Materials & Design 208 (2021) 109915 Contents lists available at ScienceDirect Materials & Design journal homepage: www.elsevier.com/locate/matdes Mechanical, magnetic and magnetostrictive properties of porous Fe-Ga films prepared by electrodeposition Aliona Nicolenco a,b, Yu Chen a, Natalia Tsyntsaru b,c, Henrikas Cesiulis c,d, Eva Pellicer a,⇑, Jordi Sort a,e,⇑ a Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, Spain b Institute of Applied Physics, Chisinau, Republic of Moldova c Vilnius University, Physics Department, Vilnius, Lithuania d JSC Elektronikos perdirbimo technologijos, Vilnius, Lithuania e Institució Catalana de Recerca i Estudis Avançats, Pg. 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  Fe-Ga films with variable composition and porosity levels are electro deposited.  Both hardness and Young’s modulus decrease as the porosity degree increases.  Highly tunable magnetic properties are achieved by adjusting the microstructure.  Macroporous films show larger magnetic-field-induced crystal lattice deformation. a r t i c l e i n f o a b s t r a c t Article history: Magnetostriction, known as the ability of magnetic materials to expand or contract in response to mag- Received 31 March 2021 netic field, is a key property of Fe-Ga alloys exploited in various types of transducers. Usually, thin films of Revised 24 May 2021 Fe-Ga deposited on rigid substrates suffer from a clamping effect that hinders the propagation of strain. Accepted 16 June 2021 Herein, Fe-Ga films with macroporous, not fully constrained, geometry are prepared by electrodeposition Available online 18 June 2021 on metallized silicon substrates templated with sub-micrometer-sized polystyrene spheres. For compar- ison, fully-dense and inherently nanoporous films are prepared by sputtering and electrodeposition, Keywords: respectively. The electrodeposition mechanism is discussed in terms of electrochemically active species Electrodeposition Fe-Ga alloy distribution and partial current densities. The composition of the Fe-Ga films has been tuned (2–40 at.% Porous films Ga) by varying the electrodeposition parameters. A complete assessment of the nanomechanical and Nanoindentation magnetic properties of the films with variable composition and porosity has been performed for an opti- Magnetostriction mized performance. The magnetostriction has been studied by X-ray diffraction applying an in-situ mag- netic field. The results demonstrate a larger magnetic-field-induced crystal deformation in templated (macroporous) films compared to the non-templated and fully-dense counterparts. The observed effects in porous Fe-Ga films are very appealing for the design of various strain-engineered nanomaterials, e.g., energy transducers or magnetoelectric composites. Ó 2021 The Author(s). 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 ⇑ Corresponding authors at: Departament de Física, Universitat Autònoma de Magnetostriction is a property of ferro- and ferrimagnets that Barcelona, Bellaterra, Spain (J. Sort). allows these materials to expand or contract in response to a mag- E-mail addresses: Eva.Pellicer@uab.cat (E. Pellicer), Jordi.Sort@uab.cat (J. Sort). netic field due to the reorientation of magnetic domains. Essen- https://doi.org/10.1016/j.matdes.2021.109915 0264-1275/Ó 2021 The Author(s). 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/). A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 tially, these materials can convert the electromagnetic energy into parameters in a relatively easy manner at ambient temperature vibrations or mechanical energy (and vice versa), an effect that can and pressure. Typically, electrodeposited Fe-Ga has a polycrys- be exploited in sensors and actuators. The ability to induce strain talline structure with a magnetostrictive coefficient up to (k) in the material by using an external magnetic field is widely 150 ppm depending on the composition, i.e., two maxima in mag- utilized in magnetoelectric (ME) composites to modulate the elec- netostriction are usually observed at around 20 at.% of Ga (also tric polarization of the adjacent ferroelectric phase (direct magne- known as ‘‘galfenol”) and at 30 at.% [5,25]. Magnetostriction can toelectric effect). Recently, this technology has been applied for be higher in (1 0 0)-single crystal Fe-Ga (3/2k100  400 ppm) [26], remote, non-invasive, electric stimulation of living cells (without but polycrystalline alloys exhibit some superior mechanical prop- implanted electrodes) [1], targeted drug delivery [2], energy har- erties (like higher resilience to yielding). It is worth mentioning vesting systems [3] and water remediation [4], among others. that although the magnetostrictive properties of electrodeposited The most magnetostrictive material known so far is Terfenol-D, Fe-Ga alloys have been studied to some extent [18–20], investiga- an alloy of terbium, dysprosium, and iron. However, Terfenol-D is tion on the mechanism of Fe and Ga co-deposition that allows for brittle and, generally, bulky in size. Having no rare-earth elements stringent control over material’s design remains rather elusive. in the composition and enhanced mechanical properties compared In this study, we propose an original way to maximize the func- to brittle Terfenol-D is highly desirable. In this sense, Fe-Ga is one tionality of Fe-Ga films grown onto rigid substrates while circum- of the most suitable alternative magnetostrictive materials for cur- venting the problem of substrate clamping: to combine rent micro-/nanofabrication technologies [5]. Moreover, Fe-Ga is electrodeposition with colloidal lithography to purposely intro- biocompatible, which makes it suitable for biomedical applications duce different porosity levels in the films during their growth. [6,7]. The interrelationships between microstructure, porosity and com- Traditionally, magnetostrictive thin films suffer from a clamp- position in the obtained alloys have been established and corre- ing effect when grown onto rigid substrates. This significantly hin- lated with the resulting mechanical, magnetic and ders effective magnetic-field induced