Some properties of ultrafine ferromagnetic particles

In an attempt to produce hard magnetic material, ultrafine particles of iron-cobalt alloys and nickel have been produced by an evaporation-condensation technique. To investigate the magnetic properties of these particles, a Faraday Balance Magnetometer was constructed, A Pulsed Field Magnetometer was also employed, These were used to measure magnetization and coercively, and to produce hysteresis loops for random assemblies of particles. The d.c. static and demagnetization remanences have also been measured X-ray diffraction techniques were used to find the structure and lattice parameters of the particles. The morphology was studied with the aid of an electron microscope. The observed values of magnetization can be explained in terms of a core of ferromagnetic material surrounded by a surface oxide layer. For cobalt this layer is antiferromagnetic and for iron it is ferrimagnetic. The results of the electron microscopy show that the particles are almost perfectly spherical and their sizes lie within a normal distribution curve. The peak in the distribution falls at approximately 400Å. The' particles show a strong tendancy to chain together. This is believed to be due to magnetic attraction. All the samples show a low remanence to saturation ratio, typically 0.20, and a difficulty to saturate. The resulting hysteresis loops can be explained in terms of fanning, coherent rotation and multi domain mechanisms. It is believed that some of the particles are genuinely single domain, and that their magnetization reverses coherently. In addition there are particles large enough to contain more than one domain. The remaining reversal mechanism which is fanning, was proposed by Jacobs and Bean for a chain of spheres, The above model would appear to be further supported by the results of electron microscopy.