Electrodeposition of Thin CoPt Films with Very High Perpendicular Anisotropy from Hexachloroplatinate Solution: Effect of Saccharin Additive and Electrode Substrate

The CoPt ﬁlms with 9.7–91 at% Co and thicknesses of 15–20 nm were obtained from a new designed stable hexachloroplatinate solution at a controlled potential deposition. The effects of the substrate (Ru and Cu) and an organic additive (saccharin) on composition, crystal structure and magnetic properties of the CoPt ﬁlms were studied. It was demonstrated that a Ru electrode substrate provides well-deﬁned surface for the epitaxial growth of hcp phase, resulting in high perpendicular anisotropy. The addition of saccharin (Sacc) as an organic additive into the plating solution caused a dramatic improvement of the epitaxial growth of CoPt ﬁlm on the Ru substrate. At the ﬁlm thickness of interest, for bit-patterned media BPM (15–20 nm), the out-of-plane coercivity showed the highest value of 6700 Oe and the squarness M r /M s ∼ 1. © The Author(s) 2016

The areal density of hard disc drives has been increasing at 25-35% per year and now it is approaching to 1 Tb/in 2 . The magnetic crystal grain volume (V) has been decreasing to several cubic nanometers which is near to the thermal instability of granular media, i.e. the superparamagnetic limit. In order to further increase the areal density thermal stability factor, K u V/kT, needs to be kept within 40-60 range. The superparamagnetic effect posses a serious challenge for continuing to increase the areal density and storage capacity of disc drives. 1 In order to solve these problems the intensive research and development efforts are currently being carried out worldwide with two major concepts. The first one is to use high magnetoanisotropy (K u ) granular media, i.e. FePt or CoPt alloys, and develop a heat-assisted magnetic recording (HAMR) writer. 2 The second is to use conventional perpendicular writer and develop a bit-patterned media (BPM) that effectively increase the grain volume (V). 3 Both concepts need to overcome numerous technical challenges in R&D in order to manufacture recording heads with areal density >1 Tb/in 2 and stability over 10 years of data storage.
The main idea of BPM technology is that each bit is stored in a single dimensionally defined magnetic switching volume, i.e. dot. Different methods-electron beam lithography (EBL), nanoimprint lithography (NIL), block co-polymer (BCP)-for the fabrication of nano-holes with the long-range ordering and diameter of sub-20 nm corresponding to the density of ∼1 Tdot/in 2 have been demonstrated. 4,5 A typical perpendicular media comprises of a multilayer structure including a substrate covered by a soft magnetic under layer (SUL), an interlayer (seed layer) and hard patterned magnetic layer (BPM). The hard magnetic layer can be a CoPt or FePt electrodeposited alloy of hcp-crystal structure with crystalline grains oriented along the c-axis (the magnetic easy axis) in the direction normal to the film. The important magnetic property at the thickness of interest in BPM (15-20 nm) is high out-of-plane coercivity (H c ) which is largely determined by magnetocrystalline anisotropy and to a lesser extent by shape anisotropy of magnetic grains (dots). Additional requirements include high magnetic anisotropy (K u ), the remanence squareness of the hysteresis loop (M r /M s ∼1), and small grains within the thermal stability limit.
The electrodeposition of CoPt alloys has been studied intensively as a possible method for fabrication of BPM and MEMS devices. The CoPt alloys obtained in as-deposited state showed relatively low perpendicular coercivity at room temperature (H c = 200-1500 Oe). 6-8 Very high coercivity (H c = 10000-13800 Oe) was achieved by electrodeposition of CoPt and FePt alloys-with near equiatomic composition-which were transformed from as-deposited cubic to the high anisotropy L1o phase upon annealing at temperatures 400-900 • C. [9][10][11][12] The major disadvantage for use of these materials in BPM is high annealing temperature, which can deteriorate the thermal stability of the whole stack in BPM, i.e. substrate, SUL, under layer and magnetic layer. Textured films of hcp CoPt alloys-without postannealing-have been obtained with high perpendicular coercivities up to 6000 Oe at thickness >100 nm. [13][14][15][16][17][18] However, these studies show that the perpendicular coercivities at the thickness of interest for BPM application (15-20 nm) appeared to be relatively low. Importantly, the formation of 10 nm thin CoPt nanodot arrays-deposited inside the nanopores with 10 nm diameters and moderate corcivities (1500-1700 Oe)-for application in ultra-high magnetic BPM was demonstrated recently. 19 This work presents the results on electrodeposition of hcp CoPt films with thicknesses of 15-20 nm from a new designed stable hexachloroplatinate solution. 20 It will be demonstrated that a Ru electrode substrate provides an interface for the epitaxial growth of hcp phase exhibiting a perpendicular anisotropy of CoPt films-as observed before in literature. 16,17,19,21 It will be shown also that the addition of saccharin, as an organic additive, into the plating solution dramatically improves the epitaxial growth of CoPt film on Ru substrate. At the film thickness of interest for BPM (15-20 nm) the out-of-plane coercivity reached as high as 6700 Oe and the squarness M r /M s ∼1.  20 The voltammetric measurements were performed in a 100 ml closed three electrode cell with a graphite rode anode and a saturated calomel electrode (SCE) as a reference electrode in the chosen solution, i.e. with and without addition of saccharin. Some experiments were carried out in three electrode cell described in Ref. 22; Fig. 1. The potential-current curves were recorded using a Gamry Instrument PC3 potentiostat. Three kinds of electrode substrates, i.e. Pt, Cu, Ru and glassy carbon electrode (GCE) were employed for cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The experiments with rotating disc electrode (RDE) were carried out with Pt (0.2 cm 2 area), and GCE (0.2 cm 2 area) disc. Prior to each experiment the Pt disc was polished on felt cloth with alumina powder, washed with water, immersed in acetone and rinsed sequentially in ethanol and water.

