Asymmetric magnetism at the interfaces of MgO/FeCoB bilayers by exchanging the order of MgO and FeCoB

TL;DR

This study investigates the asymmetric magnetic and structural properties at the interfaces of MgO/FeCoB bilayers, revealing differences in electron density, hyperfine fields, and magnetic anisotropy due to interface structure and exchange coupling.

Contribution

It provides a detailed analysis of interface asymmetry in MgO/FeCoB bilayers using XSW and NRS, highlighting the impact of boron diffusion and exchange coupling on magnetic properties.

Findings

Asymmetric electron density and hyperfine fields at interfaces.

High-density FeCoB layer formation at MgO/FeCoB interface.

Unusual angular-dependent magnetic properties explained by exchange coupling.

Abstract

Interfaces in FeCoB/MgO/FeCoB magnetic tunnel junction play a vital role in controlling their magnetic and transport properties for various applications in spintronics and magnetic recording media. In this work, interface structures of a few nm thick FeCoB layers in FeCoB/MgO and MgO/FeCoB bilayers are comprehensively studied using x-ray standing waves (XSW) generated by depositing bilayers between Pt waveguide structures. High interface selectivity of nuclear resonance scattering (NRS) under the XSW technique allowed measuring structure and magnetism at the two interfaces, namely FeCoB-on-MgO and MgO-on-FeCoB, yielding an interesting result that electron density and hyperfine fields are not symmetric at both interfaces. The formation of a high-density FeCoB layer at the MgO/FeCoB (FeCoB-on-MgO) interface with an increased hyperfine field (~34.65 T) is attributed to the increasing volume of FeCo at the interface due to boron diffusion from 57FeCoB to the MgO layer. Furthermore, it caused unusual angular-dependent magnetic properties in MgO/FeCoB bilayer, whereas FeCoB/MgO is magnetically isotropic. In contrast to the literature, where the unusual angular dependent in FeCoB based system is explained in terms of in-plane magnetic anisotropy, present findings attributed the same to the interlayer exchange coupling between bulk and interface layer within the FeCoB layer.