Surface and Coatings Technology 169 –170 (2003) 76–80 0257-8972/03/$ - see front matter  2003 Elsevier Science B.V. All rights reserved. PII: S0257-8972 Ž 03 . 00156-7 Microelectrodeposition of cobalt and cobalt alloys for magnetic layers P.L. Cavallotti*, A. Vicenzo, M. Bestetti, S. Franz Dipartimento di Chimica, Materiali e Ingegneria Chimica ‘‘Giulio Natta’’, Politecnico di Milano, Via Mancinelli, 7-20131 Milan, Italy Abstract Structure and growth characteristics of electrodeposited cobalt are discussed with regard to its electrokinetic behaviour. Three main growth modes are pointed out: outgrowth or perpendicular growth, in conditions of low surface inhibition; lateral growth, with complete coverage of the electrode by hydrolytic species giving a high degree of surface inhibition; cluster growth, in the presence of surface complex species stabilised by boric acid. The structure influence upon the magnetic properties is discussed with reference to the electrodeposition of thick layers of Co–Pt–W alloys, with Pt and W content, respectively in the range 8–15 and 1–5 at.%, having high coercivity and high magnetic moment.  2003 Elsevier Science B.V. All rights reserved. Keywords: Cobalt electrocrystallisation; Cobalt alloys electrodeposition; Permanent magnets 1. Introduction Potential applications of permanent magnets in micro- technologies calls for manufacturing alternative methods to conventional metallurgical techniques: in this regard, electrodeposition is an election technique thanks to its forming capability and tailoring of material properties through growth control even at high thickness. Structure control plays a fundamental role in electro- deposition of magnetic layers. Cobalt can be obtained with different phase structure and preferred orientations (POs) from simple sulphate or sulphamate electrolytes, depending on bath pH and deposition current density: at low pH, fcc Co is deposited; increasing pH, hcp Co with most densely packed plane (0 0.1) either perpen- dicular or parallel to the surface can be obtained. Among Co alloys, electrodeposited (ECD) Co–P and Co–W(P), which show strong POs depending on the bath pH w1,2x, were investigated for magnetic and wear resistant coatings; ECD Co–Pt and Co–Pt(P) alloys are candidate materials for permanent micromagnets w3–5x. In this work, the electrodeposition of Co–Pt–W alloys, with reduced Pt content and improved magnetic prop- erties at high deposit thickness, is presented. *Corresponding author. Tel.: q39-2-23993149; fax: q39-2- 23993180. E-mail address: pietro.cavallotti@polimi.it (P.L. Cavallotti). 2. Experimental Plating solutions were prepared from chemicals of analytical grade and distilled water. Cobalt sulphamate solutions were prepared from cobalt carbonate and sulphamic acid (in the following, s stands for NH SO ). Boric acid was added in concentration 0.4– y 2 3 0.5 M. Electrolytes were pre-treated with active charcoal (1 h, ;90 8C) and electrolysed at low current density (cd-1 mA cm ). The pH of the baths was adjusted y2 by cobalt carbonate or sulphamic acid addition. High purity Co was used as anode and brass sheets, also coated with a 3-mm thick chemically deposited NiP layer, as substrate. Deposition was carried out at 30 8C and cd in the range 10–40 mA cm . y2 Electrodeposition of Co–Pt–W alloys was performed using an electrolyte having the following composition: Co(NH SO ) 0.1 M, Pt(NH )(NO ) 0.01 M, 2 32 32 22 (NH ) CHO 0.1 M, NH CH COOH 0.1 M, 42 6 6 7 2 2 Na WO Ø2H O 0.27 M, NaH PO 0.05 M, at 40–60 8C 2 4 2 2 2 and cd 5–20 mA cm . y2 X-ray diffractometry (XRD) with Cu Ka radiation and a powder goniometer was used for structure char- acterisation; the composition of Co–Pt–W deposits was measured by energy dispersion spectroscopy; laser pro- filometry and scanning electron microscopy were used for thickness measurements and morphological exami- nation. Magnetic measurements were performed with a vibrating sample magnetometer.