nanomaterials Article Efficiency of Magnetostatic Protection Using Nanostructured Permalloy Shielding Coatings Depending on Their Microstructure Tatiana Zubar 1,2, * , Sergey Grabchikov 1 , Anna Kotelnikova 1 , Egor Kaniukov 3 , Maksim Kutuzau 4 , Karin Leistner 4,5, * , Kornelius Nielsch 4,5 , Tatiana Vershinina 6 , Daria Tishkevich 1,2 , Oleg Kanafyev 1 , Artem Kozlovskiy 7 , Maxim Zdorovets 7,8,9 , Valery Fedosyuk 1 and Alex Trukhanov 1,2,3 1 Laboratory of Magnetic Films Physics, Scientific-Practical Materials Research Centre of National Academy of Sciences of Belarus, 220072 Minsk, Belarus; gss@physics.by (S.G.); anna.kotelnikova.98@mail.ru (A.K.); dashachushkova@gmail.com (D.T.); olegkan96@mail.ru (O.K.); fedosyuk@physics.by (V.F.); truhanov86@mail.ru (A.T.) 2 Laboratory of Single Crystal Growth, South Ural State University, 454080 Chelyabinsk, Russia 3 Department of Technology of Electronics Materials, National University of Science and Technology MISiS, 119049 Moscow, Russia; ka.egor@mail.ru 4 Leibniz IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany; m.kutuzau@ifw-dresden.de (M.K.); k.nielsch@ifw-dresden.de (K.N.) 5 Institute of Material Science, TU Dresden, 01062 Dresden, Germany 6 Joint Institute for Nuclear Research, 141980 Dubna, Russia; vershinina@nf.jinr.ru 7 The Institute of Nuclear Physics, Almaty 050032, Kazakhstan; artem88sddt@mail.ru (A.K.);  mzdorovets@gmail.com (M.Z.)  8 Engineering Profile Laboratory, L.N. Gumilyov Eurasian National University, Nur-Sultan 010008, Kazakhstan 9 Department of Intelligent Information Technologies, The Ural Federal University, 620002 Yekaterinburg, Russia Citation: Zubar, T.; Grabchikov, S.; * Correspondence: fix.tatyana@gmail.com (T.Z.); k.Leistner@ifw-dresden.de (K.L.) Kotelnikova, A.; Kaniukov, E.; Kutuzau, M.; Leistner, K.; Nielsch, K.; Abstract: The effect of microstructure on the efficiency of shielding or shunting of the magnetic flux Vershinina, T.; Tishkevich, D.; by permalloy shields was investigated in the present work. For this purpose, the FeNi shielding Kanafyev, O.; et al. Efficiency of coatings with different grain structures were obtained using stationary and pulsed electrodeposition. Magnetostatic Protection Using The coatings’ composition, crystal structure, surface microstructure, magnetic domain structure, and Nanostructured Permalloy Shielding Coatings Depending on Their shielding efficiency were studied. It has been shown that coatings with 0.2–0.6 µm grains have a Microstructure. Nanomaterials 2021, disordered domain structure. Consequently, a higher value of the shielding efficiency was achieved, 11, 634. https://doi.org/10.3390/ but the working range was too limited. The reason for this is probably the hindered movement of the nano11030634 domain boundaries. Samples with nanosized grains have an ordered two-domain magnetic structure with a permissible partial transition to a superparamagnetic state in regions with a grain size of less Academic Editor: Alexander Tselev than 100 nm. The ordered magnetic structure, the small size of the domain, and the coexistence of ferromagnetic and superparamagnetic regions, although they reduce the maximum value of Received: 21 January 2021 the shielding efficiency, significantly expand the working range in the nanostructured permalloy Accepted: 1 March 2021 shielding coatings. As a result, a dependence between the grain and domain structure and the Published: 4 March 2021 efficiency of magnetostatic shielding was found. Publisher’s Note: MDPI stays neutral Keywords: permalloy; pulsed electrodeposition; nanostructured coating; microstructure; magneto- with regard to jurisdictional claims in static shielding published maps and institutional affil- iations. 1. Introduction Copyright: © 2021 by the authors. The issue of creating effective electromagnetic shields for protection from both mag- Licensee MDPI, Basel, Switzerland. netostatic fields and electromagnetic radiation is critically essential nowadays [1–6]. The This article is an open access article level of the electromagnetic background, caused by the rapid development of electri- distributed under the terms and cal, radio-electronic, transport, information, and military technology, has significantly conditions of the Creative Commons increased, the range of used frequencies of electromagnetic radiation has expanded, and Attribution (CC BY) license (https:// their amplitude has increased. On the other hand, the development of radio-electronic creativecommons.org/licenses/by/ and information technology, research, and intelligent complexes is moving toward mini- 4.0/). mizing the mass and dimensions as well as increasing the density of their arrangement. Nanomaterials 2021, 11, 634. https://doi.org/10.3390/nano11030634 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 634 2 of 13 All this significantly complicates the problem of the formation of an electromagnetic en- vironment that provides the regular functioning and electromagnetic compatibility of electrical and radio-electronic complexes, which requires the development of new special- ized materials that ensure reliable