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Journal of The Electrochemical Society, 163 (7) D287-D294 (2016) The controlled potential electrodeposition (CPE) on sputtered Cu and Ru films (200 nm) onto oxidized Si wafers with a 5 nm Ta adhesion layer, were done by exposing the substrate (1 cm 2 area) to the quiescent plating solution adjusted at pH 5.5. The cyclic voltammetry on sputtered Ru and Cu seed layer (0.754 cm 2 area) was performed within an electrochemical cell described recently. 22 Experiments were performed in a sealed cell under nitrogen after purging 25 min.
The average thickness of the CoPt films were obtaintained from two point measurements, i.e. center and edge. A Dek Tek profilometer was used to take the step height as thickness. The plating rate is expressed as a ratio thickness (nm)/time (s). The film structure of CoPt films were determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The magnetic properties of the samples were measured using a vibrating sample magnetometer (VSM).
The elemental composition and total weights of the CoPt films were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Teledyne Leeman Labs "Prodigy" ICP spectrometer. The deposited films were dissolved into approximately 5 ml solution of 50% (v/v) nitric acid and 1% (v/v) hydrochloric acid. The solutions were then diluted as necessary for analysis into volumetric flask. The analysis of light elements in CoPt films were determined using dynamic secondary-ion mass spectrometry (SIMS) depth prophile analysis. The SIMS analysis was obtained from Charles Evans & Associates through their in-house system. SIMS results were calibrated using the sample where major elements were determined by X-ray fluorescence (XRF). The chemical state of elements was determined by using X-ray photoelectron spectroscopy (XPS) technique.

