Nanoscale This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Tailoring magnetic and mechanical properties of Cite this: Nanoscale, 2020, 12, 7749 mesoporous single-phase Ni–Pt films by Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. electrodeposition Konrad Eiler, *a Jordina Fornell, a Cristina Navarro-Senent, a Eva Pellicer a and Jordi Sort *a,b Homogeneous mesoporous Ni-rich Ni–Pt thin films with adjustable composition have been synthesised by one-step micelle-assisted electrodeposition. The films exhibit a face-centred cubic solid solution (single phase) and their magnetic and mechanical properties can be tuned by varying the alloy compo- sition. In particular, the Curie temperature (TC) is shown to decrease with the Pt content and thin films with a TC close to room temperature (i.e. Ni58Pt42) and below can be produced. Hysteresis loops show a decrease of saturation magnetisation (Ms) and coercivity (Hc) with decreasing Ni content. A comparison of porous and dense films reveals significantly lower saturation magnetic field strength for porous films. Concerning mechanical properties, mainly two trends can be observed: a decrease of the Young’s Received 21st December 2019, modulus of the nanoporous films with respect to dense films by 10% in average and a progressive increase Accepted 6th March 2020 of Young’s modulus with the Ni content from 4.2 GPa to 5.7 GPa in both types of films. The tunability of DOI: 10.1039/c9nr10757f properties and facility of synthesis make this alloy a promising material for microelectromechanical rsc.li/nanoscale systems (MEMS). 1 Introduction Electrodeposited soft magnetic alloys are widely used as writing heads for hard disks, where the main requirements The ability to adjust the magnetic properties of a system include corrosion resistance, low stress and thermal stability.4 without drastically changing other physical properties—such For many applications, soft ferromagnetic materials with Curie as electrical, mechanical or thermal properties—is beneficial temperature (TC) close to room temperature (RT) are of interest for application in magnetic micro- and nanoelectromechanical in order to reduce energy consumption during storage and systems (MEMS/NEMS), where para- and ferromagnetic phases access of data. need to be in electrical contact while ensuring mechanical A TC close to RT also allows to study the magnetic behaviour compatibility between different components.1 If there is a low in this temperature region without applying heat, helping to difference in composition between the paramagnetic and the advance in the theoretical models to understand the magnetic ferromagnetic phase, there will be only little difference in elec- phase transition effects. In general, TC can be tailored by trical conductivity—thus avoiding local heating—, a low mis- varying the content of a ferromagnetic element in a solid solu- match in thermal expansion—avoiding excessive thermally tion comprising ferromagnetic and paramagnetic elements. In induced interfacial stresses—and good mechanical integrity.2 this work, Ni–Pt is used to that purpose. While Pt is an excel- Single-phase alloys have the potential to prevent these pro- lent corrosion resistant material, Ni provides ferromagnetic blems from occurring intrinsically, and—in the case of full properties. Moreover, Ni has a high abundancy and provides miscibility—do not undergo phase transition and avoid temp- mechanical stability and electrical conductivity. This makes erature-induced formation of secondary phases.3 Additionally, Ni–Pt a multifunctional alloy whose properties can be tuned an electrical contact between two phases of similar compo- by the composition, rendering it ideal for application in mag- sition is less susceptible to atmospheric corrosion caused by netic MEMS or NEMS. Ni and Pt promote the formation of humidity. single-phase alloys due to their full miscibility, given their identical lattice structure (fcc) and their comparable atomic radii.5 a Compared to coarse-grained materials, nanocrystalline Departament de Física, Universitat Autònoma de Barcelona, 08193 Bellaterra (Cerdanyola del Vallès), Spain. E-mail: konrad.eiler@uab.cat, jordi.sort@uab.cat metals and alloys exhibit larger yield stress and hardness due b Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain to enhanced dislocation pile-up at grain boundaries (Hall– This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 12, 7749–7758 | 7749 View Article Online Paper Nanoscale Petch strengthening).6 Using suitable conditions for electrode- philic) part of the micelles. The latter are thus co-deposited position, nanocrystalline films can be readily obtained, thus when the metal ions are reduced at the working electrode. The improving the mechanical properties without compromising polymer can later be easily dissolved, leaving a mesoporous any other properties.7,8 metallic film behind. