HIGH-RESOLUTION STRUCTURE IMAGING OF STACKING SEQUENCES IN Y2FE17CX (0.6-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.6) MAGNETIC MATERIAL

High-resolution electron microscopy has been used to investigate planar crystal defects in magnetic materials of the type Y2Fe17C(x) (x greater-than-or-equal-to 0.6), which have the rhombohedral Th2Zn17 type of structure. The basal plane stacking sequences are imaged in the [100] orientation. Edge-on and inclined coherent twin boundaries are observed, the latter giving rise to a characteristic coincidence site pattern. The high-resolution images further reveal a high density of conservative and non-conservative antiphase boundaries, which can mutually combine into an antiphase boundary cross-link. Non-conservative antiphase boundaries are also seen to make cross-links between successive twin boundaries. In local areas, a periodic antiphase boundary structure with Y3Fe29C(x') stoichiometry is observed. Y2Fe17C(x) compounds resulting from an annealing treatment at low temperature exhibit an order-disorder phenomenon, based upon the rhombohedral structure. High-resolution structure imaging allows us to confirm the microdomain model that was previously derived on the basis of electron diffraction results only. However, some particular restrictions on the disordered stacking are further derived from the high-resolution images. In Gd2Fe17 compounds without C, an identical order-disorder phenomenon is observed depending on the annealing temperature. The relation of the observed defects with respect to coercivity mechanisms and magnetic anisotropy is discussed.