Strain-tunable magnetic and magnonic states in Ni-dihalide monolayers

TL;DR

This study explores how biaxial and uniaxial strain influence the magnetic and magnonic states of NiX2 monolayers, revealing strain-dependent phase diagrams, the importance of biquadratic exchange, and tunable magnonic properties.

Contribution

It provides the first detailed analysis of strain effects on magnetic phases and magnonic behavior in Ni-dihalide monolayers, emphasizing the role of biquadratic exchange interactions.

Findings

Strain induces various magnetic phases including ferromagnetic, helimagnetic, and skyrmionic states.

Biquadratic exchange significantly impacts magnetic ground states and critical temperatures.

Skyrmionic phase acts as a magnonic crystal with strain-tunable magnon modes.

Abstract

Monolayer NiI$_2$ garners large research interest due to its multiferroic behavior stemming from the interplay between its non-collinear magnetic order and the spin-orbit coupling. This prompts an investigation into the stability of the magnetic order in NiI$_2$ and similar materials under external stimuli. In this work, we report the effect of biaxial and uniaxial strain on the magnetic ground state, the critical temperature, and the magnonic properties of the NiX$_2$ (X = I, Br, Cl) monolayers. For all three materials, we reveal intricate strain-dependent phase diagrams, including ferromagnetic, helimagnetic, and skyrmionic phases. Moreover, we discuss the necessity of considering the biquadratic exchange interaction in the latter analysis. We reveal that the biquadratic exchange significantly alters both the magnetic ground state and the critical temperature of the magnetic order, and we demonstrate that its importance becomes even more explicit when monolayer Ni-dihalides are strained. Finally, we calculate the magnonic dispersion for the predicted magnetic states, showing that the skyrmionic phase functions as a magnonic crystal, and demonstrate the presence of strain-tunable soft magnon modes at finite wavevectors in the helimagnetic phase.