Ferrimagnetism in stable non-metal covalent organic framework

TL;DR

This study reports the synthesis of a stable, purely organic covalent organic framework exhibiting room-temperature ferrimagnetism, a rare magnetic property in non-metal organic materials, potentially linked to flat-band electron correlation effects.

Contribution

First demonstration of room-temperature ferrimagnetism in a stable, purely organic covalent organic framework synthesized via Schiff base chemistry.

Findings

Exhibits strong magnetic hysteresis at 300 K.

Shows large magnetic susceptibility (chi/mol) up to 0.028 at 300 K.

Supports ferrimagnetism based on magnetic measurements and Curie temperature analysis.

Abstract

We synthesized a pure organic non-metal crystalline covalent organic framework TAPA-BTD-COF by bottom-up Schiff base chemical reaction. And this imine-based COF is stable in aerobic condition and room-temperature. We discovered that this TAPA-BTD-COF exhibited strong magneticity in 300 K generating magnetic hysteresis loop in M-H characterization and giant chimol up to 0.028. And we further conducted zero-field cooling and field-cooling measurement of M-T curves. The as-synthesized materials showed a large chi/mol up to 0.028 in 300 K and increasing to 0.037 in 4.0 K with 200 Oe measurement field. The TAPA-BTD-COF 1/chimol~T curve supported its ferrimagnetism, with an intrinsic delta temperature as -33.03 K by extrapolating the 1/chimol~T curve. From the continuously increasing slope of 1/chimol~T, we consider that this TAPA-BTD-COF belongs to ferrimagnetic other than antiferromagnetic materials. And the large chimol value 0.028 at 300 K and 0.037 at 4.0 K also supported this, since common antiferromagnetic materials possess chimol in the range of 10-5 to 10-3 as weak magnetics other than strong magnetic materials such as ferrimagnetics and ferromagnetics. Since this material is purely non-metal organic polymer, the possibility of d-block and f-block metal with unpaired-electron induced magnetism can be excluded. Besides, since the COF does not involve free-radical monomer in the processes of synthesis, we can also exclude the origin of free-radical induced magnetism. According to recent emerging flat-band strong correlated exotic electron property, this unconventional phenomenon may relate to n-type doping on the flat-band locating in the CBM, thus generating highly-localized electron with infinite effective mass and exhibiting strong correlation, which accounts for this non-trivial strong and stable ferrimagneticity at room-temperature and aerobic atmospheric conditions.