Neutron-Irradiation Induced Magnetization and Persistent Defects at High Temperatures in Graphite

TL;DR

This study investigates how neutron irradiation induces persistent defects and ferromagnetism in graphite at high temperatures, revealing defect types and magnetic behavior through experiments and ab-initio calculations.

Contribution

It provides new insights into defect structures and magnetic properties of neutron-irradiated graphite, combining experimental and theoretical approaches.

Findings

Defects in irradiated graphite are not fully annealed even above 653 K.

Irradiated graphite exhibits ferromagnetism at room temperature.

Magnetism is localized on 2-coordinated carbon atoms near vacancies.

Abstract

Structural as well as magnetization studies have been carried out on graphite samples irradiated by neutrons over 50 years in the CIRUS research reactor at Trombay. Neutron diffraction studies reveal that the defects in irradiated graphite samples are not well annealed and remain significant up to high temperatures much greater than 653 K where the Wigner energy is completely released. We infer that the remnant defects may be intralayer Frenkel defects, which do not store large energy, unlike the interlayer Frenkel defects that store the Wigner energy. Magnetization studies on the irradiated graphite show ferromagnetic behavior even at 300 K and a large additional paramagnetic contribution at 5 K. Ab-initio calculations based on the spin-polarized density-functional theory show that the magnetism in defected graphite is essentially confined on to a single 2-coordinated carbon atom that is located around a vacancy in the hexagonal layer.