On the origin of incoherent magnetic exchange coupling in MnBi/Fe$_x$Co$_{1-x}$ bilayer system

TL;DR

This study investigates the magnetic exchange coupling in MnBi/FeCo bilayers, revealing how interface structure, composition, and layer thickness influence the coherence and effectiveness of magnetic coupling, combining experimental and computational methods.

Contribution

It provides a comprehensive analysis of incoherent exchange coupling origins in MnBi/FeCo bilayers, integrating HR-TEM, DFT, and micromagnetic simulations to understand interface effects.

Findings

Co-rich FeCo layers enhance coherent exchange coupling.

Optimal soft layer thickness is about 1 nm.

Interface roughness and layer thickness significantly affect exchange effectiveness.

Abstract

In this study we investigate the exchange coupling between the hard magnetic compound MnBi and the soft magnetic alloy FeCo including the interface structure between the two phases. Exchange spring MnBi-Fe$_x$Co$_{1-x}$ (x = 0.65 and 0.35) bilayers with various thicknesses of the soft magnetic layer were deposited onto quartz glass substrates in a DC magnetron sputtering unit from alloy targets. Magnetic measurements and density functional theory (DFT) calculations reveal that a Co-rich FeCo layer leads to more coherent exchange coupling. The optimum soft layer thickness is about 1 nm. In order to take into account the effect of incoherent interfaces with finite roughness, we have combined a cross-sectional High Resolution Transmission Electron Microscopy (HR-TEM) analysis with DFT calculations and micromagnetic simulations. The experimental results can be consistently described by modeling assuming a polycrystalline FeCo layer consisting of crystalline (110) and amorphous grains as confirmed by HR-TEM. The micromagnetic simulations show in general how the thickness of the FeCo layer and the interface roughness between the hard and soft magnetic phases both control the effectiveness of exchange coupling in an exchange spring system.