Local-moment magnetism in Mn-based pnictides

TL;DR

BaMn$_2$Pn$_2$ (Pn = P, As, Sb, Bi) are Mn-based pnictides where magnetism and correlation effects can be tied to a Mott-like local moment formation. The authors combine density-functional theory with slave-spin mean-field (SSMF) to construct a multi-orbital Hubbard model for Mn $d$-orbitals and to diagnose the paramagnetic and G-type antiferromagnetic phases at zero temperature. They locate an interaction-driven itinerant-to-localized moment crossover (ILMC) and show that all MnPns reside on the strong-coupling side of the crossover; the closer a compound is to ILMC, the higher its Néel temperature, in qualitative agreement with experiments. The results support a magnetically ordered Mott-insulator picture for Mn-based pnictides and offer a scalable framework for studying Mn pnictides and related materials upon doping.

Abstract

We report a comprehensive study of electronic-correlation effects in Manganese-based antiferromagnetic pnictides BaMn$_2$Pn$_2$ (Pn=P,As,Sb,Bi). Our density functional theory plus slave-spin mean-field simulations indicate that all the compounds lie on the strong-coupling side of an itinerant-to-localized moment crossover, corresponding to the critical interaction strength for the Mott transition in the high-temperature paramagnetic phase. We also show that the experimental Néel temperature of each compound scales with the distance from this crossover.

Figures (3)

• Figure 1: Charge fluctuations for the antiferromagnetic (red circles) and paramagnetic (blue triangles) phase, as a function of the interaction, for BaMn$_2$Pn$_2$ (Pn=P,As,Sb,Bi). Within DFT+SSMF method, the vanishing of the charge fluctuations in the PM corresponds to the Mott transition ($U_c$). The green diamonds report the staggered magnetization (in unit of $\mu_B$) of Mn, becoming finite at the first-order magnetic transition ($U_m$), and rapidly saturating with $U$. Both $U_m$ and $U_c$ decrease along the series of 122 MnPns.
• Figure 2: Kinetic ($\Delta E_{\mathrm{kin}}= E_{\mathrm{kin}}^{AF}-E_{\mathrm{kin}}^{PM}$), potential ($\Delta E_{\mathrm{pot}}= E_{\mathrm{pot}}^{AF}-E_{\mathrm{pot}}^{PM}$) and total ($\Delta E_{\mathrm{tot}}= E_{\mathrm{tot}}^{AF}-E_{\mathrm{tot}}^{PM}$) energy differences between the AF and PM phases for the family of MnPns. The onset of $\Delta E_\mathrm{tot} \neq 0$ indicates the beginning of the weak-coupling antiferromagnetic phase. The second jump signals the entrance into the local-moment antiferromagnetic region.
• Figure 3: Total energy difference between AF and PM phase, for all the studied compounds. Once established, AF is always the ground state. For weak-coupling regime, the compounds display a trend opposite to that of the experimental Néel temperaturesJacobs_BaMn2Pn2-exp. The experimental behavior is correctly captured at strong-coupling instead.