Materials and Design 193 (2020) 108826 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes Unraveling the properties of sharply defined submicron scale FeCu and FePd magnetic structures fabricated by electrodeposition onto electron-beam-lithographed substrates Evangelia Dislaki a,⁎, Matteo Cialone b, Federica Celegato b, Paola Rizzi c, Paola Tiberto b,⁎, Simone Vadilonga d, Daniel Többens d, Jordi Sort a,e,⁎⁎,⁎⁎⁎, Eva Pellicer a a Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain b INRIM, Strada delle Cacce 91, 10135 Torino, Italy c Dipartimento di Chimica, Università di Torino, Via Giuria 7, 10125 Torino, Italy d Helmholtz-Zentrum Berlin für Materialen und Energie GmbH, Albert-Einstein-Str. 15, D-12489 Berlin, Germany e Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain H I G H L I G H T S G R A P H I C A L A B S T R A C T • Well-defined FeCu and FePd submicron-scale magnetic structures are prepared by electrodeposition onto e-beam lithographed substrates. • Sharply resolved geometries (cylindri- cal, rectangular and cruciform) are ap- proximately 200 nm in height. • The chemical composition and micro- structure of the two systems are com- pared. • Magnetic force microscopy for the cylin- drical and cruciform structures suggest magnetic curling effects during magne- tization reversal. a r t i c l e i n f o a b s t r a c t Article history: In this work, Fe–X (X = Cu, Pd) submicron-scale structures were electrodeposited onto pre-patterned substrates Received 7 March 2020 prepared by e-beam lithography. The FeCu and FePd (with reduced Pd content) systems were investigated as at- Received in revised form 21 May 2020 tractive candidates for a variety of potential applications in magnetic data storage and biomedicine. Confined Accepted 23 May 2020 growth in the restricted cavities resulted in a nanoscale grain size leading to well-defined geometries with Available online 26 May 2020 sharp edges and corners and an average height of up to 215 nm. Specifically, nine 100 μm × 100 μm arrays of Keywords: three geometries (cylindrical, rectangular and cruciform) in three different sizes were created. In addition, the Electrodeposition total deposition time ranged from 3.5 s (FeCu) to 200 s (FePd), i.e. much faster than by traditional physical Submicron structures vapor deposition approaches and was performed at ambient conditions. Magnetic force microscopy for the cylin- Electron-beam lithography drical and cruciform structures revealed virtually no contrast at zero field, suggesting magnetic curling effects Ferromagnetic (instead of coherent rotation) during magnetization reversal. These curling effects result in low values of rema- nent magnetization, which is advantageous in minimizing dipolar interactions between the structures either ⁎ Corresponding authors. ⁎⁎ Correspondence to: J. S, Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain ⁎⁎⁎ Correspondence to: J. S, Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys 23, 08010 Barcelona, Spain. E-mail addresses: evangeliadislaki@gmail.com (E. Dislaki), p.tiberto@inrim.it (P. Tiberto), Jordi.Sort@uab.cat (J. Sort). https://doi.org/10.1016/j.matdes.2020.108826 0264-1275/© 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2 E. Dislaki et al. / Materials and Design 193 (2020) 108826 when they are deposited onto the substrate or eventually dispersed in a liquid (e.g. in biomedical applications, as drug delivery carriers, where particle agglomeration is undesirable). © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction both from the magnetic and mechanical point of view can be derived, while Pd content is kept relatively low. Indeed, in its martensitic Since the discovery of giant magnetoresistance and its implementa- phase, the alloy possesses a ferromagnetic shape memory effect tion in magnetic field sensors [1], the field of nanomagnetism has expe- [23,24], which makes it an excellent candidate for wirelessly actuated rienced rapid developments in the last decades given the novel mechanical systems or strain sensors [25]. Moreover, FePd is a promis- properties that have been discovered in the field of spintronics with ing alloy in the field of spintronics where it has recently been used in the major implications for data processing and storage [2,3] as well as the field of artificial spin-ice [26] and its ability to stabilize skyrmions at highly promising and fascinating functionalities of flexible electronics room temperature has been proven by ab-initio calculations [27]. On [4]. The growing trend towards miniaturization faces challenges related the other hand, the presence of palladium in the alloy makes it an ap- to spin wave guidance [5] and the superparamagnetic limit which need pealing candidate for hydrogen absorption [28,29] and catalysis to be addressed in a holistic approach that combines several traditional [30,31]. Finally, FePd alloys can be easily integrated into biomedical disciplines under the umbrella of nanomagnetism [6]. The vast majority and bioengineering applications, given that biocompatibility assess- of existing studies have focused on two-dimensional nanomagnetism ments have clearly established that many different cell types can spread where single- or multi-layer films and structures are confined to a thick- and proliferate well on FePd single crystal