Annotation of the results obtained in 2023
The single-layer films and multilayer magnetic systems studied during the reporting period were deposited onto glass substrates using magnetron sputtering of targets of appropriate compositions. Magnetic anisotropy in the layer plane was induced by an external magnetic field present during film deposition or due to oblique deposition. The structure of the films was studied by X-ray diffraction. The magnetic properties of the samples were studied using various vibrating sample magnetometers and a magneto-optical Kerr microscope. In Fe20Ni80 film elements with a diameter of 3 to 7 mm, obtained by magnetron sputtering in the presence of a constant magnetic field of various configurations, induced magnetic anisotropy occurs in the plane of the samples. The orientation of the easy magnetization axis is determined by the orientation of the magnetization of the film during its deposition (M-induced anisotropy). The presence of a field created by a cylindrical permanent magnet, the magnetization of which is oriented perpendicular to the plane of the substrate, leads to the formation of a radial distribution of easy magnetization axes in the film plane, which contributes to the formation of a vortex-like magnetic structure in the element. In this case, the magnitude of the magnetic field in the range of fields created by permanent magnets has practically no effect on the features of the magnetic structure formed in the element. The only limitation is the magnitude of the in-plane component of the magnet field in the substrate region, which must be greater than the magnetic anisotropy field of the deposited film. By changing the distance between the magnet and the substrate, you can vary the diameter of the element in which the radial (vortex-like) magnetic structure is realized. For multilayer film systems in which magnetic anisotropy was induced by oblique deposition, it was found that a decrease in the angle between the axes of magnetic anisotropy of adjacent layers (accompanied by an increase in the number of layers) promotes the formation of helical magnetic anisotropy in the sample along the thickness of the sample. Based on an analysis of the magnetic properties of FeNi/NiCu/FeNi film systems, in which the intensity of the exchange interaction between the ferromagnetic FeNi layers was varied by changing the chemical composition of the NiCu alloy or temperature, it was found that in the nominally paramagnetic state of the NiCu interlayer, a three-layer system with parallel axes of magnetic anisotropy in FeNi layers behaves as a single whole, while its coercivity turns out to be less than that of a single-layer FeNi film, the thickness of which is equal to the thickness of the FeNi layer in a three-layer system. Magnetization reversal of a sample with orthogonal axes of anisotropy of FeNi layers occurs in a complex manner, which is most likely a manifestation of an inhomogeneous magnetic structure throughout the thickness of the sample, formed under the influence of the magnetic anisotropy of individual FeNi layers and interlayer interaction through the weakly magnetic NiCu interlayer, and the anisotropy of individual layers has a greater influence on the nature of the magnetization reversal of the sample than the interlayer interaction. Using the example of four-layer permalloy films containing FeNi layers (40 nm) with orthogonal orientation of the anisotropy axes, separated by non-magnetic Cr spacers, the thickness of which varied in the range from 0.2 nm to 5 nm, the influence of the intensity of direct exchange interaction between ferromagnetic layers on the formation of effective magnetic anisotropy of multilayer systems. The intensity of the direct exchange interaction was varied by changing the thickness of the Cr interlayer, and both the degree of continuity of the interlayer (the area of direct contact between the ferromagnetic layers) and the probability of the formation of a ternary ferromagnetic FeNiCr alloy of variable composition changed. It was found that with a decrease in the intensity of the direct exchange interaction, the effective anisotropy of the multilayer system is formed under the influence of a set of interlayer interactions: direct exchange, magnetostatic interaction due to the roughness of the interlayer boundaries, and interaction of the RKKY type. The result of this is a nonmonotonic dependence of the features of the magnetic anisotropy of the multilayer system on the thickness of the Cr interlayer. In the process of working on the project, the magnetic properties of single- and multilayer samples of multilayer structures with parallel and multidirectional easy axes magnetization of individual layers were prepared in the form of rectangles and squares with side lengths varying in the range from 3 mm to 22 mm, strips with lengths from 5 mm to 10 mm and width from 0.5 mm to 2 mm, circles with diameter from 3 mm to 7 mm. The shape and dimensions of the samples were specified using substrates of various sizes, deposition through masks, and also using the method of optical lithography. A comparative analysis of the magnetic and high-frequency properties, domain structure of both single-layer films and multilayer elements showed that the magnetic anisotropy of the shape of a sample of a multilayer film system as an additional parameter has a weak effect on the possible formation of a chiral magnetic structure. The main role in this is played by the magnetic anisotropy of individual layers and the intensity of interlayer interaction. A comparative analysis of the results of high-frequency properties (ferromagnetic resonance, spin-wave resonance and magnetic impedance) of anisotropic multilayer systems showed that a multilayer film system can be considered as an effective medium with a certain dispersion of average parameters, while the dispersion of parameters in such a complex layered structure should be considered as something natural and inevitable. Total combination of the results obtained in the project shows the promise of creating multilayer film systems with a chiral (helical) magnetic structure by varying the anisotropic properties of individual layers obtained by inclined deposition and interlayer interaction.