Results and Discussion
Voltammetry study.- Figure E : Co +2 +2e → Co [5] Notably, the addition of CoSO 4 into the H 2 PtCl 6 solution-which we call here CoPt solution-does not affect the half wave potentials, E 1/2 , of the A, B, and C waves. The onset of Co +2 reduction (wave E)-observed in CoSO 4 solution-is shifted to more positive potential by about 250 mV-observed in CoPt solution-(wave D), which was attributed to the negative enthalpy of CoPt formation. 6 The addition of saccharin (Sacc) into CoPt solution shifted the half-wave potential of the wave B and the potential of wave D toward more negative values. This effect could be attributed to the adsorption of Sacc inside of the potential range from −0.4 to −1.0 V/SCE.
The shape of the cyclic voltammogram at the Ru electrode, obtained in CoPt solution without Sacc ( Fig. 2-left), resembles the same CV from Fig. 7 in Ref. 20 with the characteristic A, B, C, and D cathodic peaks corresponding to the electrode processes described in Eqs. 1-4, respectively. On the reverse scan two anodic peaks were observed, i.e. C' peak around −540 mV/SCE due to the hydrogen oxidative desorption and peak D' around −350 mV/SCE due to the oxidation of CoPt alloy. Addition of Sacc changes the position and height of all peaks ( Fig. 2-right). Notably, the potential of the peak B is shifted toward more negative potential in solution with the addition Sacc in the CoPt solution, which is similar to the effect of Sacc on RDE voltammogram in Fig. 1. The CV at the Cu electrode (not shown here) exhibits oxidation in the range of potentials from 500 to −200 mV/SCE and could not be compared with the voltammograms at Ru or Pt substrates. In the range of potentials from -200 to −1000 mV/SCE CV exhibits the cathodic currents due to the electrodeposition of CoPt alloys.    revealed that the (10.0) and (00.2) peaks had a hcp structure with the (00.2) phase parallel to the film plane. This conclusion was confirmed by TEM measurements.

Effect of electrode substrate and saccharin additive on crystal
The comparison of hysteresis loops (Fig. 4) recorded in the plane (parallel) and out-of-plane (perpendicular) direction clearly show that the CoPt film deposited on Ru seed layer favors hcp phase formation resulting in higher perpendicular coercivity (H c = 1220 Oe) compared to the CoPt film depsited on Cu seed layer (H c = 254 Oe). This result is expected since it is known that a Ru underlayer exhibits an improvement of the microstructure and enhancement of perpendicular anisotropy of CoPt films obtained either by vacuum deposition 21,[24][25][26] or electrodeposition. 16,17,19,21 The dramatic improvement of crystal structure and perpendicular coercivity is illustrated in Fig. 5-left for a Co 71 Pt 29 film produced by potentiostatic deposition at −0.8 V/.SCE in a solution containing Sacc additive. The XRD scan of the film produced in the presence of Sacc showed only hcp (00.2) peak, while hcp (10.0) peak-observed without Sacc-was absent due to the specific influence of Sacc additive. Saccharin presumably adsorbs preferentially at the (10.0) plane hindering its growth and promotes the hetero-epitaxial growth of (00.2)  The improvement of hcp crystal structure in Co 71 Pt 29 filmdeposited on Ru seed layer in the presence of Sacc-resulted in improved perpendicular coercivity. Figure 5-right illustrates this dramatic effect of Sacc additive on perpendicular coercivity of the ∼15 nm thin CoPt films, obtained on Ru seed layer, with (H c = 6700 Oe) and without (H c = 1220 Oe) Sacc. Our results clearly demonstrate that the electrodeposited CoPt films with the hcp (00.2) plane -parallel to the film plane and good c-axis orientation perpendicular to the film plane-are due to the epitaxial growth of CoPt on Ru seed layer and even more due to the specific adsorption of Sacc which hinders growth of hcp (10.0) plane. Figure 6 shows the magnetic hysteresis loop of as-deposited ∼15 nm Co 71 Pt 29 thin film obtained in parallel and perpendicular direction. According to the best knowledge of literature, the hysteresis loop exemplifies the highest perpendicular coercivity (H c = 6700 Oe) observed for CoPt films obtained by electrodeposition. The other magnetic properties of the ∼15 nm Co 71 Pt 29 film like parallel coercivity (H c < 300 Oe), negative nucleation field (H n = −440 Oe) and high saturation field (H k = 9400 Oe) are almost ideal for application as a material in BPM at the thickness of interest (15-20 nm). 27 It is worth to point out that these samples are continues films and their perpendicular coercivity can be significantly higher once patterned into isolated islands. CoL + + e → CoL [7] The results in Table I show that the amounts of O, H, C, N, and S light elements are larger in CoPt film obtained in the presence of the saccharin additive. The origin of these light elements is certainly from the inclusion of saccharin molecule and its reduction by-products together with metal sulfides according to the mechanism discussed in our earlier paper. 28 The chemical composition of CoPt films with a thickness of ∼150 nm, obtained by electrodeposition at the controlled potential (E = −0.8 V/SCE) in the presence of Sacc, was determined by X-ray photoelectron spectroscopy (XPS). The high resolution XPS spectra for Co 2p, Pt 4f, C 1s, and O 1s are shown in Fig. 7. The samples were sputtered for 12 min using 1 kV Ar + beam and 5 min 500 eV beam. The sputtering removed about 30 nm of material, which is much higher than the thickness of surface oxides and hydroxides species (1-2 nm) at the CoPt surface. The samples were tilted and analysis was  performed at take-off angle of 75 • to analyze material damaged by ion beam. The standard binding energy of different chemical states of each element were analyzed according to the literature data. 29 The Co spectra exhibit a doublets with the binding energy at 778 and 793 eV corresponding to the Co 2p 3/2 and Co 2p 1/2 in metallic state. 29 The Pt 4f peaks located at 71.5 and 74.6 eV are very close to the values reported for metallic Pt. 29 The C 1s spectrum shows intense peak around 284 eV which can be attributed to the carbon no matter what hybridization. Oxygen bonded to carbon-in saccharin molecule or its reduction by products-shifts the peak to the higher binding energy by 1.5 eV per one C-O bond. 29 The O 1s spectrum shows peaks typical of metal hydroxides (CoOH + , Pt (OH) 2 ) at 532 eV and metal oxides (CoO, PtO) at 530 eV. 29,30 The metal oxides are formed possibly from the corresponding hydroxides through acid-base reactions according to Eqs. 8 and 9.