as well as polycrystalline ness that leads to monodomain magnetic states in the vertical direction films [32]. and interesting functionalities are achieved by capitalizing on interfacial Considering the relevance of FeCu and FePd systems, the goal of the effects [7–10]. Nevertheless, new fabrication and characterization tech- work was to show the feasibility of electrodepositing these alloys in ar- niques enable the expansion of nanomagnetism into three dimensions rays of confined cavities and to unveil the interplay between their crys- where unconventional spin textures bring about remarkable new phe- tallographic structure, the geometry and size of the motifs, and their nomena involving geometry, topology and chirality. Indeed, patterned magnetic response. The structures were very well-resolved and rela- 3D nanostructures can revolutionize sensing technology, ultrahigh den- tively smooth with a thickness ranging between 200 and 300 nm sity magnetic recording and the Internet-of-Things while 3D nano- and while plating time was as low as 3.5 s (FeCu) and 200 s (FePd), i.e. micro-scale magnetically actuated motors are essential for mechano- much faster than is typically feasible with PVD techniques. Overall com- biology applications [11]. position was initially assessed by energy dispersive X-ray spectroscopy From a synthesis point of view, electrodeposition is widely used due (EDX)/SEM while local elemental maps were obtained by EDX/scanning to its conformal coverage, control of grain size and crystallinity and fast TEM (STEM) and electron energy loss spectroscopy (EELS). The FeCu growth rates. When combined with ultra-high-resolution electron- structures exhibited a graded composition with a rather homogeneous beam lithography, well-defined nano-motifs can be created. The height Fe60Cu40 stoichiometry at the center region while the FePd motifs of the submicron motifs produced by physical vapor deposition (PVD) showed an even distribution of the Fe and Pd elements. The magnetic approaches is typically below 50 nm [7,12,13] and in order to reach an properties were investigated using magnetic force microscopy (MFM) approximate thickness of 200 nm several successive time-consuming as well as using a vibrating sample magnetometer (VSM) for large steps, including sputter etching, can be involved [14]. Contrarily, it is single-feature arrays of three selected geometries and dimensions easily possible to achieve a high aspect ratio with electroplating. Never- (small- and large-sized disks and medium crosses). The MFM results theless, existing literature on electrodeposited 3D structures has thus suggested curling of the magnetization rather than coherent rotation. far been focused on arrays of nanowires [15–17]. Herein, submicron FeCu and FePd structures organized in nine 100 μm × 100 μm arrays of three select shapes (disks, stripes and crosses) 2. Experimental and of three sizes each have been prepared by electrodeposition onto e-beam lithographed substrates. With respect to the FeCu system, the 2.1. Fabrication details optimized electrolyte and plating conditions formerly determined for the growth of FeCu films were adapted as necessary for the deposition The lithography patterns were created with a FEI Quanta 3D dual of Fe-rich miniaturized patterns. Moreover, it was shown that the satu- beam FEG which combines a focused ion beam (FIB) with a high- ration magnetization and coercivity of electroplated FeCu films can be resolution field emission gun scanning electron microscope (FEG- tuned by a facile adjustment of the Fe content [18]. It was also demon- SEM). Silicon/silicon dioxide (Si/SiO2) substrates were used with a strated that the coercivity of hierarchically porous FeCu thin films 15 nm Ti adhesion layer and an 80 nm Au seed layer, both deposited with ultrathin pore walls was successfully reduced up to 25% by an ap- by e-beam evaporation. The process was controlled by the nanometer plied voltage uncovering the potential for applications in patterning generation system (NPGS) software. The information on magnetoelectrically driven devices [19]. Interestingly, Chien et al. re- the dimensions and positions of the different structures, the sizes of ported that, while the magnetic moment of Fe often rapidly declines the arrays and their reciprocal position on the sample surface were with alloying due to the magnetic weakness of Fe, it remains nearly con- contained in a computer-aided design (CAD) file managed by the stant up to high concentrations of Cu in the FeCu system [20]. The inclu- NPGS. The features were designed with sufficient spatial separation so sion of Cu can also boost corrosion resistance and confer antibacterial as to avoid the proximity effect. The electron dose was 400 μC cm−2 properties [21]. With sustainability presenting an increasing concern for the smallest-sized feature arrays and 375 μC cm−2 for the rest. The on a global scale, Fe and Cu are among the most abundant and low- operating voltage was 30KeV. Electrodeposition was carried out in a cost materials with excellent recyclability and are non-toxic to the envi- three-electrode cell connected to a Metrohm/Eco Chemie Autolab ronment. In the case of the FePd system, the electrolyte composition PGSTAT302N potentiostat/galvanostat. A Pt wire served as the counter was directly