Room Temperature Ferromagnetism in D-D Neutron Irradiated ZnO Single Crystals

Abstract

Room temperature ferromagnetism (RTFM) was observed in unirradiated ZnO single crystal owing to zinc vacancy (VZn) defects. D-D neutron irradiation was used to regulate the defect concentration and types avoiding interference of impurity elements. D-D neutron mainly collides elastically with ZnO single crystals, producing VZn, oxygen vacancy (VO), and other point defects, the density and kind of defect associated with neutron irradiation fluence. It can be found the saturation magnetization (Ms) first increases and then decreases with the irradiation fluence increases, meaning there is an optimum value for VZn concentration. When the neutron fluence is greater than 5× 109 n/cm2, more unstable VZn combined with VO to form zinc-oxygen divacancy (VZnO), indicating neutron irradiation changes the defect types. VZnO induces a smaller magnetic moment; thus, the Ms decreased. To further verify the origin of RTFM, the CASTEP module was employed to calculate the magnetic and structural properties of native vacancy defects in ZnO. VZn induces 2.12 μB magnetic moment, and only VZnO formed by the nearest VZn and VO has 2.09 μB magnetic moment, while 3VO cluster introduces a magnetic moment of $\sim $1 μB. Combined experiment and CASTEP calculation results, VZn is the most likely origin of RTFM.