Spectroscopic study of emergent electronic phases in transition metal based compounds

Abstract

Antiferromagnets with non-relativistic spin splitting are outstanding candidates as the next generation of spintronic materials owing to their electron-volt (eV) scale spin splitting, ultrafast spin dynamics and nearly vanishing stray fields. Achieving voltage-based control of spin polarization in antiferromagnets is of great interest for realizing energy-efficient and compact devices for information storage and processing. Spin spiral type-II multiferroics exhibit an inversion-symmetry-breaking antiferromagnetic order which directly induces ferroelectric polarization, allowing for symmetry protected cross-control between spin chirality and polar order. This intrinsic coupling between the magnetic and dipolar order parameters results in record-strength magnetoelectric effects. Two-dimensional materials possessing such intrinsic multiferroic properties have been long sought for harnessing magnetoelectric coupling in nanoelectronic devices. The recent discovery of intrinsic magnetic order in atomically-thin van der Waals (vdW) materials has created new opportunities for the study of collective spin phenomena in free-standing two-dimensional (2D) systems and nanoscale devices. Among possible multiferroic vdW materials, several families have been identified, and of particular promise is the magnetic semiconductor NiI₂. The multiferroic state of NiI₂ is characterized by a proper-screw spin helix with given handedness, which couples to the charge degrees of freedom to produce a chirality-controlled electrical polarization. We use a suite of optical technique which reveal an ordered magnetic, polar state that persists down to the ultrathin limit of monolayer NiI₂.

Recent development of spin-group formalism has identified a new class of magnets with nontrivial spin textures, including even-parity d, g, or i-wave altermagnet and odd-parity p-wave antiferromagnets. The chiral magnetic order in NiI₂ breaks Inversion-Time-Reversal-Translation (P Tτ ) symmetry, and Spin-Rotation-Translation (Uτ ) symmetry, allowing for spin splitting even in the absence of spin-orbit-coupling (SOC). We provide direct evidence that the spin polarization in a spin spiral type-II multiferroic exhibits p-wave (odd-parity) character and directly couples to the spin chirality, enabling electrical control of non-relativistic spin splitting. Our findings represent the first observation of a p-wave antiferromagnet, and open a new frontier of voltage-based switching of non-relativistic spin splitting in vdW antiferromagnets.