nanomaterials Article Structural, Wetting and Magnetic Properties of Sputtered Fe70Pd30 Thin Film with Nanostructured Surface Induced by Dealloying Process Gabriele Barrera 1, * , Federica Celegato 1 , Matteo Cialone 2 , Marco Coïsson 1 , Paola Rizzi 3 and Paola Tiberto 1 1 Advanced Materials Metrology and Life Sciences, INRiM, Strada delle Cacce 91, I-10135 Torino, Italy; f.celegato@inrim.it (F.C.); m.coisson@inrim.it (M.C.); p.tiberto@inrim.it (P.T.) 2 Department de Fisica, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain; matteo.cialone@uab.cat 3 Chemistry Department and NIS, University of Turin, via Pietro Giuria, 7, I-10125 Torino, Italy; paola.rizzi@unito.it * Correspondence: g.barrera@inrim.it Abstract: FePd alloys in the thin film form represent a multipurpose and versatile material with relevant chemical and physical properties studied in different research fields. Moreover, the ability to manipulate and fine-tune the film surface with nanometric scale precision represents a degree of freedom useful to adapt these thin film properties to the demands of different desired applications. In this manuscript, Fe70 Pd30 (at. %) thin films are prepared with a thickness of 50 and 200 nm by means of the widely used co-sputtering deposition technique. Subsequently, selective removal of the  iron element from the alloy and the consequent surface diffusion of the palladium was induced by a  dealloying treatment under free corrosion conditions in hydrochloric acid. The size and shape of Citation: Barrera, G.; Celegato, F.; the grains of the as-deposited thin films determine the dissolution rate of the iron element with a Cialone, M.; Coïsson, M.; Rizzi, P.; direct consequence not only on the surface morphology and the stoichiometry of the alloy but also on Tiberto, P. Structural, Wetting and the wetting and magnetic properties of the sample. X-ray diffraction, Scanning Electron Microscopy Magnetic Properties of Sputtered (SEM) images, contact angle and magnetic measurements have been performed to provide a thorough Fe70 Pd30 Thin Film with characterisation of the fundamental properties of these nanostructured bimetallic thin films. Nanostructured Surface Induced by Dealloying Process. Nanomaterials Keywords: FePd thin film; dealloying process; wettability; magnetic properties 2021, 11, 282. https://doi.org/ 10.3390/nano11020282 Received: 17 December 2020 Accepted: 19 January 2021 1. Introduction Published: 22 January 2021 Bimetallic alloys with a magnetic transition element (Fe, Co, Ni) have attracted great attention in the field of scientific research for their peculiar and interesting properties [1–4]. Publisher’s Note: MDPI stays neutral The transition metal can be exploited directly for its intrinsic magnetic properties (e.g., with regard to jurisdictional claims in magnetic separation) or to improve the alloy ability (e.g., enhancement of catalytic effi- published maps and institutional affil- ciency [5,6]) or as a sacrificial metal in the dealloying process to promote the formation of a iations. porous nanomaterial with a large surface-to-volume ratio [7–9]. Among the thin-film bimetallic alloys of the iron group, FePt is intensively studied be- cause it is recognized as a promising magnetic material for prospective applications in high- density magnetic storage devices and micro-electromechanical systems (MEMS) [10,11] Copyright: © 2021 by the authors. because the ordered L10 FePt possesses a very high magnetocrystalline uniaxial anisotropy. Licensee MDPI, Basel, Switzerland. At the same time, FeCu is an attractive material to investigate for several devices and This article is an open access article applications (e.g., as an electrode coating material for solar cell device [12]) both because it distributed under the terms and is magnetically soft properties with high magnetic moment per Fe atom and constituted by conditions of the Creative Commons sustainable elements [4,13,14]; whereas co-sputtered FeAg granular alloy, composited of Attribution (CC BY) license (https:// two non-mixing elements [15], have attracted much attention in research and device areas creativecommons.org/licenses/by/ because of its giant magnetoresistance (GMR) properties [16,17]. 4.0/). Nanomaterials 2021, 11, 282. https://doi.org/10.3390/nano11020282 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 282 2 of 14 Recently, FePd alloy have also attracted a growing interest in the scientific community because of their possibility to combine the properties of magnetic (Fe) and noble (Pd) metal elements in a multifunctional alloy [18–22]. The chemical composition of the FePd binary alloy system strongly influences its structure and magnetic properties allowing the use in a variety of applications [21,23–26]. These properties include magnetic shape memory effect for the Fe70 Pd30 alloys [27–31], high perpendicular magnetic anisotropy for Fe50 Pd50 [32,33], as well as hydrogen absorption characteristics for the palladium-rich FePd material [34,35], catalytic activities [36,37] and SERS devices [26]. Such bimetallic alloy materials are successfully produced in thin film form using different deposition technologies [38] to meet the current trends towards the miniatur- ization of materials and the reduction of the use of expensive and exhaustible element (e.g., Pd atoms). In thin films, distinct advantages over bulk materials [39] arise from their small thickness, large surface/volume ratio and unique structural properties that derive directly from the growth process. The unique properties of thin films have paved the way for new applications in the fields of integrated circuits, information technologies and sensors [38,40,41]. In addition, the ability to manipulate and fine-tune the film surface with nanometric scale precision has opened new opportunities in materials science [42]. Among the several physical and chemical methods used to nanostructure the film surface, the dealloying treatment is a common, fast and cost-effective process [43–46], fully suitable for the FePd alloy. This process is based on the selective removal of the less noble atoms from the alloy leading to structural, compositional and morphological transformations of the film surface with a consequent variation of its physical and chemical properties [9,47,48]. Generally, adequate control during the growth process and the subsequent dealloying process results in a fundamental role in determining the resulting properties of thin film. This manuscript is focused on the study of the interplay between the wetting and magnetic properties of Fe70 Pd30 thin film and the stoichiometry, morphology and spatial confinement of its surface which continuously evolve as a function of the dealloying treatment time. These observed multiple degrees of freedom can be used to adapt the thin film properties to the needs of different desired applications. 