Effect of Secondary Deposition Parameters on the Magnetic Properties of Electrodeposited Cobalt Nickel Phosphorus Alloys
Authors: Bernasconi Roberto , Carrara Elena , Rinaldi Christian , Magagnin Luca 
Published: December 2020

Electrodeposited cobalt nickel phosphorus alloys are characterized by interesting magnetic properties, which motivate their potential applicability in micro-electromagnetic devices like sensors and actuators. CoNiP is suitable for MEMS fabrication due to its relatively high coercivity and maximum magnetic energy product. Moreover, the columnar grain structure typical of electrodeposited layers introduces a high magnetic anisotropy, which is desirable for devices that require high magnetic flux and energy product along a specific direction [1]. In addition to their magnetic properties, CoNiP alloys are also characterized by high hardness, which makes them good candidates for the production of anti-wear coatings, and by good corrosion resistance. Finally, CoNiP coatings show a notable catalytic activity, which can be exploited in water splitting applications [2].
One of the most remarkable advantages of using electrodeposition for CoNiP production is the possibility to easily tune magnetic properties by varying deposition parameters. Physical and chemical parameters like bath composition, temperature, pH, presence of additives have a great role on final properties determination [3]. Magnetic behavior can be soft or semi-hard according to the conditions employed. Magnetic properties, however, depend not only on the chemistry of the electrolyte and the primary plating parameters, but also on other secondary deposition aspects. These include, for example, the substrate on which the alloy is plated [4] and its roughness [5]. Furthermore, the application of an external magnetic field during the deposition can influence final properties of the deposited layer. Finally, also the presence of inert particles codeposited within the CoNiP layer can change the magnetic behavior of the coatings. In this context, CoNiP has been poorly characterized in the existing literature.
The aim of the present work is to explore the influence of some secondary deposition parameters on CoNiP magnetic properties. In detail, a CoNiP electrolyte was selected and its chemical composition was kept constant during the experimentation. The same was done with the deposition parameters. CoNiP was deposited on different substrates, characterized by different materials (brass, copper, silver) and by different degrees of roughness. The alloy was also codeposited with inert (TiO2) nanoparticles and in presence of an external magnetic field. The influence of these variables on final magnetic properties was investigated with vibrating sample magnetometry (VSM). Surface morphology and phase composition were characterized by mean of SEM and XRD. CoNiP layers were also annealed, and the effect of the heat treatment on magnetic properties was studied.
Finally, the observed substrate sensitivity of CoNiP magnetic properties was exploited to introduce a surface magnetic anisotropy in electrodeposited CoNiP layers. Brass substrates were patterned with electrodeposited copper and CoNiP was deposited on top. Magnetic force microscopy (MFM) was employed to characterize the result, evidencing differences in magnetic properties between the zones where CoNiP was plated on brass and the zones where CoNiP was plated on electrodeposited copper. The final result was a magnetically patterned CoNiP coating, which may potentially find application in microelectronics and sensors production.
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[5] S. J. Steinmuller et al., J. Appl. Phys. 101, 09D113 (2007)