Coercivity Changes in Sm2Fe17N3 Powders by Direct Coating with Nonmagnetic Metals on Oxide-free Powder Surfaces

Abstract

Coercivity of Sm2Fe17N3 as a sintered magnet has so far been only a few percent of the huge anisotropic magnetic field. Controlling grain boundary properties is a promising strategy to improve the coercivity. In powder metallurgical processes, coating raw powders with different materials can be a powerful methodology for grain boundary control. However, if the powder surface covered with the oxide film is coated, the coating phase and the magnetic phase do not contact with each other directly, so that a sufficient effect may not be obtained. Furthermore, iron oxides contained in the surface oxide film undergo a redox reaction with Sm2Fe17N3 at elevated temperatures, precipitating soft magnetic iron phase, which causes coercivity deterioration. For these reasons, the coating should be applied to surfaces without oxide film. In this study, first we have established a versatile technique for coating powders with an oxide-free surface. By using the technique, we coated Sm2Fe17N3 powders with 20 non-magnetic metals, and investigated the effects on the coercivity. A simple heat treatment was applied to the coated powders instead of sintering, to clarify pure effects of coating excluding inter-grain coupling issues. Sm2Fe17N3 fine powders were prepared by jet-milling coarse powders. Coating was performed by arc-plasma deposition (APD) and DC magnetron sputtering. The powder was continuously stirred during deposition to ensure uniform coating. The whole process of the sample preparation was undertaken in a low-oxygen atmosphere. After the heat treatments, the sample powders were subjected to evaluations. Magnetic properties of the powders were measured as bonded magnets by using a vibrating sample magnetometer. Scanning electron microscopy with energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy (XPS) suggested that an arc-plasma deposited Zn layer on Sm2Fe17N3 powder was highly uniform and had nearly 100 % coverage. Moreover, even the particles inside an agglomerate were fully coated with Zn, which suggested the agglomerates dynamically changed its constituent particles during the coating. An XPS depth-profile analysis confirmed that the powder had an oxide-free direct metal-metal interface between the coating layer and the main phase, in case of coating on a Sm2Fe17N3 powder prepared in the low-oxygen atmosphere. The effects of coating on the coercivity were evaluated first with Zn, Ti, and Al as the coating materials and both with APD and DC sputtering as the coating methods. With all the three coating metals, the sputter-coated powders showed higher coercivity than the raw powders. The APD powders had lower coercivity than the sputter-coated powders, and in the case of Ti, coercivity was even lower than that of the raw powder. Transmission electron microscopy and nano-beam electron diffraction suggested that the high energy particles deposited by APD damaged the magnetic phase. We therefore exclusively used the sputtering method for the rest of the experiments. It was found that sputter-coating as thick as several nm makes the coercivity increase, almost regardless of the coating element. However, further coercivity changes resulted from subsequent heat treatment at 500 °C depended strongly on the element. The mechanism of the former phenomenon is supposed to be a common and universal one. The latter one seems, on the other hand, to be related directly to the chemical details of each element. We are currently conducting research to clarify the factors controlling the coercivity change by coating and heat treatment.