Structural, electronic, and magnetic properties of bulk and epitaxial LaCoO3 through diffusion Monte Carlo

Abstract

Magnetism in lanthanum cobaltite (LCO, LaCoO3) appears to be strongly dependent on strain, defects, and nanostructuring. LCO on strontium titanate (STO, SrTiO3) is a ferromagnet with an interesting strain relaxation mechanism that yields a lattice modulation. However, the driving force of the ferromagnetism is still controversial. Experiments debate between a vacancy-driven or straindriven mechanism for epitaxial LCO's ferromagnetism. We found that a weak lateral modulation of the superstructure is sufficient to promote ferromagnetism. Our research also showed that ferromagnetism appears under uniaxial compression and expansion. Although earlier experiments suggest that bulk LCO is nonmagnetic, our Diffusion Monte Carlo calculations found that magnetic phases have a lower energy ground state for bulk LCO. This article discusses recent experiments indicating a more complicated picture for the bulk magnetism and closer agreement with our calculations. The role of defects are also discussed through excited-state calculations. arXiv:1908.02811v3 [cond-mat.mtrl-sci]

Intrinsic Ferromagnetism of Two-Dimensional (2D) MnO2 Revisited: A Many-Body Quantum Monte Carlo and DFT+U Study

“…As previously mentioned, we used the geometry obtained with SCAN (FM and U = 0) for the subsequent DMC calculations because the calculated lattice constant (for the 2 × 2 × 1 supercell) of 5.696 Å is identical with the experimental value of 5.69(1) Å. DMC has the zero-variance property which means that as the trial wave function approaches the exact ground state (i.e., the exact nodal surface), the statistical fluctuations in the energy reduce to zero . There have been instances where various sophisticated, oftentimes expensive methods have been used to optimize the nodal surface of the trial wave function. − However, similar to other DMC studies of correlated magnetic materials, ,,,, we used a PBE+ U approach where the Hubbard U value was used as a variational parameter to optimize the nodal surface by using DMC (for the FM and AFM states of 2D MnO 2 ). The fact that we can determine the optimal U parameter variationally by using DMC makes our DMC results more reliable than DFT+ U , where in DFT+ U the U parameter is usually arbitrarily chosen or fitted to experimental data.…”

“…19 There have been instances where various sophisticated, oftentimes expensive methods have been used to optimize the nodal surface of the trial wave function. 68−71 However, similar to other DMC studies of correlated magnetic materials, 23,31,33,43,72 we used a PBE+U approach where the Hubbard U value was used as a variational parameter to optimize the nodal surface by using DMC (for the FM and AFM states of 2D MnO 2 ). The fact that we can determine the optimal U parameter variationally by using DMC makes our DMC results more reliable than DFT +U, where in DFT+U the U parameter is usually arbitrarily chosen or fitted to experimental data.…”

“…Diffusion Monte Carlo (DMC) is a correlated electronic structure method that has had demonstrated success for the electronic and magnetic properties of a variety of 2D and bulk systems. − This method has a weaker dependence on the starting Hubbard parameter and density functional and can successfully achieve results with an accuracy beyond the mean-field approximation . For example, DMC has successfully been used to calculate the spin superexchange in the correlated cuprate Ca 2 CuO 3 , has been used to successfully predict the magnetic structure in FeSe when DFT methods disagreed, has been applied to bulk polymorphs of MnO 2 to achieve band gap and lattice constant values in excellent agreement with experiment, and has been applied to study the excitation energies of Mn 4+ doped phosphors (both Mn-based studies used the same RRKJ pseudopotentials we used in our work , ).…”

Tunable Ferromagnetism in LaCoO3 Epitaxial Thin Films

“…Recently, diffusion Monte Carlo calculations showed that the magnetic ground state of the LCO thin film highly depends on the La–La distance, with the rocksalt‐type HS/LS‐FM order becoming the most stable state at a specific compressive strain. [ 59 ] In terms of the exchange mechanism, this order mainly results from correlation superexchange, which refers to the magnetic interaction whereby two electrons of O 2− mediate the exchange coupling between neighboring HS and LS Co 3+ . More specifically, the double occurrence of AFM superexchange coupling along the HS Co 3+ d x2−y2 – O 2− p σ – LS Co 3+ d x 2 −y2 – O 2− p σ – HS Co 3+ d x2−y2 pathway could facilitate the FM ordering (Figure S6, Supporting Information).…”

DFT+U and Quantum Monte Carlo study of electronic and optical properties of AgNiO$_2$ and AgNi$_{1-x}$Co$_{x}$O$_2$ delafossite

“…As there exists as of yet no reported values for an optimal value of U, U opt , we first estimated U opt . We used a procedure established in previous works 24,[43][44][45][46] that has proven to be a reliable and un-biased way to estimate U opt for transition-metal oxides. In this procedure, we minimize the DMC total energy of the PBE+U trial wavefunction as a function of U.…”