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Mesoporous materials, due to their high surface-to-volume Since mostly magnetic properties of the Ni–Pt system are ratio, are generally of interest in biomedicine, water remedia- investigated here, this work focuses on Ni-rich alloys of the Ni– tion, catalysis, and energy storage and conversion devices such Pt system. A dependence of TC, Ms and Hc on the composition as batteries and fuel cells.9–13 In the case of nanoporous mag- (NixPt1−x) is observed, as well as progressive variation in their netic materials, the coercivity can be reduced by applying mechanical properties. The nanocrystalline, single-phase voltage, due to large electric charge accumulation effects on material is characterised for both mesoporous and dense mor- the surface (converse magnetoelectric effect).14 Again, this can Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. phology, revealing that the mesoporosity influences both the reduce the energy consumption during writing of information magnetic and mechanical behaviour. in magnetic storage media and other magnetically actuated devices.15 Furthermore, the interest in the biomedical field and bio- MEMS is growing.4 Here, mesoporous materials are appealing 2 Experimental for application in drug delivery since the pores may act as The electrodeposition of mesoporous Ni–Pt films was carried reservoirs for the drugs to be delivered.16 out using an aqueous solution containing 200 mM NiCl2, Nanoporous materials are often superhydrophobic (and 3 mM Na2PtCl6·6H2O, 200 mM H3BO3, 25 mM NH4Cl, and oleophilic), a property that may be utilised in water remedia- 10 g l−1 Pluronic P-123 (average molecular mass Mn = 5800). tion (i.e. oil decontamination). If the material is at the same HCl was added to adjust the pH to 2.7. For the deposition of time ferromagnetic, it can easily be retrieved from the water dense films, the block copolymer P-123 was omitted while and thereby recycled for multiple usage.17,18 keeping the concentration of all other chemicals constant. The formation of a single-phase alloy generally depends on A three-electrode set-up with an Ag|AgCl 3 M KCl reference elec- the thermodynamics of the system; the phase diagram and the trode and a platinum wire as counter electrode was used while the mixing enthalpy for the element pair being suitable indicators temperature during deposition was kept at 30 °C. All given poten- as to whether a single-phase alloy can be obtained. The Ni–Pt tials refer to the Ag|AgCl electrode. De-aeration with nitrogen gas phase diagram shows full miscibility with the existence of was performed before each deposition, and the deposition was ordered phases, and an approximately linear dependence of TC performed while stirring the electrolyte at 100 rpm. on composition, reaching 373 K (100 °C) at 26 at% Pt.19 Ni–Pt For electrodeposition, silicon substrates were metallised exhibits a negative mixing enthalpy,20 favouring the formation with a 10 nm Ti adhesion layer and a 200 nm Cu seed layer, of a single phase. However, especially by electrodeposition the both deposited by sputter deposition. The back of the sub- stabilisation of metastable phases is possible. strates was insulating due to the presence of SiO2. Another requirement for alloy electrodeposition is that the Potentiostatic deposition, using potentials between −1.3 V and deposition potentials of the two elements need to be −0.6 V and deposition times from 30 s to 180 s, were applied sufficiently close together; complexing agents are commonly using an Autolab 302N potentiostat/galvanostat to deposit used to shift the deposition potential for one of the elements films with different Ni to Pt ratios. Since the deposition rate in order to approach the deposition potential of the other and was higher for more negative potentials, the deposition time thus achieve their co-deposition.7 The use of a polymeric sur- was adjusted for each deposition potential in order to obtain factant to introduce nanoporosity can interfere with the crystal