and stable functioning under conditions of intentional and unintentional electromagnetic influences [7–12]. Beyond that, the importance of the problem task of electromagnetic protection of biological objects from the effects of various kinds of electromagnetic radiation and permanent magnetic field is becoming increasingly obvious [13–18]. Analytical calculations assume that magnetic materials with the highest value of magnetic permeability provide better protection. Magnetostatic shielding usually shunts the magnetic field via a ferromagnetic material [11,19]. It essentially closes the force lines through the material with low resistance to the magnetic flux. The Rm resistance value in the magnetic flux uses a shield with µr magnetic permeability, l average length of the magnetic induction lines through the material, and S cross-section in a perpendicular direction to the magnetic flux: Rm = l/µr S. (1) For magnetostatic protection, the shielding efficiently (SEms ) in first principle could be described by equation: SEms = 1 + µr d/D (2) where µr —relative magnetic permeability; d—thickness of the shield; and D—diameter of the cylindrical or spherical shield [11,12]. However, earlier, it was noted [19,20] that the theoretical equations cannot accurately describe the efficiency of the shield; it only characterizes the dependence of the SE on certain parameters. It should be pointed out that growing the shield thickness is always a faultless option in terms of enhancing SE. However, the large size and weight of the protection shield is often unacceptable from a design point of view. Therefore, it is necessary to search for other approaches to increasing the SE. The most commonly used material for creating magnetostatic and electromagnetic shields is soft magnetic alloy of Ni and Fe or permalloy. Ni-rich permalloy has high perme- ability, low coercivity, and small magnetic anisotropy [11,17,19,21–29]. Electrodeposited coatings are promising due to the high economic viability of the electrodeposition pro- cess [29–32]. The ability to deposit a protection coating on the substrates with a complex shape should also be noted among the main advantages of electrodeposition. This is im- portant for using electromagnetic shields, when it is necessary to cover part of the package or complex shape part. In this work, we studied the effect of permalloy structure on the efficiency of shielding (or shunting) of the magnetic flux. No studies were found on this topic in the earlier literature. We assumed that the features of the process of magnetization of ferromagnetic permalloy in a constant magnetic field can have a significant effect on the shielding ef- ficiency. One of the simplest ways to control the magnetization stages of ferromagnetic materials (changing the size of domains, absorbing domains, and ordering the vectors of magnetic moments over the field) is to change the structure of the material. Therefore, we formed FeNi shields with different grain structures and studied their magnetic domain structure using magnetic force microscopy. As a result, a relationship between the domain and grain structure and the efficiency of magnetostatic shielding was found. In addition, the fundamental possibility of controlling the width of the working range and the value of the efficiency of the magnetostatic permalloy shield was demonstrated. 2. Materials and Methods Electrodeposition was used to obtain FeNi shielding coatings. The substrates for the electrodeposition were aluminum alloy cylinders. The cylinder’s length was 100 mm and its external diameter was 30 mm. Aluminum alloy was chosen as more suitable because it has low weight, high mechanical and corrosive characteristics, as well as high electrical Nanomaterials 2021, 11, 634 3 of 13 conductivity required for electrodeposition. The disadvantage is poor adhesion of Al-alloy to coating. For this reason, complex surface preparation was carried out. At the first stage, the surface was cleaned from organic contaminants using trichloroethylene and a mixture of calcium oxide (90 wt %) and magnesium oxide. Then, degreasing and cleaning was carried out in a solution of H25 Na2 O16 P—40g/L, Na2 CO3 —40 g/L, and NaOH—40 g/L at a temperature of 70 ◦ C. After that, the surface was chemically etched in the nitrogen (300 mL/L) and hydrofluoric (100 mL/L) acid solution to remove sludge. The obligatory stage of Al surface preparation is the creation of a thin adhesive layer of Zn. To obtain a uniform and compact layer, zinc is applied chemically in two stages: chemical coating in solution (ZnSO4 —250 g/L, NaOH—450 g/L, and KNaC4 H4 O6 —7 g/L), removal of the upper loose zinc layer in 50% solution of HNO3 , and the deposition of a second Zn layer. The last stage of the surface preparation is the electrolytic deposition of a copper sublayer with a thickness of 3 µm. The solutions for Cu deposition contains cuprous sulfate—30 g/L, potassium phosphate—140 g/L, sodium phosphate trisubstituted dodecahydrate—90 g/L, and sodium potassium tartrate—20 g/L. The Cu-electrolyte temperature was 35 ◦ C, current density was 8 mA/cm2 , and