Self-nanoscaling of the soft magnetic phase in bulk SmCo/Fe nanocomposite magnets

Fabrication of bulk nanocomposite materials, which contain a magnetically hard phase and a magnetically soft phase with desired nanoscale morphology and composition distribution has proven to be challenging. Here we demonstrate that SmCo/Fe(Co) hard/soft nanocomposite materials can be produced by distributing the soft magnetic a-Fe(Co) phase particles homogenously in a hard magnetic SmCo phase matrix through a combination of high-energy ball milling and a warm compaction. Severe plastic deformation during the ball milling results in nanoscaling of the soft phase with size reduction from micrometers to *15 nm. Up to 35% of the soft phase can be incorporated into the composites without coarsening. This process produces fully dense bulk isotropic nanocomposite materials with remarkable energy-product enhancement (up to 300%) owing to effective inter-phase exchange coupling. Introduction Despite tremendous efforts dedicated to searching for new permanent magnetic materials, no single compound or alloy has yet been discovered which possesses all the properties of an ideal permanent magnet. These properties include high magnetization, high Curie temperature, and high anisotropy. It is also desirable that these excellent properties are achieved at a low cost. Sm–Co intermetallic compounds (including SmCo5, Sm2Co17, and Sm2Co7) have the highest magnetocrystalline anisotropy (*10 erg/ cc) and the highest Curie temperatures (up to 1190 K) among all the permanent magnets discovered to date [1]. For these reasons, Sm–Co based magnets are the permanent magnets of choice for high temperature applications. One outstanding challenge of the currently available Sm–Co based magnets is their relatively low saturation magnetization, Ms, as compared to Nd–Fe–B based magnets. Consequently utilizaton of Sm–Co has been restricted in high power density applications, such as wind power turbines and electric motors in hybrid vehicles. Since Nd–Fe–B based magnets have a relatively low Curie temperature (*580 K), utilization of these materials in high power density applications requires cooling, which in turn reduces the overall system efficiency. In order to take advantage of the considerably higher operating temperature attainable with Sm–Co magnets, the Ms value of this composite needs to be raised to boost the energy product, (BH)max, the figure of merit of a permanent magnet. To reduce the materials costs, it is desirable to lower the concentration of Sm and Co in the magnetic material since the elements are expensive. One solution to both the Ms and the material cost problems is to fabricate nanocomposite magnets that contain a magnetically hard phase (e.g., Sm–Co) and a C. Rong Y. Zhang N. Poudyal J. P. Liu (&) Department of Physics, University of Texas at Arlington, Arlington, TX 76019, USA e-mail: [email protected] Y. Zhang M. J. Kramer Division of Materials Science and Engineering, Ames Laboratory, Iowa State University, Ames, IA 50011, USA I. Szlufarska Department of Materials Science & Engineering, University of Wisconsin at Madison, Madison, WI 53706, USA R. J. Hebert Department of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, USA 123 J Mater Sci (2011) 46:6065–6074 DOI 10.1007/s10853-011-5568-7