An investigation of the physical and magnetic properties of transition-metal alloy nanotubes prepared via electroless deposition

The unique magnetic characteristics and impressive efficiencies associated with nanostructured magnetic materials significantly increase their potential as active materials in many modern applications including integrated magnetics. Morphological and compositional control and uniformity are crucial for efficient device operation in such applications. In this study, we aim to fabricate magnetic nanostructured alloys made from Co, Ni, Fe and B, via electroless deposition, and subsequently study their magnetic properties with respect to composition. We prepared Co-B, Co-Fe-B, Co-Ni-B and Co-Ni-Fe-B nanotube arrays of length 20 μm and outer diameter of 400 nm, in porous polycarbonate membranes via electroless deposition using dimethylamine borane (DMAB) as a reducing agent. We systematically investigated the effect of varying the electrolyte parameters to determine those which ensure a controlled deposition process to yield continuous, good-quality nanotubes.

We have demonstrated significant compositional control for all of the alloys investigated by tailoring the electrolyte chemistry. We prepared amorphous Co-B nanotubes with compositions of Co100-xBx (13 ≤ x ≤ 22). The specific saturation magnetisation is found to reduce with increasing amount of boron due to the dilution of the magnetic matrix by boron as the non-magnetic alloying element. Values ranged from 116 to 134 J T1kg-1. Coercive field values increases in proportion to boron content, and ranged between 3.3 and 5.75 mT. In order to achieve higher values of magnetisation while maintaining low coercive field values, we alloyed the Co-B with Fe. The relative metal content of Fe in the Co-Fe-B deposits ranged between 5 and 28 at. %, while the B content ranged between 7 and 18 at. %. An increase in Fe yielded a decrease in B. The specific saturation magnetisation of the deposits varies linearly with Fe content with a maximum value of 178 J T-1 kg-1 obtained for Co71Fe21B8. Coercive fields as low as 4.2 mT were achievable for Co-Fe-B. We observed an overall increase in coercivity with increasing Fe content. 

Analysis of our results indicates that the magnetisation reversal mechanism evident in both Co-B and Co-Fe-B relies on two separate switching modes – the vortex mode and the transverse mode. The mechanism by which the magnetic moments reverse within the nanotubes changes from one mode to another at some critical angle (θS) because they are presented with a more energetically favourable route. In other words, the dominant mode depends on the orientation of the tubes to the applied magnetic field. The critical switching angle corresponds to the orientation at which maximum coercivity is measured.