pH level was 8.5. The cupper deposition rate with such parameters is 9 µm/h. The FeNi coatings were deposited from a complex electrolyte containing NiSO4 7H2 O—250 g/L, NiCl2 6H2 O—20 g/L, H3 BO3 —25 g/L, MgSO4 7H2 O—110 g/L, FeSO4 7H2 O—35 g/L, D (+) Glucose—85 g/L, HC6 H7 O6 —3 g/L, and additive NiB1—3 g/L. The temperature was kept at 35◦ C and pH was kept at 2.0. The current density was 35 mA/cm2 . Under these conditions, the deposition rate was 35 µm/h. The described electrodeposition technology makes it possible to obtain permalloy coatings with a nickel and iron ratio of 50–50 at % and satisfactory mechanical and aesthetic characteristics. A description of the technological features of obtaining samples of shields in direct current and pulse modes [33,34] is presented in Table 1. Sample P0 was obtained at direct current for 85 min. Samples P1, P0.1, and P0.01 were obtained in pulsed modes with pulse durations of 1, 0.1, and 0.01 s, respectively. The pause time was equal to the pulse time. Figure 1 shows graphs of changes in current density and potential for pulse modes. Table 1. Technological features of obtaining FeNi shields in direct current (DC) and pulse deposi- tion modes. Pulse Pause Full Deposition Effective Deposition Shield Name Current Duration, s Duration, s Time, Min Time, Min P0 direct - 0 85 85 P1 pulsed 1 1 170 85 P0.1 pulsed 0.1 0.1 170 85 P0.01 pulsed 0.01 0.01 170 85 AZtecLive Advanced with Ultim Max 40 (Oxford Instruments, Bognor Regis, UK) in- vestigated the chemical composition using energy-dispersive X-ray spectroscopy. The crys- tal structure investigation was carried out by X-ray structural analysis on an EMPYREAN (PANalytical, Malvern Instruments, Malvern, UK) powder diffractometer using Cu-Kα radiation in the Bragg–Brentano geometry focusing in the angle range 2θ = 40–100◦ . The sizes of the coherent scattering regions (CSR) were estimated using the Williamson–Hall method for all peaks from the fcc solid solution. The surface microstructure was studied using the scanning electron microscope Zeiss EVO 10 (Zeiss, Oberkochen, Germany) and atomic force microscope (AFM) Bruker Demension Icon (Bruker Corporation, Billerica, MA, USA). Magnetic force microscopy (MFM) imaging was performed in a dual-scan mode with Bruker Demension Icon microscore. The first scan pass was used for the morphological imaging; the second pass was operated at constant height from the surface (100 nm). The advantage of the applied MFM method, in addition to its large lateral resolution, is that long-range magnetic interactions can be virtually excluded [35]. A silicon probe with CoCr thin coating was used. The probe coercivity was about 400 Oe and the magnetic moment was about 10−13 EMU. The AFM tip diameter was about 20 nm. The lateral resolution of Nanomaterials 2021, 11, 634 4 of 13 AFM and MFM images can be higher than the tip’s curvature due to non-contact scanning and the regularity of the surface [36–40]. The resonant frequency during scanning was 4 of 14 Nanomaterials 2021, 11, x FOR PEER REVIEW 80 kHz, and the force constant = 2.8 N/m [41]. The magnetic tip was magnetized by an external magnet and tested with a magnetic calibration grid prior to measurements. Figure 1. Change in current density and potential during pulsed deposition of the (a) P1, (b) P0.1, and (c) P0.01 shields, as Figure 1. Change in current density and potential during pulsed deposition of the (a) P1, (b) P0.1, and (c) P0.01 shields, as well as (d) a schematic representation of pulsed deposition. well as (d) a schematic representation of pulsed deposition. For shielding efficiency (SE) measurements, the cylindrical sample was placed in a AZtecLive uniform Advanced magnetic with by field created Ultim a pairMax 40 (Oxford of Helmholtz Instruments, coils Bognor (Figure 2a). The Regis, UK) calculation investigated theon of SE is based chemical composition measurements of the Hallusing energy-dispersive potential in the protectedX-ray region.spectroscopy. The SE was The determined crystal structureas the ratio magnetic investigation fieldcarried was strengthoutwithout Hext and by X-ray with an H structural int shieldon analysis using an EM- Equations (3) and (4). PYREAN (PANalytical, Malvern Instruments, Malvern, UK) powder diffractometer us- SE= Hext /Hint (3) ing Cu-Kα radiation in the Bragg–Brentano geometry focusing in the angle range 2θ = 40–100°. The sizes of the coherent SE=20 scattering lg(Hext /Hintregions ) [in dB].