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Spherical Fe–Ni (50–50) single crystals were ordered by neutron irradiation at 295°C in the presence of a magnetic field. Magnetization curves and magnetic anisotropy energy measurements demonstrate the very important directional effect of the irradiation magnetic field on the bulk anisotropy of the single crystal. When the magnetic field is applied along a fourfold axis, the bulk magnetic anisotropy is tetragonal with a very large value for K3. When the magnetic field is applied along a three fold axis, the bulk magnetic anisotropy is cubic and the threefold axis becomes an easy axis of magnetization; the magnetization measured along the easy direction reaches saturation with a certain law of approach, while the magnetization measured along the [100] direction reaches saturation immediately. The model proposed by Néel, in which three types of quadratic ordered domains are assumed to be strongly coupled, fits well the experimental results. An electron microscopy image shows the different type of ordered domains and the superlattice spots.

2014 Des expériences d'irradiation aux neutrons sous champ magnétique d'alliages Fe-Ni (50-50) ont permis de découvrir une transition ordre-désordre du type Au Cu dont la température critique est 320 °C. L'ordre ne peut s'établir que sous irradiation étant donné la valeur trop faible du coefficient de diffusion auteur de 300°C. Un effet directeur du champ magnétique appliqué, dû aux couplages magnétiques des liaisons avec l'aimantation favorise le développement de l'axe tétragonal dans la direction du champ pour les cristallites dont un axe [100] est voisin de la direction du champ. Une étude sur des monocristaux montre que cet axe tétragonal est le seul axe de facile aimantation de la structure ordonnée.

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Calculations of the resonance fields as a function of temperature and of resonance frequency as a function of field for temperatures near the compensation temperature for lithium-chromium ferrite and gadolinium garnet are given. For the latter case, a comparison is made between the assumption of zero anisotropy and an assumed negative anisotropy.

The measurements of magnetic after-effects on irradiated iron point to the presence of three families of defects : after 25°C irradiation, the first one has a mean activation energy of 1.2 eV ; after 78°K irradiation, the second and third one have respectively 0.3 and 0-6eV. The irradiation in a magnetic field of a 50 % iron+nickel alloy, make