derived from the work of Konczak et al. [22] and the plating electrode and a double junction Ag|AgCl (E = +0.210V/SHE) electrode parameters were tailored towards a particular stoichiometry, namely (Metrohm AG) with a 3 M KCl inner solution and a 1 M Na2SO4 outer so- the Fe70Pd30 alloy. At this particular composition, interesting properties lution was used as a reference. E. Dislaki et al. / Materials and Design 193 (2020) 108826 3 2.2. Sample preparation and post-process treatments at the KMC-2 beamline at the BESSY II facility [35]. Radiation energy was selected as 8048 eV for a wavelength of 1.5406 Å, equivalent to Prior to lithography, the substrates were cleaned successively with Cu-Kα1. An area detector with an angular aperture of 12° was used. acetone, isopropanol and de-ionized water. The substrates were subse- The sample was mounted on a cradle allowing for movement and rota- quently spin-coated at 2000 rpm for 1 min with a layer of poly(methyl tion of the sample along the x, y and z directions. Moreover, the inci- methacrylate) (PMMA) polymeric resist. This ensured that a homoge- dence and reflection angle could be controlled separately. The nous layer of PMMA was formed on top of the conductive substrates. incidence angle of the beam was 21.5° both in the case of the FeCu The thickness of the resist was around 300 nm, as measured by and of the FePd sample. Under these conditions, it was possible to ellipsometry. After coating, the PMMA was heat treated for 5 min at limit the beam footprint on the sample surface to an area smaller than 165 °C. Upon completion of the patterning procedure, the PMMA in 100 μm × 100 μm, enough to achieve the desired lateral resolution to the exposed regions was removed by placing the samples for 60 s in a distinguish among the different arrays of structures. For both the FePd 1:3 solution of 4-methyl-2-pentanone with isopropanol, followed by and FeCu sample, 6 fine scans of 10 min each were recorded for every 20 s in isopropanol and, finally, 20 s in de-ionized water. Subsequent array. The different patterns were averaged to reduce the noise of the to electrodeposition, the samples were placed in acetone for 10 min to measurement and then analyzed using the Fityk software [36]. Atomic remove the remaining PMMA leaving behind the created structures of and magnetic force microscopy were performed with a Bruker Multi- different geometries attached to the substrate. This was followed by a mode V Nanoscope 8 microscope equipped with a fully non-magnetic final rinsing in isopropanol and Milli-Q water. head and scanner and with an electromagnet capable of applying fields The TEM lamellae preparation was made on a dual beam Carl Zeiss up to 1000 Oe in the sample plane. The tip used for the magnetic char- Neon 40 station equipped with a Schottky FE-SEM and Ga + ion col- acterization was s a commercial Bruker MESP-HR10 cantilever coated umns (CANION31, Orsay). The lamellae, about 60 nm in thickness, orig- with Co/Cr hard magnetic alloy. Both the in-plane and perpendicular co- inated from a section of the medium-sized cruciform structures. The ercivity of the tip are in the order of 900 Oe. The magnetic domain evo- region of interest was first covered with a thin Pt layer of approximately lution was studied at room temperature under application of an in- 200 nm using e-beam assisted Pt deposition. On top of the thin Pt layer, plane magnetic field with values up to 600 Oe. All the samples were a thicker Pt layer (≈500 nm) was added by ion-beam assisted Pt depo- demagnetized along the in-plane direction prior to the MFM character- sition. Both Pt layers were used in order to protect the underlying struc- ization. For further magnetic characterization, samples patterned with a tures from ion beam damage during lamella preparation. large array of a single geometry and size were fabricated and placed in a vibrating sample magnetometer from LOT-Quantum Design. Hysteresis loops were recorded at room temperature, along the in-plane direction 2.3. Electrolytes and plating conditions with an applied field range of −20 kOe to 20 kOe. The electrolytes were prepared with Millipore Milli-Q water and ACS Reagent grade chemicals purchased from Sigma-Aldrich. For the deposi- 3. Results and discussion tion of the FeCu structures, the solution contained ammonium ferrous sulfate hexahydrate (58.8 g L−1), copper (II) sulfate pentahydrate 3.1. Morphology of patterned structures (0.625 g L−1), sodium gluconate (22.9 g L−1), sodium dodecyl sulfate (0.2 g L−1) and saccharine (0.46 g L−1). The as prepared pH was 4.1 While existing literature on lithographed patterns mainly focuses on and the temperature was adjusted to 45 °C through the use of an exter- the fabrication of cylindrical arrays [37–40], this work targets more nal thermostatization jacket. Electrodeposition was carried out in complex geometries. The dimensions of the various motifs are given in potentiostatic mode at an applied potential of −1.1 V for 3.5 s with mag- Table 1. As can be viewed in Figs. 1 and 2, the electrodeposited FeCu netic stirring at 300 rpm. and FePd arrays of different shapes and dimensions have a well- For the electrodeposition of the FePd structures, a reduction of the defined geometry, with sharp edges in the case of the medium- and gap in the standard electrode