2. Materials and Methods FePd thin film was deposited at room temperature by dual-source radio-frequency RF sputtering (Cinquepascal, Milan, Italy) equipped with elemental iron (purity 99.99%) and elemental palladium (purity 99.98%) targets; using a confocal configuration, the iron target was driven at radio frequency whereas the palladium target with a continuous current. The deposition parameters were: base pressure of the sputtering at 4.0 × 10−7 mbar and, Ar gas pressure at 1.2 × 10−2 mbar. The deposition rate was experimentally evaluated (1.35 Å/s) and then later used to set the deposition time in order to obtain two samples with the desired thickness of 50 and 200 nm to obtain as-deposited samples with a marked difference in structural properties, see paragraph 3.1 below. The FePd thin films were deposited on a Si substrate covered by a SiO2 oxide (500 nm). The adhesion of FePd thin film on the SiO2 surface was ensured by a thin Ti(8 nm)/Au(80 nm) bilayer deposited between them. Room-temperature grazing incidence X-ray diffraction (GIXRD) with Cu-K α radiation was used to study the crystal structure of the samples. The diffractometer was made by Panalytical in Malvern (UK). The as-deposited FePd samples were chemically treated for selected times in a 2 molar aqueous solution of hydrochloric acid under free corrosion condition in order to induce the dealloying process. The solution was prepared from reagents grade chemicals and de-ionised water. Scanning Electron Microscopy (SEM) equipped by an energy dispersive x-ray spec- trometer (EDS) was used to investigate the sample morphology and to evaluate the sto- Nanomaterials 2021, 11, x FOR PEER REVIEW 3 of 14 Nanomaterials 2021, 11, 282 3 of 14 Scanning Electron Microscopy (SEM) equipped by an energy dispersive x-ray spec- trometer (EDS) was used to investigate the sample morphology and to evaluate the stoi- chiometry of the FePd alloy as a function of the dealloying treatment time. The SEM ac- ichiometry of the FePd celeration voltage alloy was set as a to 20 kV.function of the dealloying The stoichiometry resultstreatment time. are affected by The SEM an uncer- acceleration voltage was set to 20 kV. The stoichiometry tainty of 2%. The SEM was made by FEI company in Hillsboro (USA). results are affected by an uncer- taintyInvestigation of 2%. The SEM waswettability on the made by FEI companyofinthe properties Hillsboro surface(USA). of the FePd samples was Investigation on the wettability properties of the surface performed at room temperature by means of contact angle measurements of the FePd samples with was per- a home- formed at room temperature by means of contact angle measurements with made-built setup. Each value results from the average of at least six measurements of a a homemade- built setup. droplet Each valuewater of de-ionized results from with the average a volume of at of 1 μL in least six measurements different of a droplet points of the sample of surface. de-ionized water with a volume of 1 µL in different points of the sample Room temperature hysteresis loops were performed using an Alternating Gradient surface. Room temperature hysteresis loops were performed using an Alternating Gradient Field Magnetometer (AGFM) applying the magnetic field in the film plane in the interval Field Magnetometer (AGFM) applying the magnetic field in the film plane in the interval −18 kOe ≤ H ≤ 18 kOe. The diamagnetic signal of the sample holder and film substrate was −18 kOe ≤ H ≤ 18 kOe. The diamagnetic signal of the sample holder and film substrate was evaluated and adequately subtracted. The values of the coercive field were extrapolated evaluated and adequately subtracted. The values of the coercive field were extrapolated from hysteresis loops; an error bar of 2 Oe was associated to take into account the uncer- from hysteresis loops; an error bar of 2 Oe was associated to take into account the uncer- tainty resulting from the measurement of the loop and from their subsequent extrapola- tainty resulting from the measurement of the loop and from their subsequent extrapolation. tion. The AGFM was made by Princeton Measurements Corporation in Princeton (USA). The AGFM was made by Princeton Measurements Corporation in Princeton (USA). Atomic and magnetic force microscopy (AFM/MFM) to image the surface morphol- Atomic and magnetic force microscopy (AFM/MFM) to image the surface morphology ogy and the magnetic domain configuration was performed in lift-mode using a commer- and the magnetic domain configuration was performed in lift-mode using a commercial cial tip coated with a ferromagnetic CoCr alloy. The MFM images were acquired at the tip coated with a ferromagnetic CoCr alloy. The MFM images were acquired at the rema- remanence state after the application of a saturating in-plane magnetic field. The nence state after the application of a saturating in-plane magnetic field. The AFM/MFM AFM/MFM was microscope microscope made bywas made Bruker inby Bruker(USA). Billerica in Billerica (USA). 3. Results 3. Results and and Discussion Discussion 3.1. As-Deposited FePd Thin Film 3.1. The The Fe:Pd Fe:Pdratio ratioininthe thealloy alloywas wasdetermined determinedusingusingEDS EDStechnique techniqueresulting resultingininFeFe Pd 7070Pd3030 for for both both samples. samples. This This evidence evidence indicates indicates that that the the stoichiometry stoichiometry of of the the alloy alloy remains remains the the same during the deposition independently of the sample same during the deposition independently of the sample thickness. thickness. The The x-ray diffraction patterns patterns ofofthe theas-deposited as-depositedS50 S50and andS200 S200samples samples are are shown shown in in Figure Figure 1. Discerned 1. Discerned the substrate the substrate peaks,peaks, both patterns both patterns indicate indicate the formation the formation of a of a metasta- metastable supersaturated ble supersaturated solid solution solid solution of α-(Fe,Pd) of α-(Fe,Pd) [26,49]. [26,49]. The slightThe slight broadening broadening of the reflec-of the tionreflection peaks indicates peaks indicates overlappingoverlapping of diffraction of diffraction peaks at peaks slightlyat different slightly different angles angles induced induced by a non-uniformly by a non-uniformly distribution distribution of the elements of the elements in the crystalline in the crystalline grains [26]. grains [26]. Figure X-raydiffraction Figure 1. X-ray diffraction(XRD) (XRD) patterns patterns of the of the as-deposited as-deposited S50 S50 (green (green line)line) and (blue and S200 S200 (blue line) samples. line) samples. The The surface surface morphology morphology of of both both as-deposited as-deposited samples samples isis shown shown inin Figure Figure 2a,b; 2a,b; the the investigated investigated area in the SEM images is the same to make easier the comparison. The area in the SEM images is the same to make easier the comparison. The S50 S50 film film surface surface appears appears conformal conformal constituted constituted byby small, small, irregularly irregularly shaped shaped and and uniformly uniformly distributed distributed grains grains with with well-defined well-defined boundaries boundaries which which are are figured figured out out as as darker darker line line around the grains [50,51]. The S50 sample cross-section observation (see Figure around the grains [50,51]. The S50 sample cross-section observation (see Figure 2c) shows2c) shows a homogeneous and dense growth of the thin film along the entire thickness. No crack is visible in the entire region under investigation. Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 14 Nanomaterials 2021, 11, 282 4 ofis a homogeneous and dense growth of the thin film along the entire thickness. No crack 14 visible in the entire region under investigation. Figure Figure2.2.Top Topand andcross crosssection sectionscanning scanningelectron electronmicroscopy microscopy(SEM) (SEM)images imagesof ofas-deposited as-depositedS50 S50((a,c) ((a,c)panels), panels),S200 S200((b,d) ((b,d) panels) samples. panels) samples. By Byincreasing increasingthe thefilm filmthickness thicknessup upto to200 200nm, nm,thethe surface surfacemorphology morphologyshows showssome some narrow narrowand andshort shortcracks cracksinduced inducedby bythetheaccumulation accumulationof ofgrowing growingstrain strainduring duringthe thefilm film deposition [52,53]. deposition [52,53]. The The S200 sample cross-section shows columnar crystals sample cross-section shows columnar crystals with well-visible with well-vis- ible boundaries boundaries oriented oriented perpendicularly perpendicularly to theto the substrate substrate [53];[53]; see see Figure Figure 2d. 2d. The Thecrystalline crystalline grains’ grains’ dimension is is estimated estimatedusing usingthetheScherrer Scherrerformula, formula,considering consider- ing thethe fullfull widthwidth at at halfhalf maximumofofthe maximum the110110peak peakof ofthe the α-(Fe, α-(Fe, Pd) phase, obtaining obtaining for for the theS50 S50sample samplea a 50 = 50(13 = ± (132)±nm2) and nm for and the forS200 the sample S200 a sample a = 200 (12 ± 200 2) = nm. (12 ±Hence, 2) nm. in the range Hence, in theofrange thickness considered of thickness in this work, considered in thisthe dimensions work, of the crystalline the dimensions grains of the crystalline grains are constant. Note that microstrains also contribute are constant. Note that microstrains also contribute to peak broadening and thus theto peak broadening and thus the Scherrer’sformula Scherrer’s formulaunderestimates underestimatescrystal crystalsize.size. Root-mean-squareroughness Root-mean-square roughness (R (Rqq))values values evaluated from the the AFM AFMimagesimagesofofFigure Figure3b,c 3b are ~0.5 and c arenm ~0.5and nm~4.3 andnm ~4.3for nm S50 forand S50S200andsamples, respectively. S200 samples, These These respectively. valuesvalues confirm the con- highthe firm flatness of the surface high flatness of theof the two surface of as-deposited samples. samples. the two as-deposited The higher TheRqhigher value R observed q value in the S200 sample denotes the more complex surface structure observed in the S200 sample denotes the more complex surface structure induced by co- induced by columnar growth,growth, lumnar as already observed as already in SEMin observed images. SEM images. The wettability property of The wettability property of the as-depositedthe as-depositedsamples sampleshas hasbeen been investigated investigatedby by the the contactangle contact angle(CA)(CA)values valuesbetween betweenaadrop dropofofdeionized deionizedwaterwaterandandthethesample samplesurface surfaceitself. itself. Themeasured The measured CA CA values values are 94◦ ±±3°3◦and are 94° and97° 97◦±± ◦ 1°1forfor S50S50 andand S200samples, S200 samples,respectively, respectively, seeFigure see Figure6.6.TheseTheseresults resultsindicate indicateaaweak weakhydrophobic hydrophobicbehaviour behaviourfor forboth bothas-deposited as-deposited samples where the effect of the different surface morphologies samples where the effect of the different surface morphologies appears almost negligible. appears almost negligible. This evidence is probably because the observed differences This evidence is probably because the observed differences in surface characteristics be-in surface characteristics be- tween the samples, in particular their very low roughness (Rq), tween the samples, in particular their very low roughness (Rq), determine only a minimal determine only a minimal influenceon influence onthe theshape shapeof ofthe thewater waterdrop dropso, so,presumably, presumably,only onlyaaslight slightvariation variationof ofthetheCACA value is induced, which is included in the measurement value is induced, which is included in the measurement uncertainty. uncertainty. Basedon Based on the the Fowkes-Girifalco-Good Fowkes-Girifalco-Good (FGG) (FGG)theory, theory,thethesurface surface free freeenergy energy (γs ) for for the as-deposited FePd thin films can be calculated by the expression γs = 14 γl (cosθCA + 1) the as-deposited FePd thin films can be calculated by the expression = +1 where γl is the liquid surface tension [54]. By using the deionized water (γl = 72.8 mJ/m2 ) where is the liquid as tested liquid, surface the surface tension free energy [54]. of By theusing the deionized as-deposited S50 and water = 72.8 are S200( samples mJ/m 2) 16.9 and 16.0 mJ/m2 , respectively. as tested liquid, the surface free energy of the as-deposited S50 and S200 samples are 16.9 and 16.0 mJ/m2, respectively. Nanomaterials 2021, 11, 282 Room temperature hysteresis loops of the as-deposited S50 and S200 samples are 5 of 14 shown in Figure 3a; the magnetic field is applied parallel to the film plane and the curves are normalized to the magnetization value at H = 5kOe. Figure 3. (a) (a) Room Room temperature temperature hysteresis loops of as-deposited samples; samples; Atomic/Magnetic force microscopy (AFM/MFM) (AFM/MFM) images images of the as-deposited (b)/(d) S50 and (c)/(e) S200 samples. The MFM images are taken at parallel magnetic remanence. of the as-deposited (b)/(d) S50 and (c)/(e) S200 samples. The MFM images are taken at parallel magnetic remanence. In Roomthe S50 sample, the temperature reversalloops hysteresis process of the of the magnetization as-deposited S50 andis characterized S200 samplesbyare a single irreversible jump that occurs in a narrow field range; the coercive field (Hc) and the shown in Figure 3a; the magnetic field is applied parallel to the film plane and the curves are normalized normalized to the magnetization magnetization remanencevalue (Mr/M ats)Hare = 5kOe. Hc = 44 Oe and Mr/Ms = 0.89. The