Atom probe study on the bulk nanocomposite SmCo/Fe permanent magnet produced by ball-milling and warm compaction X.Y. Xiong a,b,n , C.B. Rong c , S. Rubanov d , Y. Zhang e , J.P. Liu c a Monash Centre for Electron Microscopy, Monash University, Vic. 3800, Australia b Department of Materials Engineering, Monash University, Vic. 3800, Australia c Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA d Electron Microscopy Unit, Bio21 Institute, University of Melbourne, Vic. 3052, Australia e Division of Materials Science and Engineering, Ames Laboratory, Iowa State University, Ames, IA 50011, USA article info Article history: Received 12 May 2011 Received in revised form 11 June 2011 Available online 29 June 2011 Keywords: SmCo permanent magnet Nanocomposite Microstructure Atom probe tomography TEM Ball-mill abstract The microstructure and compositions of the bulk nanocomposite SmCo/Fe permanent magnet were studied using transmission electron microscopy and 3-dimensional atom probe techniques. The excellent magnetic properties were related to the uniform nanocomposite structure with nanometer a-Fe particles uniformly distributed in the SmCo phase matrix. The a-Fe phase contained  26 at% Co, and the SmCo phase contained 19 at% Fe, confirming that the interdiffusion of Fe and Co atoms between the two phases occurred. The formation of the a-Fe(Co) phase explained why the saturation magnetization of the nanocomposite permanent magnet was higher than that expected from the original pure a-Fe and SmCo 5 powders, which enhanced further the maximum energy product of the nanocomposite permanent magnet. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanocomposite permanent magnets have been the focus of research for two decades with a view to producing more powerful and more economic permanent magnets [1,2]. The extensive research work has produced some successful results [3–7], although their maximum energy products were not as high as predicted by the modeling work [8,9]. As SmCo-based permanent magnets have the highest coercivity and highest Curie tempera- ture among all permanent magnets [10], the nanocomposite SmCo/Fe permanent magnets are expected to have better mag- netic properties than the nanocomposite NdFeB/Fe permanent magnets, especially for the high temperature applications. How- ever, for bulk material, how to make an appropriate nanocompo- site structure with the a-Fe phase present in nanometer scale and uniformly distributed in the SmCo phase, as suggested by the modeling work [8,9], remains very challenging. Some research groups tried to use powder metallurgy and sintering to produce bulk nanocomposite SmCo/Fe permanent magnets, but failed to achieve good magnetic properties [11,12]. Recently we reported a successful work on fabricating bulk nanocomposite SmCo/Fe permanent magnets, which used ball-milling and warm compac- tion of powder mixtures [5]. The maximum energy product was increased more than double compared with that of the single- phase counterparts. The results demonstrated a promising way to produce high temperature and high performance permanent magnets at low cost. The purpose of this paper is to report in detail the micro- structure and composition distributions in the high performance nanocomposite SmCo/Fe permanent magnet, together with some issues of the characterization technique. The composition of the nanometer scale particles has been determined quantitatively using a 3-dimensional atom probe (3DAP) technique. In combin- ing with the transmission electron microscopy (TEM) results, the excellent magnetic properties are well understood by relating to the unique uniform nanocomposite structure achieved in this alloy. 2. Experimental Fully dense bulk nanocomposite permanent magnet samples were prepared by ball-milling and warm compaction of hard magnetic and soft magnetic powders. The raw powder materials, commercial SmCo 5 and a-Fe powders with particle sizes of 1–20 mm, were mixed as per 80 wt% SmCo 5 and 20 wt% a-Fe in a high-energy ball-milling machine and milled for 4 h, followed by Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.06.035 n Corresponding author at: Monash Centre for Electron Microscopy, Monash University, Vic. 3800, Australia. Tel.: þ61 3 99051727; fax: þ61 3 99053600. E-mail address: xiangyuan.xiong@mcem.monash.edu.au (X.Y. Xiong). Journal of Magnetism and Magnetic Materials 323 (2011) 2855–2858