Metal-coated Sm2Fe17N3 magnet powders with an oxide-free direct metal-metal interface

Abstract

We successfully prepared Sm2Fe17N3 magnet powders uniformly coated with non-magnetic metals without oxides in the interface between the magnet phase and the coating layer. The whole process from powder preparation by jet milling to coating was undertaken in a low-oxygen atmosphere. Powder coating was investigated using two different dry thin-film preparation methods, arc-plasma deposition (APD) and DC magnetron sputtering. The powder was continuously stirred during coating in order to ensure the uniformity of the coating layer. Scanning electron microscopy with energy dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy demonstrated that an arc-plasma deposited Zn layer on Sm2Fe17N3 powder had nearly 100% coverage and an oxide-free direct metal-metal interface with the underlying magnet phase. The effects of metal-coatings and coating methods on the coercivity of the magnet powder were evaluated with Zn, Ti, and Al as the coating materials and with APD and DC magnetron sputtering as the coating methods. With all three coating metals, the sputter-coated Sm2Fe17N3 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 with EDS and nano-beam electron diffraction suggested that the high energy particles deposited by APD damaged the magnetic phase. This study demonstrated that nanometer-thick coatings of non-magnetic metals without oxides in the interface with the magnet phase have the effect of enhancing the coercivity of Sm2Fe17N3 powders, provided the proper coating method is selected.