mag- netic In the S50 domain sample, theatreversal configuration process state the remanence of the magnetization taken by the MFMis(Figurecharacterized by a 3d) appears single as irreversible a slight jumpofthat modulation theoccurs in a narrow detectable field range; out-of-plane the coercive component of the field (Hc ) and the magnetization; it normalized covers several magnetization grains resulting remanence (Mr /Ms ) are in a magnetically softHbehavior c = 44 Oewith and aMhigh r /MsM =r/M 0.89. The s value magnetic and domain configuration fast approach to saturation. at the remanence state taken by the MFM (Figure 3d) ap- pearsBy asincreasing a slight modulation the sample of thickness, the detectable out-of-plane component the magnetization jump appears of theless magnetization; intense and the reversal process covers a wider magnetic field range inducing a reductionMofr /M it covers several grains resulting in a magnetically soft behavior with a high thesM value r/Ms =and0.43fastand approach to saturation. an increase of Hc = 73 Oe. In the MFM image of the S200 sample (Figure 3e), a moreBy increasing complex the sample magnetic domainthickness, the magnetization structure with a more evidentjump appears light-dark less intense contrast points out an out-of-plane component of the magnetization that periodically changes itsofsign. and the reversal process covers a wider magnetic field range inducing a reduction the M /M = 0.43 and an increase of H = 73 Oe. In the MFM Such magnetic domains appear with an irregular pattern and its size is smaller than the r s c image of the S200 sample (Figure one 3e), a in observed more the complex S50 sample. magnetic domain As a matter structure of fact, with a more the columnar grainsevident light-dark (see Figure 2b,d) contrast points out an out-of-plane component of the magnetization induce a local crystal anisotropy energy resulting in a non-negligible effect on the orien- that periodically changes tation its sign. of the Such magnetic magnetization at thedomains remanentappear witha an state with irregular increase consequent pattern and of theitsout-of- size is smaller plane than the one component. observed ina the In particular, S50 randomly mostly sample. Asorientation a matter ofinfact, theofcolumnar space grains the magnetiza- tion, characterized by a low Mr/Ms value, is obtained by a balance between the localeffect (see Figure 2b,d) induce a local crystal anisotropy energy resulting in a non-negligible ani- on the orientation sotropy energy within of theeach magnetization columnar grain at theagainst remanentthe state shapewith a consequent anisotropy energy increase effectingof the out-of-plane component. In particular, a mostly randomly in the film plane. Moreover, increasing the applied field performing the hysteresis loop, orientation in space of the magnetization, characterized by a low M /Ms value, is obtained by a balance between the the field energy must overcome the local rcrystal anisotropy inducing a reversible rotation local anisotropy energy within each columnar grain against the shape anisotropy energy processes to bring the sample magnetization to saturation. effecting in the film plane. Moreover, increasing the applied field performing the hysteresis This interpretation about the magnetic behavior of the as-deposited samples, which loop, the field energy must overcome the local crystal anisotropy inducing a reversible is obtained from the analysis of the in-plane hysteresis loops and the MFM image, is also rotation processes to bring the sample magnetization to saturation. confirmed by the measured out-of-plane hysteresis loops shown in Figure S1 of Supple- This interpretation about the magnetic behavior of the as-deposited samples, which mentary Materials. is obtained from the analysis of the in-plane hysteresis loops and the MFM image, is also confirmed by the measured out-of-plane hysteresis loops shown in Figure S1 of Supplementary Materials. Furthermore, the observed difference in Hc values between the two samples can also be related to the difference in crystalline structure and grain shape [55]. The counterplay between the local magnetic anisotropy inside the grain and the exchange interactions Furthermore, the observed difference in Hc values between the two samples can also be related to the difference in crystalline structure and grain shape [55]. The counterplay Nanomaterials 2021, 11, 282 between the local magnetic anisotropy inside the grain and the exchange interactions6 be- of 14 tween adjacent grains determines the characteristics of the structure of the magnetic do- mains in the thin film, see Figure 3d–e. The grain boundaries acting as pinning sites [56] hinder between theadjacent movement of the grains domain wall determines the during the magnetization characteristics inversion of the structure of the process magneticby increasing coercivity. It follows that the S200 sample, characterized by domains in the thin film, see Figure 3d–e. The grain boundaries acting as pinning sites [56] columnar grains, see Figure hinder the3b,d, is characterized movement by a larger of the domain coercive wall during thefield with respect magnetization to the S50 inversion sample process by which displays increasing a denserIt structure coercivity. follows thatwiththeless marked S200 sample,grains boundaries,bysee characterized Figure 3a,c. columnar grains, The main see Figure characteristics 3b,d, is characterizedof thebyhysteresis loops observed a larger coercive field with in the as-deposited respect to the S50S50 and sample S200 samples are in good agreement with those already published which displays a denser structure with less marked grains boundaries, see Figure 3a,c. in the literature about similar FePd The mainor Fe thin films. The characteristics of soft the magnetic hysteresisbehavior characterized loops observed in theby the single irre- as-deposited S50 versible and S200 samples are in good agreement with those already published inby magnetization jump in the inversion process was already observed theCialone et literature al. [26] and about by Liu similar FePd et al. or [57] in FePd Fe thin films.thinThefilm softwith a thickness magnetic of 100characterized behavior and 30 nm, respec- by the tively. singleInstead, the progressive irreversible magnetization increase jump of in thethe magnetic inversion fieldprocess intervalwas in which already theobserved reversal process by Cialonetakesetplaces al. [26]andandthe by consequent Liu et al. [57] emergence in FePd thin in thefilmhysteresis loop of reversible with a thickness of 100 and mechanisms leading Instead, 30 nm, respectively. to the magnetic saturation the progressive as a of increase function of filmfield the magnetic thickness interval were pre- in which viously observed by Salaheldeen et al. in FePd thin film on a glass substrate the