(CSR) were estimated using (4) the Williamson–Hall method for all peaks from the fcc solid solution. The parameters SEmax , Hmax , and WWR (width of working range) were used to The surface micro- structure quantifywas studied efficiency the shielding using the scanning of the samples.electron Figure 2bmicroscope Zeiss EVO shows schematically how 10 (Zeiss, these Oberkochen, Germany) parameters were and The determined. atomic force value maximum microscope (AFM)efficiency of the shielding Bruker SE Demension max and the Icon (Bruker Corporation, strength of the magneticBillerica, MA, field Hmax USA). , which Magnetictoforce corresponds microscopy this maximum, are(MFM) determinedimaging as the extremum of the function SE(H). WWR is the width of the peak was performed in a dual-scan mode with Bruker Demension Icon microscore. The first of the experimental graph scan passat was half height. used forThe the WWR value characterizes morphological the width imaging; theofsecond the shield’s passworking range. at was operated constant height from the surface (100 nm). The advantage of the applied MFM method, in addition to its large lateral resolution, is that long-range magnetic interactions can be virtually excluded [35]. A silicon probe with CoCr thin coating was used. The probe co- ercivity was about 400 Oe and the magnetic moment was about 10−13 EMU. The AFM tip diameter was about 20 nm. The lateral resolution of AFM and MFM images can be higher than the tip’s curvature due to non-contact scanning and the regularity of the surface [36– 40]. The resonant frequency during scanning was 80 kHz, and the force constant = 2.8 N/m [41]. The magnetic tip was magnetized by an external magnet and tested with a The parameters SEmax, Hmax, and WWR (width of working range) were used to quantify the shielding efficiency of the samples. Figure 2b shows schematically how these parameters were determined. The maximum value of the shielding efficiency SEmax and the strength of the magnetic field Hmax, which corresponds to this maximum, are deter- Nanomaterials 2021, 11, 634 mined as the extremum of the function SE(H). WWR is the width of the peak of5the of 13 ex- perimental graph at half height. The WWR value characterizes the width of the shield’s working range. a b Hall sensor Shield SEmax WWR is width of working range Shielding Efficiency, SE, dB (width on half of height) WWR Protected area H>0 Helmholtz coils Hmax Magnetic FIeld Strength, H, Oe Figure2.2.Schematic Figure Schematicrepresentation representationofofthe theshielding shieldingefficiency efficiencymeasurement: measurement:(a)(a)installation installation forfor shielding shielding efficiency efficiency (SE) (SE) measurement of the cylindrical shields and (b) schematic representation of graph of the change measurement of the cylindrical shields and (b) schematic representation of graph of the change in SE. in SE. 3.3.Results Resultsand andDiscussion Discussion. Theresults The resultsof of the the analysis analysis by by energy-dispersive X-ray spectroscopy spectroscopy are presented in the in theform formofofspectra spectrain in Figure Figure 3. The percentage percentage of ofnickel nickeland andiron ironobtained obtained from from thethe analysisof analysis ofthe thespectra spectraisispresented presentedas asaacaption captionon onthe thecorresponding correspondingspectra. spectra.There Thereisis a aslight slight(less (lessthan than11atat%) %)but butstable stabledecrease decrease in in the the concentration concentration of of Fe Fe with a decrease in in thethe Nanomaterials 2021, 11, x FOR PEER REVIEW pulse pulse duration. duration. TheThe reason reason of of thethe Fe Fe content content decrease decrease is aissignificant a significant change change in in 6 oftech- 14 technological parameters: nological parameters: thethe transitionfrom transition fromstationary stationary to pulsed pulseddeposition depositionmode mode and anda a sharp decrease in the pulse duration. A change in technological parameters sharp decrease in the pulse duration. A change in technological parameters can lead to a can lead to a shift shiftininthe theredox redoxreaction reaction studied [47,48]. This led to an increase Me2+in the→roughness + 2e Me0 and a decrease in the current (5) density during the formation of the coating. Me + 2e → Me0 2+ (5) Different electrodeposition parameters can influence the Fe/Ni ratio in opposite di- a b the oxidation Fe48.3Ni51.7 For example, an increase in the rections. Fe electrolyte temperature intensifies 47.4 Ni 52.6 of Fe ions (transition from Fe2+ to Fe3+). The iron content in the film decreases as a result of oxidation [42,43]. It was previously shown by many authors that during the deposition of binary and ternary alloys [44–46], a composition gradient is observed from the substrate to the coating surface (for the FeNi alloys, a decrease in the nickel content is observed). It is also known that iron is predominantly deposited on the irregularities and boundaries of the substrate and grains in electrodeposited alloys. The reason for the high concentra- tion of iron in “ridge” is the higher current density in this area [47]. It is also reported that keV keV a high substrate roughness leads to a decrease in the iron content, since the current den- sity decreases in terms of the actual surface area (taking into account the roughness). c Thus, the chemical composition is determined by the competition of many d phenomena. A Ni53.2 in the pulse time contributedFe Fe46.8decrease to45.9 Ni54.1 in the Fe content in the considered a decrease case. This is probably due to the structure of the coating surface. It will be shown below that the grain size decreases with decreasing pulse duration, which is natural and widely keV keV Figure 3. Nominal chemical composition of permalloy permalloy shielding shielding coating: coating: (a) P0, (b) (b) P1, P1, (c) (c) P0.1, P0.1, and and (d) (d) P0.01. P0.01. Different Figure 4 andelectrodeposition parameters Table 2 demonstrate results can influence of XRD the Fe/Ni investigations ratio of the in shielding FeNi opposite directions. For example, an increase in the electrolyte temperature intensifies coating obtained by the different modes. The number of well-distinguished peaks can be the oxidation of Fe ions on observed (transition the XRDfrom Fe2+ toThe patterns. Fe3+ ). The most iron content intense in the film decreases and characteristic peaks areas43–44 a result of deg., oxidation 50–51 deg.