  Quantum effects fundamentally engender exotic physical phenomena in
macroscopic systems, which advance next-generation technological applications.
Rotational tunneling that represents the quantum phenomenon of the librational
motion of molecules is ubiquitous in hydrogen-contained materials. However, its
direct manifestation in realizing macroscopic physical properties is elusive.
Here we report an observation of reentrant ferroelectricity under low pressure
that is mediated by the rotational tunneling of ammonium ions in molecule-based
(NH$_4$)$_2$FeCl$_5 \cdot$H$_2$O. Applying a small pressure leads to a
transition from spin-driven ferroelectricity to paraelectricity coinciding with
the stabilization of a collinear magnetic phase. Such a transition is
attributed to the hydrogen bond fluctuations via the rotational tunneling of
ammonium groups as supported by theoretical calculations. Higher pressure lifts
the quantum fluctuations and leads to a reentrant ferroelectric phase
concomitant with another incommensurate magnetic phase. These results
demonstrate that the rotational tunneling emerges as a new route to control
magnetic-related properties in soft magnets, opening avenues for designing
multi-functional materials and realizing potential quantum control.
