Increase of Gilbert damping in Permalloy thin films due to heat-induced structural changes

Frank Schulz,Filip Lisiecki,Hubert Głowiński,Nick Träger,Joachim Gräfe,Eberhard Goering,Piotr Kuświk,Robert Lawitzki,Gisela Schütz

doi:10.1063/5.0049804

Abstract

Spin-wave based computing requires materials with low Gilbert damping, such as Ni80Fe20 (Permalloy) or yttrium iron garnet, in order to allow for spin-wave propagation on a length scale comparable to the device size. Many devices, especially those that rely on spin–orbit effects for operation, are subject to intense Joule heating, which can exacerbate electromigration and induce local phase changes. Here, the effect of annealing on the Gilbert damping coefficient α of 36 nm Py thin films grown on a Si substrate is examined. Ferromagnetic resonance measurements, high resolution transmission electron microscopy, as well as energy dispersive x-ray spectroscopy have been employed to determine α while also studying structural changes in the thin films. The Gilbert damping parameter was found to increase sixfold when annealed at 350 °C, which was linked to the diffusion of Ni atoms into the Si substrate on a length scale of up to 50 nm. The results demonstrate that magnonic devices have to be treated with caution when Joule heating occurs due to its detrimental effects on the magnonic properties, but the effect can potentially be exploited in the fabrication of magnonic devices by selectively modifying the magnonic damping locally.

Highlights

The results demonstrate that magnonic devices have to be treated with caution when Joule heating occurs due to its detrimental effects on the magnonic properties, but the effect can potentially be exploited in the fabrication of magnonic devices by selectively modifying the magnonic damping locally
A more than sixfold increase of the Gilbert damping has been observed in 36 nm thin Ni80Fe20 layers grown on Si when the samples are annealed above 300 C, as well as a reduction of the effective magnetization by 28%. These findings have been linked to structural changes of the sample, studied by TEM and energy dispersive x-ray (EDX) measurements
These revealed that Ni atoms migrate from the Ni–Fe layer into the Si substrate, forming a Ni–Si layer of thicknesses up to 61 nm, while Fe atoms are pushed back from the Ni–Fe/Si interface

Summary

INTRODUCTION

The emerging field of magnonics has attracted great attention due to the possibility of wave-based information processing, utilizing the amplitude, as well as the phase of spin waves, without the drawback of heat production caused by moving electrons.[1,2,3,4] This may give spin-wave based computing an edge over conventional CMOS technology for certain applications, such as neuromorphic computing and low energy consumption devices.[5,6]. When combining magnonics with spintronics, which offers a large additional tool set for the control and manipulation of spin waves, the required current densities are usually very high, heating up the sample significantly.[12] This rise in temperature can lead to irreversible structural changes within the magnetic thin films rendering the device unusable, which will be discussed in detail in this work. While the effects of vacuum annealing on magnonic devices have not received much attention in the literature, which this work attempts to address, the structure of Ni thin films grown on a Si substrate has previously been studied by Julies et al.[13] They observed the formation of nickel silicides, with Ni–Si forming at approximately 350 C, well below the eutectic temperature[14] of the system. This could be done using a focused laser spot or the tip of an atomic force microscope (AFM) probe to obtain structures on a nanometer length scale, opening up many possibilities for new magnonic devices that require alternating magnonic properties, such as magnonic crystals.[17]

EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION

SUMMARY