GIANT MAGENTORESISCANCE AND MAGNETIC PROPERTIES OF ELECTRODEPOSITED Ni-Co-Cu/Cu MULTILAYERS

G. Nabiyouni Department of Physics, University of Arak, Arak 38156, Iran I.Bakonyi Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences. H-1525 Budapest, P.O.B. 49, Hungary W. Schwarzacher, H. H. Wills Physics Laboratory, Bristol BS8 1TL, UK,

Multilayer structures are most commonly prepare by vacuum based techniques such as evaporation, sputtering and molecular beam epitaxy (MBE). However, it has been well established that nanoscale metallic multilayers can also be successfully produced by electrodeposition and at appropriate element and layer thickness combinations, they can exhibit a significant giant magnetoresistance (GMR) effect as demonstrated for the systems Ni-Cu/Cu, Co-Cu/Cu and Ni-Co- Cu/Cu.

Ni-Co-Cu/Cu multilayers were electrodeposited from a single bath under potentiostatic control on (100)-textured polycrystalline Cu substrates (this texture proved to be superior to the (110) and (111) textures for the GMR effect). The films consist of 100 repeats of Ni-Co- Cu(3nm)/Cu(1nm) and Ni-Co- Cu(3nm)/Cu(2nm) respectively.

The starting electrolyte composition (in units of g/litre H 2 0) was as follows: 742 g/ Ni-sulphamate (135 g/ Ni as metal), 13.5 g/ CuSO 4 (3.5 g/ Cu as metal) and 30 g/ H 3 BO 3.

Co metal concentration in solution was as follow: X = 0, 1, 2, 3, 5, 7, 10, 14, 18, 21 and 24 g/ The Co-content of the solution is denoted by X referring to the amount of Co metal (in g/ units) in the solution To introduce Co, CoSO 4 was added to the solution. (1 g/ Co metal corresponds to 4.46 g/ CoSO 4 ).

The magnetoresistance (MR) measurements were performed at room temperature for magnetic fields up to H = 8 kOe in the current-in-plane/field-in plane (CIP/FIP) configuration. The MR ratio was defined as  R/R 0 = (R H - R 0 )/R 0 where R 0 = R(H=0) is the resistance without a magnetic field and R H = R(H) that in a magnetic field H.

The magnetoresistance was measured with four point-probes arranged in a square making pressure contacts with the sample. The MR measurements were performed with the current either predominantly parallel (“longitudinal magnetoresistance,” LMR) or perpendicular (“transverse magnetoresistance,” TMR) to the applied magnetic field

In the multilayers prepared by sputtering the room- temperature GMR increases continuously from about 7 to 55 % when varying the Co-content of the magnetic layers in Ni-Co/Cu multilayers from 0 to 100 %. For sputtered Ni-Co/Cu multilayers an optimum GMR behaviour was found when the Co-content of the magnetic layer was around 20 to 40 at.% Co. In the GMR data for electrodeposited multilayers, also a significant increase has been found when going from: Ni-Cu/Cu multilayers to the Co-Cu/Cu system.

Due to the differences in the electrochemical behaviour of Ni and Co atoms, the electrodeposition of Co-Cu/Cu multilayers is less favourable than that of Ni-Cu/Cu multilayers. This is because the exchange reaction between Co and Cu is stronger than between Ni and Cu.

All these considerations led us to perform the present work aimed at investigating the influence of Co- content on the GMR and magnetic properties of Ni-Co-Cu/Cu multilayers by systematically increasing the Co ion concentration in the electrolyte.

It was also of interest to correlate the magnetic properties and GMR parameters for a series of samples with widely differing magnetoresistance behaviour.

The overall chemical composition (C Ni, C Co and C Cu ) of the electrodeposited multilayers was established for both A and B type samples by electron probe microanalysis (EPMA) in a scanning electron microscope (SEM) after removing the substrates (table 1&2).

The overall Cu-content didn’t show a systematic dependence on electrolyte Co-content (X) and C Cu was 40  3 at.% and 51  4 at.% for series A and B, respectively

The higher Cu-content in series B reflects the larger Cu-layer thickness (d Cu = 2 nm) in comparison with series A (d Cu = 1 nm).

Acknowledgement G.Nabiyouni (corresponding author) acknowledges the University of Arak for financial supports.