Simultaneous enhancement in coercivity and remanence of Nd 2 Fe 14 B permanent magnet by grain boundary diffusion process using NdH x Moonhee Choi a , Seungchan Cho b , Yeonghwan Song a , Sungkyun Park c, * , Yangdo Kim d, * a Opto & Mechatronics Solution Division, Samsung Electro-Mechanics, Suwon 443-743, Republic of Korea b Composites Research Center, Korea Institute of Materials Science, Changwon 642-831, Republic of Korea c Dept. of Physics, Pusan National University, Busan 609-735, Republic of Korea d Dept. of Materials Science and Engineering, Pusan National University, Busan 609-735, Republic of Korea article info Article history: Received 5 November 2014 Received in revised form 15 December 2014 Accepted 20 January 2015 Available online 4 February 2015 Keywords: Nd 2 Fe 14 B Permanent magnet Grain boundary diffusion HDDR NdH x abstract We have successfully developed a Dy-free grain boundary diffusion process with neodymium hydride (NdH x ) alloy to the permanent magnet Nd 2 Fe 14 B powders using hydrogenation e disproportionation e desorption e recombination (HDDR) method. All the diffusion treatments were performed at 700 e800  C for various annealing time under the high vacuum with rotating diffusion method that effec- tively control the abnormal grain growth. The coercivities of Dy-treated Nd 2 Fe 14 B powders were varied from 9.5 kOe to 13.2 kOe but the remanence was decreased to 8.1 kG (10% reduction) depending on dysprosium hydride (DyH x ) content and diffusion treated time. However, the coercivity and remanence of Dy-free diffusion treated powder have been increased to 12.2 kOe (28.5% enhancement) and 11.1 kG (22% enhancement) at the optimal diffusion treatment (800  C for 3 h), respectively. This unique simultaneous enhancement is to isolate the magnetic coupling between Nd 2 Fe 14 B grains by creating non- magnetic Nd grain boundaries and enhance the alignment of the Nd 2 Fe 1 4B hard magnetic phase, fabricated by optimal diffusion conditions. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Rare-earth based permanent magnets have been used in main application such as the motors of hybrid and electric vehicles and IT devices due to a higher maximum energy density product, (BH) max [1,2]. Among them, Nd 2 Fe 14 B ternary magnets are exhibiting excellent magnetic properties since, it can be produced into fine crystalline grains (ca 0.3 mm), closed to a single domain size using hydrogenation e disproportionation e desorption - recombination (HDDR) process [3], which was first reported by Takeshita and Nakayama [4]. However, Nd 2 Fe 14 B magnetic powders show lower coercivities after HDDR treatments because of the coupling effect that occurred by irregular coarse grains of the Nd 2 Fe 14 B hard magnetic phase. To overcome this huddle, the substitution of the heavy rare earth (HRE) elements such as Dy and Tb were incorpo- rated to form the diffusion barrier at the grain boundaries of Nd 2 Fe 14 B hard magnet phase [5]. As results, the coercivity increased substantially with the same microstructure, resulting in the suppression of the coupling effect as well as reversing the domain nucleation [6]. On the other hand, the remanence of diffusion treated Nd 2 Fe 14 B powders (in the case of Dy substitution) decreased to ~10% due to 1) the reduction of the Fe volume fraction in the Nd 2 Fe 14 B phase and 2) the antiparallel alignment of magnetic moments between Dy and Nd 2 Fe 14 B [7e9]. Furthermore, the cost of Dy alloys used for the substitution is too expensive to be used in the mass-production for the industrial applications. Therefore, it is desired to develop HRE free diffusion process without suffering magnetic properties. In this article, we have demonstrated that the diffusion process using NdH x alloy powders enhanced the coercivity as well as the remanence of HDDR treated Nd 2 Fe 14 B powders. The mechanism of magnetic properties enhancement was discussed based on the detailed both microstructure and magnetic characterization by transmission electron microscopy (TEM), energy dispersive spec- troscopy (EDS) and vibrating sample magnetometer (VSM). 2. Experimental procedures The raw Nd-Fe-B-Nb materials with Nd content of 27.5 wt% were prepared by strip casting method [10,11]. The raw materials were * Corresponding authors. E-mail addresses: psk@pusan.ac.kr (S. Park), yangdo@pusan.ac.kr (Y. Kim). Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/locate/cap http://dx.doi.org/10.1016/j.cap.2015.01.025 1567-1739/© 2015 Elsevier B.V. All rights reserved. Current Applied Physics 15 (2015) 461e467