[Paper Review] Noncollinear magnetic sampling method for paramagnetic Mott insulator MnO

Sangmoon Yoon, Seounghun Kang+5 more|arXiv (Cornell University)|Oct 12, 2018

Magnetic and transport properties of perovskites and related materials61 references4 citations

TL;DR

This paper proposes a noncollinear magnetic sampling method combined with DFT+U (NCMSM+U) to accurately model the paramagnetic Mott insulating state of MnO at room temperature. By incorporating spin noncollinearity and canonical ensemble sampling into the disordered local moment framework, the method captures crucial biquadratic exchange interactions, yielding improved electronic structure and Néel temperature (120 K) matching experiment, outperforming standard collinear approaches.

ABSTRACT

We present a new approach based on the static density functional theory (DFT) to describe paramagentic MnO, which is a representative paramagnetic Mott insulator. We appended the spin noncollinearity and the canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT$+U$ represents a highly favorable computational method called NCMSM$+U$ to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We revealed that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM$+U$ approach may be used as an alternative to the $ extit{ab initio}$ framework of dynamic mean field theory based on DFT in the simulation of the high-temperature properties of Mott insulators.

Motivation & Objective

• To address the failure of standard DFT in describing paramagnetic Mott insulators like MnO due to self-interaction errors.
• To improve the description of high-temperature properties in MnO by accounting for spin noncollinearity and biquadratic exchange interactions.
• To develop a computationally efficient alternative to DFT+DMFT for simulating paramagnetic Mott insulators in complex systems like nanostructures.
• To validate the NCMSM+U approach by comparing its predictions with experimental data, especially the Néel temperature.

Proposed method

• The NCMSM+U method extends the disordered local moment (DLM) model by introducing spin noncollinearity in the magnetic sampling process.
• It uses a canonical ensemble to sample spin configurations, ensuring proper statistical averaging at finite temperature.
• The method combines DFT+U with noncollinear spin quantization to account for biquadratic spin correlations.
• Spin correlation functions ⟨Φα⟩ and ⟨Ψα⟩ are calculated to quantify bilinear and biquadratic exchange interactions.
• Supercell calculations are performed with identical computational parameters across collinear and noncollinear models for direct comparison.
• The Néel temperature is estimated from the temperature dependence of spin correlations in the collinear model.

Experimental results

Research questions

• RQ1 How does spin noncollinearity affect the electronic structure and energetics of paramagnetic MnO?
• RQ2 Can the NCMSM+U method accurately predict the Néel temperature of MnO compared to experiment?
• RQ3 What is the role of biquadratic exchange interactions in determining the physical properties of paramagnetic MnO?
• RQ4 How does the NCMSM+U approach compare to conventional collinear MSM+U and DFT+DMFT in predicting paramagnetic Mott insulator behavior?
• RQ5 To what extent does spin noncollinearity improve the description of the valence band structure in MnO?

Key findings

• The NCMSM+U method successfully reproduces the delocalized valence band characteristic of paramagnetic MnO, unlike the conventional MSM+U approach which shows a spurious peak at the top of the valence band.
• The inclusion of spin noncollinearity reduces the Néel temperature estimate to 120 K, closely matching the experimental value of 120 K, whereas the collinear MSM+U method overestimates it at 189 K.
• The biquadratic spin correlation function ⟨Ψα⟩ is found to be 1/3 in the noncollinear model, indicating significant biquadric interactions, which are absent in the collinear model.
• The NCMSM+U approach provides a more accurate description of the electronic profile and precise energetics of paramagnetic MnO by capturing noncollinear spin fluctuations.
• The method is scalable to large supercells and offers a computationally feasible alternative to DFT+DMFT for paramagnetic Mott insulators in materials design.
• The antiferromagnetic state of MnO is insensitive to spin noncollinearity, as both collinear and noncollinear models yield identical results due to its type-II ordering.