On the relationship of high coercivity and L1(0) ordered phase in CoPt andFePt thin films

The microstructure and the room-temperature hysteretic magnetic properties of sputtered, 10 nm thin films of equiatomic binary alloys of CoPt and FePt were characterized using transmission electron microscopy (TEM) and a supe rconducting quantum interference device (SQUID) magnetometer. A transformat ion from an atomically disordered, face-centered-cubic structure to the L1( 0) ordered structure occurred during postdeposition annealing and was chara cterized using digital analysis of dark-field TEM images. The transformatio n was observed to follow first-order nucleation and growth kinetics, and th e ordered volume fraction transformed was quantified at numerous points dur ing the transformation. The ordered volume fraction was then compared to th e magnetic coercivity data obtained from the SQUID magnetometer. In contras t to the relationship most commonly described in the literature, that the h ighest coercivity corresponds to a two phase ordered/disordered mixture, th e maximum value for coercivity in this study was found to correspond to the fully ordered state. Furthermore, in samples that were less than fully ord ered, a direct relationship between ordered volume fraction and coercivity was observed for both CoPt and FePt. The proposed mechanism for the high co ercivity in these films is an increasing density of magnetic domain wall pi nning sites concurrent with an increasing fraction of ordered phase. (C) 19 99 American Institute of Physics. [S0021-8979(99)00720-3].