Magnetic molecular orbitals in MnSi

TL;DR

This study reveals that in MnSi, the fundamental magnetic units are extended molecular orbitals of three Mn atoms, challenging the traditional ionic model and providing new insights into magnetic quantum materials.

Contribution

It demonstrates, through neutron scattering and ab initio calculations, that MnSi's magnetism arises from molecular orbitals rather than individual ions, introducing a new perspective on magnetic interactions.

Findings

Magnetic units in MnSi are extended molecular orbitals of three Mn atoms.

Spin wave wavelength is comparable to the size of molecular orbitals.

Contrasts with the ionic picture of magnetism.

Abstract

A large body of knowledge about magnetism is attained from models of interacting spins, which usually reside on magnetic ions. Proposals beyond the ionic picture are uncommon and seldom verified by direct observations in conjunction with microscopic theory. Here, using inelastic neutron scattering to study the itinerant near-ferromagnet MnSi, we find that the system's fundamental magnetic units are interconnected, extended molecular orbitals consisting of three Mn atoms each, rather than individual Mn atoms. This result is further corroborated by magnetic Wannier orbitals obtained by ab initio calculations. It contrasts the ionic picture with a concrete example, and presents a novel regime of the spin waves where the wavelength is comparable to the spatial extent of the molecular orbitals. Our discovery brings important insights into not only the magnetism of MnSi, but also a broad range of magnetic quantum materials where structural symmetry, electron itinerancy and correlations act in concert.