SUBMILLIMETER DYNAMICS OF THE NEW SPIN-REORIENTATION HOFE1-XALXO3 INDUCED BY MAGNETIC VACANCIES

Spin excitations in HoFe1-xAlxO3 orthoferrites involving Al impurity i ons (magnetic vacancies) have been investigated at submillimeter wavel engths (v=2 - 25 cm(-1)). Three resonance modes are detected, two of w hich are identified as antiferromagnetic resonance modes in the Fe-sub system, and the third one - as a rare-earth mode induced by electron t ransitions inside the ground quasidoublet of the Ho3+ ion. The observe d temperature dependences and anomalies in the behaviour of these mode s at point T-M = 60 K indicate that in Hofe(1-x)Al(x)O(3) there is a n ew first-order spin-reorientation phase transition (SRPT) from the wea k ferromegnetic phase Gamma(4)(F(z)G(x)), to the antiferromagnetic one Gamma(1)(G(y)), which is induced by the magnetic vacancies.