Magnetic circular dichroism of f-f electron transitions in Ho3+ and Nd3+ ions in antiferromagnetic crystal Ho0.75Nd0.25Fe3(BO3)4 in the region of spin-reorientation transition

Absorption and magnetic circular dichroism (MCD) spectra have been studied in the region of ⁵ I ₈ → ⁵ F ₂, ⁵ S ₂, ⁵ F ₃, and ⁵ F ₅ absorption bands of the Ho ³⁺ ion and in the region of ⁴ I _9/2→ ⁴ G _7/2, ² K _13/2, ⁴ S _3/2, ⁴ F _7/2, ⁴ F _5/2 and ² H _9/2 absorption bands of the Nd ³⁺ ion in an antiferromagnetic trigonal crystal Ho _0.75Nd _0.25Fe ₃(BO ₃) ₄ in the temperature range from 5 to 90 K. A qualitative change in the shape of the MCD spectra of Ho ³⁺ 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 Ho ³⁺ 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 C ₃ axis, quenches the usual paramagnetic and diamagnetic MCD in Ho ³⁺ ion, but creates a condition for the appearance of a large paramagnetic MCD of mixing ( B-term). With temperature increasing, the MCD spectrum of Ho ³⁺ 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 Nd ³⁺ 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 Nd ³⁺ ion does not make a reorientation transition simultaneously with the Fe ³⁺ and Ho ³⁺ ions of the rest of the crystal. This phenomenon is accounted for by a strong local magnetic anisotropy of the Nd ³⁺ ion and by a weak exchange interaction Fe-Nd, which is not enough for rotation of the Nd ³⁺ ion moment synchronously with that of the Fe ³⁺ ion. The magneto-optical properties of neodymium are controlled by the easy-axis anisotropy of neodymium.