Composition dependence of the magnetooptical properties of HoIG:Al single crystals

The Faraday rotation of aluminum substituted holmium iron garnets single crystals of composition Ho₃Fe_5-zAl_z O₁₂ has been investigated at 1152 nm wavelength in the 6 K<T<T_Neel temperature range. A magnetic field up to 20 kOe was applied along the [111] easy axis of five platelets of composition z=0.45, 0.57, 1.15, 2.05 and 2.5 obtained from as grown single crystals. The experimental data were extrapolated to H=O to get the spontaneous Faraday rotation Φ_s(z, T) and linear susceptibility χ_F(Z, T) values. In contrast to the absolute values of Φ_s, the temperature variation of χ _F shows a discontinuity at the compensation temperature. χ_F is Al-concentration dependent particularly when T<100 K due to the decrease of the Ho³⁺-F³⁺ interactions. The gyroelectric coefficient of the Ho³⁺ ions due to electric dipole transitions C_e(z, T) deduced from the experimental values is independent of z for z<1.15, and is linearly temperature dependent when T>50 K