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Abstract
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 4I9/2→4G7/2, 2K13/2, 4S3/2, 4F7/2, 4F5/2 and 2H9/2 absorption bands of the Nd3+ ion in an antiferromagnetic trigonal crystal Ho0.75Nd0.25Fe3(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 TR = 6.9 К. Below TR, 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 TR, 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.
Published Version
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