Ordering and Grain Growth Kinetics in CoPt Thin Films
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ABSTRACT Ordering and grain growth have been studied in a 10 nm thick CoPt alloy film of equiatomic composition annealed in the temperature range 550-700"C by quantifying ordered domain size, volume fraction ordered, grain size, and grain size distribution. Ordering occurs by nucleation and growth of LIo ordered domains, with a mean size of 3 nm at 550*C and 19 nm at 700"C. The volume percent ordered shows a dramatic increase from h. For the 650°C annealed sample, where d = h, the assumption that half the volumes were spherical and half cylindrical was used. For grain size measurements, a series of bright field images of the same area were taken with the specimen holder tilt angle varying by 1 or 2 degrees. A single, high-contrast composite tracing of the grain boundaries was made from the image series for each specimen and grain size measurements were made by the image analyzer. Several areas of each specimen were sampled in this manner until between 1100-1700 grains were measured. By varying the magnification of the images and/or enlargements, grains below I nm diameter could be reliably measured.
Figure I - Selected area diffraction pattern of CoPt thin film as-deposited (a) and annealed 700" C
(b).
RESULTS
Analysis of the selected area electron diffraction pattern of the as-deposited film, presented in figure la, reveal that the film is composed of small polycrystals (broad rings) with the FCC crystal structure. Following ex situ anneals the grains are larger (sharp rings), as can be seen for the 700"C annealed sample in figure lb, and superlattice reflections in the diffraction pattern indicate the presence of LIo ordered phase. The diffraction pattern also shows the presence of strong fiber texturing in the annealed films when the sample is tilted. Conical dark field TEM images using the ordered reflection (110) showed numerous small regions of the LIo phase
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for the lower annealing temperatures. The ordered domains increased in size, while their number decreased as annealing temperature was increased, consistent with a first order transformation that proceeds by a nucleation and growth mechanism. Figures 2a and 2b show the dark field images for the 600 and 700°C annealed samples, respectively. From the profusion of ordered domains at the lower temperatures we expect that each grain will have at least one ordered domain, whereas at 700°C we observe evidence for grains which are free of ordered domains. Mean domain size increased from 3 nm for 550°C to 19 nm for 700°C anneals, as compared to mean grain size of 27 nm and 55 nm, respectively. The volume percent ordered, see table 1, showed a dramatic increase from 20 times the film thickness. On the other hand, we find that a significant portion of the grains resist shrinking leading to a wide distribution in grain sizes. CONCLUSIONS We have quantitatively determined ordered domain size, volume fraction ordered, grain size, and grain size distribution in CoPt thin films in as-deposited and annealed states and have related these factors to the coercivit
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