Effects of doping on the magnetic properties of La-Ce misch-metal based (La,Ce)2Fe14B melt spun ribbons: 2:14:B phases for cost effective permanent magnets.

Rare earth (RE)-based permanent magnets with high energy product are at the core of modern technology; they enable efficient electric engines and power generation from renewable sources. The urgent issue facing these materials is the resource risk. RE based ores can be efficiently retrieved only in a limited number of extraction sites, making such raw materials vulnerable to wild cost fluctuations. On top of this, a growing use of RE in various hi-tech applications is expected to rapidly increase their market demand. In this contribution we will report on RE intermetallic MM2Fe14B compounds, in which a blend of 66 at.% Ce and 33 at.% La Misch-Metal (MM) has been used as the RE element. In this way it is possible to maintain the 2:14:i type of crystal structure, which provides a strong lattice contribution to the onset of uniaxial magnetic anisotropy, while using the least expensive and most abundant REs. Melt spun ribbons have been prepared with different doping elements, both magnetic and non-magnetic (Cu, Co, Zr, Ga). Compositions with excess MM prevented the formation of the competitive Ce2Fe soft magnetic phase. Nanocrystalline and amorphous ribbons have been cast by melt spinning at different wheel speeds; heat treatments in the range 670-1070 K ensured homogenization and crystal growth. Heat treated samples show hard magnetic character, with saturation magnetization and coercivity dependent on composition: Cu doping is detrimental on both the coercivity and saturation magnetization. Both Zr and Co addition improve saturation magnetization at the expenses of coercivity, while Ga doping enhance both coercivity and magnetization. The maximum energy product in ribbons increases from 4.3 MGOe for (Ce12La6.5)Fe75.5B6 up to 5.5 MGOe for (Ce12La6.5)Fe74.5GaB6. We will present a thorough magnetic and structural characterization, including magnetic anisotropy measurements directly performed on polycrystalline ribbons by means of the singular point detection technique in pulsed magnetic field up to 20 T [1,2]. The results and possible improvement strategies will be discussed by taking into account intrinsic and extrinsic effects. Work was supported by Ford Motor Company and by the EU funded project PIRSES-GA-2011-295190 [1] G. Asti and S. Rinaldi Phys. Rev. Lett. 28, 1584-15