strain generation and magnetostrictive properties. The obtained results can lead to new transfer (in the case of ME composites) [8]. Therefore, the ability strain-engineered nanomaterials concepts, with potential applica- to design magnetostrictive thin film-based architectures at the tions in diverse fields such as magnetoelectric materials for data micro-/nanoscales (e.g., high aspect ratio patterned structures storage, energy conversion or biomedicine. where the interface contact area with the substrate is minimized) has become extremely valuable. Nonetheless, while the existence of homogeneous strains under the action of magnetic field in thin 2. Experimental magnetostrictive films grown onto rigid substrates is a priori not expected (due to the clamping), a recent study from our group Fe-Ga films were electrodeposited from an electrolytic bath demonstrated the occurrence of strain-gradients (instead of homo- with the following composition: 0.015 M FeSO4, 0.038 M Ga2(SO4)3, geneous strains) in mm-thick Fe-Ga films prepared by electrodepo- 0.5 M Na2SO4 and 0.065 M C6H8O7 (citric acid). The pH of the bath sition onto metalized silicon [9]. The observed effect originated was varied between 3 and 5, by adding NaOH to the solution. Elec- from the residual nanoporosity in between the columnar Fe-Ga trodeposition was performed in a standard single compartment grains that allowed each grain to exhibit a lateral compression, double jacketed cell connected to a Metrohm/Eco Chemie Autolab progressively larger with the increase in distance from the sub- PGSTAT302N potentiostat/galvanostat. Silicon substrates with an strate, where the grains are physically clamped. e-beam evaporated 10 nm Ti adhesion layer and a 90 nm Au seed In spite of their potential interest, porous magnetostrictive layer (highly textured along (1 1 1) direction) were used as a work- materials have not been systematically studied yet, although the ing electrode. A double junction Ag/AgCl reference electrode investigation of such materials is interesting for several reasons: (Metrohm AG) was used with 3 M KCl inner solution and 1 M Na2- (i) the magnetostriction of porous films deposited on rigid supports SO4 outer solution. A platinized titanium mesh served as a counter can be enhanced due to the absence of lateral constraints, ulti- electrode. Prior to use, the substrates were cleaned in acetone, mately leading to strain gradients instead of homogeneous strains, ethanol and Milli-Q water. To prepare the templated (macrop- which might be interesting for, e.g., generation of flexoelectric or orous) films, amidine functionalized polystyrene (PS) spheres of flexomagnetic effects [10,11]; (ii) porosity may offer several advan- 400 nm in diameter (purchased from Thermo Fisher) were elec- tages when it comes to mechanical properties of the films, e.g., trophoretically deposited on the substrate in a custom-made 2- increase of the plasticity index, making the material eventually electrode cell [23]. A constant potential of 30 V was applied for suitable for damping applications [12], or reduction of the Young’s 2 min to allow for multilayer assembly of the spheres. Following modulus, which is highly desirable in biomaterials targeted for the electrophoretic deposition, the samples were promptly placed orthopedic applications [13]; (iii) porous magnetostrictive matri- on a hot plate and heated at 50 °C for 15 min to evaporate the sol- ces can accommodate guest materials inside their voids, e.g., ferro- vent and stabilize the assembled sphere layers. Electrodeposition electric oxides or polymers, thus producing large surface area ME of Fe-Ga films was performed potentiostatically in the range from composites [14] (this, so far, has been achieved mainly by loading 1.20 to 1.35 V (vs. Ag/AgCl) for 15 min, at 25 °C. Mild stirring nanoparticles into 3D porous matrices [15,16]), or exchange- conditions were applied to enhance the mass transport of species coupled systems (e.g., spring magnets) [17]. from the bulk electrolyte towards the electrode surface. In the case Thin films of Fe-Ga can be deposited either by physical (e.g., of macroporous films, after Fe-Ga electrodeposition, PS spheres sputtering) or, more rarely, electrochemical methods i.e., by elec- were removed by immersing the samples in tetrahydrofuran for trodeposition from aqueous (citrate, gluconate, acetate, phosphate, 1 h followed by a final rinsing in acetone, ethanol, and Milli-Q etc.) or ionic liquid electrolytes [5,18–21]. Interestingly, compared water. Schematic drawings of the synthetic routes used to produce to physical deposition methods, electrodeposition offers several templated and non-templated Fe-Ga films are shown in Fig. 1. Con- significant advantages such as scalability and low cost (particularly tinuous, fully dense, Fe-Ga films with 20 at.% Ga were also pre- when aqueous electrolytes, e.g., based on citrates, are used). Fur- pared by sputtering and used as control samples whenever thermore, electrodeposition is one of the most suitable methods needed. For this, an individual Fe80Ga20 (at.%) target was evapo- for the fabrication of porous micro-/nanostructures [22–24]. Alloys rated using a 150 W DC gun for 25 min. with variable composition, crystallographic structure, and thick- The distribution of metal species in the electrolyte as a function ness (nm–mm) can be obtained by varying the electrodeposition of pH was calculated by solving an equation set, which included 2 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 where P is the porosity degree, Eporous is the experimentally obtained reduced elastic modulus of the films, Ebulk is the elastic modulus of the fully dense material, and n is a density exponent that depends on the deformation mechanism of the cell (n  2 for a material with an open cell porosity) [27]. Ebulk of Fe-Ga films was determined using the rule of mixtures for isostrain conditions (Eq.4): Ebulk ¼ fEFe þ ð1  f ÞEGa ð4Þ where f is the atomic fraction of Fe in the deposit, and EFe and EGa are the elastic moduli of bulk Fe and Ga, respectively. All the extracted parameters were statistically