Core loss in magnetic rings made of Fe16N2-like iron nitride powder

Abstract

The soft magnetic properties of α-Fe fine powders were investigated before and after low temperature ammonolysis together with the effect of alumina presence in the grain boundaries to clarify the contribution of these during the preparation process. The crystallite size was controlled to be approximately 50 nm and 30 nm before and after ammonolysis, respectively, by changing the preparation conditions to eliminate the effect of particle size on the magnetic properties. The ammonolysis product was a mixture of approximately 50 wt% of an α″-Fe16N2-like compound (hereafter denoted as “α″-Fe16N2”) with residual α-Fe, which had the largest magnetization among the ammonolysis products reported in our previous works. The core loss of magnetic rings made from this powder was first studied after sealing in Delrin® capsules to avoid decomposition by oxidation and the presence of humidity. The core loss was decreased to one third and the permeability was decreased to approximately 80% by the ammonolysis process, independent of the presence of alumina at the grain boundaries. The hysteresis contribution in the core loss was significantly decreased due to enhanced magnetic domain interaction in the anisotropic crystal lattice of “α″-Fe16N2”. The covalent chemical bonds of Fe-N caused the compound to be more electrically resistive and also resulted in a slight decrease of the eddy current loss contribution. Alumina precipitation at the grain boundaries caused a doubling of core loss increase and an increase of the permeability to approximately 130% because the magnetic interaction between grains was reduced by the presence of alumina in the composite products. Combing the decreased core loss by nitridation and the increased magnetic permeability by alumina precipitation will be useful to develop a new soft magnetic material.