  Aguilar introduced isotropic almost complex structures $J_{\delta , \sigma}$
on the tangent bundle of a Riemannian manifold $(M, g)$. In this paper, some
results will be obtained on the integrability of these structures. These
structures with the Liouville 1-form define a class of Riemannian metrics
$g_{\delta , \sigma}$ on $T M$ which are a generalization of the Sasaki metric.
Moreover, the notion of a harmonic unit vector field is introduced with respect
to these metrics like as the Sasaki metric and the necessary and sufficient
conditions for a unit vector field to be a harmonic unit vector field are
obtained.
