[Paper Review] Structural transitions and transport-half-metallic ferromagnetism in LaMnO3 at elevated pressure

Jiangang He, Mingxing Chen+2 more|arXiv (Cornell University)|May 7, 2012

Magnetic and transport properties of perovskites and related materials3 citations

TL;DR

This study uses hybrid density functional theory to demonstrate that under high pressure, LaMnO3 undergoes a structural transition from an antiferromagnetic insulator to a ferromagnetic transport half-metal, driven by suppression of Jahn-Teller distortions and a high-spin to low-spin transition. The resulting half-metallic state exhibits spin polarization up to 92%, suggesting stoichiometric LaMnO3 can host colossal magnetoresistance behavior without doping.

ABSTRACT

By means of hybrid density functional theory we investigate the evolution of the structural, electronic and magnetic properties of the colossal magnetoresistance (CMR) parent compound LaMnO$_3$ under pressure. We predict a transition from a low pressure antiferromagnetic (AFM) insulator to a high pressure ferromagnetic (FM) transport half-metal (tHM), characterized by a large spin polarization (~ 80-90 %). The FM-tHM transition is associated with a progressive quenching of the cooperative Jahn-Teller (JT) distortions which transform the $Pnma$ orthorhombic phase into a perfect cubic one (through a mixed phase in which JT-distorted and regular MnO6 octahedra coexist), and with a high-spin (S=2, m_Mn=3.7 mu_B) to low-spin (S=1, m_Mn=1.7 mu_B) magnetic moment collapse. These results interpret the progression of the experimentally observed non-Mott metalization process and open up the possibility of realizing CMR behaviors in a stoichiometric manganite.

Motivation & Objective

• To investigate the pressure-driven structural, electronic, and magnetic evolution of the CMR parent compound LaMnO3.
• To determine whether stoichiometric LaMnO3 can exhibit transport half-metallic ferromagnetism under high pressure.
• To resolve the limitations of standard DFT in describing strong electron correlations and Jahn-Teller distortions in manganites.
• To explore the feasibility of realizing colossal magnetoresistance (CMR) phenomena in undoped, pristine manganites.
• To provide a theoretical basis for experimental exploration of CMR in stoichiometric oxides using high-pressure techniques.

Proposed method

• Employed hybrid density functional theory (HSE) with varying exact-exchange mixing parameters (α = 0.10 to 0.35) to accurately describe strong electron correlations and Jahn-Teller distortions.
• Calculated energy-volume curves and structural parameters to identify phase transitions under pressure, using the Vienna Ab initio Simulation Package (VASP).
• Tracked the evolution of Jahn-Teller parameters Q2 and Q3 to quantify lattice distortions and their suppression with increasing pressure.
• Computed spin-dependent density of states (DOS), Fermi velocities (vF), and spin polarization (Pn) using BoltzTrap interfaced with VASP to assess transport half-metallicity.
• Applied the Mazin formula (Pn = [N↑vF↑n − N↓vF↓n]/[N↑vF↑n + N↓vF↓n]) to calculate spin polarization for different transport regimes (n=0,1,2).
• Monitored orbital occupancies of t2g and eg bands to correlate electronic structure changes with magnetic moment collapse.

Experimental results

Research questions

• RQ1 Can stoichiometric LaMnO3 exhibit transport half-metallic ferromagnetism under high pressure?
• RQ2 What is the role of Jahn-Teller distortions in the insulator-to-metal transition and magnetic phase evolution of LaMnO3 under pressure?
• RQ3 How does the high-spin (S=2) to low-spin (S=1) transition influence the electronic and transport properties of LaMnO3?
• RQ4 To what extent does the suppression of cooperative Jahn-Teller distortions lead to a cubic symmetry and half-metallic behavior?
• RQ5 Can the spin polarization in the high-pressure phase match or exceed that of doped CMR manganites?

Key findings

• LaMnO3 undergoes a transition from an antiferromagnetic insulator to a ferromagnetic transport half-metal at pressures above ~32 GPa, with a critical volume ratio V/V₀ ≈ 0.70.
• The transition is driven by the progressive quenching of Jahn-Teller distortions, leading to a fully cubic perovskite structure (Pnma → ideal perovskite) at V/V₀ ≈ 0.76.
• A high-spin (S=2, mMn=3.7 μB) to low-spin (S=1, mMn=1.7 μB) magnetic moment collapse occurs between V/V₀ = 0.70 and 0.65, accompanied by electron transfer from eg↑ to t2g↓ orbitals.
• Spin polarization reaches P₀ = 92% at V/V₀ = 0.65 and P₁ = 87% at V/V₀ = 0.65, indicating strong transport half-metallicity across multiple transport regimes.
• The half-metallic gap closes at V/V₀ < 0.53 (P > 300 GPa), signaling a transition to a metallic state, but the FM-tHM regime persists up to ~150 GPa.
• The calculated spin polarization values are comparable to those in doped CMR manganites like La₀.₇Sr₀.₃MnO₃, confirming the potential for CMR-like behavior in undoped LaMnO3 under pressure.