Investigations on the Magnetic Properties of High-Coercivity Nd 40 Fe 30 Co 15 Al 10 B 5 Bulk Amorphous Alloys
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Investigations on the Magnetic Properties of High-Coercivity Nd40 Fe30 Co15 Al10 B5 Bulk Amorphous Alloys L. Bessais1,2 , C. Dj´ega-Mariadassou1 , and N. X. Phuc3 . 1 LCMTR, UPR209, CNRS, 2/8 rue Henri Dunant, B.P. 28 F-94320 Thiais, France 2 IUFM de Cr´ eteil, F-94861 Bonneuil sur Marne, France, 3 Institute of Materials Science, NCST, Hoang Quoc Viet Road, Viet Nam. ABSTRACT Nd40 Fe30 Co15 Al10 B5 bulk amorphous prepared by high energy milling shows a coercivity of 8.1 kOe with a Curie temperature of 645 K. The controlled nanocrystallization enhances the coercivity to 20 kOe and the remanence ratio is equal to 0.59. The coexistence of two crystalline magnetic phases, ferromagnetic Nd2 (Fe,Co,Al)14 B and antiferromagnetic Nd6 (Fe,Co,Al)14 are revealed by x-ray diffraction, high-resolution transmission electron microscopy, magnetization measurements, and M¨ossbauer spectrometry. The grain size for optimal magnetic properties is around 30 nm. The nucleation process may play a leading role in the high magnetic behavior. INTRODUCTION Rare-earth based R-Fe-Al bulk amorphous alloys have attracted much attention mainly due to their large glass-forming ability, their room temperature hard magnetic properties coupled to high fracture strength and good corrosion resistance. After the first investigation of A. Inoue et al. [1], lot of works have been devoted to Nd-Fe-Al alloys obtained by melt-spinning [2] or suction casting [3] with various compositions. The substitution of B for Al leads to an enhancement of the hard magnetic properties via Nd2 Fe14 B [3]. More recently, Kumar et al. [4] obtained with mold-cast samples Nd40 Fe40 Co5 Al8 B8 a coercivity of 3.37 kOe at RT, in contrast, their mechanically alloyed amorphous samples are soft magnetic with maximum coercivity of 0.13 kOe. It is therefore attractive to reinvestigate the high-energy milling route and perform a controlled nanocrystallization treatment to favor the nanostructure responsible for improved high coercivity and remanence. Moreover, mechanical alloying yields a large amount of homogeneous sample. The subsequent annealing offers the possibility to refine the nanostructure by a judicious choice of the annealing temperature combined to an adequate duration [5]. Our objective is to create an amorphous state by mechanical alloying and perform a controlled in-situ nanocrystallization of the amorphous as-milled alloys in order to improve their magnetic properties required in many applications especially for magnetic recording media and permanent magnets. In the present work are presented results from x-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) combined with energy dispersive x-ray analysis (EDX), Curie temperature (TC ) and coercivity (HC ) measurements coupled with M¨ossbauer spectroscopy, on transition metal rich composition Nd-Fe-Co-Al-B.
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EXPERIMENTAL DETAILS The samples Nd40 Fe30 Co15 Al10 B5 were prepared by high-energy ball milling. Milling was performed during 5h under high purity Ar atmosphere with a
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