Chemical composition, morphology and TEM structural characterization of CoPt films.-The
Saccharin additive is known as a smoothening and stress relieving agent. Figure 8 shows SEM images of two CoPt films with thickness ∼150 nm deposited at the controlled potential (E = −0.9 V/SCE) without Sacc (Fig. 8-left)-exhibiting a rough surface and the cracks  due to the high tensile stress-and with Sacc showing a smooth surface without cracks (Fig. 8-right). Figure 9 shows the cross-sectional and planar bright field (BF) TEM images of CoPt films grown on Ru substrate in the presence of Sacc. The cross-sectional BF image ( Fig. 9-left) shows columnar growth in which CoPt hcp (00.2) lattice plane grows continuously on Ru hcp (00.2) plane-indicating the epitaxial growth-with the oxide/hydroxide material segregating to the grain boundaries. The columnar CoPt diameters are 7-18 nm with the median value of 10 nm.
The qualitative detection of oxygen at the grain boundaries was determined by EDS elemental profile analysis across the grain boundary (18 nm) between the grains (Fig. 10). It was demonstrated that the content of both Co and Pt metals decreases across the grain boundary and O-content increases.
The plan-view of BF TEM image ( Fig. 9-right) shows well isolated grains by non-ferromagnetic or weakly ferromagnetic Pt-rich phase localized at the grain boundaries. Such structure of CoPt nano grains would pin domain motion and also inhibit exchange interactions among the grains, 31 which would enhance the perpendicular magnetic anisotropy and increase the coercivity of the CoPt film deposited in the presence of Sacc on Ru substrate. Figure 11A shows a high resolution bright-field transmission electron microscopy (BF-TEM) image of the selected area diffraction (SAD)-about 200 nm diameters-from a region of the CoPt/Ru interface. The Co 71 Pt 29 hcp (002) layer grows continuously on Ru hcp (00.2) seed layer, indicating the epitaxial growth. The SAD of CoPt patterns shown in Fig. 11B is consistent with a strong (00.2) reflection. The diffraction from Ru seed (not shown here) and the CoPt film show a good epitaxial alignment of (00.2) planes with growth direction. Figure 11C shows transmission high-energy electron diffraction (THEED)-obtained from the CoPt region of cross-section SAD regionrevealed hcp structure with a strong (00.2) and weak (01.1) diffraction patterns. The same patterns were observed by XRD with the Co 71 Pt 29 film obtained in the presence of saccharin (see Fig. 5-left).