treated, and average values are reported. The magnetic properties of the Fe-Ga films were studied at room temperature using a VSM from MicroSense (LOT-Quantum Design). Hysteresis loops were recorded along the in-plane (with the substrate) and the out-of-plane directions with a maximum applied field of ± 20 kOe. For proper normalization of the hysteresis Fig. 1. Schematic illustration of the synthetic routes used to produce the Fe-Ga loops, the films were digested in aqua regia. The resulting solutions films: (a) sputter deposition, (b) potentiostatic electrodeposition onto a rigid were analyzed by inductively coupled plasma mass spectrometry substrate, and (c) two-step process involving electrodeposition through colloidal (ICP-MS) in a NexION350D spectrometer. template followed by the template removal. ‘‘SP” – sputtering, ‘‘ED” – electrode- To study the structural and magnetostrictive properties of tem- position, ‘‘PS” – polystyrene spheres. plated and non-templated Fe-Ga films, XRD patterns were acquired on a Philips X’Pert diffractometer using Cu Ka radiation in h-2h the following relations and quantities: (i) the equilibrium con- configuration. A constant magnetic field was applied in-situ and stants for all compounds added to or formed in solution, using val- along the out-of-plane direction of the films using a NdFeB perma- ues listed in Table S1; (ii) the mass balance equations for all forms nent magnet (Ø = 2 cm, 4000 Oe) placed inside the diffractometer. in the equilibrium mixture (Eq. (1)), and (iii) the charge balance Fe-Ga samples with an area of 0.5 cm  0.5 cm were placed on top equation, where ‘‘Cat” and ‘‘An” denote cation and anion, respec- and at the center of the permanent magnet and the xyz position of tively (Eq. (2)). the sample in the XRD diffractometer was kept the same for the X nþ= measurements without and with the magnetic field. Variations in ½Jtot ¼ ½J i  ð1Þ structural parameters caused by the magnetic field were evaluated X  nþ  X  n  by Rietveld refinement using the ‘Materials Analysis Using Diffrac- ni Cat i ¼ ni Ani ð2Þ tion’ (MAUD) software. where J is the species and n is the charge. For this purpose, a com- plete system of equations related to all equilibria in the solutions 3. Results and discussion was solved using Maple6 (Waterloo Maple Software and University of Waterloo). 3.1. Fe-Ga co-deposition mechanism The morphology of the deposited films was studied by scanning electron microscopy (SEM) using a Zeiss MERLIN field-emission Electrodeposition of Fe-Ga alloys is relatively difficult due to the SEM at 5 keV. Their chemical composition was determined with high tendency of both metal precursor ions to hydrolyze in aque- an energy dispersive X-ray spectroscopy (EDX) analysis tool ous solutions. Electrodeposited Fe-Ga alloys are prone to passivate attached to the SEM at an acceleration voltage of 20 keV. or even incorporate oxygen [28] which may alter the mechanical The mechanical properties of the Fe-Ga films were studied by and magnetic performance of the films. Understanding the bath nanoindentation. The indentations were performed on a Nanoin- chemistry is a crucial step towards the optimization of the elec- denter NHT2 from Anton Paar equipped with a Berkovich trolyte performance. For this purpose, the distribution of various pyramidal-shaped diamond tip under load-control mode. An array hydroxo- and organometallic species in the electrolyte was primar- of 50 indentations was performed at the top surface of the films. ily estimated. The nanoindentation function consisted of a loading segment of Fig. 2a provides the calculated mole fractions of Ga(III) contain- 30 s, followed by a load holding segment of 10 s and an unloading ing species vs. pH in an aqueous Ga2(SO4)3 solution. As it can be segment of 30 s. The maximum applied load was set to 1 mN to seen, Ga(III) ions form various mono- and polynuclear hydroxo- avoid the influence from the substrate. The reduced elastic modu- species over the entire range of pH. The distribution of various lus (Er) and hardness (H) were derived from the initial part of the Fe2+ hydroxo-species is shown in Supporting Figure S1. However, unloading–displacement curve by applying the method of Oliver the species distribution changes in the presence of the complexing and Pharr. The plasticity index (Wpl/Wtot) was calculated as a ratio agent used to stabilize the electrolyte (i.e., citrate) due to the between plastic and total (plastic + elastic) energies during nanoin- reduction of the concentration of ‘‘free” Fe2+ and Ga3+ ions. dentation. The plastic energy (Wpl) was derived as the area Fig. 2b shows the calculated species distribution in the citrate elec- between loading and unloading curves, while the total energy trolyte utilized in this work to deposit Fe-Ga films. The addition of (Wtot) was calculated as the area between the loading curve and the complexing agent to the bath significantly shifts equilibria the displacement axis. From the obtained nanomechanical data, towards the formation of complex compounds with citrate at the porosity degree in the Fe-Ga films was estimated using Eq. (3): pH > 2.5. The reduction of metal-containing species on the elec- trode gives rise to the respective shifts in the different chemical Eporous 1=n equilibria built in the electrolyte which involve complexes, ligands, P ¼1ð Þ ð3Þ Ebulk hydronium ions, etc. Keeping this in mind, all processes in the sys- tems under consideration should proceed through, at least, two 3 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Fig. 2. Calculated fraction of (a) Ga(III) hydroxo-complexes in water, and (b) Fe(II) and Ga(III) species at the following total concentrations of electrolyte constituents: [C6H8O7] = 0.065 M, [Fe(II)] = 0.015 M and [Ga(III)] = 0.038 M. In panel (b), ‘‘Cit” stands for the citrate anion C6H5O3- 7 ; dotted line represents the sum of equilibrium concentrations [FeCitH+2] + [FeOH+]. Equilibrium reactions and their constants used for the calculation are given in Table S1. steps: a chemical step followed by an electrochemical one (charge tively charged Ga(III) specie, i.e., Ga4(OH)+11 