Effect of titanium addition on structure and magnetic properties of hard magnetic Nd-Fe-B nanocomposites
Marzena Spyra, Marcin Leonowicz*
Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland
e-mail: * mkl@inmat.pw.edu.pl
Quarterly No. 1, 2007 pages 9-12
DOI:
keywords: hard magnetic nanocomposites, Nd-Fe-B magnets, Nd-Fe-B-Ti magnets
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abstract Among a variety of magnetic materials, in early 90 s of the XXth century, appeared a new class of alloys called magnetic nanocomposites. Such materials combine hard and soft nanostructured magnetic phases giving rise for the property enhan¬cement. These materials take advantage of the correlations between the exchange and anisotropy energy. When the grains are small enough (for the Nd-Fe-B magnets this critical size is close to 30 nm) the exchange length L from crystallites oriented favourably with their easy magnetisation axes to the external magnetic field, covers substantial part of the neighbouring gra-ins. The exchange length is proportional to the exchange constant A and reversibly proportional to average anisotropy con-stant . The latter coefficient strongly depends on the grain size dz. Thus, in nanocrystalline magnets the exchange length covers substantial volume of neighbouring crystallites. As a result the material gains some proportion of magnetic anisotropy in a crystallographically isotropic state. In this study the nanocomposite Nd9Fe77-xB14Tix system, containing hard magnetic Nd2Fe14B phase and soft magnetic Fe phase, was investigated. The effect of titanium addition on the structure and magnetic properties was studied. It was found that 24 at. % Ti addition leads to substantial increase of the coercivity and maximum energy product maintaining the remanence unchanged. The highest properties: Jr = 0.81 T, JHc = 907 kA/m, (BH)max =
= 99 kJ/m3 have been achieved for the Nd9Fe73B14Ti4 alloy. This effect we attribute to the fine and homogeneous grain struc-ture in Ti containing materials. The hysteresis loops for Ti containing alloys are smooth and characteristic of a single phase alloys. The initial magnetization curve indicates change of the coercivity mechanisms giving rise to pinning of domain walls, which is caused by the reduction of the crystallite size.