Reentrance of spin-driven ferroelectricity through rotational tunneling of ammonium

TL;DR

This study demonstrates how rotational tunneling of ammonium ions can induce reentrant ferroelectricity in a molecular magnet, revealing a quantum mechanism to control magnetic and electric properties in soft materials.

Contribution

It provides the first experimental evidence linking rotational tunneling to reentrant ferroelectricity in a molecular compound, supported by theoretical calculations.

Findings

Reentrant ferroelectric phase observed under specific pressure conditions.

Transition from spin-driven ferroelectricity to paraelectricity with pressure.

Quantum fluctuations influence magnetic and electric phase stability.

Abstract

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.