Effects of nonmagnetic overlay metals on coercivity of Sm2Fe17N3 magnet powders

The coercivity of Sm2Fe17N3 as a sintered magnet has remained at several % of its huge anisotropy field to date. Control of grain boundary properties has proven to be effective for improving coercivity of modern permanent magnets, and in powder metallurgical processes, overlaying the raw powder surface with a different material can be a powerful methodology for the grain boundary control. Recently, we established a versatile technique for coating powders with minimal surface oxides. In this study, we coated Sm2Fe17N3 powders with 20 nonmagnetic metals by using the technique, and investigated the effects of coating and heat treatment on the coercivity. A simple non-sintering heat treatment was applied to the coated powders to clarify the pure effects of coating excluding intergrain coupling issues. The whole process from powder preparation to coating and heat treatment was undertaken in gloveboxes with a low-oxygen atmosphere. Powder coating was performed by a DC magnetron sputtering method. The powder was continuously stirred during deposition to ensure uniform coating. The magnetic properties of the powders were measured as bonded magnets by using a vibrating sample magnetometer. It was found that a sputter-coating as thick as several nm increased coercivity with most coating elements in varying degrees. However, subsequent heat treatment at 500 °C caused further coercivity changes which depended strongly on the element. The mechanism of the former effect is supposed to be a universal one, whereas the latter seems to be related directly to the chemical details of each element.