Correlation strength, orbital-selective incoherence, and local moments formation in the magnetic MAX-phaseMn2GaC

H J M Jönsson,I Leonov,M Ekholm,J Rosen,I A Abrikosov,M Dahlqvist

doi:10.1103/physrevb.105.035125

Abstract

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase Mn$_2$GaC with the main focus given to the origin of magnetic interactions in this system. Using the density functional theory+dynamical mean-field theory (DFT+DMFT) method we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically-ordered phases of Mn$_2$GaC. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the DFT+DMFT method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of Mn$_2$GaC, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of Mn$_2$GaC, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that Mn$_2$GaC can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by the Hund's exchange interaction, and not the Coulomb interaction.

Highlights

MAX-phases are a promising class of functional materials with the generic chemical formula Mn+1AX n and hexagonal crystal structure, which consists of layers of C or N (X ) and transition metal (M) atoms, interconnected by layers of the A-group atoms
Our density functional theory [10] (DFT)+DMFT results reveal a complex magnetic behavior of Mn2GaC with strongly competing FM and AFM states at low temperatures. This suggests a high sensitivity of the magnetic state of Mn2GaC to fine details of its crystal structure, lattice volume, pressure, etc., in agreement with recent experiments
We performed a theoretical study of the electronic structure and magnetic properties of the PM, FM, and AFM states of the prototypical MAX-phase Mn2GaC using the DFT+DMFT and DFT-disordered local moment (DLM) methods

Summary

Introduction

MAX-phases are a promising class of functional materials with the generic chemical formula Mn+1AX n and hexagonal crystal structure, which consists of layers of C or N (X ) and transition metal (M) atoms, interconnected by layers of the A-group atoms. First discovered in the 1960s [1,2] and later rediscovered in 1996 [3], these materials possess intriguing physical behavior combining the properties typical for ceramics, such as high hardness, and those of metallic systems, e.g., good electrical and thermal conductivity [4]. This combination of physical and mechanical properties, as well as being machined [5], makes MAX phases promising for numerous applications, such as thin-film coatings for low friction surfaces, electrical contacts, and heat exchangers [4,5]

Results

Discussion

Conclusion