  Heavy metal-ferromagnet bilayer structures have attracted great research
interest for charge-to-spin interconversion. In this work, we have investigated
the effect of the permalloy seed layer on the Ta polycrystalline phase and its
spin Hall angle. Interestingly, for the same deposition rates the crystalline
phase of Ta deposited on Py seed layer strongly depends on the thickness of the
seed layer. We have observed a phase transition from $\alpha$-Ta to
($\alpha$+$\beta$)-Ta while increasing the Py seed layer thickness. The
observed phase transition is attributed to the strain at interface between Py
and Ta layers. Ferromagnetic resonance-based spin pumping studies reveal that
the spin-mixing conductance in the to ($\alpha$+$\beta$)-Ta is relatively
higher as compared to the to $\alpha$-Ta. Spin Hall angles of to $\alpha$-Ta
and to ($\alpha$+$\beta$)-Ta are extracted from inverse spin Hall effect (ISHE)
measurements. Spin Hall angle of the to ($\alpha$+$\beta$)-Ta is estimated to
be $\theta$_SH=-0.15 which is relatively higher than that of to $\alpha$-Ta.
Our systematic results connecting the phase of the Ta with seed layer and its
effect on the efficiency of spin to charge conversion might resolve ambiguities
across various literature and open up new functionalities based on the growth
process for the emerging spintronic devices.