[42,43]. It was previously (corresponding shown to the atomic by many plane (111)),authors and 74 that deg. during the deposition (the atomic of plane (220)). A decrease in the width of the peaks was observed as the pulse duration was shortened. The evaluations carried out by the Williamson–Hall method showed that the main factor causing the change in the width of the X-ray peaks is the size of the CSR. Table 2 shows that the CSR was 3.9 and 3.3 nm for the samples obtained in the P0 and P1 modes. Then, Nanomaterials 2021, 11, 634 6 of 13 binary and ternary alloys [44–46], a composition gradient is observed from the substrate to the coating surface (for the FeNi alloys, a decrease in the nickel content is observed). It is also known that iron is predominantly deposited on the irregularities and boundaries of the substrate and grains in electrodeposited alloys. The reason for the high concentration of iron in “ridge” is the higher current density in this area [47]. It is also reported that a high substrate roughness leads to a decrease in the iron content, since the current density decreases in terms of the actual surface area (taking into account the roughness). Thus, the chemical composition is determined by the competition of many phenomena. A decrease in the pulse time contributed to a decrease in the Fe content in the considered case. This is probably due to the structure of the coating surface. It will be shown below that the grain size decreases with decreasing pulse duration, which is natural and widely studied [47,48]. This led to an increase in the roughness and a decrease in the current density during the formation of the coating. Figure 4 and Table 2 demonstrate results of XRD investigations of the FeNi shielding coating obtained by the different modes. The number of well-distinguished peaks can be observed on the XRD patterns. The most intense and characteristic peaks are 43–44 deg., 50–51 deg. (corresponding to the atomic plane (111)), and 74 deg. (the atomic plane (220)). A decrease in the width of the peaks was observed as the pulse duration was shortened. The evaluations carried out by the Williamson–Hall method showed that the main factor causing the change in the width of the X-ray peaks is the size of the CSR. Table 2 shows that the CSR was 3.9 and 3.3 nm for the samples obtained in the P0 and P1 modes. Then, an increase in CSR to 5.1 and 5.0 nm is observed for samples P0.1 and P0.01, respectively. It was noted that coatings deposited in pulsed modes have a common feature: the X-ray diffraction patterns show the change in the ratio of the peaks (111) and (200) when switching to pulsed modes. If we take nickel as the starting point, then the ratio of the integral intensities (111) and (200) for a completely disordered state is 46%. As can be seen in Table 1, the peak of the crystallographic plane (200) for sample P0 has a ratio I(111) /I(200) , which is close to the intensity of the completely isotropic material (57%). The most intense peak (111) of the fcc lattice is taken as 100%. The contribution of the peak (200) increases and the ratio I(111) /I(200) reaches 84, 91, and 95% for P1, P0.1, and P0.01 coatings, respectively. It should also be noted that a decrease in the pulse duration leads to an increase in the detected effect. The unit cell parameter (a) and cell volume (V) decrease nonlinearly from 3.578 to 3.567 Å and from 45.80 to 45.38 Å3 with transition from stationary to pulsed electrodeposition and with the pulse duration decreasing. The reason for the compression of the crystal lattice can be two factors: (1) surface compression of grains with a decrease in their size [27,49,50], and (2) the effect of the chemical composition (a decrease in the Fe content with an atomic radius bigger (rFe = 0.156 nm) than Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of that 14 of Ni (rNi = 0.124 nm)). a b c 5000 d Figure 4. Figure 4. XRD XRDpatterns patternsofof FeNi shielding FeNi coatings: shielding (a) P0, coatings: (a)(b)P0, P1,(b) (c)P1, P0.1, (c)and (d)and P0.1, P0.01. (d) P0.01. Table 2. Crystal structure parameters of FeNi shielding coatings obtained with different electrodeposition modes. Shielding Coating CSR, nm I(111)/I(200), % a, Å V, Å3 P0 3.9 57 3.578 45.80 P1 3.3 84 3.570 45.50 Nanomaterials 2021, 11, 634 7 of 13 Table 2. Crystal structure parameters of FeNi shielding coatings obtained with different electrodepo- sition