The effect of neutron irradiation on remanent magnetization in multidomain iron and kamacite

The effects of neutron irradiation on remanent magnetization in multidomain iron and kamacite (3wt% Ni) samples of varying initial coercivities were investigated in order to determine whether cosmic ray exposure significantly affects the natural remanent magnetization (NMR) of lunar samples and chondritic meteorites. Irradiation to 1017 fast neutrons/cm2 increased bulk coercive force (Hc) by an average of 15%. Annealing for 1 hour at 200-300°C returned Hc to its pre-irradiation value. The dominant effect of irradiation on remanent magnetization was to decrease the total intensity by 5-45%. AF demagnetization spectra of both isothermal and thermal remanence before and after irradiation indicate that neutron bombardment slightly increases the resistance to AF demagnetization. No similar increase in resistance to thermal demagnetization was indicated. The changes in Hc and AF demagnetization spectra are attributed to radiation induced point defects rather than to pinning of domain walls by vacancy clusters and dislocations. The NRM in chondritic meteorites and most lunar samples is too stable to be accounted for by exposure of isothermal remanence to cosmic rays. Thus the NRM must be some form of thermal or thermochemical remanence.