XPS and XAS investigations of multilayer nanostructures based on the amorphous CoFeB alloy
Introduction
By the early 90′s experimental evidence of quantum-dimensional effects in the optical and electrical properties of multilayer nanostructures (MLNS) with the thickness of layers of amorphous hydrogenated silicon a-Si:H ≤ 5 nm was presented in [1].
Around the same time, there was an interest in granulated nanocomposites and MLNS with magnetic metals, due to their new electromagnetic properties, which differ significantly from the properties of similar substances in the macroscopic state [2].
The new characteristics are associated not only with a decrease in the size of the metal clusters themselves, but also with the wave nature of the transfer processes, as well as interatomic interactions on the interphase boundaries and interlayer interfaces, and other factors. In particular, in multilayer structures of composite layers with d-metals and interlayers of non-magnetic materials, transitions are observed from the superparamagnetic and antiferromagnetic states of an individual layer to the ferromagnetic ordering of the entire multilayer system, as well as a periodic change in the magnetic interaction from antiferromagnetic to ferromagnetic between metal layers with varying interlayers thicknesses [[3], [4], [5]].
Undoubtedly, the element base, based on the use of various low-dimensional structures, is the most promising for electronic technology of new generations [6]. The main question is the development of technological processes of production, allowing to obtain with high precision low-dimensional materials with a given structure at the atomic level. Thus, in order to achieve progress in this area, it is necessary to have information both on the atomic structure of the investigated nanosystems and on the nature and hierarchy of chemical bonds formed in the formation process.
One of the first studies of interatomic interactions in the surface layers of MLNC (Co45Fe45Zr10/a-Si)40 and (Co45Fe45Zr10/SiO2)32 using XPS (X-ray photoelectron spectroscopy) was our work [7].
The XPS spectra in this work were obtained on the Russian-German channel of the BESSY II synchrotron (Berlin). A MUSTANG station equipped with a SPECSPHOIBOS 150 analyzer and equipped with an OMICRON cassette sample loading system was used to register MLNS XPS. Measurements were made at photon energies of 800 eV and photon flux of 1012–1013photon/s. XPS studies of Co and Fe metals were represented by the Co3p1/2,3/2 and Fe 3p1/2,3/2 spectra, since we were not able to get the Co2p1/2,3/2 spectra on this channel.
The results obtained showed that in the surface layers of both MLNS metal clusters are surrounded by oxide shells, regardless of the nominal composition of the interlayers of amorphous silicon or silicon oxide. Therefore, the system is in a superparamagnetic state, in which negative magnetoresistance can be manifested.
Then we went on to study two types of MLNS [(Co40Fe40B20)60C40/SiO2]200 and [(Co40Fe40B20)34(SiO2)66/C]46, consisting of alternating metal-composite layers and non-metal interlayers.
Using various non-destructive research methods, such as X-ray diffraction (XRD), small-angle X-ray diffraction (SAXD), ultrasoft X-ray emission spectroscopy (USXES), and infrared (IR) spectroscopy, we obtained data on the electronic structure and phase composition of the studied samples [8,9]. Both MLNS [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 were found to have an amorphous structure, regardless the thickness of the metal-composite layers and nonmetal interlayers. At the same time, the samples with SiO2 interayers and carbon in composite layers with metal clusters had better planarity of interfaces than in MLNS with carbon interlayers and silicon dioxide in metal- composite layers. Modeling of USXES SiL2,3-spectra of silicon showed the formation of suboxide phase in both MLNSs without noticeable formation of metal silicides on the interfaces, which may be associated with the carbon blocking action.
In [10] we investigated interatomic interactions in the same MLNSs by method of IR spectroscopy, which showed that in the MLNS [(CoFeB)60C40/SiO2]200 the SiO2 interlayer makes the main contribution to the formation of infrared spectra. The formation of other bonds with oxygen and silicon blocks carbon in the composition of the composite layers (CoFeB)60C40, as evidenced by the presence of the most intense of composite bond mode corresponding to boron carbide B-C, and a low intensity SiC mode, related to bonds on the interface composite layer/interlayer SiO2.
In MLNS [(CoFeB)34(SiO2)66/C]46, a significant redistribution of the intensity of the IR spectra in the low-frequency region is caused by the formation intra–composite of CoSiO and CoFeO bonds at the interphase boundaries of metal clusters with the SiO2 matrix. At the same time, carbon, being in interlayers between composite layers, much less interacts with boron atoms of composite layers.
The purpose of this work is to obtain data on the local environment and chemical bonds of the Co and Fe atoms of metal clusters in the MLNS [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 depending on the matrix composition of the composite layers in the form of carbon or silicon dioxide and inverse change of the atomic composition of the interlayers.
Section snippets
Experimental
MLNSs [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 were obtained by ion-beam sputtering [2] (with a layer/interlayer thickness gradient) on a glass-ceramic substrate of two targets [6], one of which consisted of an amorphous Co40Fe40B20 alloy with graphite or quartz inserts, depending on the desired composition of the metal-containing composite layer, and the second was a quartz or graphite plate for sputtering interlayers. To create a gradient in the thickness of composite layers and
XPS studies of the MLNS [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46
The main purpose of the XPS study of two MLNSs [(CoFeB)60C40/SiO2]200 and [(CoFeB)34(SiO2)66/C]46 was to determine the nature of the chemical bonds of metal clusters with atoms in the composite layers and between composite layer/interlayer atoms in the interface. The latter influence formation of the CoFeB magnetic clusters in both structures.
In the Fig. 1, Fig. 2 shows the survey XPS spectra of the two MLNSs, registered from non-etched samples and after removal of the surface layers within
Conclusions
Information obtained using the XPS spectra of core levels of all elements of two MLNSs with the same elemental composition but inverse arrangement of nonmetallic phases C and SiO2 in metal-composite layers or interlayers showed that, as a result of self-organization, metal clusters of CoFeB form chemical bonds primarily on interfacial boundaries with the elements of the surrounding matrix.
As a result, predominantly carbide/carboboride shells in a carbon matrix or mainly oxide/oxyboride shells
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The work is supported by the Ministry of Science and Education of the Russian Federation under State Task for Higher Education Organizations in Science for 2020-2022 (Project FZGU-2020-0036).
This study was performed using equipment of the Shared Use Centre “Centre of Physical and Physicochemical Methods of Analysis and Study of the Properties and Surface Characteristics of Nanostructures, Materials, and Products” UdmFRC UB RAS" supported bythe Ministry of Science and Education of the Russion
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