Investigation of Magnetic Properties of Ni-B Nanotubes at Low Temperatures

We have investigated the magnetic properties of Ni_1-xB_x nanotubes as a function of composition, temperature, and magnetic fields. The nanotubes were prepared by electroless deposition in polycarbonate membranes using dimethylamine borane as a reducing agent. The nanotube arrays are formed with a length of 20 μm, an outer diameter of 400 nm, and an inner diameter of 110 nm. The composition, x = 12, 20, 27, and 35, was varied by controlling the pH during the preparation process. A range of low temperature magnetic measurements show both superparamagnetic and ferromagnetic behaviors. The saturation magnetization dependence on temperature cannot be directly represented by Bloch's law. However, we introduce a modified relationship taking into account Bloch's law and Langevin's equation to fit the experimental data from 2 to 380 K for samples with 27 at.% of B. Good agreement between the model and data is observed for this composition. The mathematical model is the summation of superparamagnetism and ferromagnetism.