* Corresponding author. Fax: #91-11-6581114. E-mail addresses: kant4p@hotmail.com, dkpandya@phys- ics.iitd.ernet.in (D.K. Pandya). Journal of Magnetism and Magnetic Materials 219 (2000) 309}316 Structure and giant magnetoresistance in electrodeposited granular Cu}Co "lms G.R. Pattanaik, S.C. Kashyap, D.K. Pandya* Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India Received 16 March 2000 Abstract Cu}Co alloy "lms have been electrodeposited galvanostatically from a single sulphate bath. XRD reveals the as-deposited "lms to be single phasic metastable FCC alloy of Cu and Co with a grain size of 5}20 nm. Phase separation on annealing forms "ne Co granules at the core of Co-rich shells. The grain size grows to 40}60 nm after annealing at 4003C for 1 h. A total phase separation into Cu (FCC) and Co (FCC) is observed after annealing at 7003C. Maximum room-temperature magnetoresistance (MR) of 4% at a magnetic "eld of 10 kOe was observed for "lm annealed at 4503C for 1 h. A MR ratio of 7.8% was observed at 20 K in a magnetic "eld of 3 kOe. The MR was observed to increase with "lm thickness. Coexistence of typical superparamagnetic and ferromagnetic features is observed in the room-temperature magnetization curves of as-deposited and 4003C annealed "lms.  2000 Elsevier Science B.V. All rights reserved. PACS: 75.70.Pa Keywords: Giant magnetoresistance; Electrodeposited granular "lm; Structure Cu}Co "lm 1. Introduction Magnetoresistive materials have recently attrac- ted a great deal of attention due to their potential application in magnetic data storage and reading. In the presence of magnetic "eld these materials exhibit large drop in resistivity. This so-called giant magnetoresistance (GMR) [1,2] e!ect, "rst ob- served in Fe/Cr [3] multilayers, has made these materials attractive due to their application in mag- netoresistive devices. Metallic granular "lms [4}6] consisting of nanometric ferromagnetic granules (e.g., Fe, Co, etc.) embedded in a non-ferromagnetic matrix (e.g., Ag, Cu, Au, etc.) were subsequently found to exhibit the GMR e!ect. It is easier to prepare granular "lms than metallic multilayers. The simplicity of fabrication and that of the tech- nique are key to the technological viability. In equilibrium state at room temperature Cu}Co have almost zero miscibility and thus make an excellent combination for GMR investigations. In general, conventional PVD techniques have been employed to deposit Cu}Co granular thin "lms [4,5,7}9]. Electrodeposition is a relatively simple, low cost and precise technique capable of produ- cing metastable alloy "lms of various compositions. It is only recently that few workers [10}14] in- itiated the use of electrodeposition to produce 0304-8853/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 4 5 3 - 4