전기화학적으로 제조한 Co3W/Co-W 필름의 마그네틱 특성

고장면,  *Ken. Nobe

한밭대학교, University of California, Los Angeles, U.S.A.

Magnetic Properties Of Electrodeposited Nano-Structured Co3W/Co-W Multilayers

Jang Myoun Ko and *Ken. Nobe

Department of Chemical Technology, Hanbat National University of Technology, Korea. *Department of Chemical Engineering, University of California, Los Angeles, U.S.A.

Co-W thin film alloys have been electrodeposited from the citrate bath with different current densities to understand their magnetic properties. Tungsten contents in deposited film at 70oC were independent of current density while magnetic properties shifted from hard (HC,// = 145 Oe) to soft (HC,// = 22 Oe)  with increasing a current density from 10 to 100 mA cm-2.  X-ray diffraction data indicated that hcp Co3W phase was dominantly electrodeposited at 10 mA cm-2 while fcc Co-W solid solution dominantly formed at 100 mA cm-2.  At 25 and 50mA cm-2, mixed hcp Co3W and fcc Co-W solid solution were observed. This is responsible to the change from soft to hard magnetic properties. On the other hand, nano-structured Co3W/Co-W multilayers were fabricated by applying alternative current between 10 and 100 mA cm-2 with variation of individual layer and proportion of hard magnetic layer while keeping total thickness to be constant.  Magnetic properties of Co3W/Co-W multilayers were strongly dependent on layer thickness and portion of hard layers. Co3W/Co-W multilayers showed from soft (HC,// = 3 Oe) to hard magnetic (HC,// = 250 Oe) with increasing the portion of HCP Co3W phase deposited at 10 mA cm-2, suggesting there is a strong coupling effect between multilayers.

Introduction

Co-W thin film alloys have been investigated for potential applications including magnetic recording media, computer drives, microelectromechanical systems (MEMS) due to favorable magnetic properties, wear resistance, corrosing resistance, and good lubrication properties (1-5). Co-W alloy films are of special interest because they combine favorable magnetic properties with wear resistance, wear being a major problem in magnetic recording disks. These alloys possess a high Curie point and display endurance to hostile environments or extreme vacuum conditions, features of value in space technology. Several researchers have studied the magnetic properties of Co-W alloy films and tried to understand the relationship between electrodeposition parameters and their magnetic properties (1-5). But there are still lacks of understanding due to their complex relations, even though Co-W alloy film exhibits a variety of highly useful properties.

In the present study, we have electrodeposited Co-W alloy thin films from citrate bath, and discuss the relationship between the magnetic properties and film structures, comparing magnetic properties for Co-W alloy thin films deposited at current density of 10 mA/cm2 and 100 mA/cm2, respectively.

Experimental

The Co-W alloy films were produced by electrodeposition at 70 oC. The chemical composition of the electroplating bath was 75.6 g/l ammonium citrate (NH4C6H8O7.xH2O) as a complex agent, 50.0 g/l NH4Cl as a conducive agent, 12.0 g/l Na2WO4, 30.0 g/l CoCl2.6H2O and NH4OH as pH control agent, The substrates for electrodeposition were brass. Electrodeposition was carried out with a current density of 10, 25, 50, 100 mA/cm2, respectively, in plating solution, maintained at a pH value of 8.7.  The thickness of the deposited film was controlled by film deposition charge of 60C. The magnetic properties of samples were measured by a vibrating sample magnetometer (VSM). The structures of samples were examined by X-ray diffractometer. Elements of Co and W in the samples were analyzed using atomic absorption spectroscopy (AA) and energy dispersive X-ray spectroscopy (EDXS).

Results

Co-W alloy thin film has approximately 40 % tungsten content. For the Co-Ws alloy film deposited at 100 mA/cm2, the magnetization curve shows soft magnetic while for the film deposited at 10 mA/cm2 the magnetization curve shows hard magnetic properties. For the of Co-W alloy film deposited at 100 mA/cm2, hcp Co3W phase become pronounced while at 10 mA/cm2 fcc Co phase become more pronounced. This is responsible to the change from soft to hard magnetic properties. The Co3W/Co-W multilayers showed gradually from soft (HC,// = 3 Oe) to hard magnetic (HC,// = 250 Oe)  properties with increasing the portion of HCP Co3W phase deposited at 10 mA cm-2, suggesting there is a strong coupling effect between multilayers.

References

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Fig. 1. Current density dependence of the element compositions

Fig. 2. Current density dependence of the magnetic hysteresis loop of Co-W alloy thin films.

Fig. 3. Magnetic hysteresis loops obtained from the alternative multilayers of CoW alloy consisting of  CoW layers with a fixed 30 nm thick and a varied Co3W layer thick from 2 nm to 26 nm.