reversal process takes places and the consequent emergence in the hysteresis loop of with thickness inreversible the rangemechanisms 20–80 nm [58] and bytoPrida leading et al. in Fe the magnetic thin filmas saturation with a thickness a function in the of film range thickness 30–100 nm [59]. observed by Salaheldeen et al. in FePd thin film on a glass substrate with were previously thickness in the range 20–80 nm [58] and by Prida et al. in Fe thin film with a thickness in 3.2. theDealloying range 30–100 Process nm FePd [59]. Thin Film The effect of the dealloying process on the stoichiometry of the FePd alloy as a func- 3.2. Dealloying Process FePd Thin Film tion of the treatment time is observed to be different for the two studied samples (see FigureThe 4). effect of the dealloying process on the stoichiometry of the FePd alloy as a function of the treatment time is observed to be different for the two studied samples (see Figure 4). Figure 4. Evolution of Fe content as a function of the dealloying time in S50 and S200 samples. Figure 4. Evolution of Fe content as a function of the dealloying time in S50 and S200 samples. An almost linear dissolution rate of the Fe element is observed in the S50 sample up to 15An almost min linear (dashed dissolution green rate of the Fethe line), subsequently, element is observed dissolution in the is stopped andS50the sample achievedup toalloy 15 min (dashed green stoichiometry (Fe60line), Pd40 )subsequently, remains unaltered the dissolution is stopped anduncertainty within the measurements the achieved up alloy to 60stoichiometry min. This stop(Fe Pd40 of60the Fe) remains unaltered dissolution effect iswithin ascribedthetomeasurements the formation uncertainty up of a passivation tolayer 60 min. richThis in Pdstop onofthethefilm Fe dissolution surface which effect is ascribed hinders to the formation the further dealloyingofprocess a passivation on the layer rich in Pd on the film surface which hinders the further dealloying FePd solid solution still presents. The formation of this passivation layer is favored process on by thea FePd high solid solution diffusion still rate of presents. the Pd atoms Theonformation film surface of during this passivation layer process the dealloying is favored by a [26,60]. high diffusion rateS200 Instead, the of the Pd atoms sample showson afilm surface slower during the dissolution ratedealloying followingprocess [26,60]. a quadratic law Instead, (dashed bluethe S200 line) andsample covers shows a slower the entire timedissolution interval up rate to following 60 min in awhich quadratic law the alloy composition (dashed is Fe50and blue line) Pd50covers proving theaentire dealloying time process interval more up toeffective. 60 min in However, which the foralloy treat- ment time longer than 60 min, the dealloyed FePd thin film detaches from the substrate preventing any further characterization, as a consequence the formation of a complete Pd passivation layer is not observed. composition is Fe50Pd50 proving a dealloying process more effective. However, for treat- ment time longer than 60 min, the dealloyed FePd thin film detaches from the substrate Nanomaterials 2021, 11, 282 preventing any further characterization, as a consequence the formation of a complete7 of Pd14 passivation layer is not observed. The evolution of the sample morphology as a function of the dealloying time is shown in Figure 5a–d; the magnification of the reported SEM images is the same than the The evolution ones reported of the in Figure 2 sample morphology as for the as-deposited a function samples of theto in order dealloying timethe make easier is shown com- in Figure parison. 5a–d; the magnification of the reported SEM images is the same than the ones reported in Figure The grains 2 for of the the as-deposited dealloyed samples S50_15 sample in order appear with to make more easierboundaries evident the comparison. (see The grains of the dealloyed S50_15 sample appear with more evident boundaries (see Figure 5a) with respects to the as-deposited S50 sample. These features indicate that, in Figure 5a) with respects to the as-deposited S50 sample. These features indicate that, in this this sample, the dealloying process preferentially starts removing the Fe element from the sample, the dealloying process preferentially starts removing the Fe element from the grain grain boundaries whereas the core remains almost unaltered. Therefore, it can be deduced boundaries whereas the core remains almost unaltered. Therefore, it can be deduced that that the grain boundaries are less noble than the core of the grains, that is, they have a the grain boundaries are less noble than the core of the grains, that is, they have a higher higher Fe content disposable for removal. The further increase in treatment time no longer Fe content disposable for removal. The further increase in treatment time no longer affects affects the surface morphology (see Figure 5b for the sample S50_60) confirming the dealloy- the surface morphology (see Figure 5b for the sample S50_60) confirming the dealloying ing process is stopped after 15 min, as already observed by the EDS characterization (see Fig- process is stopped after 15 min, as already observed by the EDS characterization (see ure 4). Figure 4). Instead, the very slow Fe dissolution rate at the beginning of the dealloying treatment Instead, the very slow Fe dissolution rate at the beginning of the dealloying treatment observed by the EDS measurements of the S200 sample (Figure 4 blue line) indicates a observed by the EDS measurements of the S200 sample (Figure 4 blue line) indicates a ho- homogenous stoichiometry mogenous stoichiometry andand high high electrochemical electrochemical stability stability of itsofsurface its surface and, conse- and, consequently, quently, a slow evolution of its morphology a slow evolution of its morphology is observed. is observed. Figure Figure5.5.To-view To-viewSEM SEMimages: images:(a) (a)S50_15, S50_15,(b) (b)S50_60, S50_60,(c) (c)S200_15 S200_15and and(d) (d)S200_60 S200_60sample. sample. InInfact, fact,the themorphology morphologyofofthetheS200_15 S200_15sample sample(Figure (Figure5c) 5c)appears appearsalmost almostunchanged unchanged with withrespect respectto tothe the one one of of the S200 as-deposited as-deposited sample sample (Figure (Figure2b); 2b);the theslight slightreduction reductionof ofthe theFeFe amount amount observed observed by EDS by EDS cannot cannot revealed revealed bydirect by the the direct observation observation of sample of sample surface. surface. On the On the other other hand, hand, after after 60 min of 60 min oftreatment, chemical chemical the treatment, the short narrow and narrow andalready cracks short visible in the as-deposited S200 (Figure 2b) and S200_15 (Figure 5c) samples appear wider and deeper (Figure 5d). Therefore, it