similar film thickness for all compositions. In practice, this growth of the metallic film,21 eventually changing the micro- meant increasing the deposition time when moving towards structure and leading to phase separation. A general issue in more positive potentials (Fig. 1). After deposition of the porous the electrodeposition of metallic films is also the incorpor- ation of oxygen and hydrogen, leading to the formation of oxide/hydroxide phases22,23 and hydrogen embrittlement, to which Ni is rather susceptible.24 Single-phase dense Ni–Pt films have already been electrode- posited from K2PtCl4 containing solution at acidic pH for the purpose of oxygen reduction reaction in the full compositional range.25 Here, the synthesis of Ni–Pt thin films is accomplished using the well-described mechanism of micelle-assisted elec- trodeposition to introduce porosity.26–28 In this one-step depo- sition process, a block copolymer forms micelles when its con- centration in water is above the critical micellar concentration Fig. 1 Deposition rate and faradaic efficiency of mesoporous Ni–Pt (cmc), and the metal ions assemble at the exterior (hydro- films as a function of the applied deposition potential. 7750 | Nanoscale, 2020, 12, 7749–7758 This journal is © The Royal Society of Chemistry 2020 View Article Online Nanoscale Paper films, the block copolymer was removed by ultrasonic cleaning load while the material is penetrated in order to determine the in ethanol for 10 min. continuous stiffness, which is then further processed to calcu- In order to determine the faradaic current efficiency, the late H and Er.31 theoretical mass (mtheoretical ) of a NixPt (1−x) film was deter- Er can be seen as a combined modulus of the tested sample This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. mined assuming that all current measured during the depo- and the indenter (eqn (2)).32 sition process corresponds to the reduction of the metal ions 1 1  νs 2 1  νi 2 (eqn (1)). ¼ þ ð2Þ Er Es Ei  ð 1 MðNiÞ MðPtÞ t where ν is the Poisson’s ratio and the indices r, s and i stand mtheoretical ¼ x þ ð1  xÞ i  dt ð1Þ F zðNiÞ zðPtÞ 0 for reduced, sample and indenter, respectively. Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. The nanoindentation tests were carried out with a where F is the Faraday constant, x is the atomic fraction of Ni Nanoindenter XP from MTS using a Berkovich-shaped in the deposit, M(Ni) and M(Pt) are the molar masses of Ni diamond tip with Ei = 1140 and νi = 0.07. Due to the large and Pt, z(Ni) = 2 and z(Pt) = 4 are the numbers of electrons Young’s modulus of the indenter in comparison with most supplied for the reduction of a Ni, and a Pt ion, respectively. i metals, the difference between Er and Es is usually within a few is the measured current and t is the deposition time. per cent. The CSM mode was applied with a harmonic displa- Together with the real masses of the deposits determined cement of 2 nm and a harmonic frequency of 45 Hz. The by ICP-MS (see below for the procedure), which also allowed experiments were performed in displacement control mode, the determination of the atomic fractions of Ni and Pt, the far- with a strain rate of 0.05 s−1 up to a maximum penetration adaic efficiency is determined by the ratio of the real and the depth of 100 nm. The Poisson’s ratio ν was assumed to be 0.3. theoretical mass mreal/mtheoretical. Sixteen indents, separated by 10 μm each, were performed on For determination of the deposition rates, the real masses each sample from on top (on the films’ surfaces). of the deposits were used to estimate their density, from which Magnetic hysteresis loops were recorded at RT in-plane and the total thickness was calculated. Thus the deposition rates out-of-plane by vibrating sample magnetometry (VSM) on an reported show the total thickness divided by the deposition LOT-QuantumDesign MicroSense VSM up to 20 000 Oe. VSM time. was also used for measuring the temperature dependence of The synthesised Ni–Pt thin films were analysed by scanning the magnetisation in the saturated regime at 1000 Oe in order electron microscopy (SEM) coupled with energy-dispersive to determine TC, using nitrogen gas flow to control the temp- X-ray spectroscopy (EDX) on a Zeiss Merlin electron micro- erature. After first measurements of TC, the deposition poten- scope. Imaging was done at an acceleration voltage of 1–2 kV tial was refined in the range between −0.70 V and −0.60 V to using an InLens