potentials of Fe and Pd can be achieved large-sized crosses and stripes, thanks to the small grain size achieved through complexation of the Pd2+ ions with ammonia [33]. The electro- by electrodeposition. This is significant considering the submicron di- lyte composition consisted of tetraamminepalladium(II) chloride (0.01 mensions of the structures and, especially, the complex geometry of M), 5-sulfosalicylic acid dihydrate (0.06 M), iron(III) sulfate heptahydrate the cross-shaped motifs which are characterized by several edges and (0.05 M) and ammonium sulfate (0.3 M) [22]. Sulfosalicylic acid (SSA) has corners separating them into segments along their axes of symmetry. been proposed in literature as a means to stabilize the Fe-ions and to pre- Notably, the FePd structures appear very dense while the FeCu struc- vent hydroxide formation and incorporation [34]. In particular, the ratio tures display a cauliflower-like morphology. Differences in surface mor- between the concentration of the Fe3+ ions and the SSA was kept at [Fe3 + phology stem from the different electrolyte formulations and ]:[SSA] = 1:1.2 and the pH was adjusted to 5 with addition of ammo- parameters used (see Experimental section), since each electrolyte nium hydroxide. To obtain the desired Fe70Pd30 alloy composition, depo- had been previously optimized for the production of continuous dense sition was performed potentiostatically at −0.95 V for 200 s at room films on planar substrates. Note that the growth rates were, therefore, temperature and without stirring the electrolyte. not equivalent. Importantly, electrodeposition was carried out at a pH of 4.1–5.0, hence avoiding the use of environmentally unfriendly, highly 2.4. Characterization acidic electrolytes. The morphology of the patterned structures was observed using a Zeiss MERLIN field emission scanning electron microscope (FE-SEM) and the elemental composition was determined by EDX at an accelera- Table 1 tion voltage of 15 kV. Additional EDX mappings were acquired with a Detailed dimensions of the nine arrays of patterned structures. FEI Magellan 400 L XHR SEM. Further chemical analysis at nanometer Geometry Dimensions level and elemental mappings as well as study of crystalline structure Small Medium Large by selected area electron diffraction (SAED) was conducted with a FEI Tecnai G2 F20 200 kV field emission gun high resolution and analytical Stripes 150 nm × 450 nm 300 nm × 900 nm 450 nm × 1.35 μm TEM/STEM. The crystalline structure of the submicron FeCu and FePd Disks d: 200 nm d: 500 nm d: 1 μm Crosses 300 nm × 500 nm 600 nm × 1 μm 900 nm × 1.5 μm structures was also investigated by XRD using synchrotron radiation 4 E. Dislaki et al. / Materials and Design 193 (2020) 108826 Fig. 1. SEM micrographs of FeCu electrodeposited arrays of a) stripes, b) disks and c) crosses in three distinct sizes (increasing from left to right). The average height and roughness of the FeCu and FePd structures, 3.2. Chemical and structural analysis of structures as determined by analysis of the AFM images acquired, are shown in Tables 2 and 3, respectively. These values are consistent considering For the deposition of the FeCu submicron structures, the optimized that the deposition time was the same (3.5 s and 200 s for FeCu and plating parameters previously established for the fabrication of Fe-rich FePd, respectively) for all structures. Interestingly, the FeCu medium- FeCu films [18] were utilized. However, in the case of the structures, sized features were generally at the upper range limit for the height, the very short plating time of 3.5 s into submicron cavities and the use while the larger stripes were the shortest. In the case of the FePd struc- of potentiostatic rather than galvanostatic deposition mode led instead tures, the larger features displayed more roughness due to increased de- to a Cu-rich composition, namely 20 at.% Fe and 80 at.% Cu in average. In position around the edges while the smallest geometries were also the order to enhance Fe deposition, the Cu salt amount was then halved. Ini- shortest. The occurrence of donut-like structures due to an uneven dis- tial elemental analysis by SEM/EDX after this modification of the Cu salt tribution of the current density in the cavities can occur when concentration showed an average value of 45 at.% Fe and 55 at.% Cu electroplating dot arrays [41,42]. In the case of the FeCu and FePd which varied slightly for the different arrays. Notably, a compositional disks, it can be easily observed that the structures are fully plated in gradient can occur when depositing a system with low mutual solubility the center. This means that current distribution is homogeneous within such as FeCu [43] into cavities of high aspect ratio, and thus the compo- the cavities, which can be partly due to the beneficial effect imparted by sition determined by SEM/EDX represents only an overall value with no the additives present in the FeCu and FePd electrolytes. further insights into deposit growth mechanisms. Therefore, a lamella In Fig. 3, AFM images of the large-sized FeCu and medium-sized FePd was prepared from a section of the array of the medium-sized crosses structures are shown where the very small grain size is visible. This re- for STEM/EELS characterization and the results of the