  Hopfions are three-dimensional (3D) topological textures characterized by the
integer Hopf invariant $Q_H$. Here, we present the realization of a
zero--field, stable hopfion spin texture in a magnetic system consisting of a
chiral magnet nanodisk sandwiched by two films with perpendicular magnetic
anisotropy. The preimages of the spin texture and numerical calculations of
$Q_H$ show that the hopfion has $Q_H=1$. Furthermore, another non-trivial state
that includes a monopole--antimonopole pair (MAP) is also stabilized in this
system. By applying an external magnetic field, hopfion and MAP states with the
same polarization can be switched between each other. The topological
transition between the hopfion and the MAP state involves a creation
(annihilation) of the MAP and twist of the preimages. Our work paves the way to
study non-trivial 3D topological spin textures and stimulates more
investigations in the field of 3D spintronics.