is presumed to act as transfer reaction), in other words, a certain chemical process gen- EAC, therefore the electroreduction mechanism involves the fol- erates the particle (i.e., the electrically active complex, EAC) that lowing relatively slow formation of adsorbed particle: further takes part in the charge transfer. Although there is no ther- modynamic limitation on the EAC composition (the same equilib- Ga4 ðOHÞþ11 þ e ! Ga4 ðOHÞ11;ads ð7Þ rium potential is attributed to any charge transfer process), single As it is seen from Fig. 3a, the partial current densities for Ga(III) electrically active species can be often defined, whereas all kinds of slightly increase with an increase in pH. The different effect of pH species participate in mass-transfer to the electrode. It is assumed on partial current densities for Fe(II) and Ga(III) electroreduction in this case that these species supply the total current in the sys- results in an increase of the Ga content in the alloys with increase tem, and the complex predominating in the solution does not need in pH. to be electrically active. Kinetic factors and the activation energy Electrodeposition conditions were first optimized for the non- are responsible for the electrochemical activity of the complexes templated Fe-Ga films. Fig. 3b shows the Ga content determined [29]. In addition, the electroreduction of Fe(II) and Ga(III) com- by both EDX and ICP-MS in films obtained by electrodeposition pounds to metallic state on the electrode involves multi-electron onto rigid Si/Au substrates. As it can be observed, Ga charge transfer reactions. This implies an intricate electroreduction content in the deposit increases with pH for a fixed deposition mechanism involving a sequence of intermediate stages preferen- potential, in accordance with the proposed co-deposition tially consisting of a single-electron transfer step and formation of mechanism. The alloys are iron-rich because of the more positive adsorbed species. In the case of Fe(II) electroreduction from acidic Nernst potential of Fe in the studied electrolyte i.e., EFe2þ =Fe ¼ solutions, three adsorbed species should at least be considered, and the corresponding intermediates must contain the OH– anion in 0:51V vs: EGa3þ =Ga ¼ 0:67V at pH 3. Remarkably, the experimen- their structure [30]. According to the model of Bocris el al. tal results suggest that below this value, i.e., for pH < 3, only [31,32] the intermediate in Fe(II) electroreduction from non- Fe (Ga-free) deposits can be obtained. An increase in cathodic complexed solution is Fe(OH)+. However, not only Fe(OH)+ but also polarization also leads to an increased Ga content in the Fe(HCitH2)+, are present in the citrate solution used in our study deposits, that can reach up to 40 at.% under the investigated (see Fig. 2b). Thus, the electroreduction mechanism involves for- conditions. mation of adsorbed zero-charged species followed by the single- electron transfer electrochemical reaction, and the electroreduc- 3.2. Morphological characterization tion to Fe0 during a slow (rate determining) subsequent stage, as follows: The morphology of the non-templated Fe-Ga films changes sig- FeLþ þ e ! FeðIÞLads ð5Þ nificantly with the Ga content, Fig. 4a-d. Multifaceted grains with some intragrain nanoporosity particularly visible for 2–20 at.% Ga FeðIÞLads þ e ! Fe þ L ð6Þ films (as it is indicated with the white arrows) is observed. This nanoporosity probably originates from the highly distorted colum- where L is either OH–, or CitH–2 species. nar growth promoted by accompanying hydrogen evolution, as it The calculated partial current densities for both Fe(II) and Ga can be observed in the cross-section SEM image in Fig. 4g (the (III) electroreduction are presented in Fig. 3a. As it can be seen, white arrows point to large pores). A controllable degree of poros- the partial current density for Fe(II) electroreduction decreases sig- ity in Fe-Ga films was achieved using colloidal crystal-templated nificantly with increasing pH, which correlates well with a substrates. Fig. 4e,f shows the representative SEM images of the decrease in the sum of equilibrium concentrations [FeCitH+2] + [- top surface of the templated Fe-Ga films with varying Ga content, FeOH+]. This reveals that both these species act as EAC in Fe(II) i.e. 20 and 33 at.% respectively. As expected, the pore diameter electroreduction. (~400 nm) matches the diameter of the latex spheres. A multilayer Investigations into gallium electroreduction mechanism are structure with an open-cell porosity and pore wall width of about elusive. Nonetheless, in an analogous manner to Fe(II), a similar 50–100 nm can be directly observed from the micrographs. Fig. 4h mechanism can be assumed for Ga(III) electroreduction. A posi- shows the fracture surface of templated Fe-Ga film having 20 at.% 4 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Fig. 3. (a) Partial current densities for Fe and Ga electrodeposition at various pH and potentials, and (b) composition of electrodeposited Fe-Ga alloys as a function of deposition potential and pH of electrolyte. Solid lines represent the composition obtained by EDX analysis and the dashed lines show the composition determined by ICP-MS analysis. Ga in composition with macropores homogeneously distributed mented and it is given by Eq. (3), from where the porosity of the along the film thickness. specimens can be calculated based on the measured mechanical Remarkably, the composition of templated Fe-Ga alloys is dif- properties [27,36]. The obtained values of porosity are reported ferent from the non-templated layers (i.e., films directly electrode- in Table S2. The results suggest that the non-templated films con- posited onto metallized Si substrates) produced under the same tain about 10–20% fraction of pores. This correlates well with the conditions (Fig. 3b), i.e., more Ga is incorporated when deposition morphological features revealed in Fig. 4. As the Ga content in occurs at the narrow regions between the close-packed spheres as the films increases, the shape of the grains becomes more regular compared to flat surfaces. This can be due to the hindered Fe spe- allowing them to pack closer. This contributes to a reduction in cies diffusion through the cavities that favors preferential Ga depo- porosity in the films. sition [33]. In addition, the alkalization of the near-electrode The porosity degree in templated Fe-Ga films reaches up to 35% surface can be more pronounced in the case of templated sub- according to the nanoindentation data. Nevertheless, according to strates, thus favoring the formation of Ga hydroxo-species that the literature, the porosity in colloidal templated films can reach presumably facilitate Ga deposition as compared to citrate mole- up to 65%, as it was demonstrated by ellipsometry measurements cules (Fig. 2b). It is also worth mentioning that the Fe-Ga films nor- in electrodeposited Fe-Cu films templated with 200 nm PS spheres mally contain up to 10 at.