detector to reveal the existence of obtain films with a TC in the desired range. All thin films were mesoporosity. subsequently dissolved in aqua regia for chemical analysis by Transmission electron microscopy (TEM) was performed on ICP-MS using an Agilent 7500ce spectrometer to obtain the a Jeol JEM-2011 electron microscope with an acceleration exact compositions of the Ni–Pt films, and to normalise the voltage of 200 kV working in bright field and diffraction mode. measured magnetic moment by the total mass of Ni and Pt for Sample preparation for TEM was performed by making a each sample. TC was determined by the two-tangent method, cross-cut of a mesoporous sample with a diamond saw. By applying tangents to the linear parts of the M–T (magnetisa- grinding and polishing with a final diamond polish of 1 μm tion over temperature) curve both below and above the mag- particle size, the thickness was reduced to about 30 μm. netic transition and taking TC at the point of their Finally, Ar ion milling with an energy of 5 keV was performed intersection.33 at an angle of 8° for several hours to reduce the thickness locally to a few nanometers. For the preparation of the cross- section of a dense Ni–Pt film, a lamella was cut using a 3 Results and discussion focused ion beam SEM (FIB-SEM). Grazing incidence X-ray diffraction (GIXRD) was conducted 3.1 Chemical composition on a Malvern-PANalytical Xpert Pro MRD diffractometer using The composition of the synthesised thin films is determined Cu-Kα radiation for phase analysis of the films in a 2θ range by the electrodeposition parameters, mainly the deposition from 38° to 62°. Rietveld refinement was performed using the potential. Since Pt(IV) deposits at less negative potentials than software MAUD29 in order to determine the lattice parameter Ni(II), the Ni content increases when the potential is more and crystallite size for each composition.30 negative (Table 1). The same trend was observed by Liu et al. The mechanical properties of the films, i.e. hardness (H) during the electrodeposition of dense Ni–Pt films from a and reduced Young’s modulus (Er), were evaluated by nanoin- different bath formulation.25 Progressive enrichment in Ni as dentation. The continuous stiffness measurement (CSM) tech- the applied potential was made more negative was nique was used since it allows the assessment of depth-depen- accompanied by an increase in the deposition rate up to −1.2 dent properties of materials in a single step. The method V (Fig. 1). The compositions of the dense thin films are similar involves applying a dynamic (oscillatory) load on top of a static to their porous counterparts deposited under the same con- This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 12, 7749–7758 | 7751 View Article Online Paper Nanoscale Table 1 Composition of dense and mesoporous Ni–Pt films produced at given deposition potentials and determined by ICP-MS Composition This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Deposition potential [V] Porous Dense −1.3 Ni99Pt1 −1.2 Ni98Pt2 Ni98Pt2 −1.0 Ni95Pt5 Ni93Pt7 −0.9 Ni92Pt8 Ni91Pt9 −0.8 Ni84Pt16 −0.7 Ni76Pt24 Ni79Pt21 −0.6 Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. Ni61Pt39 ditions, i.e. the composition is not influenced by the presence of the P-123 triblock copolymer for a given potential. All per- centages of Ni and Pt in this work, ranging from 61 at% Ni to 99 at% Ni, are given in atomic percentage. For the film compo- sitions, oxygen or other impurities are not taken into account, however those were monitored by EDX and the oxygen content never exceeded 10 at% (note that in EDX, it is not possible to discriminate whether the oxygen originates from the film or the substrate layers). Current efficiencies determined for the mesoporous Ni–Pt films were between 65% and 85% for all deposition potentials except −0.6 V, where the current efficiency was 47% (Fig. 1). 