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

Abstract

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 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 magneti...

Controlled Electrodeposition and Magnetic Properties of Co35Fe65Nanowires with High Saturation Magnetization

“…Role of malonic acid as an organic additive in electrodeposition of Co 35 Fe 65 nanowire arrays.-The most important parameter in the electrodeposition of transition metals (TM) and their alloys (CoFe, NiFe, CoNi, CoNiFe) is, arguably, an organic additive in the plating solution. It has been demonstrated that under otherwise the same experimental conditions, merely changing the organic additive, will result with TM alloys deposit varried in: composition, crystal structure, and properties (magnetic, electrical, mechanical, and morphological and corrosion).…”

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Among all transition metals magnetic alloys, Co 35 Fe 65 possesses the highest saturation magnetization B S = 2.45 T at room temperature given by the so-called "Slater-Pauling limit". For controlled electrodeposition of Co 35 Fe 65 nanowire arrays the following parameters were found to be optimal: electrolyte solution with 1-2 mM malonic acid (MA), ionic ratio Fe +2 /Co +2 = 2.0, growth rate, and pulsed potential deposition with time-on (2.5 s) at the potential of −1.15 V/SCE and time-off (1.0 s) at −0.70 V/SCE.

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“…For controlled electrodeposition of Co 35 Fe 65 nanowire arrays the following parameters were found to be optimal: electrolyte solution with 1-2 mM malonic acid (MA), ionic ratio Fe +2 /Co +2 = 2.0, growth rate, and pulsed potential deposition with time-on (2.5 s) at the potential of −1.15 V/SCE and time-off (1.0 s) at −0.70 V/SCE. These arrays were deposited inside anodic aluminum oxide (AAO) templates that contained columnar nanopores with diameters either 35 or 200 nm.…”

“…Fe, deposits preferentially. 32 The present work describes the method for electrodeposition of Co 35 Fe 65 nanowire arrays with controlled targeted composition, the high saturation magnetization, and deposition uniformity. From the magnetic behavior of resulting Co 35 Fe 65 nanowire arrays, it was determined that the shape and magnetostatic anisotropies dominate the effective anisotropy, while the impact of magnetocrystalline and magnetelastic anisotropies is very small.…”

“…4a) shows uniform deposition into AAO templates with the lengths, L = 20 μm and diameters d = 35 nm. The composition of the CoFe nanowires-taken inside of the alumina template-was found to be Co 35 Fe 65 ± 1.5 at. % by EDX (Fig.…”