% of oxygen on the top surface, [23]. A priori, one may assume that the porosity degree in the tem- independently of the applied electrodeposition conditions (Fig- plated deposits is directly dictated by the size of the spheres ure S2). Nevertheless, EDX analysis on the cross-section of the (which are much larger than the size of the pores produced during films evidenced that there is only < 5 at.% in the interior of the lay- electrodeposition). This would give the same porosity level to all ers. This means that the films undergo superficial surface oxidation templated films, independently of their composition. However, after fabrication, while inclusion of oxides and hydroxides during the porosity degree for the templated samples increases with the electrodeposition is not promoted. Ga content (contrary to what happens when depositing onto a flat surface). This is most likely caused by the inherent ‘‘second-order” 3.3. Mechanical properties porosity in Fe-Ga attributed to the Fe-Ga grain structure, i.e., larger grains of Ga-rich alloy are more difficult to accommodate in nar- The influence of both porosity and chemical composition of row spaces between the PS spheres, thus producing more voids, electrodeposited Fe-Ga films on their mechanical properties was as compared to the smaller grains of more Fe-rich deposits. As a investigated by nanoindentation. Dense film of Fe-Ga with 20 at. result, colloidal templated films show dual or hierarchical porosity, %Ga produced by sputtering was used as a reference material. i.e., macropores caused by the PS spheres and nanopores in the Fig. 5a shows representative load–displacement curves from pore walls. where various nanomechanical parameters were extracted after Yet, the porosity degree obtained from nanoindentation data is applying the method of Oliver-Pharr [34]. It can be observed that likely underestimated due to the compaction of pores that occurs the maximum and final penetration, the slope at the beginning of under the indenter tip during loading. This can be inferred from the unloading segment and the areas under the loading and/or the shape of characteristic load–displacement curves of the highly unloading curves vary depending on the composition and the porous templated samples where a pronounced ‘‘flattening” of the degree of porosity of the films. Indeed, concerning the reduced loading segment is observed at low forces. That is, as the applied Young’s modulus (Er), for samples with similar compositions (i.e., load is increasing slowly the indentation depth changes signifi- 20 at.%), the dense sample has the highest value, while Er decreases cantly due to the densification of the porous microstructure [37]. significantly for the non-templated and even further for the tem- Supporting Figure S3 shows an indentation imprint obtained at plated electrodeposited Fe-Ga layers (Fig. 5b, Table S2). Remark- 20 mN and clearly evidences the pore compaction. Note that even ably, the elastic modulus of the reference sample is very close to at 1 mN several pores of the templated Fe-Ga films are embraced the bulk value estimated from the rule of mixtures (Eq. (4). taking since the indentation contact area (estimated by the model of Oli- EFe = 211 GPa, EGa = 9.6 GPa [35]), confirming that the sputtered Fe- ver and Pharr) is around 0.6–0.8 lm2 for the templated sample Ga films are truly dense and virtually free of porosity. The correla- (thus larger than the size of the macropores). tion between elastic modulus and porosity degree is well docu- 5 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Fig. 4. Representative SEM images of the Fe-Ga films. Panels a-d show the top surface of the non-templated films: (a) 2 at.% Ga; (b) 15 at.% Ga; (c) 20 at.% Ga; and (d) 33 at.% Ga. Panels e,f show the top surface of the templated films: (e) 20 at.% Ga; and (f) 33 at.% Ga. Panels g,h show the fracture surfaces (cross-sections) of the Fe-Ga films with 20 at. % Ga: (g) non-templated; (h) templated. White arrows indicate the pores. The presence of pores in electrodeposited Fe-Ga films is also the to the other two (i.e., porosity degree and Fe:Ga ratio in the films) most likely factor responsible for the observed reduction in hard- [39]. ness of non-templated and templated films as compared to the Finally, the elastic–plastic response of the films can be also fully dense reference sample. Hardness is directly related to the studied from the nanoindentation data. The load–displacement yield stress, which, in turn, is directly proportional to the porosity curves of electrodeposited templated and non-templated Fe-Ga degree [36]. Furthermore, taking into account that the macroscopic films (Fig. 5a) feature so-called ‘‘pop-in” events, i.e., sudden bursts hardness of Ga is generally smaller than that of Fe (by a reported during the loading of the indenter onto the sample. Generally, in factor of at least 3.5 [35]), the decrease in hardness for larger Ga fully dense polycrystalline materials such plastic instability may contents (Table S2) can be understood again based on the rule of arise from the dislocation nucleation, formation of stacking faults, mixtures argument. Grain size can also play a role in the reduction cracks, etc. (not noticed