Atomistic approaches on 2D magnetic materials

Abstract

The discovery of magnetism in two dimensional materials, first found in CrI₃ and Cr₂Ge₂Te₆, has opened up many possibilities to explore, concerning novel phenomena and potential applications in spintronics. Prior to these discoveries, it was thought that magnetism in two dimensions was impossible due to the conditions of the Mermin-Wagner theorem. In this work, the ferromagnet CrI₃ is examined in the monolayer to understand how its magnetism may be utilised, potentially in racetrack memory. Ab initio methods provides a way to study CrI₃’s nature by calculating individual exchange values and using them to find an appropriate spin model. In the initial literature, CrI₃ was identified as an Ising magnet based on experimental data. By using a Monte Carlo atomistic simulation approach, the Curie temperature (T_C) of the material can be calculated. Thus, the suitability of the Ising model can be assessed. The atomistic spin model shows that the Ising model overestimates the T_C, and we show that by using a modified Heisenberg model, a value of T_C much closer to experiment can be calculated. Using this model and our calculated exchange parameters, the spin dynamics in monolayer CrI₃ can be examined with the Landau-Lifshitz-Gilbert equation. Spins in monolayer CrI₃ appear to fluctuate in zero-field cooling simulations, seemingly as a result of the system’s high anisotropy. Zero-field cooling simulations are also performed on bulk CrI₃ revealing a multi-step transition in the material. To consider CrI₃’s potential in domain wall motion based technology, simulations of a moving domain wall are carried out on a short section of nanowire to assess domain wall motion under applied field and with the use of a spin-transfer torque, applied current. High velocities of 1020 m/s are reached under applied current. At this point the material undergoes a Walker breakdown collapse of the domain wall, so the limits of CrI₃ domain wall motion capabilities are shown.

Date of Award	Dec 2022
Original language	English
Awarding Institution	Queen's University Belfast
Supervisor	Myrta Grüning (Supervisor) & David Wilkins (Supervisor)