Submillimeter dynamics of the new spin-reorientation in HoFe/sub 1-x/Al/sub x/O/sub 3/ induced by magnetic vacancies

Spin excitations in HoFe/sub 1-x/Al/sub x/O/sub 3/ orthoferrites involving Al impurity ions (magnetic vacancies) have been investigated at submillimeter wavelengths (v=2-25 cm/sup -1/). Three resonance modes are detected, two of which are identified as antiferromagnetic resonance modes in the Fe-subsystem, and the third as a rare-earth mode induced by electron transitions inside the ground quasidoublet of the Ho/sup 3+/ ion. The observed temperature dependences and anomalies in the behavior of these modes at point T/sub M/=60 K indicate that in HoFe/sub 1-x/Al/sub x/O/sub 3/ there is a new first-order spin-reorientation phase transition (SRPT) from the weak ferromagnetic phase Gamma /sub 4/(F/sub z/G/sub x/), to the antiferromagnetic one Gamma /sub 1/(G/sub y/), which is induced by the magnetic vacancies. >