is evident that the electrolyte penetrated the cracks and removed the Fe atoms by opening gradually wider voids and inducing a consequent increase of the Fe dissolution rate (Figure 4). Eventually, the fast iron removal will allow the electrolyte to penetrate the entire thickness of the film until it reaches the interface with cracks already visible in the as-deposited S200 (Figure 2b) and S200_15 (Figure 5c) samples appear wider and deeper (Figure 5d). Therefore, it is evident that the electrolyte pene- Nanomaterials 2021, 11, 282 trated the cracks and removed the Fe atoms by opening gradually wider voids and induc- 8 of 14 ing a consequent increase of the Fe dissolution rate (Figure 4). Eventually, the fast iron removal will allow the electrolyte to penetrate the entire thickness of the film until it reaches the interface with the underlying gold. Here the corrosion process will determine thedetachment the underlying of gold. the Here the corrosion film from process the substrate for will timedetermine longer than the60detachment min. of the film fromThethesurface substrate for time such properties longer asthan 60 min. stoichiometry, pattern and morphology play an im- portant The surface role properties to determine such the as stoichiometry, wettability propertiespattern and morphology [54,61–63]. The drop placedplay anon impor- the tant role to determine the wettability properties [54,61–63]. The drop sample surface and the measured contact angle (CA) values as a function of the dealloying placed on the sample surface time and the measured are reported in Figure contact 6 panel angle a and(CA) values as a afunction b, respectively; of the behavior hydrophobic dealloying is time ob- are reported served in Figure in almost 6 panel a and b, respectively; a hydrophobic behavior is observed in all samples. almost allbe It can samples. observed that the hydrophobic property increases with increasing the deal- loying time up observed It can be to 15 and that 30 min the for hydrophobic S50 and S200 property sample,increases with increasing respectively. In the casetheof deal- S50 loying time up to 15 and 30 min for S50 and S200 sample, respectively. samples, the maximum value of CA corresponds to the moment in which the dissolution In the case of S50 ofsamples, the maximum the Fe suspends, value of as observed inCA corresponds Figure to the moment 4 (green curve). in which For a longer the dissolution treatment time, the of the Fe suspends, as observed in Figure 4 (green curve). For measured CA values slowly decreases in S50 samples while strongly drops in S200 a longer treatment time, sam-the measured CA values slowly decreases in S50 samples ples until to reach a hydrophilic behavior for the S200_60 sample. while strongly drops in S200 samples until to reach a the Obviously, hydrophilic behaviorbehavior of the CA forvalues the S200_60 sample. is deeply related to the evolution of the Obviously, the behavior of the CA values stoichiometry and morphology of the sample surface [54,61–63] is deeply related to thecontinuously which evolution ofde- the stoichiometry and morphology of the sample surface [54,61–63] which velop during the dealloying process. Although disentangle such contributes is difficult, continuously de- in agreement with previous studies [54,62,64], the initial increase of the CA can be relatedin velop during the dealloying process. Although disentangle such contributes is difficult, agreement with previous studies [54,62,64], the initial increase of the CA can be related to to an increase in the surface patterning induced, in these studied samples, by the dealloy- an increase in the surface patterning induced, in these studied samples, by the dealloying ing process. In particular, the surface patterning provides gaps where a large amount of process. In particular, the surface patterning provides gaps where a large amount of water water (Wenzel theory) or air (Cassie-Baxter theory) is trapped by changing the propensity (Wenzel theory) or air (Cassie-Baxter theory) is trapped by changing the propensity of the of the drop to spread on the sample surface [62]. drop to spread on the sample surface [62]. Following the FGG theory already used for the as-deposited samples [54], the surface Following the FGG theory already used for the as-deposited samples [54], the surface free energy ( ) of dealloyed samples can be easily calculated from the measured CA val- free energy (γs ) of dealloyed samples can be easily calculated 2from the measured CA values. ues. The values are included in the range 12.2–14.7 mJ/m for the S50 samples and in The γs values are included in the range 12.2–14.7 mJ/m2 for the S50 samples and in the the range 13.6–19.9 mJ/m 2 2 for the S200 samples. range 13.6–19.9 mJ/m for the S200 samples. Figure 6. (a) Water drop on the sample surface; (b) contact angle evolution as a function of the dealloying time. The evolution of the room-temperature magnetic properties as a function of the dealloying time for the two studied samples is summarized in Figure 7. The chemical treatment implies some important changes in the shape of the hysteresis loops and in the coercive field value [26]. Figure 6. (a) Water drop on the sample surface; (b) contact angle evolution as a function of the dealloying time. The evolution of the room-temperature magnetic properties as a function of the deal- loying time for the two studied samples is summarized in Figure 7. The chemical treat- Nanomaterials 2021, 11, 282 ment implies some important changes in the shape of the hysteresis loops and in the9 co- of 14 ercive field value [26]. Room-temperaturehysteresis Figure7.7.Room-temperature Figure hysteresisloops loopsfor forthe thein-plane in-plane direction direction of of the the field field of of (a) (a) S50 S50 samples sam- andand ples (b) (b) S200 samples; S200 (c) evolution samples; of H (c) evolution asc aasfunction ofc H of the a function dealloying of the dealloyingtime. time. InIngeneral, general,the thedealloying dealloyingprocess processbroadens broadensthe therange rangeofofthe themagnetic magneticfieldfieldininwhich which complete magnetic saturation of the sample occurs. The irreversible complete magnetic saturation of the sample occurs. The irreversible processes are distrib- processes are dis- tributed over a larger portion of the hysteresis loop and the reversible uted over a larger portion of the hysteresis loop and the reversible rotation mechanisms rotation mechanisms occuratathigh occur highfield. field.Consequently, Consequently,the thevalue valueofofthetheHH increases in the treated sample c cincreases in the treated sample whereas the one of M /M whereas the one of Mr/Mr s decreases. s decreases. These overall changes These overall changes in magnetic in magnetic behaviourbehaviour induced in- by chemical treatment are in good agreement with those already