Determination of the Structure of Lattice Defects by Magnetic Anisotropy

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Radiation damages have been an important problem that the materials which are used as the environmental materials for an atomic or a fission reactor are bombarded by fast neutrons, alpha, beta or atom beams. The materials which are considered to be suitable, at present, are iron-base alloys, it is necessary to understand the physical properties of defects in iron. The structure and the physical properties of interstitial atoms which were generated by irradiations of fast particles made clear by using magnetic techniques such as magnetic anisotropies, magnetization and magnetic domains.At first, a wide-range torque magnetometer was constructed, and induced magnetic anisotropy in single crystals of Fe and Ni by fast particle irradiation was detected. By analyzing the observed torque curves it was shown that in low temperatures the structure of interstitials in iron is 110-dumbbell and in nickel 100-dumbbell. At elevated temperatures the interstitials aggregate and form cluster loops on (111) plane in the case of iron and on (110) plane in the case of nickel.Then, by observing the motion of magnetic domain walls on electron-irradiated single crystal specimens by a high voltage electron microscope, it was found that the mobility in iron is smaller than in nickel. From this experiment it is concluded that the volume of an iron interstitial atom becomes small by reducing its magnetic moment.Theoretically, the situation of an interstitial atom is considered to be similar to the case of materials under high pressure. The stability and thermal properties are investigated theoretically by using a model d-band or the LMTO band calculations by including the magnetic energy. The phydical properties of interstitial atoms have been explained by the present method. The results of this study is the base of the development of a new useful alloy for reactor materials.