INFLUENCE OF MAGNETIC VACANCIES ON SPIN EXCITATIONS IN THE RARE-EARTHORTHOFERRITES RFE1-XALXO3 (R=GD, HO)

We have observed a strong influence of non-magnetic impurity Al3+ ions (magnetic vacancies) on spin excitations in GdFe0.9Al0.1O3 and HoFe0. 925Al0.075O3 at frequency nu = 2-33 cm-1. A significant growth of the frequencies of both AFMR modes nu1 and nu2 at low temperatures was fou nd in GdFe0.9Al0.1O3 compared to pure GdFeO3. The anomalies observed i n the behaviour of the AFMR modes for HoFe0.925Al0.075O3 at T(M) = 60 K give evidence of a new first-order spin reorientational phase transi tion from the GAMMA4(GxFz) phase to the antiferromagnetic phase GAMMA1 (G(y)) induced by the magnetic vacancies.