analysis are sulted in the growth of well-resolved geometries, despite the narrow- shown in Figs. 4a and 5. ness of the cavities, especially at the corners. Due to the high aspect The bright-field images of the FeCu lamella, as seen in Fig. 4a, show a ratio of the structures and the dimensions of the AFM/MFM tip used, dendritic growth typically encountered in electrodeposited Fe and Cu the convolution between the two gives an inaccurate representation of [44,45]. Electrodeposited pure Fe has shown to form dendrites from the structures on the xy plane. This effect is more visible along the fast acidic sulfate electrolytes, even on planar substrates. The dendrites scan direction (in this case, the y direction). This is especially true as fea- were reported to have cauliflower endings as in our case (see inset of ture size decreases and, consequently, the AFM images of the smallest- Fig. 1b). On the other hand, the low concentration of Cu(II) depositing sized structures were less reliable for subsequent surface analysis. There- ions (0.625 g L−1) in the electrolyte might enhance the occurrence of fore, the SEM micrographs offer a direct representation of geometrical ramified growth. In Fig. 5a, the indexed SAED pattern collected from a aspects while the AFM images act in a complementary manner to further spot in the middle of the lamella displays the typical Debye-Scherrer investigate morphology and estimate height and roughness since very rings with small bright spots confirming the nanocrystalline nature of accurate measurements can be obtained in the z-axis direction. the deposited structures. The presence of two phases was detected, E. Dislaki et al. / Materials and Design 193 (2020) 108826 5 Fig. 2. SEM micrographs of the nine FePd arrays of a) stripes, b) disks and c) crosses in three sizes (increasing from left to right). namely a body-centered cubic (bcc) and a face-centered cubic (fcc), starting from the substrate and extending beyond the surface, show a which correspond to Fe-rich and Cu-rich solid solutions, respectively. rather uniform distribution of Fe in the middle of the lamella with a sud- The measured interplanar distances indicate phase separation with par- den decrease at the uppermost section while copper deposition is fa- tial alloying. From the high-resolution (HR) TEM image in Fig. 5b, we vored near the top. Compositionally graded materials are utilized in can observe the sharply defined lattice planes with distances of perpendicular magnetic media as a way of achieving a graded anisot- 0.207 nm. This distance corresponds to the bcc (110) planes of Fe with ropy which leads to a reduction in coercivity, and thus in the field re- an increase in value due to the incorporation of Cu atoms into the lattice. quired to record data [46–48]. The advantage of the electrochemical In Fig. 5c, the quantitative STEM/EDX chemical maps, taken vertically approach herein is the possibility to create a graded composition Table 2 Average height and RMS surface roughness values for the nine arrays of FeCu patterned Table 3 structures as determined by AFM. Average height and RMS surface roughness values for the nine arrays of FePd patterned structures as determined by AFM. FeCu structures FePd structures Geometry Dimensions Geometry Dimensions Small Medium Large Small Medium Large Stripes – RMS = 23 RMS = 23 Zavg = (180 ± 6) nm Zavg = (168 ± 10) nm Stripes RMS = 9 RMS = 14 RMS = 20 Disks RMS = 23 nm RMS = 18 RMS = 24 Zavg = (112 ± 2) nm Zavg = (215 ± 7) nm Zavg = (214 ± 15) nm Zavg = (187 ± 13) Zavg = (188 ± 4) nm Zavg = (189 ± 3) nm Disks RMS = 10 nm RMS = 6 RMS = 15 nm Zavg = (145 ± 2) nm Zavg = (166 ± 3) nm Zavg = (140 ± 2) nm Crosses RMS = 19 RMS = 16 RMS = 12 Crosses RMS = 10 RMS = 16 RMS = 21 Zavg = (209 ± 5) nm Zavg = (212 ± 15) nm Zavg = (209 ± 18) nm Zavg = (188 ± 1) nm Zavg = (180 ± 6) nm Zavg = (196 ± 7) nm 6 E. Dislaki et al. / Materials and Design 193 (2020) 108826 Fig. 3. AFM images of the a) large-sized FeCu and b) medium-sized FePd disks, crosses and stripes. Fig. 4. STEM bright field images of the a) FeCu and b) FePd lamellae. E. Dislaki et al. / Materials and Design 193 (2020) 108826 7 Fig. 5. a) Indexed SAED pattern taken at a middle point of the FeCu medium-sized cross, b) HR-TEM image, c) EDX/STEM compositional maps and d) EELS mixed elemental map at selected area. through a simple and fast process without the need of high vacuum. copper is very low in equilibrium conditions despite their similar atomic With respect to oxygen content, O counts were concentrated within radii due to the positive enthalpy of mixing, ΔHmix (calculated value of the Fe-rich region close to the substrate and in the area above the sur- ΔHmix ≈ +13 kJ mol−1 in the liquid phase) [49], which encourages clus- face of the lamella. Since it is known that oxides/hydroxides during Fe tering of the atoms into Fe-rich and Cu-rich groups to reduce the inter- electrodeposition often form at the working electrode due to local pH nal energy [50]. changes, the co-presence of Fe and O in regions close to the substrate A similar analysis was performed for the FePd crosses to study the is thus anticipated. Likewise, passivation of the structures upon standing compositional and structural characteristics originating from the in air leads to the accumulation of oxygen at the surface of the lamella. growth in restricted patterns. In comparison, the microstructure was These trends were further confirmed by vertical extreme high- shown to be very fine, as seen in Fig. 4b. Additionally, as seen by juxta- resolution (XHR) SEM/EDX line scans where the Fe content was found posing Fig. 5a with Fig. 6a, the FePd structures were more nanocrystal- to be roughly 60 at.