published in the litera-in duced by chemical treatment are in good agreement with those already published the on ture literature similar on similarSun systems: systems: et al. inSun Ni70et Cual. in Ni70 CuRobbennolt 30 film [65], 30 film [65],etRobbennolt al. in Fe63Cu et37al. [13]in Fe 63 Cu 37 [13] and in Fe 58 Cu 42 thin films [66] show a development of reversible and in Fe58Cu42 thin films [66] show a development of reversible mechanisms in the rever- mechanisms in the reversal magnetization process, an increase of Hc values and a reduction of Mr /Ms sal magnetization process, an increase of Hc values and a reduction of Mr/Ms value in- value induced by dealloying process performed in different conditions. duced by dealloying process performed in different conditions. In particular, the single and sharp irreversible magnetization jump observed in the In particular, the single and sharp irreversible magnetization jump observed in the as-deposited S50 sample is completely disappeared after 10 min of dealloying treatment as-deposited S50 sample is completely disappeared after 10 min of dealloying treatment (Figure 7a); in fact, the loop branches of the S50_10 sample remain separated over a larger (Figure 7a); in fact, the loop branches of the S50_10 sample remain separated over a larger field interval indicating that the irreversible processes are now spread over a wider portion field interval indicating that the irreversible processes are now spread over a wider por- of the hysteresis loop. After the merging of loop branches, the complete magnetic saturation tion of the hysteresis loop. After the merging of loop branches, the complete magnetic is reached at a higher applied magnetic field with a dominant role of the reversible rotation saturation is reached at a higher applied magnetic field with a dominant role of the re- process. A further significant change compared to the S50 as-deposited sample is the versible rotation process. A further significant change compared to the S50 as-deposited increase of the coercive field for a treatment time up to 10 min, as shown in Figure 7c, with sample is the increase of the coercive field for a treatment time up to 10 min, as shown in the arising of a non-negligible component of magnetization in the perpendicular direction with respect to the film plane as results from out-of-plane hysteresis loop shown in the Figure S2 of the Supplemental Materials. Increasing the dealloying time, the shape of the hysteresis loop of S50_60 sample recovers some features more similar than the ones of the as-deposited sample: around the zero applied field region, a single, narrow although less marked irreversible jump of magnetization is visible; the coercive field and the merging field of the loop branches decrease their values close to the ones of the continuous thin film, as shown in Figure 7a. Nanomaterials 2021, 11, 282 10 of 14 Such evolution of the magnetic properties of the S50 samples as a function of the treatment time, which ends with the partial recovery of some magnetic features belong- ing to the as-deposited sample, allows formulating a hypothesis about the dissolution mechanism of Fe element during the dealloying process in this sample. As previously dis- cussed, the dealloying process in the S50 sample preferentially starts from the boundaries of the grains by increasing their separation distance and making patterned the sample surface. Consequently, the magnetic decoupling among grains increases and local magnetic anisotropies, which hinder the domain wall motion during the magnetization reversal process, are originated. Such anisotropies favour the increase of the coercive field, the spreading of the irreversible processes over wider field range, the magnetization rotation process at high field, as observed for the hysteresis loop of the S50_10 sample and, the arising of a non-negligible component of magnetization in the perpendicular direction with respect to the film plane as results from out-of-plane hysteresis loop shown in the Figure S2 of the Supplemental Materials. For longer treatment time, the EDS characterization has revealed that the formation of a passivation layer rich in Pd on the film surface stops the dealloying process. At this treatment time, the sample can be magnetically modelled as a bi-layered thin film: the upper layer is the passivation layer under which an almost unaltered Fe70 Pd30 layer is still present; obviously, the interface between the layers is not clear and sharp but should be considered as a progressive transition region. As the upper rich in Pd layer can be considered magnetically inactive [67], the measured magnetization reversal of the S50_60 sample takes place in the interface region and mainly in the lower continuous Fe70 Pd30 layer. This may result in a recovery of some magnetic features belonging to the as-deposited FePd thin film especially around at zero field range. In the S200_30 sample, the decrease in Mr /Ms and the increase in Hc compared to the as-deposited sample (see Figure 7b) are due to the patterning of the sample surface induced by the Fe dissolution process which involves mainly the surface cracks. Such patterning process arises local magnetic anisotropies that compete to the local crystal ones associated with the columnar grains hindering the domain wall motion during the magnetization process and making more randomly oriented in space the magnetization configuration at the remanence state as confirmed by the appearance of an out-of-plane component of magnetization in the out-of-plane hysteresis loop shown in the Figure S2 of the Supplemental Materials. Although the EDS characterization of the S200 samples does not show a clear stop of the dealloying process, the magnetic properties of the S200_60 sample partially recover some features of the as-deposited S200 sample such as the value of Hc and the merging field of the loop branches (see Figure 7b,c). Similarly, to the S50 samples, also in the case of the S200_60 sample, it can be inferred that a passivation layer rich in Pd and magnetically inactive is arranging on the sample surface and the main contribution to the magnetic reversal process is due to the lower FePd layer almost unaltered by the dealloying process. The order-of-magnitude estimation of the thickness of the passivation layer can be inferred under some rough assumptions: a net interface between the unaltered Fe70 Pd30 and the upper passivation layer, which is now considered ideally constituted by pure Pd atoms only. Under these assumptions, the EDS data reveal the ratio between the number of Fe atoms located only in the lower Fe70 Pd30 layer and the number of Pd atoms present in both layers. During the dealloying process, the number of Fe atoms reduces due to the dissolution mechanism whereas the number of Pd atoms never changes. Therefore, the progressive