3.2 Microstructure All films deposited from the electrolyte containing P-123 exhibit a homogeneously distributed mesoporosity (Fig. 2). The thin films containing 99% and 98% Ni have a clearly visible grain structure with an appreciable roughness (Fig. 2a and b). For 95% Ni, a rough surface without perceptible grain structure is observed (Fig. 2c), while the films ranging from 94% to 61% Ni are smoother, since no topographic contrast other than the one caused by the porosity is visible. Furthermore, those films do not show any apparent grain structure on the nanoscale, and they are highly mesoporous with a narrowly distributed pore size on the order of 10 nm. The microstructures of the dense films show the continuous surfaces with an appreciable, low roughness (Fig. 3). On a cross-section of a Ni92Pt8 thin film, the porosity over the full film thickness is observed by TEM, together with the Fig. 2 SEM micrographs of mesoporous Ni–Pt thin films (a) Ni99Pt1, (b) Cu seed layer (Fig. 4a). Underneath the Cu layer, the Ti layer Ni98Pt2, (c) Ni95Pt5, (d) Ni94Pt6, (e) Ni92Pt8, (f ) Ni84Pt16, (g) Ni76Pt24, (h) and Si substrate are visible. The irregular shape of the film Ni61Pt39 acquired by InLens detector. suggests that the film surface has been partially removed during ion polishing. SEM micrographs of this cross-section porous film (Fig. 5), its nanocrystalline morphology is revealed reveal the film’s homogeneity at surface level and the film in high resolution conditions (Fig. 6b). thickness is around 280 nm (Fig. 4b). At high resolution, crystal planes become visible and indi- cate the nanocrystalline structure of the thin film (Fig. 5). A 3.3 Phase analysis few nanometre thick layer covers the surface of the thin film, The diffraction patterns obtained by GIXRD show an fcc which may be a surface oxide, or a damaged layer caused by single-phase solid solution with a cell parameter between amorphisation of the metal due to the ion beam polishing those for fcc Ni and fcc Pt (as indicated by the discontinuous during preparation. lines in Fig. 7) for all compositions. The diffraction patterns of In the cross-section of the dense Ni91Pt9 film, a film thick- the Ni–Pt films are superposed on the reflections of the fcc Cu ness of ca. 250 nm is observed (Fig. 6a). Similar to the meso- seed layer (Fig. 7). 7752 | Nanoscale, 2020, 12, 7749–7758 This journal is © The Royal Society of Chemistry 2020 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Fig. 3 SEM micrographs of dense Ni–Pt thin films (a) Ni98Pt2, (b) Fig. 6 TEM micrographs of dense Ni91Pt9 thin film (a) over the full Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. Ni91Pt9 acquired by InLens detector. cross-section and (b) under high-resolution conditions. Fig. 4 (a) TEM and (b) SEM micrograph of a cross-section of meso- porous Ni92Pt8 thin film. Fig. 7 GIXRD diffraction patterns of Ni–Pt thin films showing the deconvolution of the Ni–Pt (green) and Cu (red curve) phases after Rietveld refinement for (a1) Ni98Pt2, (a2) dense Ni98Pt2, (b) Ni92Pt8, (c) Ni84Pt16, (d) dense Ni79Pt21, (e) Ni76Pt24, (f ) Ni61Pt39. With exception of (a2) and (d) all films are mesoporous. the structure (cf. Fig. 7, by comparing curves a1 and a2 or d and e). Upon increasing Pt content in the alloy, the diffraction peaks shift towards lower diffraction angles, indicating a pro- gressive increase in the cell parameter. The cell parameter of the Pt–Ni phase obtained by Rietveld refinement scales linearly with the alloy composition, thus following Vegard’s law.34 However, there is a tendency towards cell parameters slightly higher than expected (Fig. 8). This effect was also observed by Kumar et al.,35 and related to the size mismatch due to the different atomic radii of Ni and Pt. Similar deviations, ascribed Fig. 5 High resolution TEM micrograph of a cross-section of meso- to the presence of tensile microstrain, have been documented porous Ni92Pt8 thin film with selected crystal planes indicated. The for electrodeposited Ni films.36 corresponding fast Fourier transform (FFT) image for the entire zone is shown as an inset on the top left. With the exception of the film containing 98 at% Ni, the reflections of the Ni–Pt phase are significantly broadened due to the small crystallite size. The Rietveld refinement returned crystallite sizes on the order of 5 nm for Ni contents between Remarkably, there is no apparent difference in the diffrac- 61% and 92%, and 50 nm in the case of Ni98Pt2. Similar tion patterns between dense and porous films of identical values are obtained for dense films, hence the nanocrystalli- composition, i.e. the porosity has very little or no influence on nity resulted entirely from the electrodeposition parameters This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 12, 7749–7758 | 7753 View Article Online Paper Nanoscale This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Fig. 8 Dependence of cell parameter a on composition of Ni–Pt thin Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. films as determined in this work