modes. Shielding Coating CSR, nm I(111) /I(200) , % a, Å V, Å3 P0 3.9 57 3.578 45.80 P1 3.3 84 3.570 45.50 P0.1 5.1 91 3.570 45.50 P0.01 5.0 95 3.567 45.38 The surface microstructure of the permalloy shielding coating investigated using SEM is shown in Figure 5. As expected, the grain size decreased upon transition to pulse electrodeposition and the decrease in the pulse duration. While increasing the grain size is more energetically beneficial than the formation of a new growth nucleus, the coating grains grow continuously in stationary electrolysis [34,51]. The coating with a grain size of 500–600 nm was formed (Figure 5a) by stationary deposition. When switching to the pulse mode P1, the grains grow for 1 s. Growth stops when the current is turned off. Then, when the current is switched on again, the growth of predominantly new grains begins [52–54]. The size of grains formed within 1 s ranges from 150 to 300 nm (Figure 5b). A decrease in the pulse duration leads to a decrease in the grain size. Thus, the grain size of the P0.1 Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 14 shield is in the range from 100 to 200 nm, and almost all grains are less than 100 nm for the P0.01 shield. a b 500 nm 500 nm c d 500 nm 500 nm Figure 5. Surface Figure microstructure 5. Surface of FeNi microstructure shields of FeNi shieldsinvestigated using investigated using SEM: SEM: (a) (a) P0, P0, (b)(c) (b) P1, P1,P0.1, (c) and P0.1,(d)and (d) P0.01. P0.01. Figure Figure 6a 6a showsthe shows thechange change in inthe themicrostructure microstructureobtained usingusing obtained AFM. AFM. Figure Figure 6b is 6b is a 3D rendering of the corresponding image of surface topography. The results (surface a 3D rendering of the corresponding image of surface topography. The results (surface structure and grain size) are in good agreement with the SEM results (Figure 5). A number structure and(Figure of images grain6c) size) showarethe in good results agreement of studying with thedomain the magnetic SEM results structure(Figure of the 5). A number ofregions surface imagescorresponding (Figure 6c) to show the images. the AFM results Comparison of studyingof the magnetic Figure domain 6a,c allows us to struc- turedraw of the surface several regions corresponding to the AFM images. Comparison of figures 6a comments. and 6c allows us to draw several comments. 1. There is practically no ordered domain structure for the P0 and P1 modes. High values of magnetization (perpendicular to the coating plane in modulus) correspond to the central regions of grains, and low values correspond to grain boundaries. 2. A two-domain structure (magnetization vectors up and down) is formed within Nanomaterials Nanomaterials 11,11, 2021, 2021, 634 PEER REVIEW x FOR 8 of 1314 9 of Figure Figure6.6.Surface Surfacemicrostructure microstructure of FeNi shielding of FeNi shieldingcoating coatinginvestigated investigated using using atomic atomic force force microscope microscope (AFM)(AFM) and mag- and magnetic netic domain structure investigated using magnetic force microscopy (MFM, scan sizes are 2.3 μm × 2.3 μm): domain structure investigated using magnetic force microscopy (MFM, scan sizes are 2.3 µm × 2.3 µm): (a) 2D surface (a) 2D sur- face microstructure, (b) 3D surface microstructure, (c) 2D domain structure, (d) domain structure superimposed microstructure, (b) 3D surface microstructure, (c) 2D domain structure, (d) domain structure superimposed as a colored as a colored skin on the 3D image of the surface microstructure. skin on the 3D image of the surface microstructure. Figure 6d is a 3D visualization of the surface topography with a contrasting image 1. There is practically no ordered domain structure for the P0 and P1 modes. High superimposed on top as a skin, demonstrating the magnetic structure. This is done to values of magnetization (perpendicular to the coating plane in modulus) correspond better understand the correspondence of the domain structure to the surface micro- to the central regions of grains, and low values correspond to grain boundaries. structure. 2. A two-domain structure (magnetization vectors up and down) is formed within grains Figure 7a 150–200 with size shows thenm)change in the for the P0.1 SE of the cylindrical shields with permalloy coating. coatings 3. One domain is formed in grains with a increase obtained in different modes with an in the magnetic size of 100–150 nm with the fieldmagnetization strength up to 90 Oe.vectors Significant differencestobetween perpendicular the shields the coating disappear plane (purple in a magnetic regions). Anotherfield of more oppositely than 50directed Oe; however, in the range of 0–50 Oe, all shields show different domain is formed near but outside the grain to compensate (red regions). efficiency char- acteristics. The maximum SE (SE ) of the This is observed for coatings P0.1 and P0.01. max P0 shield is 20 dB, and the SE max for the P0.01 shield 4. is 25.5 The dB, or 30 and magnetization 19 times, vectors respectively of grains with a size(Figure 7b, black less than 100 nm line). The lie in themaximum plane of field