reduction of the Fe content as a function of the dealloying process, as observed in the EDS data in Figure 4, can be interpreted as the progressive reduction of the thickness of the Fe70 Pd30 substrate in favour of the formation of the passivation layer Pd. In this framework, from the EDS results obtained after 1 h of dealloying treatment (see Figure 4), the thickness of Pd passivation layer can be roughly estimated: ~20 nm and ~100 nm for S50 and S200 samples, respectively. Nanomaterials 2021, 11, 282 11 of 14 Although this estimation procedure is severely limited by the assumptions described above, it nevertheless confirms that the dealloying process is more effective in the S200 sample in which the electrolyte penetrated the cracks among the columnar structure compared to the S50 sample in which the deep Fe dissolution is hindered by the denser and more compact film structure. The affinity of both samples in the structural evolution during the dealloying process, which ends with the formation of a passivation layer rich in Pd, is also underlined by the analogous evolution of Hc , see Figure 7c. The initial increase in Hc is ascribed to the patterning of the sample surface induced by the Fe dissolution process, while the subsequent decrease in Hc corresponds to a progressive recovery of the characteristics of the as-deposited sample induced by the formation of the passivation layer. However, the comparison of the two curves shows how the rate of the dealloying process is different in the two samples, confirming the evidence obtained from the EDS characterization, see Figure 4. Although the maximum Hc value reached by the two samples is comparable, it is reached at much higher rates in sample S50 confirming the faster dissolution of Fe atoms in this sample rather than in S200. Similarly, after the Hc maximum, the decrease in Hc values is much more rapid in the S50 sample, indicating a faster formation of the passivation layer. These differences in the Hc curves can be mainly attributed to the initial difference in the structural properties of the as-deposited samples. 4. Conclusions Continuous Fe70 Pd30 thin films with thickness of 50 and 200 nm are deposited by a confocal sputtering allowing to easily control the film thickness by simply picking the desired deposition time. The as-deposited films display the same metastable supersaturated solid solution of α-(Fe,Pd) with a non-uniformly distribution of the Pd element in the crystalline grains. The film thickness does not affect the grains size but only their shape. The S50 sample appears homogeneous in thickness with a surface constituted by uniformly distributed grains with an average size of about ~13 ± 2 nm. Instead, the S200 sample displays columnar crystals with an average size of about ~12 ± 2 nm; moreover, the accumulation of growing strain during the film deposition induces the formation of narrow and short cracks. Such morphological differences between the as-deposited samples do not affect the wettability properties which results in weak hydrophobic behaviour for both samples with similar surface free energy values. The magnetic properties of the continuous thin films are in good agreement with their observed morphology: the columnar grains of the S200 sample induce a more randomly orientation in space of the magnetization than the more uniform structure of the S50 sample resulting in a lower Mr /Ms value, a smaller magnetic domain size and a wider magnetic field range in which the magnetization reversal occurs. As-deposited Fe70 Pd30 samples show different responses to the dealloying treatment under free corrosion condition in hydrochloric acid (2 M aqueous solution of HCl) as a function of film thickness indicating a different distribution of the elements in the crystalline grains. The combination between the removal of the Fe and the diffusion of the Pd atoms has allowed modifying the surface features, which turns out to play a fundamental role in determining the resulting properties of the samples such as the wetting and magnetic ones. The Fe element was preferentially removed from the grain boundaries in the S50 sample while the electrolyte preferentially acts inside the cracks opening gradually wider and deeper voids in the S200 sample. In the S50 sample, the high diffusion rate of the Pd atoms on the film surface favors the formation of a passivation layer rich in Pd which stops the dealloying process. The wettability properties of the samples are deeply related to the evolution of the stoichiometry and morphology of the sample surface during the dealloying process. The surface patterning induced by the dealloying treatment provides gaps which change the propensity of the drop to spread on the sample surface with the surface free en- ergy values included in the range 12.2–14.7 mJ/m2 for the S50 samples and in the range Nanomaterials 2021, 11, 282 12 of 14 13.6–19.9 mJ/m2 for the S200 samples. The dealloying treatment also implies some impor- tant changes in magnetic properties. The irreversible processes of magnetization are spread over a larger portion of the hysteresis loop, the complete magnetic saturation is reached at a higher applied magnetic field and the coercive field increases. However, a recovery of some magnetic features more similar to those of the as-deposited samples is observed in the samples chemically treated for a longer time. In these samples, the main contribution to the magnetic reversal process stems from a portion of FePd thin-film almost unaltered respect to the as-deposited one because it is covered by the passivation layer rich in Pd and magnetically inactive arranged on the sample surface during the dealloying process. The development of the Pd-rich passivation layer can also be exploited as a protective layer allowing the underlying FePd alloy to maintain its magnetic properties stable over time even if the sample will be used in an acid environment. Supplementary Materials: The following are available online at https://www.mdpi.com/2079-4 991/11/2/282/s1, Figure S1: Out-of-plane room temperature hysteresis loops of the as-deposited S50 and S200 samples, Figure S2: Out-of-plane room temperature hysteresis loops of the S50_10 and S200_30 samples. Author Contributions: Conceptualization, P.T., M.C. (Marco Coïsson) and P.R.; preparation of the samples, F.C. and M.C. (Matteo Cialone); XRD characterization and structural data analysis M.C. (Matteo Cialone) and P.R.; DC-magnetic measurements and data analysis, G.B., M.C. (Marco Coïsson), M.C. (Matteo Cialone) and F.C.; contact angle measurements, F.C. and G.B.; SEM investigation, F.C.; MFM investigation F.C. and M.C. (Matteo Cialone); writing—original draft preparation, G.B.; supervision, P.T., P.R. and M.C. (Marco Coïsson). All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. 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