as well as by Kumar et al.35 Values for pure Ni and Pt are included as a reference.37 Table 2 Crystallite sizes of representative Ni–Pt thin films obtained by Rietveld refinement of their respective GIXRD patterns Composition Morphology Crystallite size [nm] Ni98Pt2 Porous 41 Ni98Pt2 Dense 53 Ni92Pt8 Porous 7 Ni84Pt16 Porous 6 Ni79Pt21 Dense 5 Ni76Pt24 Porous 5 Ni61Pt39 Porous 6 and electrolyte composition (Table 2). The presence of the P-123 surfactant did not provoke or influence the formation of a nanocrystalline structure. It can be concluded that differ- ences in behaviour between mesoporous and dense films are entirely related to effects of porosity, since there are no other structural differences apparent. Fig. 10 Representative curves of (a) the reduced Young’s modulus and (b) hardness as a function of the nanoindenter’s penetration into the The nanocrystalline nature of the films is also demon- surface for selected dense and porous thin films. strated by TEM under diffraction conditions, showing that crystals are oriented in all possible directions within the cross- section (Fig. 9). into the surface for selected samples (Fig. 10). Nanoindentation measurements may have some influence 3.4 Mechanical properties from the substrate. It is commonly accepted that if the maximum penetration depth is kept lower than one tenth of the Representative curves obtained from nanoindentation tests overall film thickness, the contribution of the substrate can be display the dependences of Er and H on the penetration depth disregarded.31 Considering the experimentally determined film thickness of approximately 280 nm for the Ni–Pt films, the con- tribution of the substrate may not be neglected for penetration depths higher than 30 nm. Nevertheless, since the film thick- ness is similar for all compositions, the influence of the sub- strate on the obtained mechanical properties may be considered similar for all samples, and thus the observed trends in Er and H are representative of the films’ properties. For a given composition, the dense films show a higher Young’s modulus than their porous counterparts, as expected due to their higher density (Fig. 10a). The decrease of hardness and Young’s modulus with porosity is a well-documented effect.38–40 The relation between the Young’s moduli of the porous and bulk materials is given by Fig. 9 Selected area electron diffraction pattern taken at a cross-  2 ρporous section of mesoporous Ni92Pt8 with the Miller indices of the diffraction Eporous ¼ C1 Ebulk ð3Þ rings indicated. ρbulk 7754 | Nanoscale, 2020, 12, 7749–7758 This journal is © The Royal Society of Chemistry 2020 View Article Online Nanoscale Paper where C1 is a geometry constant close to 1.41 In turn, hardness by the surfactant is constant over all compositions, however, is directly related to the yield stress σ according to H = 3σ for Ni98Pt2 may have some additional porosity due to voids in- metals, which has been confirmed to hold for a nanoporous, between the grains (cf. Fig. 2). For Ni98Pt2, the relative density single-phase metallic material.42 The relation between the yields 61%, i.e. a porosity of 39%. Contrarily, for the pair This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. yield stress of the porous and dense counterparts is the Ni94Pt6 ( porous) and Ni93Pt7 (dense), the determined relative following density is 92% (and thus corresponding to a porosity of 8%). The relative densities of the two remaining porous/non-porous  32 ρporous pairs (Ni92Pt8/Ni91Pt9 and Ni76Pt24/Ni79Pt21) are 82% and 83%, σ porous ¼ C2 σ bulk ð4Þ respectively. ρbulk An interplay between porosity, Ni/Pt ratio and grain size Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. where C2 is equal to 0.3.41 Therefore, porosity has a more may be appreciated in the Young’s modulus. As observed, E is drastic influence on the Young’s modulus than on hardness always larger in bulk solid films than in their porous counter- since, as shown in the equations above, Eporous/Ebulk is pro- parts. Also, the Young’s modulus tends to increase with the Ni portional to (ρporous/ρbulk)2 (where ρporous/ρbulk is the relative content in agreement with the rule of mixture of two different density of the material), whereas hardness is proportional to elements where ENi = 200 GPa and EPt = 172 GPa.5 Grain size (ρporous/ρbulk)3/2. Comparing the results for mesoporous and might also have an influence on E but its effect should be neg- dense Ni–Pt films, the