 

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Annotation of the results obtained in 2022
A laboratory technology has been developed and adjusted for based on two sputtering equipments: URM3.279 and ORION 8 UHV. It was possible to obtain multilayered magnetic structures by ion-plasma sputtering of the targets of appropriate compositions with controlled relative orientation of the axes of induced magnetic anisotropy in layers. It is shown that oblique deposition does not introduce noticeable changes of the crystal structure of the films in comparison with the structure of the films obtained by vertical deposition. At the same time, there were differences in the microstructure of the samples deposited by various methods. In particular, the root-mean-square roughness of FeNi films obtained by oblique sputtering is approximately two times higher than that one for the films prepared by vertical sputtering deposition. In addition, the images of the surface of the samples obtained using an atomic force microscope indicate that an extra “strip” relieve is formed in the films obtained by oblique deposition, the intensity of which increases with an increase of the film thickness. The presence of a magnetic field applied during film deposition also does not noticeably affect the structural state of the films. It has been established that for the films obtained by ion-plasma sputtering in the presence of a magnetic field, regardless their composition and crystal structure, the axis of the induced magnetic anisotropy in the plane coincides with the axis of the applied magnetic field. The exception was the films with high crystallographic magnetic anisotropy. These facts fit into the model of the so-called M-induced anisotropy, in which the sample magnetization and its orientation during film deposition are considered to be an origin of the magnetic anisotropy. For soft magnetic permalloy films, it was found that the field of induced magnetic anisotropy decreases by about a factor of two for the films deposited in a rotating magnetic field with a frequency of 60 rpm in comparison with films deposited in a stationary magnetic field. For FeNi films obtained by oblique deposition, a non-monotonic dependence of the induced magnetic anisotropy field on the film thickness was found. It was due to the changes in the microstructure and type of the domain structure of the films appeared with the change of the thickness. It was shown that for the films deposited in the presence of two sources of induced magnetic anisotropy (a magnetic field parallel to the substrate plane and oblique deposition), the main factor determining the orientation of the axis of the induced magnetic anisotropy of films was the magnetic field applied during the deposition. The source of the perpendicular magnetic anisotropy in the studied FeNi films obtained by magnetron sputtering is their columnar microstructure. It has been experimentally established that the thickness dependence of the induced magnetic anisotropy field of such films can be described by the Murayama formula. In this case, the value of the perpendicular magnetic anisotropy constant does not depend on the film thickness. Nanostructuring of the relatively thick “transcritical” FeNi films by Fe and Co interlayers having a crystal structure different from permalloy prevents the transition of the film to the “transcritical” state only for Co interlayers with a thickness above 50 nm. In this case, a significant increase in the coercive force of the multilayer sample is observed. For four-layered FeNi films, in which the EMAs of neighbouring layers are orthogonal, an effective magnetic anisotropy and the EMA oriented at an angle of 45° to the EMA of individual layers were observed. An estimate was made for the interlayer coupling constant of such films (103 erg/cm2). The value of the magnetic anisotropy field of such a sample turned out to be smaller than that of a single-layer film, the thickness of which is equal to the thickness of the layer of a multilayer sample. This result gives the prospect for obtaining the quasi-isotropic multilayer films by increasing the number of layers and decreasing the angle between the magnetic anisotropy axes of neighbouring layers. For Co/W/Co films, a non-monotonic dependence of the saturation field Hs on the interlayer thickness W was found. The value of the interlayer exchange interaction constant (J = 0.1 erg/сm2) corresponding to the first antiferromagnetic maximum in the dependence of Hs on the interlayer thickness W was determined. This value is in good agreement with the value of J observed for the Co/Cu/Co system. The tendency of Hs to decrease with an increase of the W interlayer thickness is described by a function inversely proportional to the square of the interlayer thickness, which is a sign of interlayer interaction of the RKKI type. An increase in Hs at an interlayer thickness of more than 3 nm is most likely associated with structural changes in the W interlayer, with an increase in the thickness of which the transition of the -phase (A15 type structure) to the W α-phase was observed, and the roughness of interlayer boundaries up to the formation of regions of a high coercive CoW solid solution, which, in turn, leads to an increase of the value of Hs. For Co multilayered films with a Gd interlayer, no change in the structure of the samples was observed with a change of the thickness of the interlayer. It was shown that, at an interlayer thickness of less than 3 nm, the Co/Gd/Co system remagnetises as a whole, and at large interlayer thicknesses, a layer-by-layer magnetization reversal of the sample occurs. A sharp change in the appearance of the domain structure obtained using a magneto-optical microscope in the range of interlayer thicknesses of 2–3 nm confirms this fact. For FeNi/Cr multilayered films, X-ray reflectometry showed that the introduction of even ultrathin (less than 1 nm) Cr interlayers ensures the formation of clear interlayer boundaries between FeNi layers. During oblique deposition and when the substrate holder is rotated by 90 degrees before deposition of each subsequent layer of permalloy, with a thickness of Cr interlayers of more than 2 nm, an individual axis of easy magnetization is formed in each individual FeNi layer. At smaller interlayer thicknesses, a complex system of local anisotropy axes is formed in multilayer films. Ferromagnetic resonance (FMR) studies showed the possibility to analyse and reveal the inhomogeneity of magnetic anisotropy in the films under consideration. Some of the results obtained during the project development were mentioned in the article published in the newspaper of the Siberian Branch of the Russian Academy of Sciences "Science in Siberia" (https://www.sbras.info/sites/default/files/2022-07/28.pdf).

 

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