Laser induced crystallization of Co\u2013Fe\u2013B films

Maria Almeida,Sandra Busse,Olav Hellwig,Patrick Matthes,Apoorva Sharma,Dietrich R T Zahn,Georgeta Salvan,Stefan E Schulz,Matthias Müller,Alexander Horn,Nicole Köhler

doi:10.1038/s41598-021-93009-x

Abstract

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices. In the case of Co–Fe–B TMR devices, used most successfully so far in applications and devices, Co–Fe–B layers are initially deposited in an amorphous state and annealed post-deposition to induce crystallization in Co–Fe, thereby increasing the device performance. In this work, first direct proof of locally triggered crystallization of 10 nm thick Co–Fe–B films by laser irradiation is provided by means of X-ray diffraction (XRD) using synchrotron radiation. A comparison with furnace annealing is performed for benchmarking purposes, covering different annealing parameters, including temperature and duration in the case of furnace annealing, as well as laser intensity and scanning speed for the laser annealing. Films of Co–Fe–B with different stoichiometry sandwiched between a Ru and a Ta or MgO layer were systematically assessed by XRD and SQUID magnetometry in order to elucidate the crystallization mechanisms. The transformation of Co–Fe–B films from amorphous to crystalline is revealed by the presence of pronounced CoFe(110) and/or CoFe(200) reflexes in the XRD θ-2θ scans, depending on the capping layer. For a certain window of parameters, comparable crystallization yields are obtained with furnace and laser annealing. Samples with an MgO capping layer required a slightly lower laser intensity to achieve equivalent Co–Fe crystallization yields, highlighting the potential of laser annealing to locally enhance the TMR ratio.

Highlights

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices
Co–Fe–B alloys have found a wide range of applications in spintronic devices, such as for biomedical sensing platforms, magnetic recording, and communication a pplications[1,2,3,4]. These alloys have proven to be fundamental in the development of magnetic tunnel junctions (MTJs), providing large tunneling magnetoresistance (TMR) ratios using an MgO barrier
First direct proof of the locally triggered crystallization of Co–Fe–B thin films is provided by X-ray diffraction (XRD) measurements performed with synchrotron radiation at BESSY II endstation

Summary

Introduction

Local crystallization of ferromagnetic layers is crucial in the successful realization of miniaturized tunneling magnetoresistance (TMR) devices. In the context of TMR devices, this annealing step is typically employed to induce the Co–Fe–B crystallization and to pin the magnetization of one of the ferromagnetic layers via the exchange bias effect with an additional antiferromagnetic layer[11]. The non-destructive nature of XRD measurements and flexibility in measurement geometries present at KMC-2 enabled a detailed comparison over a wide range of oven and laser annealing parameters This allowed a range of laser intensities and laser scanning speeds to be determined, where comparable levels of crystallization of Co–Fe–B are achieved, and to identify differences arising from a dynamic heating process with inherently large thermal gradients as common for fast, localized laser annealing, paving the way for better and more flexible future applications with TMR devices

Methods

Results

Conclusion