value (Hmax) changes the coating in the same uniformly and are magnetized way as theorSEnot max;magnetized a linear decrease (light from green8color to 4.6for Oe is observed coatingswith P0.1transition and P0.01).from stationary to pulsed electrodeposition and with de- creasing the pulse Figure 6d is aduration to 0.01 sof(red 3D visualization the line on Figure surface 7b). Nevertheless, topography with a contrastingthe width imageof the working range or WWR (blue line on Figure 7b) of the shields increases superimposed on top as a skin, demonstrating the magnetic structure. This is done to better nonlinearly for the same transition understand of modes of the correspondence from the the P0 coating domain to the structure P0.01 to the coating. surface Thus, the width microstructure. is 24.5,Figure 25.6, 30.5, and 32.8 Oe for the P0, P1, P0.1, and P0.01 samples, respectively. 7a shows the change in the SE of the cylindrical shields with permalloy coatings obtained in different modes with an increase in the magnetic field strength up to 90 Oe. Significant differences between the shields disappear in a magnetic field of more than 50 Oe; however, in the range of 0–50 Oe, all shields show different efficiency characteristics. The maximum SE (SEmax ) of the P0 shield is 20 dB, and the SEmax for the P0.01 shield is Nanomaterials 2021, 11, 634 9 of 13 25.5 dB, or 30 and 19 times, respectively (Figure 7b, black line). The maximum field value (Hmax ) changes in the same way as the SEmax ; a linear decrease from 8 to 4.6 Oe is observed with transition from stationary to pulsed electrodeposition and with decreasing the pulse duration to 0.01 s (red line on Figure 7b). Nevertheless, the width of the working range or WWR (blue line on Figure 7b) of the shields increases nonlinearly for the same transition Nanomaterials 2021, 11, x FOR PEER REVIEW of modes from the P0 coating to the P0.01 coating. Thus, the width is 24.5, 25.6, 30.5, 10 and of 14 32.8 Oe for the P0, P1, P0.1, and P0.01 samples, respectively. a 30 25 29 P0 25 P1 28 P0.1 20 P0.01 27 20 SE (dB) SE (dB) SE (dB) 26 15 25 10 15 24 5 23 0 22 0 10 20 30 40 50 60 70 80 90 0 1 2 0 2 4 6 8 10 12 14 H (Oe) H (Oe) H (Oe) WWR (Oe) b Smax(dB) Smax Hmax(Oe) 34 Hmax 29 9 32 WWR 28 8 30 27 7 28 26 6 26 25 5 24 24 P0 P1 P0.1 P0.01 Shield type of FeNi Figure 7. SE of FeNi shields: shields:(a) (a)dependence dependenceofofSE SEon onthe thestrength strength ofof the the applied applied magnetic magnetic field field in the in the range range from from 0 to090 toOe, 90 Oe,the (b) (b) maximum the maximumvaluevalue of shielding of the the shielding efficiency efficiency (SE—black), (SEmax max—black), thethe strength strength of the of the magnetic magnetic field field that that corresponds corresponds to to max SE SEmax (H(H max—red) and the width of the shield’s working range (WWR—blue). max —red) and the width of the shield’s working range (WWR—blue). The behavior of the SE is directly dependent on the magnetic domain structure. It is known known that the magnetization magnetization of of a ferromagnetic ferromagnetic material,material, such such as as permalloy, occurs in several stages. stages. (1)(1)The Theferromagnetic ferromagnetic initially initiallyis unmagnetized is unmagnetized without the magnetic without field; the magnetic (2) in a(2)small field; in a magnetic field, domains small magnetic that arethat field, domains collinear with thewith are collinear fieldthe increase in size and field increase in uncollinear size domains decrease and uncollinear domains in sizes; (3)incollinear decrease sizes; (3)large domains collinear large absorb small domains uncollinear absorb small ones with aones uncollinear furtherwithincrease a further in the field strength; increase in the field and (4) all magnetization strength; vectors are and (4) all magnetization orderedare vectors according orderedtoaccording the magnetic to the field. magnetic field. Studies of of the themagnetic magneticdomaindomain structure structure with MFM with MFM in ain small magnetic a small magneticfield field inducedin- by a CoCr duced by aprobe CoCrhave probeshown havethatshownthe size thatof thea domain size of aordomain a regionorwith uniform a region with magnetiza- uniform tion decreases with magnetization a decrease decreases with in the grainin a decrease size. theThe grainestimated size. Themagnetic estimated field of the probe magnetic field does not exceed 1 Oe. This corresponds to the starting point on of the probe does not exceed 1 Oe. This corresponds to the starting point on the SE curves the SE curves (Figure 7a). At the H = 0.5 Oe, the SE is 13.4, 15.8, 18.5, and 19.6 dB for the (Figure 7a). At the H = 0.5 Oe, the SE is 13.4, 15.8, 18.5, and 19.6 dB for the P0, P1, P0.1, P0, P1, P0.1, and P0.01 shields, and P0.01 respectively. An increaseAn shields, respectively. in the magnetic increase field in the leads to field magnetic an increase leads toin an theincrease size of the in domains the size ofdirected collinear the domains to the collinear directed field. Previously, many to the field. experts reported Previously, that moving