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

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.

💡 Research Summary

The paper presents a comprehensive first‑principles investigation of the manganese pnictide series MnX (X = N, P, As, Sb, Bi) with the aim of identifying materials that combine strong magnetocaloric effect (MCE) and pronounced magneto‑optical (MO) activity for next‑generation magnetic refrigeration and multifunctional devices. After a concise introduction that highlights the environmental and economic drawbacks of conventional vapor‑compression cooling and rare‑earth based magnetocaloric materials, the authors motivate the search for inexpensive 3d‑transition‑metal compounds, emphasizing the large spin moments attainable in Mn‑based intermetallics.

Structural optimization is carried out using the PAW method within VASP and the PBE‑GGA functional, employing a 600 eV plane‑wave cutoff and dense k‑point meshes (up to 8 × 8 × 6). The relaxed structures confirm that MnN adopts an antiferromagnetic A‑type configuration, MnP crystallizes in an orthorhombic cell, while MnAs, MnSb and MnBi share a hexagonal NiAs‑type lattice. To capture strong on‑site Coulomb interactions, the authors apply GGA + U corrections (U = 0.9 eV for MnSb, U = 0.7 eV for MnBi), which are essential for reproducing experimental Curie temperatures.

Exchange interactions J_ij are extracted via the Liechtenstein formalism using the full‑potential linear muffin‑tin orbital (FP‑LMTO) code RSPt. A fine sampling of at least 5 000 k‑points ensures convergence. The calculated J_ij values reveal ferromagnetic coupling for all compounds except MnN, which is antiferromagnetic with a Néel temperature around 660 K. The magnitude of J_ij and the resulting Curie temperatures increase systematically from P to Bi, reflecting the growing hybridization between Mn‑3d and the heavier pnictogen p‑states.

Monte‑Carlo simulations (20 × 20 × 20 supercell, 5 × 10⁵ Monte‑Carlo steps per temperature) are performed with the UppASD package, using the ab‑initio J_ij as input. Temperature‑dependent magnetization curves M(T, H) are generated for magnetic fields up to 5 T. Applying Maxwell’s thermodynamic relation, the isothermal magnetic entropy change ΔS_m is computed. MnBi exhibits the largest ΔS_m ≈ 4.5 J kg⁻¹ K⁻¹ at 5 T, while MnSb reaches ≈ 3.8 J kg⁻¹ K⁻¹, both values comparable to or exceeding those of many rare‑earth based refrigerants, confirming the suitability of these materials for room‑temperature magnetic cooling.

The optical and magneto‑optical properties are evaluated using a combination of FP‑LMTO and APW+lo methods. Both interband and intraband (Drude) contributions to the complex dielectric tensor ε(ω) are included; a Drude scattering parameter ħ/τ = 0.2 eV is adopted. Spin‑orbit coupling (SOC) and orbital‑polarization (OP) corrections are explicitly incorporated, restoring Hund’s second rule and providing accurate orbital moment estimates. The off‑diagonal components of ε(ω) yield the Kerr rotation θ_K(ω) and Faraday rotation θ_F(ω). Results show a clear trend: heavier pnictogens produce stronger SOC, leading to larger off‑diagonal conductivity. MnBi displays a maximum Kerr rotation of about –1.6° at 1.8 eV, while MnAs reaches –0.5° at 1.75 eV. MnP and MnN, with weak SOC, exhibit negligible MO response. Correspondingly, the calculated Faraday rotation for MnBi reaches ≈ 0.9°, indicating potential for transparent magneto‑optical devices.

Element‑specific magnetic moments are extracted from X‑ray magnetic circular dichroism (XMCD) sum‑rule analysis and directly from the DFT calculations. Mn sites carry spin moments of 3.3–4.0 µ_B and orbital moments of 0.05–0.08 µ_B, with excellent agreement between the two approaches, validating the computational methodology.

In summary, the study establishes a unified computational workflow that links electronic structure, exchange interactions, thermodynamic magnetocaloric performance, and magneto‑optical activity. The systematic variation across the pnictogen series demonstrates that increasing atomic number simultaneously enhances exchange splitting (raising Curie temperatures) and SOC (amplifying Kerr/Faraday effects). MnBi and MnSb emerge as the most promising candidates, offering high Curie temperatures (> 600 K), sizable magnetic entropy changes, and strong magneto‑optical signals. The authors conclude that such multifunctional Mn‑based pnictides can serve as cost‑effective, environmentally friendly refrigerants while also enabling magneto‑optical applications such as optical isolators, magnetic sensors, and spin‑tronic components. The presented framework can be readily extended to explore other transition‑metal‑based compounds for combined caloric and optical functionalities.