  The aim of this paper is to define a chain level refinement of the
Batalin-Vilkovisky (BV) algebra structure on the homology of the free loop
space of a closed, oriented $C^\infty$-manifold. For this purpose, we define a
(nonsymmetric) cyclic dg operad which consists of "de Rham chains" of free
loops with marked points. A notion of de Rham chains, which is a certain hybrid
of the notions of singular chains and differential forms, is a key ingredient
in our construction. Combined with a generalization of cyclic Deligne's
conjecture, this dg operad produces a chain model of the free loop space which
admits an action of a chain model of the framed little disks operad, recovering
the string topology BV algebra structure on the homology level.
