Thickness-dependence of the Dzyaloshinskii-Moriya interaction in Co2FeAl ultrathin films: effects of the annealing temperature and the heavy metal material

TL;DR

This study investigates how the Dzyaloshinskii-Moriya interaction in ultrathin Co2FeAl films varies with thickness, annealing temperature, and heavy metal layers, revealing sign changes and interface effects.

Contribution

It provides a detailed analysis of the thickness and annealing temperature dependence of iDMI and interface anisotropy in Co2FeAl films with different heavy metal buffers.

Findings

iDMI sign varies with heavy metal layer (negative for Pt/Ir, positive for W)

Effective iDMI constant shows two regimes related to interface degradation

Annealing temperature affects iDMI and anisotropy differently, indicating interface electronic structure changes

Abstract

Interfacial Dzyaloshinskii-Moriya interaction (iDMI) has been investigated in Co2FeAl (CFA) ultrathin films of various thicknesses (0.8 nm<tCFA<2 nm) grown on Si substrates, using Pt, W, Ir and MgO buffer or/and capping layers. Vibrating sample magnetometry revealed that magnetization at saturation (Ms) for the Pt and Ir buffered films is higher than the usual Ms of CFA due to the proximity induced magnetization (PIM) in Ir and Pt, estimated to be 19% and 27%, respectively. The presence of PIM in these materials is confirmed using x-ray resonant magnetic reflectivity. Moreover, while no PIM is induced in W, higher PIM is obtained with Pt when it is used as buffer layer rather than capping layer. Brillouin light scattering (BLS) in the Damon-Eshbach geometry has been used to investigate iDMI constants and the perpendicular anisotropy field versus the annealing temperature. The DMI sign has been found to be negative for Pt/CFA and Ir/CFA while it is positive for W/CFA. The thickness dependence of the effective iDMI constant for stacks involving Pt and W shows the existence of two regimes similarly to that of the perpendicular anisotropy constant due to the degradation of the interfaces as the CFA thickness approaches the nanometer. The surface iDMI and anisotropy constants of each stack have been determined for the thickest samples where a linear thickness dependence of the effective iDMI constant and the effective magnetization has been observed. The interface anisotropy and iDMI constants, investigated for Pt/CFA/MgO system, showed different trends with the annealing temperature. The decrease of iDMI constant with increasing annealing temperature is probably due to the electronic structure changes at the interfaces, while the increase of the interface anisotropy constant is coherent the interface quality and disorder enhancement.