Morphology and Magnetic Properties of Co-rich Co-Pt Thin Films Electrodeposited on Cr Seed Layers

Abstract

Co-Pt ͑Pt ϳ 20 atom %͒ alloy films were electrochemically grown at constant current density on Cr seed layers in order to study the influence of the Cr surface on their morphology and magnetic properties. The Cr seed layer favors the growth of a hexagonal close-packed phase, as revealed by X-ray diffraction. Morphological investigation using a scanning electron microscope revealed the electrocrystallization of Co-Pt on Cr to be proceeding through the nucleation and growth of three-dimensional ͑3D͒ clusters with columns growing outward in all directions from isolated nucleation centers. The 3D growth is likely due to the presence of a thin oxide layer on the Cr seed. At lower current density, individual micrometer-sized clusters were formed with distinct hexagonally shaped faces. Increasing current density results in a finer grain microstructure. At early stages of growth and low current density, the clusters exhibit a faceted morphology similar to that of the substrate. Samples prepared at higher current density or with larger thickness show larger coercivities, reaching values as high as 3490 Oe ͑278 kA/m͒ and 2275 Oe ͑181 kA/m͒ in the perpendicular and parallel directions, respectively, for a 500-nm-thick film deposited at 50 mA/cm 2 .Co-rich Co-Pt alloy thin films with composition about 20-25 atom % Pt are known to exhibit high magnetic anisotropy ͑MA͒ and high coercivity in the as-deposited state. These magnetic properties are reportedly induced either by the coexistence of a face-centered cubic ͑fcc͒ and a hexagonal close-packed ͑hcp͒ phase, 1 a microstructure composed of small hcp grains with nonmagnetic matter precipitated at the grain boundaries, 2 or by the growth-induced formation of a partially ordered metastable phase. 3,4 These alloys exhibit much lower anisotropies and coercivity than those typical of equiatomic face-centered tetragonal ͑fct͒ Co-Pt or Fe-Pt alloys; they do not require the postdeposition annealing at temperatures above 600°C that is necessary to start recrystallization and achieve formation of the fct phase. As such, Co-rich Co-Pt alloys constitute an attractive class of materials, in particular for permanent magnet components in microelectromechanical systems ͑MEMS͒, 5-7 where post-deposition annealing at high temperature is generally unacceptable from a processing standpoint.In previous work by our group 8,9 we have demonstrated that the electrochemical deposition ͑ECD͒ of Co-Pt ͑Pt ϳ 20 atom %͒ onto specific underlayers, e.g., fcc Cu ͑111͒, results in heteroepitaxial growth and favors the formation of a hexagonal phase, yielding single-phase hcp films with their c axis perpendicular to the film plane and high perpendicular anisotropy. ECD is an important technique in MEMS fabrication, since ECD through lithographic masks is a unique method for the growth of high aspect ratio threedimensional structures, providing also an outstanding control of the morphology, microstructure, and magnetic properties of the resulting components. ECD from an electrolyte with suitable chemistry has al...

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Electrodeposition of hard magnetic films and microstructures

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