% along the middle section. Horizontal line scans re- line than their FeCu counterparts. EELS elemental mapping (Fig. 6d) vealed a rather homogeneous distribution of Fe and Cu (Fig. S1 of the revealed that the Fe and Pd elements are homogeneously distributed, Supporting Information). with no evident Fe-rich or Pd-rich regions. Vertical EDX line scans also The EELS compositional map seen in Fig. 5d, which was taken at a evidenced a uniform elemental distribution (Fig. S2 of the Supporting starting region close to the center and encompassing upper parts of Information). This is expected given the high miscibility of Fe and Pd the lamella, displays Fe-rich and Cu-rich areas as well as areas with a [51] for which the enthalpy of mixing is negative (calculated value of mixed composition where partial alloying takes place. It has been well ΔHmix ≈ −4 kJ mol−1 in the liquid phase) [49]. The reduction in the in- established in literature that the mutual solid solubility of iron and ternal energy of the system is thereby achieved by increasing the 8 E. Dislaki et al. / Materials and Design 193 (2020) 108826 Fig. 6. a) SAED pattern taken at a middle point in the FePd medium-sized cross b) HR-TEM micrograph, c) bright field image of sample and d) EELS mixed compositional map. number of Fe\\Pd bonds [50]. On the other hand, oxygen is detected footprint of the X-ray beam on the sample surface and the percentage primarily at the surface of the sample and in the substrate (originating of the surface covered by the structures in the area illuminated by the from the SiO2 layer), thus confirming the metallic nature of the deposits. beam (ranging from 6% to 19%). Additionally, given the high Fe content STEM/EDX quantification resulted in an average composition of 62 at.% of both the FeCu and FePd structures, a predominant bcc phase was ex- Fe and 38 at.% Pd, indicating a slightly higher Pd content compared to pected. Indeed, it was not possible to discriminate any reflection peaks earlier SEM/EDX measurements which revealed an average overall composition of Fe70Pd30 with slight variations among the arrays as well as the individual structures. This discrepancy, therefore, could be due to the superior spatial resolution of the STEM/EDX measurement. Analysis of the SAED pattern revealed the presence of a bcc phase and an fcc phase, corresponding to the α-(Fe,Pd) and the disordered γ-(Fe, Pd) solid solutions. The lattice fringes in Fig. 6b reveal the crystalline na- ture of the deposits and the interplanar distance, as determined by re- duced fast Fourier transform (RFFT), was 0.22 nm which corresponds to the bcc (110) planes of the α-(Fe,Pd) solid solution. In the pursuit of uncovering possible microstructural variations de- pendent on size and geometry, additional characterization by XRD was performed at the BESSY II synchrotron radiation facility for each of the nine 100 μm × 100 μm FeCu and FePd arrays. In Fig. 7, the syn- chrotron X-ray diffraction patterns obtained from the FeCu and FePd medium-sized disks are shown. Similar trends were observed among the different geometries (the patterns acquired from all of the individ- ual arrays are included in Fig. S3 of the Supporting Information). The re- duced intensity of the signal was a combined result of the reduced Fig. 7. X-ray diffractograms obtained from the FeCu and FePd medium-sized disks. E. Dislaki et al. / Materials and Design 193 (2020) 108826 9 Table 4 phase well below the detection threshold of the instrument. Here, it is Values of 2θ position, d-spacing and lattice constant for the FeCu crosses of different di- important to mention the different scales in which the two experimen- mensions. The tabulated lattice constant for α-Fe is 2.8664 Å [53]. tal techniques work. Indeed, with SAED, the diffraction data are gener- Size FeCu crosses ated from a very narrow spot (in the order of hundreds of 2θ [°] d [Å] Lattice constant [Å] nanometers) in a well-defined region of a single structure. Contrarily, the XRD experiment performed at the synchrotron targeted an entire Large 44.41 ± 0.07 2.064 ± 0.002 2.918 ± 0.002 Medium 44.44 ± 0.03 2.062 ± 0.002 2.917 ± 0.002 array averaging the signal over all the structures. Small 44.45 ± 0.02 2.062 ± 0.002 2.916 ± 0.002 To determine the angular position of the reflection peaks, they were fitted using a pseudo-Voigt function. Before performing the fit, a cubic spline was used as a baseline to remove the background. The 2θ values were obtained through the fit, while the d-spacings and lattice con- Table 5 stants were calculated using Bragg's law. We considered the following Values of 2θ position, d-spacing and lattice constant for the FePd crosses of different di- tabulated atomic radii: Fe (126 pm), Pd (137 pm), and Cu (128 pm) mensions. The tabulated lattice constant for α-Fe is 2.8664 Å [53]. [52]. Only the results related to the FeCu and FePd crosses are included Size FePd crosses in Table 