in the sputtered reference Fe-Ga prepared of both, hardness and Young’s modulus of the alloys, as it is well- in this work) [40]. In the case of porous Fe-Ga films the pop-in documented in the literature [38]. Nevertheless, in the case of events most likely caused by the abrupt densification of the pores highly porous films this effect is most likely negligible as compared or their collapse due to the brittle nature of the material [41]. The 6 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Fig. 5. Mechanical properties of Fe-Ga alloys at 1 mN: (a) representative load–displacement curves for the reference (dense, sputtered) Fe-Ga sample, non-templated ‘‘NT” and templated ‘‘T” electrodeposited Fe-Ga films; (b) reduced Young’s modulus of the Fe-Ga films as a function of the Ga content; (c) hardness of the investigated Fe-Ga films. The legend in the panel (b) applies also to the panel (c), where the same but empty symbols are used instead of filled symbols. elastic strain to failure, H/Er, and the plasticity index, Wpl/Wtot, were porous samples the loops recorded in the out-of-plane direction also derived (see experimental section for details). The former indi- become less tilted with respect to in-plane, i.e., the difference in cates how much the material can be elongated to failure, and the Mr/Ms in-plane and out-of-plane directions decreases. Such behav- later shows the ability of the material to absorb plastic deforma- ior indicates that the hysteresis behavior of the denser Fe-Ga films tions. The results presented in Table S2 indicate that for a given is governed by the shape anisotropy but this effect is partially lost composition (i.e., 20 at.%), the plastic energy is larger as the poros- in the porous samples. This also leads to a reduction of the out-of- ity degree increases. Interestingly, there is a linear correlation plane saturation field in the templated samples (Table 1). It is between the two above-mentioned parameters [34,42], which is worth mentioning that at magnetic saturation, the magnetostric- also confirmed for the studied Fe-Ga system (see Figure S3). tive strain reaches a maximum, such as all the magnetic domains become aligned with the magnetic field [43]. Therefore, based on these data it can be assumed that in porous Fe-Ga samples the 3.4. Magnetic properties maximum magnetostriction along the out-of-plane direction can be induced at lower fields as compared to the fully dense, or The magnetic properties of the dense and porous Fe-Ga films non-templated samples. having 20 and ~ 30 at.% of Ga (composition corresponding to max- Remarkably, contrary to the fully dense reference (sputtered) ima in magnetostriction) were studied by VSM. Fig. 6 and Figure S5 sample, the hysteresis loops of both non-templated and templated show magnetization hysteresis loops of the Fe-Ga films with 20 at. (electrodeposited) films recorded along out-of-plane show a % of Ga but different porosity degree. The hysteresis loops of the widening when approaching coercive field. Such ‘out-of-plane films with 33 at.% Ga are shown in Supporting Figure S6. In all component’ has been previously observed in various Fe-Ga film cases, the remanence-to-saturation magnetization ratio, Mr/Ms, systems, regardless of the growth method, although to a different measured along the out-of-plane direction, is lower than for the extent, that makes it difficult to establish a direct correlation with in-plane direction (Table 1), which indicates the magnetic easy axis the structural and compositional characteristics of the films lies in the plane of the films (in spite of porosity). The overall shape [18,44]. Very recently, it has been demonstrated that the appear- of the in-plane hysteresis loops is squarer, which means the films ance of such out-of-plane magnetic component in Fe-Ga alloys become magnetically saturated at lower magnetic fields. In the Fig. 6. In plane and out of plane hysteresis loops for Fe-Ga layers with 20 at.% of Ga: (a) reference – fully dense, (b) non-templated – nanoporous Fe-Ga, and (c) templated film – macroporous Fe-Ga. Insets show the magnified central area of the corresponding loops. 7 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Table 1 Summary of the magnetic properties of the Fe-Ga films obtained from VSM measurements. ‘‘NT” – non-templated, ‘‘T” – templated. Sample Ga, at.% Saturation Saturation Coercivity, Oe Remanence-to-saturation magnetization, field \, kOe magnetization ratio Mr/Ms emu/g k \ k \ D(k-\) Reference 20 176 14.0 86 86 0.68 0.02 0.66 NT 20 187 13.7 69 926 0.74 0.24 0.50 NT 33 100 12.5 94 186 0.56 0.03 0.53 T 20 178 12.3 70 365 0.51 0.18 0.33 T 33 112 11.0 70 86 0.35 0.02 0.33 can be due to the presence of several magnetic phases with com- peting anisotropies [45]. An addition of Ga to Fe leads to a reduction of saturation mag- netization (Ms), as it can be expected from the dilution of magnetic Fe (Ms(Fe) = 217 emu/g [46]) by non-magnetic Ga atoms. Neverthe- less, the dependence of Ms follows a linear trend with the compo- sition only up to ~ 20 at.% of Ga [47]. Above this concentration, the magnetization versus Ga content changes slope and drastically decreases due to the change in structural ordering of the Ga-rich alloys [48]. This explains the considerable variation of MS between the samples with 20 and 33 Ga at. %. 3.5. Structural and magnetostrictive properties Essentially, the conversion between the mechanical and mag- netic energies in Fe-Ga is realized through magnetostriction. In this work, structural characterization and assessment of magnetostric- tion in the Fe-Ga films was performed by XRD. Fig. 7a shows the XRD pattern corresponding to the templated (macroporous) Fe- Ga film with ~ 20 at.% of Ga, while the XRD pattern of the non- templated film with the same composition is shown in Supporting Figure S7a. The patterns were recorded in a relatively narrow region capturing the strongest Fe-Ga reflection, that is (1 1 0) [9,25], and the substrate (Au (1 1 1)) peak that served as a reference for determination of magnetostriction in subsequent experiments. Iron and gallium are highly miscible (Ga solubility in body- centered cubic (bcc)-Fe is 47.5 at.