition occurs by a cooperative Jahn-Teller transition; 27 however, this distortion does not break the symmetry of the P bnm structure. Finally, a magnetic transition to an A-type antiferromagnetic (A-AFM) ordering (ferromagnetic alignment in the xy planes, with spin direction alternating from plane to plane) occurs at 135 K.It proves to be challenging to reproduce the observed A-AFM ground state of LaMnO 3 , with its half filled e g level, from first principles. If the structural parameters are fixed to the experimental values, the correct magnetic ordering is obtained with a range of density-functional-theory (DFT) methods. However, full optimization of the structure within DFT generally gives a competing FM-M phase as the ground state, 11,17 and the correct ground state is obtained only in calculations with both a generalized gradient form of the density functional and inclusion of a Hubbard U . 16 More generally, various couplings in com...

Electric Field Control of Jahn-Teller Distortions in Bulk Perovskites

“…We, then, perform a symmetry mode analysis with respect to a hypothetical P4=mmm phase (corresponding to Pm3m for unstrained bulk compounds) in order to extract the amplitude of the relevant lattice distortions [59] (see Table I). As expected, the four materials develop the required distortions, and amazingly, the magnitude of the Q þ 2 Jahn-Teller distortion is relatively large, being, for instance, of the same order of magnitude as the one developed in the prototypical Jahn-Teller system LaMnO 3 (around 0.265 Å [60]). Additionally, the values of the spontaneous polarization are rather large, reaching 76 μC cm −2 for SrTiO 3 , for instance.…”

Interfacial Control of Magnetic Properties at LaMnO3/LaNiO3 Interfaces

“…When grown as a thin film on cubic SrTiO3 (STO) substrates (a=3.905 Å), LMO typically displays an insulating ferromagnetic behavior, possibly attributed to off-stoichiometry, cubic in-plane strain imposed by the substrate, and/or development of a specific orbital order and/or electronic reconstructions due to polar discontinuity. [15][16][17][18][19] On the other hand, the least distorted member of the perovskite nickelates, LNO (rhombohedral structure with a pseudocubic lattice parameter apc=3.84 Å), is the only member of the family that in bulk is a paramagnetic metal at all temperatures. [20][21][22] A thickness-driven metal-to-insulator transition however occurs as the LNO film thickness is reduced to just a few monolayers.…”

“…16,17 In addition to strain, octahedral connectivity requirements across the interface between different perovskite materials can be another source of interfacial lattice reconstructions. 45 LMO remains an open question.…”

Uniaxial pressure-induced half-metallic ferromagnetic phase transition inLaMnO3

“…These studies were focused on providing an interpretation of experiments on epitaxially grown LaMnO 3 [36,37] and therefore, only applied small uniaxial strains of the order of 2% or less were considered. It should be mentioned that biaxial in-plane strains have also been applied with the same purpose in a strain range of up to ± 4% [38,39]. Note that larger strains will not allow high quality epitaxial growth and therefore, have not been studied.…”