Magnetic circular dichroism of f-f electron transitions in Ho³⁺ and Nd³⁺ ions in antiferromagnetic crystal Ho_0.75Nd_0.25Fe₃(BO₃)₄ in the region of spin-reorientation transition

Absorption and magnetic circular dichroism (MCD) spectra have been studied in the region of ⁵I₈ → ⁵F₂, ⁵S₂, ⁵F₃, and ⁵F₅ absorption bands of the Ho³⁺ ion and in the region of ⁴I_9/2→⁴G_7/2, ²K_13/2, ⁴S_3/2, ⁴F_7/2, ⁴F_5/2 and ²H_9/2 absorption bands of the Nd³⁺ ion in an antiferromagnetic trigonal crystal Ho_0.75Nd_0.25Fe₃(BO₃)₄ in the temperature range from 5 to 90 K. A qualitative change in the shape of the MCD spectra of Ho³⁺ ion was revealed during the spin-reorientation transition at T_R = 6.9 К. Below T_R, in the easy-axis state of the crystal, the MCD spectrum has a shape typical for paramagnets, i.e., it consists of diamagnetic and paramagnetic parts. Just above T_R, in the easy-plane state of the crystal, MCD of Ho³⁺ ion has a spectrum similar to the absorption spectrum, which is typical for the paramagnetic part of the MCD. It was shown, that in the easy-plane state the exchange field of iron, being perpendicular to the external field directed along the C₃ axis, quenches the usual paramagnetic and diamagnetic MCD in Ho³⁺ ion, but creates a condition for the appearance of a large paramagnetic MCD of mixing (B-term). With temperature increasing, the MCD spectrum of Ho³⁺ ion gradually turns into a spectrum typical for paramagnets with the predominance of the sign-changing diamagnetic part, because the influence of the exchange field decreases. The shape of the MCD spectra of Nd³⁺ ions are typical for paramagnets but does not change during the reorientation transition in the rest of the crystal. This means that the magnetic moment of the Nd³⁺ ion does not make a reorientation transition simultaneously with the Fe³⁺ and Ho³⁺ ions of the rest of the crystal. This phenomenon is accounted for by a strong local magnetic anisotropy of the Nd³⁺ ion and by a weak exchange interaction Fe-Nd, which is not enough for rotation of the Nd³⁺ ion moment synchronously with that of the Fe³⁺ ion. The magneto-optical properties of neodymium are controlled by the easy-axis anisotropy of neodymium.