Young’s modulus shows a stronger ligible compared to the other two. influence of porosity than the hardness (Fig. 11). Indeed, the Depending on the state of annealing, the hardness of a decrease in hardness due to the occurrence of porosity is only pure metal can vary significantly from a metallurgical view- apparent for the Ni98Pt2 film. For the rest of compositions, the point. Generally, Ni bulk metal can reach a Vickers hardness hardness of the porous films is equal or slightly higher than of 1.7 GPa in an annealed state, and 6.3 GPa in a hardened for their dense counterparts. state.5 Concerning electrodeposited pure Ni, nanocrystalline Comparing the reduced Young’s moduli of porous and non- films with grain sizes (d ) between 12 nm and 22 nm exhibit porous films with similar composition using eqn (3), the rela- hardness values between 3.9 (d = 22 nm) and 6.4 GPa (d = 14) tive density is different for each composition. From the micro- nm (ref. 43)—in this case, the high hardness is a result of the structures it can be assumed that the mesoporosity introduced nanocrystallinity. For Pt bulk material, the hardness values vary between 0.4 GPa (annealed) and 2.1 GPa (hardened).5 The inversion of the Hall–Petch relationship due to a change in the mechanism of plastic deformation was determined for nanocr- sytalline Pt for grain sizes lower than 10 nm.44 The Ni–Pt films possess hardness values lying between 4 GPa and 6 GPa. The generally high values are a result of the nanocrystallinity, however, internal stresses may also contrib- ute to an increased hardness. In the compositional range between 75% and 95% Ni, where the grain size is constant, a trend of increasing hardness with increasing Pt content is observed, which may be related to a solid solution strengthen- ing. For Ni98Pt2, this trend is not continued and its higher hardness may be the result of the difference in grain size (cf. Fig. 2b and Table 2). It has been shown that below a grain size of 14 nm, the breakdown of the Hall–Petch relationship is reached for Ni.43 Thus, apart from the Ni98Pt2 films, all other Ni–Pt films are subject to an inverse Hall–Petch effect, result- ing in lower hardness values for those films. An effect of the porosity on the hardness of the films in the compositional range between 75% and 95% Ni lies within the measurement uncertainty and cannot be observed (Fig. 10b and 11a). Only for the composition Ni98Pt2, the hardness is significantly lower when porosity is present. 3.5 Magnetic properties In-plane hysteresis loops conducted by VSM show highest sat- uration magnetisation Ms and coercivity Hc for Ni-rich films, Fig. 11 Averaged reduced Young’s modulus and hardness of porous and dense Ni–Pt thin films (a) and the relative changes in Er and H intro- both decreasing with Pt content (Fig. 12). A change of slope duced by porosity, in percentage of the values determined for the dense (the orientation of magnetic domains is increasingly hindered) films (b), at a penetration depth of 40 nm. occurring between the coercivity field and the saturation mag- This journal is © The Royal Society of Chemistry 2020 Nanoscale, 2020, 12, 7749–7758 | 7755 View Article Online Paper Nanoscale cally hard axis lies out of plane, where the loops are strongly tilted with respect to the in-plane direction. Therefore, the hys- teresis loops of dense and porous films are dominated by shape anisotropy. Here, the orientation of magnetic domains This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. normal to the plane is energetically unfavourable, and an increased energy input (applied field) is needed to fully mag- netise the films normal to the plane (Fig. 14). The out-of-plane coercivity is lower, i.e. it is easier to demagnetise the material in this mode compared to in-plane. Contrarily to the in-plane mode, the out-of-plane hysteresis loops show a widening when Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. approaching saturation. This effect is generally observed in Ni- rich alloy films—permalloy in particular—as the so-called tran- scritical state.46 When the film thickness is sufficiently high, magnetic stripe domains form by part of the magnetisation orienting out of the plane (when applying an external mag- netic field in plane) due to an anisotropy caused by in-plane Fig. 12 In-plane magnetic hysteresis loops for porous and dense Ni–Pt internal stresses.46 thin films with different compositions. netisation is observed for both mesoporous