Metal-Free Room-Temperature Ferromagnetism

TL;DR

This paper introduces a novel design strategy for creating stable, metal-free ferromagnetic 2D organic crystals operable at room temperature, with potential applications in spintronics and quantum technologies.

Contribution

It presents a mix-topology design approach to induce strong ferromagnetism in purely organic 2D materials, overcoming previous limitations of antiferromagnetic coupling.

Findings

Designed 32 stable ferromagnetic organic 2D crystals.

Achieved record-high magnetic coupling of 127 meV.

Demonstrated Curie temperatures above 550 K.

Abstract

Achieving robust room-temperature ferromagnetism in purely organic 2D crystals remains a fundamental challenge, primarily due to antiferromagnetic (AFM) coupling mediated by {\pi}-electron superexchange. Here, we present a mix-topology design strategy to induce strong ferromagnetic (FM) coupling in metal-free 2D systems. By covalently connecting radical polyaromatic hydrocarbon monomers (also referred to as nanographenes) with distinct sublattice topologies, this approach rationally breaks inversion symmetry and enables selective alignment of majority spins across the extended network, giving rise to metal-free ferromagnetism. Based on this strategy, we designed a family of 32 organic 2D crystals featuring spin-1/2 and mixed spin-1/2-spin-1 honeycomb lattices. Systematic first-principles calculations reveal that these materials are robust FM semiconductors with tunable spin-dependent bandgaps ranging from 0.9 to 3.8 eV. Notably, we demonstrate record-high magnetic coupling of up to 127 meV, spin-splitting energies exceeding 2 eV, and Curie temperatures surpassing 550 K, indicating thermal stability well above room temperature. The microscopic origin of the strong FM exchange stems from enhanced spin-orbital overlap and dominant direct exchange, while AFM superexchange is effectively suppressed. Our findings establish a generalizable design principle for realizing robust metal-free FM semiconductors and open new avenues for developing flexible and biocompatible magnets for next-generation spintronic and quantum technologies.