Quantification of residual stress governing the spin-reorientation transition (SRT) in amorphous magnetic thin films

Highlights

• Tensile stress caused spin-reorientation transition in amorphous multilayers.

• Degree of SRT was directly related to the temperature difference of thermal shock.

• SRT in amorphous magnetic multilayers can deteriorate uniaxial magnetic properties.

• Stress-induced SRT should be avoided in post-processing for technology applications.

• This magnetoelastic anisotropy loosely follows Landau’s phenomenological model.

Abstract

Soft magnetic thin films with in-plane uniaxial magnetic anisotropy are of significant importance for a broad range of technological applications, including high-frequency power conversion. In-plane uniaxial anisotropy in amorphous films is of particular interest for ultra-low materials loss and MHz frequency operations. The present work is focused on one of the fundamental mechanisms, i.e., residual stress, that can negate the uniaxial anisotropy in amorphous films by engendering perpendicular magnetisation and hence, undermining the soft magnetic performance. It is quantified how the nature of residual stress, compressive or tensile, transforms the magnetisation from an in-plane to an out-of-plane configuration, also well-known as spin-reorientation transition (SRT). A correlation between engineered residual stress in multilayer stacks, induced by the uneven expansion of metallic/dielectric layers following a thermal-shock scheme, and SRT mechanism demonstrates tensile stress inside the films undermines the soft magnetic performance. We suggest the magnetic softness can be retained by eluding sources of tensile stress during fabrication or post-processing of the amorphous films.

Introduction

Soft magnetic amorphous thin films are a potential candidate for the inductor/transformer core applications, especially when high flux density (B_s > 1 T) materials are the focus of interest for the miniaturisation of magnetic components for the power supplies on chip (PowSOC) concept [1], [2]. Lamination of these films with thicknesses below the skin depth, with a suitable insulator, is a well-adopted approach for an optimal material performance [2], [3]. These materials need to retain in-plane uniaxial magnetic anisotropy for minimal energy losses, attributed to the magnetisation reversal through coherent magnetisation rotation instead of domain wall displacement, and high-frequency drive operations [4], [5]. Uniaxial anisotropy of thin films can suffer from many intrinsic and extrinsic factors, such as demagnetisation effects from material dimensions (shape anisotropy), magnetostriction constant of the alloys, and the effect of deposition process/parameters [6], [7]. Consequently, uniaxial anisotropy can be suppressed by perpendicular magnetic anisotropy (PMA), which eventually redefines the magnetisation orientation to be perpendicular to the plane of films. Materials exhibiting PMA are of interest in the area of perpendicular recording media where a higher recording density can be achieved and also in novel spintronic applications such as magnetic tunnel junctions [8], [9]. Nevertheless, this work highlights the applications where PMA contributions are undesired. In such a case, the PMA significantly undermines the functionality of thin-film materials (i.e. low permeability, high materials loss, multimodal ferromagnetic resonance behaviour) for high-frequency applications. Therefore, understanding the mechanisms contributing to the PMA, in order to retain the uniaxial anisotropy in-plane is of significant importance for high-frequency applications, including electrodynamic energy conversion, magnetic sensing and magnetic shielding [5], [10].

Several mechanisms have been proposed as an origin of the perpendicular magnetisation in amorphous films, such as high degrees of atomic growth processes, large magnetostriction constants of the alloy and residual stress produced due to the nature of the fabrication process, such as magnetron sputtering [11], [12], [13]. Furthermore, perpendicular magnetisation in amorphous films depends on film thickness [11], [13], alloy composition [11], method of the fabrication process [11], [12], [13], substrate temperature [11] and post-annealing conditions. An important example is magnetron sputtered Co-Fe-Zr-Ta amorphous system where a thickness-dependent change in the residual stress at micro-scale plays an important role and, eventually, transforms the magnetisation in the perpendicular direction, well-known as a spin-reorientation transition (SRT) [10], [14]. In addition to the micro-scale stress, the strain induced at the substrate-film interface contributes significantly to the perpendicular magnetisation, being another detrimental contribution to soft magnetic properties [8], [15]. This was attributed to the interfacial anisotropy at small thickness with gradually increasing magnetoelastic anisotropy at higher thicknesses [13]. Nevertheless, interfacial anisotropy is prominent mainly on ultra-thin (<25 Å layer system; hence it can be ignored for thick amorphous films. Sharma et al. [11] and Marco et al. [13], separately, reported that the SRT in amorphous films may have origin in residual stress and if reduced by thermal annealing the magnetisation can transform back to the in-plane orientation. However, a comprehensive analysis of stress-induced SRT requires further investigations to understand how the residual stress (the magnitude as well as its nature) overcomes the in-plane uniaxial anisotropy and configures the magnetisation in perpendicular orientation.

Thermal shock in multilayer laminations, can cause a significant expansion between the magnetic layers, dielectric and the substrate, especially when laminated stack contains several layers and there is a substantial difference in the thermal expansion coefficients of the materials. This can induce a large residual stress in the system that, eventually, can work as a system under “stress-induced magnetoelastic anisotropy” in the plane of the compression or tensile stress [6], [16]. Hence, this type of system provides an opportunity to quantify how residual stress in amorphous thin films contributes to the SRT, and, consequently, undermines the material performance for high-frequency applications. Currently, soft magnetic Co-Zr-Ta-B/AlN multilayer system is considered as an excellent candidate for it its application as inductor/transformer cores for integrated power conversion applications. However, the how the residual stress induced by fabrication process or during device packaging can undermines the soft magnetic properties requires further investigations. In the present work, an engineered residual stress in an amorphous multilayer Co-Zr-Ta-B/AlN stack, induced by following a thermal shock scheme from various temperatures, was correlated to the SRT to quantify how its nature transforms the magnetisation from in-plane to out-of-plane orientation. This work demonstrates that tensile stress in thin films must be eluded to avoid its detremental effects on the soft magnetic performance for high-frequency applications.