%) and may form both solid solu- tions and intermetallic compounds, namely: Fe3Ga, Fe6Ga5, Fe3Ga4, and FeGa3 (with increasing Ga content) [49]. Among them, a disor- dered bcc a-Fe(Ga) solid solution (or A2 phase) is typically found in electrodeposited films [2,9,50,51], although the formation a-Fe3Ga phase in electrodeposits has been also reported [28]. Increasing the Ga content in the films may lead to the appearance of several ordered phases (coherent with the parent A2 cubic structure), such as cubic B2, bcc-D03, or fcc-L12. This is observed in films prepared by physical methods, e.g., arc melting or sputtering [52–54]. Some- Fig. 7. X-ray diffraction patterns of templated Fe-Ga alloy with 20 at.% Ga: (a) wide times, the ordered and disordered phases coexist (even in non- 2h range capturing the seed-layer (gold) peak used as a reference recorded with no annealed samples [55]) and so, the understanding of the structural magnetic field applied, and (b) narrow 2h range showing the two peaks of (1 1 0)-Fe properties of the films becomes more complex [45]. (Ga) solid solution with no field and with an out of plane magnetic field of 0.4 T. In Fig. 7a, two peaks at around 44-45° can be observed. The two peaks can be attributed to the (1 1 0) reflection of Fe(Ga) solid solu- tions, with dissimilar lattice parameters. No peaks of oxides or Fe- the phase ‘‘B” is more Fe-rich having ~ 12 at.% of Ga. This gives an Ga intermetallic compounds are present (in a wide 2h range). For average of 17 at.% of Ga in the templated film, in accordance with simplicity, the peak at lower 2h angle is denoted as ‘‘A” and the sec- ICP-MS results (Fig. 3). It is worth mentioning that such phase sep- ond peak separated by 0.6° from ‘‘A” is symbolized as ‘‘B”. Since Ga aration is typically not observed in as-prepared Fe-Ga films, and so has a slightly larger atomic radius than Fe, the random substitution far, remains not fully understood. The presence of two clear peaks of Fe atoms by Ga when forming a solid solution causes an increase with well distinguished cell parameters implies that the phase sep- in the lattice parameter and shifts the peaks’ position towards aration is not attributed to the compositional gradients (that may lower 2h angles. Thus, phase B is likely to have less Ga, as com- develop in some cases due to the local variations in pH near the pared to phase A. The Vegard’s law can be used to estimate the electrode [28]). Remarkably, that the non-templated Fe-Ga films composition of each phase. To obtain the cell parameters of ‘‘A” also exhibit a double (1 1 0)-peak (Figure S7a), and therefore, the and ‘‘B”, the patterns were refined by Rietveld method using MAUD observed effect is reproducible regardless of the templating software. Based on values reported in Ref. [50], the cell parameter method. The effects of substrate could not explain the double peak of the phase ‘‘A” corresponds to approximately 22 at.% of Ga, while because the substrate itself does not diffract in this angular range. 8 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 In addition to having two A2 solid solutions, the splitting of the direction [9]. The templated Fe-Ga films show an even larger peaks could also be due to the formation of ordered phases within magnetic-field induced variation of the cell parameter, i.e., an the A2 matrix, in accordance with reported metastable Fe-Ga phase increase by 0.033% for phase ‘‘A” and by 0.045% for phase ‘‘B”. diagrams [56,57]. Such metastability could be reached during elec- The increase in the cell parameter is accompanied by an increase trodeposition since the potential applied to the electrode drives the in the microstrains due to cell distortion. These results are in accor- electrochemical system far from the situation predicted by the dance with previous theoretical works from which an enhanced equilibrium phase diagram [58–63]. Indeed, according to the phase magnetostriction with porosity in materials with k111 < k100 (char- diagram reported in Ref. [57], the phase ‘‘A” may contain D03 clus- acteristic for Fe-Ga alloys [26]) was inferred [64]. Nevertheless, the ters (22 at.% Ga) and phase ‘‘B” - A2 structure (12 at.% Ga). Interest- complexity of the system does not allow to easily discern between ingly, the splitting of the Fe(Ga) peaks has been also shown in Ref. different possible magnetostriction contributions (i.e., ‘‘internal [21], although the interpretation remained elusive. Most likely magnetostriction” due to dipole–dipole interactions, versus ‘‘vol- phase separation occurs at the nanoscale and is probably governed ume magnetostriction”). by the intricate adsorption and nucleation mechanisms occurring during electrodeposition. To assess the magnetostriction in templated and non-templated 4. Conclusions Fe-Ga films, an external magnetic field was applied in-situ while recording the XRD patterns. Theoretical works involving grain Fe-Ga alloys were electrodeposited from citrate electrolyte onto averaging theory for modelling heterogeneous materials (Green’s rigid metalized silicon substrates. The co-deposition mechanism of function) have shown that the magnetostriction effect is sensitive Fe-Ga alloys is discussed based on strong interdependences from not only to the single crystal constants but also to microstructure, electrochemically active complexes, such as FeCitH+2, FeOH+ and such as crystal anisotropy and porosity [64,65]. Formally, the Ga4(OH)+11. The different influence of pH on partial current densi- macroscopic behavior of a polycrystal can be quantitatively ties for Fe(II) and Ga(III) electroreduction correlates well with the assessed using so-called ‘‘effective medium expression” (or effec- concentrations of positive single-charged species. This leads to tive medium approximation) that accounts for the effects of crystal the possibility of tuning the composition of the films (in the range constants, microstructural features, and applied fields. Thus, 2 – 40 at.