many experts do- reported mainmoving that boundaries domainis less energy-intensive boundaries than absorption domains is less energy-intensive with eachdomains than absorption other [55–58]. with Based on the MFM data, we can conclude that samples P0 each other [55–58]. Based on the MFM data, we can conclude that samples P0 and P1 areand P1 are magnetized mainly magnetized mainly due to the absorption of neighboring domains, and samples P0.1 and P0.01 are due to an increase in the size of domains during the absorption of disordered regions. In addition, the remagnetization of small domains requires a lower magnetic field. Thus, shield P0.01 is magnetized faster than the others (minimum value of Hmax = Nanomaterials 2021, 11, 634 10 of 13 due to the absorption of neighboring domains, and samples P0.1 and P0.01 are due to an increase in the size of domains during the absorption of disordered regions. In addition, the remagnetization of small domains requires a lower magnetic field. Thus, shield P0.01 is magnetized faster than the others (minimum value of Hmax = 4.6 Oe), and shield P0 is saturated last (maximum value of Hmax = 8 Oe). The high saturation magnetic field for shield P0 corresponds to a high SEmax value. However, this does not explain the fact that the WWR greatly increases with decreasing grain and magnetic domain size. Probably, the explanation for this is the disordered structure due to the large number of small grains and domains. Within the domain, the magnetization vectors are co-directional, but there is some deviation of the vectors at its boundaries. Within the nanoscale domain, the con- tribution of boundary disordering increases. There is a possibility of a partial transition to the superparamagnetic state for the P0.01 sample. It is possible that the coexistence of a ferromagnetic and superparamagnetic region in a nanograined shield can significantly increase the WWR of the shield. However, quantitative confirmation of this conclusion re- quires careful studies of the magnetic properties using SQUID (superconducting quantum interference device) techniques, which are planned to be carried out in the future in the framework of the work not related to the effectiveness of shielding. 4. Conclusions A set of permalloy coatings protecting against a permanent magnetic field was ob- tained using stationary and pulsed electrodeposition with a variable pulse duration (1, 0.1, and 0.01 s). The shields had a composition of 50 wt % Fe + 50 wt % Ni with small changes of no more than a few percent, which were caused by a change in the kinetics of the redox reaction. XRD studies of the crystal structure have shown that the unit cell parameter and cell volume decrease nonlinearly from 3.578 to 3.567 Å and from 45.80 to 45.38 Å3 with transition from stationary to pulsed electrodeposition and with decreasing the pulse duration. The reason for the compression of the crystal lattice can be two factors: the surface compression of grains with a reduction in their size, and the effect of the chemical composition. The microstructure was investigated using SEM and AFM. All coatings had a pronounced grain structure and showed a decrease in the average grain size from 0.5 µm for stationary P0 mode to 100 nm for pulsed P0.01 mode. Along with a decrease in the grain size, the domain size investigated using MFM also decreased. It is shown that samples with 0.3–0.6 µm grains have a disordered domain structure, which makes it possible to achieve higher values of the maximum shielding efficiency (SEmax = 29 dB for P0 shield), but these are characterized by a narrow working range. Samples with a grain size of less than 200 nm have an ordered two-domain magnetic structure with a possible partial transition to a superparamagnetic state in regions with a grain size of less than 100 nm. The ordered magnetic structure, the small size of the domain, and the coexistence of ferromagnetic and superparamagnetic regions, although they reduce the maximum value of the efficiency from 29 to 25.5 dB, they significantly expand the operating range of the shields from 24.5 to 33 Oe. Author Contributions: Conceptualization, T.Z. and A.T.; methodology, O.K. and E.K.; formal anal- ysis, D.T.; investigation, T.Z., A.K. (Anna Kotelnikova), M.K., T.V. and A.K. (Artem Kozlovskiy); resources, M.Z.; data curation, D.T., and K.L.; writing—original draft preparation, review and edit- ing, T.Z., A.T and V.F.; visualization, D.T.; supervision, S.G. and V.F.; project administration, V.F.; funding acquisition, S.G. and K.N. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the European Union’s Horizon 2020 research and innova- tion programme under the Marie Sklodowska-Curie grant number 861145. And The APC was funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 861145. Data Availability Statement: The data of the study is available on reasonable request from corre- sponding author. Nanomaterials 2021, 11, 634 11 of 13 Conflicts of Interest: The authors declare no conflict of interest. References 1. Panagopoulos, D.J.; Chrousos, G.P. 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