Dynamically fluctuating electric dipole moments in fullerene-based magnets

Area of Science:

• Condensed Matter Physics
• Materials Science
• Magnetism and Dielectric Properties

Background:

• Fullerene-based magnets, such as tetra-kis-(dimethylamino)-ethylene-C60 (TDAE-C60), are known for their magnetic properties.
• Previous studies observed no structural differences between the ferromagnetic (α-phase) and antiferromagnetic (α'-phase) polymorphs of TDAE-C60 at room temperature.
• Understanding the interplay between magnetic and electric properties in molecular materials is crucial for developing advanced electronic devices.

Purpose of the Study:

• To provide direct experimental evidence for the existence of a permanent electric dipole moment in both the α and α' phases of TDAE-C60.
• To investigate the origin of the permanent electric dipole moment in these fullerene-based magnetic materials.
• To explore the differences in dielectric responses between the two polymorphs and their implications for molecular ordering and local environments.

Main Methods:

• Utilized dielectric measurements to directly detect and quantify the permanent electric dipole moment in both crystal phases of TDAE-C60.
• Analyzed the dielectric responses at room temperature and during phase transitions (freezing process).
• Correlated dielectric behavior with structural and magnetic properties, including molecular displacement and orientational ordering.

Main Results:

• Direct evidence for a permanent electric dipole moment was established in both the ferromagnetic α-phase and the antiferromagnetic α'-phase of TDAE-C60.
• Distinct differences in dielectric responses were observed between the two polymorphs at room temperature and during freezing, despite no previously observed structural differences.
• The permanent electric dipole is proposed to arise from the specific pairing of TDAE molecules with surrounding C60 molecules.

Conclusions:

• The observed differences in dielectric responses indicate distinct local environments around the molecules in the two polymorphs.
• The ferromagnetism in the α-phase is attributed to homogeneous molecular displacement and orientational ordering, linked to the observed dipole.
• The formation of distinct phases is discussed in relation to different rotational states of Jahn-Teller distorted C60 molecules.