Spin fluctuations steer the electronic behavior in the FeSb$_{3}$ skutterudite

TL;DR

FeSb3 skutterudite is analyzed to understand how spin fluctuations influence its electronic structure. The authors combine Hubbard-corrected DFT (DFT+U+V), special quasirandom structures to model paramagnetism, and mapping to a Heisenberg Hamiltonian to quantify exchange and TN. They find a low-energy AFM ground state and a paramagnetic-like SQS state that opens a band gap of ~61 meV (SQS-PM) and ~38 meV in larger supercells, with features resembling a Luttinger-compensated ferrimagnet. However, the predicted Neel temperatures (roughly 163–178 K) exceed the experimental value (≈10 K), suggesting stoichiometry effects and magnetic frustration play crucial roles and that non-Heisenberg physics and static correlations may be important.

Abstract

Skutterudites are promising materials for thermoelectric and spintronics applications. Here we explore spin fluctuations in the FeSb$_{3}$ skutterudite and their effect on its electronic structure using Hubbard-corrected density-functional theory calculations. We identify multiple magnetic and charge-disproportionated configurations, with an antiferromagnetic metallic ground state. Paramagnetic fluctuations modeled through a special quasirandom spin structure open a 61 meV gap, consistent with experiments. This state features non-degenerate spin channels and band-avoided crossings, resembling a Luttinger-compensated ferrimagnet. Mapping the electronic structure to a Heisenberg Hamiltonian fails to explain the low Néel temperature ($\lesssim$10 K), suggesting that factors such as stoichiometry and magnetic exchange frustration may play an important role, calling for more detailed experimental investigations.

Figures (18)

• Figure 1: Phonon dispersions of the non-magnetic A1 (red) solution and the antiferromagnetic C3 (black) ground state in the 16-atom primitive cell of the FeSb$_{3}$ skutterudite calculated along the main symmetry directions. The grey shaded area indicates the region of imaginary phonon frequencies.
• Figure 2: Electronic band structure of the antiferromagnetic G-AFM (black) and “paramagnetic” SQS-PM (red) states (see Table \ref{['tab:magnetic_states_32_atoms']}) in the 32-atom cubic unit cell with P23 space group. The band structures are calculated along the main symmetry directions. The spin up (spin down) contribution to the band structure is given by full (dashed) lines. The shaded gray area represents the band gap of the semiconducting SQS-PM state. The zero of the energy axis has been set equal to the Fermi level and the highest occupied level for metallic and semiconducting states respectively.
• Figure 3: Band structure of the G-AFM ground state of FeSb$_3$. The solid lines correspond to the DFT results, and the dashed line to the Wannier interpolated bands.
• Figure 4: Magnetic exchange coupling $J$ as a function of distance between coupled spins $d$. Negative values, which characterize most couplings, correspond to antiferromagnetic interactions.
• Figure 5: Normalized magnetization $m$ (in blue) and the specific heat $C_v$ (in red) as a function of temperature. Their cusps are indicative of a phase transition into the paramagnetic phase with increasing temperatures.