4 and Table 5, respectively (supplementary data for the disks 2θ [°] d [Å] Lattice constant [Å] and stripes can be found in Tables S1–S4 of the Supporting Information). Although the reported trends are reliable, the defects resulting from the Large 43.09 ± 0.03 2.124 ± 0.003 2.994 ± 0.003 Medium 43.25 ± 0.01 2.116 ± 0.001 2.993 ± 0.001 ramified growth in the FeCu motifs can have an influence on the lattice Small 43.24 ± 0.01 2.118 ± 0.001 3.005 ± 0.001 constant values and, therefore, these should be considered with certain caution. In the case of the FeCu crosses, if we compare the tabulated angular originating from the fcc Cu and γ-(Fe,Pd) phases of the FeCu and FePd position of the bcc (110) planes corresponding to pure α-Fe (2θ = arrays, respectively, which had previously been detected by the SAED 44.67°) with the ones obtained from the measurements (44.05° b 2θ b analysis. Moreover, the γ-(Fe,Pd) phase was most likely a minority 44.50°), we can observe a shift towards lower angles. This is due to an Fig. 8. MFM images of the FeCu structures taken a,e) under a static field of 600 Oe, b,f) at remanence after removal of the field as well as c,g) and d,h) following the same steps upon reversal of the field direction. 10 E. Dislaki et al. / Materials and Design 193 (2020) 108826 increase in the length of the crystal lattice parameter driven by the dis- By comparison, when the field is removed, the images become rather solution of Cu into the bcc Fe phase. Indeed, at nano dimensions, sur- featureless, thus suggesting curling of the magnetization instead of co- faces and interfaces have been shown to bring about a modulation of herent rotation [57]. The MFM images taken at a static field show a sin- the solubility gap as observed in the case of systems such as Au\\Si, gle domain state with reverse polarity. Representative images for the Ge\\Si and Al\\Cu [54]. medium disks and crosses, where dipolar contrast is evident and oppo- The investigation of the FePd structures revealed the presence of the site upon field reversal, are seen in Fig. 8. In the case of the crosses, how- bcc (110) reflection of the α-(Fe,Pd) solid solution at 43.05° b 2θ b ever, a different behavior can be discerned along the long and short axis. 43.33°, for all geometries and sizes. Τhe crystal lattice decreases with in- Indeed, along the long axis the dipolar contrast is clearly reversed once creasing dimensions pointing towards a modulation of the solubility of the magnetic field is reversed. Nevertheless, the change in contrast Pd into the α-Fe phase. along the short axis is less pronounced. This behavior suggests that The change in the magnetic exchange interactions between Fe atoms the shape anisotropy promotes orientation of the magnetization along might also be playing a role when alloying Fe with a non-magnetic ele- the axis that is parallel to the direction of the magnetic field and plays ment, such as Pd or Cu. In fact, the increase of the lattice parameter a fundamental role on the magnetization reversal mechanism. when alloying Fe with Cu has been reported in the literature [55]. The Finally, since the positioning of different geometries on the same increase of the lattice parameter with the Pd content in FePd alloys is substrate did not allow acquisition of hysteresis loops of each individual also known [56]. array, three substrates with large, single-feature arrays were also pre- pared. Specifically, a 6 × 6 and a 12 × 12 pattern were created by repe- 3.3. Magnetic characterization of the FeCu and FePd motifs tition of the small- and large-sized disk array, respectively, as well as a 16 × 16 duplication of the medium cross array. In this way, two geom- In consideration of the stoichiometries of the FeCu structures as de- etries as well as three different feature sizes could be assessed. The termined by STEM/EDX elemental analysis, i.e. 60 at.% Fe and 40 at.% Cu, number of duplications for each pattern was selected so as to ensure it should be mentioned that the value of saturation magnetization for a an adequate signal by VSM. The hysteresis loops recorded at room tem- Fe39Cu61 film was previously measured at approximately 83.7 emu g−1 perature corresponding to the single feature arrays, are shown in Fig. 9. and for a Fe73Cu27 film it was 152.5 emu g−1 [18]. Consequently, despite The strong diamagnetic background originating from the substrate has the relatively high overall Cu content of these motifs, a clear magnetic been subtracted. The measured coercivities were approximately response is expected. In order to study the magnetization reversal of 81 Oe, 114 Oe and 174 Oe for the large disks, small disks and medium the fabricated structures, each of the nine arrays was exposed to an crosses, respectively. It can also be observed that saturation is reached in-plane static magnetic field of 600 Oe and images were recorded by at an applied field of N1000 Oe. At 600 Oe, the maximum field applied MFM. Additional images were taken at remanence. The field was then by MFM, the structures could not reach saturation and this held espe- applied in the opposite direction and the same process was repeated. cially true for the large-sized features. However, it was not possible to Fig. 9. Hysteresis loops measured in-plane for the FeCu large, single-feature arrays of the a) large