Irradiation damage in magnetic insulators

Using high energy heavy ion irradiation, the damage induced in magnetic insulators (Y3Fe5O12, BaFe12O19, SrFe12O19, NiFeO4, MgFe2O4, ZnFe2O4, Fe3O4) in the electronic stopping power (dE/dx) regime is studied. The amorphization cross section Ap is extracted from the paramagnetic fraction observed on Moessbauer spectra. Electronic stopping power threshold for damage creation appears. The damage efficiency ε = A/(dE/dx) is calculated and can be fitted by a general amorphization law ε = εmax(1-exp(-k(dE/dx)4)). The damage morphology has been correlated to the damage efficiency. Spherical extended defects appear for low values of ε at low values of dE/dx. When increasing dE/dx and consequently ε, the spherical defects overlap to give discontinuous cylindrical defects. Then for the higher values of dE/dx, the defects are continuous cylinders of amorphous phase. The change of the physical properties induced by the irradiation has been studied. Depending on the shape of the defects, the evolution of the electrical conductivity and the change in the orientation of the hyperfine magnetic field are different. Specific crystallographic sites in BaFe12O19 are more sensitive to the irradiation than others. Local order in the new amorphous phase is determined using X-ray absorption at Fe K-edge and Moessbauer spectroscopies. The creation of magnetization is observed in irradiated ZnFe2O4 which initially shows only a paramagnetic behavior at room temperature. (orig.)