and dense films. This effect is generally stronger for dense films—with the exception of Ni98Pt2 which shows the same behaviour as its porous counterpart—indicating that the observed behaviour may be caused by internal stresses which are less pronounced in the porous films. In the case of Ni98Pt2, it is assumed that internal stresses are minimised due to the fact that the material is almost pure Ni and therefore the effect of stresses due to dissolution of Pt (with a higher atomic radius) into the Ni lattice is smaller. The saturation magnetisation Ms follows a linear trend with the composition for all films. For a given composition, the mass-normalised Ms is identical for both dense and porous films. The same is true for the coercivity of the porous films (Fig. 13). For Ni98Pt2, Ms reaches 46 emu g−1, approaching the Fig. 14 In-plane and out-of-plane magnetic hysteresis loops for two value for pure Ni of 54 emu g−1 (ref. 45) and thus confirming selected dense and porous Ni–Pt thin films with similar composition. again that the oxygen content is negligible. Comparing the behaviour along in-plane and out-of-plane measuring directions, it is clearly observed that the magneti- Fig. 13 Dependence of saturation magnetisation Ms and coercivity Hc on composition of Ni–Pt thin films, taken from in-plane hysteresis measurements. The linear trends for the mesoporous films are displayed Fig. 15 Dependence of magnetisation on temperature of mesoporous as dotted lines. Ni–Pt thin films at 1000 Oe. 7756 | Nanoscale, 2020, 12, 7749–7758 This journal is © The Royal Society of Chemistry 2020 View Article Online Nanoscale Paper other possible effects such as a voltage-dependency of TC. Apart from TC, the saturation magnetisation and—in the case of the mesoporous films—also the coercivity scales with the composition, thus facilitating to predict the magnetic behav- This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. iour for a certain composition and to easily select a suitable composition in order to obtain the desired magnetic properties. Fig. 16 Curie temperature TC as a function of composition of meso- porous Ni–Pt thin films. TC for pure Ni is shown for reference. The Conflicts of interest Open Access Article. Published on 25 March 2020. Downloaded on 11/2/2020 3:50:46 PM. general trend is indicated by a dotted line. There are no conflicts to declare. The temperature scans reveal a phase transition of the Ni– Pt alloys from ferromagnetic to paramagnetic state (Fig. 15). Acknowledgements The fact that a single transition is observed consolidates that This work has received funding from the European Union’s the structure of the films is indeed single-phase. Horizon 2020 research and innovation programme under the As a general trend, the magnetic transition and therefore Marie Skłodowska-Curie grant agreement no. 764977. Further the Curie temperature shifts to lower temperatures for films funding has been obtained from the Generalitat de Catalunya with higher Pt content. The magnetisation curve for Ni58Pt42 (2017-SGR-292 Project) and the Spanish Ministerio de shows only the final part of the phase transition, indicating Economía, Industria y Competitividad (MAT2017-86357-C3-1-R that its TC is very close to RT (Fig. 15, violet curve). and associated FEDER Project). The European Research The resulting values for TC show a linear relationship with Council (SPIN-PORICS 2014-consolidator grant, agreement no. the percentage of Ni (Fig. 16). The TC was only determined for 648454) is also acknowledged. J. F. is grateful to MINECO for those films where the magnetic transition appeared the “Juan de la Cierva” (IJCI-2015-27030) contract. sufficiently above the start of the measurement range (298 K). Notes and references 4 Conclusions 1 G. Schiavone, M. P. Y. Desmulliez and A. J. Walton, Mesoporous and dense Ni–Pt thin films have been synthesised Micromachines, 2014, 5, 622. by electrodeposition from aqueous media, achieving the meso- 2 MEMS: A practical guide to design, analysis, and applications, porosity through micelle-assisted deposition with a block ed. J. Korvink and O. Paul, William Andrew, NY and copolymer. The composition of the thin films can be tuned in Springer-Verlag, Germany, 2006. a large range between Ni98Pt2 and Ni61Pt39 by varying the depo- 3 R. Coutu Jr., P. Kladitis, K. Leedy and R. Crane, sition potential, resulting in a single-phase Ni–Pt solid solu- J. Micromech. 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