disks, b) small disks and c) medium-sized crosses along the long axis. E. Dislaki et al. / Materials and Design 193 (2020) 108826 11 further increase the applied field without inducing a reversal in the circular and cross-shaped structures. Moreover, the straightforward magnetization of the MFM tip. Importantly, at maximum applied field, preparation technique and ferromagnetism makes them attractive no interaction between the magnetization of the structures and the candidates for a slew of prospective applications in spintronics, magnetization of the tip was observed. microelectromechanical systems and biomedicine (once these struc- Conversely, for the FePd structures, a maximum field of 400 Oe was tures would be detached from the substrate). The combination of the adequate to saturate the structures in the in-plane direction. Interest- biocompatibility of FePd with the presence of magnetization curling, ingly, when higher fields up to 600 Oe were applied, an interference be- opens up the possibility for applications in the field of mechanically tween the magnetic moment of the tip and the stray field of the induced cell destruction as a treatment for cancer. This mechanism structures was observed. Taking this into consideration, we can infer has been proven to be a successful approach using permalloy that the FePd structures are magnetically softer than their FeCu counter- (Fe80 Ni20) disks where the application of an alternating magnetic parts. As can be observed in Fig. 10, upon reversal of the magnetic field, field causes the magnetic disks to oscillate, thus compromising the dipolar contrast of the stripes reverses as well. In the case of the me- membrane integrity and initiating cell death. However, when using dium disks, the contrast once again indicates a curling of the magnetiza- non-biocompatible materials such as permalloy, an additional layer tion. With respect to the crosses, similar to the FeCu system, a different of gold as a surface coating is necessary. behavior for the magnetization reversal can be inferred from the differ- ence in contrast intensity along the long and short branches. Complex CRediT authorship contribution statement reversal mechanisms, including magnetic curling promoted at the edges of the crosses, seem to occur in these structures at zero field, ren- Evangelia Dislaki: Investigation, Data curation, Formal analysis, dering a non-homogeneous magnetic contrast, which is different from Writing - original draft. Matteo Cialone: Investigation, Data curation, the rotating dipolar contrast that would be expected if reversal of the Formal analysis. Federica Celegato: Investigation. Paola Rizzi: Concep- magnetization occurred by coherent rotation. tualization, Funding acquisition, Supervision, Resources. Paola Tiberto: Conceptualization, Funding acquisition, Supervision, Resources. Simone 4. Conclusions Vadilonga: Investigation. Daniel Többens: Investigation. Jordi Sort: Conceptualization, Funding acquisition, Supervision, Resources, Writing In this work, electrodeposition of complex FeCu and FePd geo- - review & editing. Eva Pellicer: Conceptualization, Funding acquisition, metric structures onto pre-lithographed substrates was demon- Supervision, Resources, Writing - review & editing. strated, which opens new possibilities from the viewpoint of materials and applications beyond permalloy, probably the most uti- Declaration of competing interest lized Fe alloy in electrodeposited structures so far. The fabricated features show different growth patterns and microstructures as a re- The authors declare that they have no known competing financial sult of the dissimilar electrolyte formulations. While interests or personal relationships that could have appeared to influ- compositionally-graded structures are obtained for the FeCu system ence the work reported in this paper. with a 60 at.% Fe content in the center region, compositionally homo- geneous Fe 62 Pd 38 motifs are obtained for the FePd system. In any Acknowledgements case, fast deposition rates (3.5 s for FeCu and 200 s for FePd) enable the growth of the submicron structure arrays from electrolyte for- This work was supported by the SELECTA (No. 642642) H2020- mulations at a moderate pH. X-ray diffraction and SEM analyses sug- MSCA-ITN-2014 project. Partial financial support by the Spanish Gov- gest that the size of the cavities in which the structures are grown ernment [Project MAT2017-86357-C3-1-R and associated FEDER], the induces microstructural variations most notably in the grain size Generalitat de Catalunya (2017-SGR-292) and the European Research and the degree of alloying. The magnetization reversal process at Council (SPIN-PORICS 2014-Consolidator Grant, Agreement n° room temperature for the in-plane direction of the field is similar 648454) is acknowledged. We thank Helmholtz-Zentrum Berlin (HBZ) for the two systems and dominated by the shape anisotropy of the for the allocation of synchrotron radiation beamtime. Nanofacility Pie- structures while evidence of magnetic curling is unveiled for the monte is acknowledged. Fig. 10. MFM images of medium-sized FePd motifs a) under a static field of 400 Oe, b) at remanence and c,d) following the same steps upon reversal of the applied field direction. 12 E. 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