Multi-Functional Properties of Manganese Pnictides: A First-Principles Study on Magneto-Optics and Magnetocaloric Properties

TL;DR

The paper addresses the design of Mn-based pnictides for magnetocaloric cooling and magneto-optical applications by combining first-principles density functional theory with Monte Carlo simulations and relativistic magneto-optical spectroscopy. It develops a unified framework linking Mn–X hybridization, spin–orbit coupling, and exchange interactions to magetocaloric entropy changes $\Delta S_m$, Curie/Néel temperatures, MAE, and Kerr/Faraday responses. Key findings show MnN as antiferromagnetic with a high $T_N$, while heavier pnictogens stabilize ferromagnetic order with increasing $T_C$, with MnBi delivering the strongest MO activity and MnAs offering the largest $|\Delta S_M|$ near room temperature; calculated results align well with experiments and XMCD sum rules, supporting the design of Mn-based multifunctional materials.

Abstract

Magnetic refrigeration presents an energy-efficient and environmentally benign alternative to traditional vapour-compression cooling technologies. It relies on the magnetocaloric effect, in which the temperature of a magnetic material changes in response to variations in an applied magnetic field. Optimal magnetocaloric materials are characterized by a significant change in magnetic entropy under moderate magnetic field. In this study, we systematically investigated the inter-atomic exchange interactions, magnetic anisotropy energy and magnetocaloric properties of MnX (X = N, P, As, Sb, Bi) using a combination of density functional theory and Monte-Carlo simulations. Additionally, the magneto-optical Kerr and Faraday spectra were computed using the all-electron, fully relativistic, full-potential linearized muffin-tin orbital method. The largest Kerr effect observed in MnBi can be inferred as a combined effect of maximal exchange splitting of Mn 3d states and the large spin-orbit coupling of Bi. To extract site-projected spin and orbital moments, spin-orbit coupling and orbital polarization correction are accounted in the present calculation, which shows good agreement between the moment obtained from the X-ray magnetic circular dichroism sum rule analysis, spin-polarized calculation, and experimental studies. The magnetic transition temperatures predicted through Monte-Carlo simulations were in good agreement with the corresponding experimental values. Our results provide a unified microscopic understanding of magnetocaloric performance and magneto-optical activity in Mn-based pnictides and establish a reliable computational framework for designing next-generation magnetic refrigeration materials.

Figures (12)

• Figure 1: Optimized equilibrium crystal structures of Mn$X$ ($X$ = N, P, As, Sb, Bi) compounds: (right panel) MnN in the A-type antiferromagnetic configuration, (middle panel) orthorhombic MnP, and (left panel) hexagonal MnAs/MnSb/MnBi. Red and green spheres represent Mn and $X$ atoms, respectively.
• Figure 2: Fixed–spin–moment versus total energy curves with respect to lowest energy (left panel) and magnetic moment at Mn site changes with volume (right panel) for MnAs and MnP in the hexagonal (Hex) and orthorhombic (Ortho) structures. MnAs exhibits an energy minimum only in the higher moment regime, where Hex structure energetically favoured, reflecting the large equilibrium volume that strengthens exchange splitting and suppresses the ortho distortion. In contrast, MnP shows its minimum at a low magnetic moment in its ground state Ortho structure and consistent with its reduced equilibrium volume where the $d$-band width enhanced by reduction in the bond length that stabilize low moment configuration.
• Figure 3: Exchange interaction between Mn atoms in MnN (left panel), MnP(middle panel) and Mn$Z$ ($Z$ = As, Sb, Bi) as a function of ratio between Mn-Mn distance($r_{ij}$) to the lattice parameter ($a$).
• Figure 4: Magnetic entropy change as a function of normalized temperature for various applied external magnetic field for Mn$Y$ ($Y$=P, As, Sb, Bi)
• Figure 5: Comparison of magnetic entropy change as a function of normalized temperature at 2 T of applied external magnetic field for Mn$Y$ ($Y$=P, As, Sb, Bi).