The dependence of in-plane residual stress (s) onTa is illustrated
in Fig. 4. For the as-deposited film, s was in tensile direction with a
magnitude of 0.93 GPa and almost remained for the sample after a
annealing of Ta ¼ 450 C, since no significant microstructural and
grain size alterations were observed. A minimum value of s was
found for the sample annealed at 500 C, which presumably results
from the nucleation of L10 ordering. The L10 ordering leads to a
volumetric expansion and atomic rearrangement [23,24], whichreleases the preformed tensile stress. A similar stress behavior was
also found in the FePt annealed using a conventional vacuum
furnace [17].With increasing Ta from 500 to 650 C, the s increases
from 0.73 to 1.65 GPa, while the average grain size increased from
about 5 to 8 nm, as shown in the inset of Fig. 4. Theoretically, this
grain growth can induce the tensile stress of FePt film by more than
3 GPa by annihilation of grain boundaries [25]. Our samples show a
mitigated value of about 1 GPa probably owing to the dynamic
stress relaxation during annealing [15,26]. This interpretation also
can be applied to the stress behavior of the samples as Ta was
increased from 700 to 800 C, where a measured tensile stress of
0.8 GPa is induced by a significant grain growth from about 9.1 to
25 nm. However, rather interestingly, the findings appear to be
inconsistent with that of the sample with increasing Ta from 650 to
700 C. The fact shows that a huge tensile stress of 0.9 GPa is
induced without extensive grain growth. The origin for this tensile
stress generation is not fully understood yet. This might be correlated
to the surface oxidation between L10 FePt phase and residual
atmosphere in the RTA chamber during annealing [10]. In addition,
it was suggested that the oxidation of capping layer might result in
the in-plane tensile stress on the underlying FePt layer [27]. Surface
oxidation causes a volumetric expansion of FePt lattice, which in
turn induces a compressive surface stress since the oxide FePt layer
is bonded by the unoxided FePt film. As a counterpart, the unoxided
FePt part is expected to bear a tensile stress induced by surface
oxidation layer