% of Ga) by varying the pH value of the electrolyte and the numerical simulation showed that the magnetostriction of a poly- electrodeposition potential. Alloys with different porosity levels, crystal can either decrease if |k111| > |k100|, in e.g. Fe or Terfenol-D, i.e., nanoporous vs. macroporous layers, were prepared in our or increase if the |k111| < |k100|, as in the case of Ni and Fe-Co [64]. It work. The films directly grown on silicon exhibited a certain degree should be noted that the experimental determination of magne- of inherent nanoporosity within the pore walls endowed by hydro- tostriction in porous materials remains challenging. Most of the gen coevolution, and macroporous films were produced by elec- methods to estimate the magnetostriction constant rely on bend- trodeposition through colloidal crystal templates. The induced ing the films using an underlying substrate (e.g., standard strain porosity significantly reduces the Young’s modulus and the hard- gauge method). However, in case of non-constrained geometries, ness of electrodeposited Fe-Ga films, as compared to fully dense like in highly porous films where there is a limited contact sputtered layers. Increasing the Ga content from a few at.% to 30 between the layer and the substrate, these methods are not very at.% causes the hardness to decrease from 5 GPa to ~ 1 GPa (in reliable, basically because the deformations induced by magnetic the case of Ga-rich templated film), respectively. However, the field are not effectively transferred to the substrate and the presence of pores increases the plasticity index, which is as impor- induced deformations can be accommodated through the empty tant parameter indicative of the film’s ability to accommodate space in the pores. Thus, standard magnetostriction measurements plastic deformations. In terms of magnetic properties, addition of based on the macroscopic specimen deformation cannot be applied Ga to Fe decreases the saturation magnetization, although the to study the here-prepared porous materials. Alternatively, XRD dependence does not follow a linear trend with the overall compo- with magnetic field measurements can provide information about sitional range. The shape anisotropy is partially lost in the porous the ‘‘internal magnetostriction”, i.e., magnetic field induced crys- samples and the out-of-plane saturation field is decreased, sug- tallographic deformation within each unit cell [66,67]. Upon appli- gesting that, along this direction, the maximum magnetostriction cation of a magnetic field, the reorientation of magnetic domains can be achieved at lower magnetic fields as compared to the fully can lead to an expansion or contraction of the lattice, which can dense films. Finally, the magnetic-field induced changes in lattice cause a shift of the corresponding XRD peaks [67–70]. constants of the non-templated (nanoporous) and templated Indeed, it has been observed that under an external magnetic (macroporous) Fe-Ga films were evaluated by XRD. The results field both (1 1 0)-Fe(Ga) peaks are shifted towards lower 2h angles, demonstrate that under the action of external magnetic field the as depicted in Fig. 7b. The magnetostriction of Au is negligible and macroporous Fe-Ga films exhibit a larger crystal deformation, as therefore the position of the peak can serve as a reference (Fig- compared to nanoporous and fully dense counterparts, i.e., the cell ure S7). The obtained patterns were analyzed using Rietveld parameter is increased by 0.033 – 0.045%. Thus, the reduction of refinement from where the magnetic-field induced changes in lateral constraints due to porosity minimizes the clamping effect (1 1 0)-Fe(Ga) cell parameter were calculated as a percent fraction with rigid substrates and allows for strain gradients. In addition, from the initial value. As expected, the cell parameter of the refer- the pores could accommodate second-phase materials, thus open- ence, fully constrained (dense) sample almost does not change ing new possibilities for the design of various composite materials, with the external magnetic field (i.e., an increase in cell parameter such as exchanged coupled systems or magnetoelectric from initial state is ~ 0.003%). This result could be anticipated due heterostructures integrated on silicon. to the clamping with the rigid support (metallized Si) that hinders the generation of strain. Contrarily, the cell parameter of the non- templated Fe-Ga film increases by approximately 0.033% for phase ‘‘A” and 0.012% for phase ‘‘B”. In this sample, upon application of Declaration of Competing Interest magnetic field, the residual nanoporosity in between the Fe-Ga grains allows the material to deform at a certain distance from The authors declare that they have no known competing finan- the substrate, thus significantly reducing the clamping effect cial interests or personal relationships that could have appeared through the development of strain gradients along the vertical to influence the work reported in this paper. 9 A. Nicolenco, Y. Chen, N. Tsyntsaru et al. Materials & Design 208 (2021) 109915 Acknowledgements [14] D. Chien, A.N. Buditama, L.T. Schelhas, H.Y. Kang, S. Robbennolt, J.P. Chang, S.H. Tolbert, Tuning magnetoelectric coupling using porosity in multiferroic nanocomposites of ALD-grown Pb(Zr,Ti)O3 and templated mesoporous This work has received funding from the European Union’s Horizon CoFe2O4, Appl. Phys. Lett. 109 (2016), https://doi.org/10.1063/1.4962536 2020 research and innovation programme under the Marie 112904. [15] P. Martins, R. Gonçalves, A.C. Lopes, E. Venkata Ramana, S.K. Mendiratta, S. Sklodowska-Curie grant agreement N° 892661– MAGNUS and par- Lanceros-Mendez, Novel hybrid multifunctional magnetoelectric porous tially from N° 778357- SMARTELECTRODES. Financial support by composite films, J. Magn. Magn. Mater. 396 (2015) 237–241, https://doi.org/ the European Research Council (SPIN-PORICS 2014-Consolidator 10.1016/j.jmmm.2015.08.041. [16] F. Mushtaq, H. Torlakcik, Q. Vallmajo-Martin, E.C. Siringil, J. Zhang, C. Röhrig, Y. Grant (N° 648454) and MAGIC-SWITCH 2019-Proof of Concept Shen, Y. Yu, X.-Z. Chen, R. Müller, B.J. Nelson, S. 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