Polycrystalline MgCr2O4 and CuCr2O4 samples are investigated by X‐ray and neutron diffraction methods before and after irradiation by fast neutrons (En ≧ 1 MeV) of fluence 2 × 1024 m−2. It is found that under irradiation in cubic magnesium and tetragonal copper chromites with the corresponding space groups O h7 – Fd3m and D 4h19 – I4/amd different types of cations are redistributed both, over “allowed” and “forbidden” sites of the initial spinel lattice. As a result, the lattice periods and crystal symmetries are changed. The structures of the irradiated chromites may be described in terms of the space group O h5 – Fm3m. Under annealing the irradiated samples restore their initial crystal structures. Structural changes occurring on different stages of annealing are analysed by X‐ray.

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Polyerystalline samples of yttrium iron garnet (VIG) irradiated by different fluences of fast neutrons (0 to 1.6 × 1024m−2) are investigated by X‐ray and magnetic methods. It is found that fast neutron irradiation leads to partial amorpliization of YIG. Moreover, at irradiation by high fluences (≈︁ 1024 m−2) one can obtain practically a complete amorphous state. The magnetic state of the amorphous samples is the “spin glass” one, the freezing temperature is about 70 K. Possible reasons resulting in the amorphization of YIG under neutron irradiation are analysed.

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Single crystals of gallium‐gadolinium garnet (GGG) irradiated by fluences of fast neutrons (0 to 3.8 × 1024 m−2) are investigated by X‐ray and magnetic methods. It is found that irradiation with high fluences leads to complete amorphization of the samples. Besides, the samples recover their single crystalline structure under subsequent thermal annealing (excluding the surface layer). It is shown that on amorphization the absolute value of the paramagnetic Curie temperature increases by a factor ≈ 2.