Effect of electrode potential on elemental composition and magnetic properties of ∼15 nm CoPt films.-
The potentiostatic depositions of CoPt films-obtained from the quiescent plating solution with Sacc-at potentials ranging from −0.4 V/SCE to −1.1 V/SCE, were carried out at the fixed time changing from 9.0 s to 220 s depending on the controlled potential in order to achieve the nominal thickness of 15 nm for CoPt films (Fig. 12). The plating rate (nm/min) was determined for each potential in separate experiments measuring the thickness of CoPt films after the electrodeposition. By varying the electrode potential from less negative (−0.4 VSCE) to the more negative (−1.1 V/SCE) Co-content in CoPt films increases from 9.7 to 91 at %. The shape of potential -at. % Co curve shown in Fig. 12, obtained for electrodeposition of CoPt alloys in the the quiescence solution in the presence of Sacc is very similar to the curve without Sacc, which we have discussed in more detail in our recent paper. 20

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Journal of The Electrochemical Society, 163 (7) D287-D294 (2016)  The increase of Co-content in CoPt films is expected to increase their magnetic saturation, Ms, and cause the change in coercivity. The selected out-of plane hysteresis curves in Fig. 13, obtained at different potentials, illustrate the changes of magnetic properties. The mag- The in-plane hysteresis loop of CoPt film with 71 at% Co (Fig. 6) exhibited a linear hysteresis behavior of hard axis anisotropy field (H k ). The magnetocrystalline anisotropy constant, K u , of the Co 71 Pt 29 film is calculated according to the Equations 10-12.
K eff = H k M s /2 [10] K = K eff + 1/2 N d M s 2 [11] K u = K eff + K /2 [12] where K eff is the effective anisotropy energy, M s is the saturation magnetization, N d is demagnetization factor taken as N d = 4π 32 , 1 2 N d M s 2 is the maximum possible shape anisotropy in the perpendicular direction, and K is an upper bound of anisotropy energy. 18 The estimated values of K u for ∼15 nm thin CoPt films with 71 at % Co are shown in Table II, together with out-of-plane coercivity (H c , ⊥) and saturation magnetization (M s ) which was determined experimentally from measured magnetization and volume of CoPt films. The values of the uniaxial anisotropy constant K u and saturation magnetization of the Co 80 Pt 20 films shown in Table II

Conclusions
The CoPt films with 9.7-91 at% Co and thicknesses of 15-20 nm were obtained from new designed stable hexachloroplatinate solution at a controlled potential deposition. The CoPt film with 9.7 at% Co is nonmagnetic, i.e. M = 0.0 memu, indicating that the ferromagneticparamagnetic transition for electrodeposited CoPt alloys occurs at <10 at% Co. The increase of Co-content in CoPt films-achieved through the increase of the cathodic potentials-results in the increase of their magnetic saturation, Ms, and cause the change in coercivity.
The cross-sectional BF TEM image shows columnar growth in which CoPt hcp (00.2) lattice plane grows continuously on the Ru hcp (00.2) plane-indicating epitaxial growth-with the oxide/hydroxide material segregating to the grain boundaries. The plan-view of the BF TEM image shows well isolated grains by non-ferromagnetic or weakly ferromagnetic Pt-rich phase localized at the grain boundaries.
The comparison of hysteresis loops (Fig. 6) recorded in the plane (parallel) and out-of-plane (perpendicular) direction clearly show that the CoPt film deposited on Ru seed layer favors hcp phase formation resulting in higher perpendicular coercivity (H c = 1220 Oe) The dramatic improvement of hcp crystal structure in ∼15 nm Co 71 Pt 21 film-deposited on Ru seed layer in the presence of Saccresulted inthe improved perpendicular coercivity. Our results clearly demonstrate that the electrodeposited CoPt films with the hcp (00.2) plane -parallel to the film plane and good c-axis orientation perpendicular to the film plane-are due to the epitaxial growth of CoPt on Ru seed layer and even more due to the specific adsorption of Sacc which hinders growth of hcp (10.0) plane. According to the best knowledge of literature, the hysteresis loop exemplifies the highest perpendicular coercivity (H c = 6700 Oe) observed for ∼15 nm CoPt films obtained by electrodeposition. The other magnetic properties of ∼15 nm Co 71 Pt 29 film like parallel coercivity (H c = 220 Oe), negative nucleation field (H n = −440 Oe),and high saturation field (H k = 9400 Oe) are almost ideal for application as a material at the film thickness of interest for BPM (15-20 nm).