On the Mechanism of Spin Reorientation in YIG:Si

Abstract

The magnetocrystalline anisotropy theory developed earlier by us and based on a novel energy level model for Fe2~ in YIG:Si(Ge) is used here to study the mechanism of spin reorientation in YIG:Si. Single-ion anisotropy with a uniform distribution of Fe2+ ions indicate a broad spin reorientation which turns out to be due to the higher-order cubic terms K} and KA. It is found that the completion temperature (T2) of the spin reorientation is strongly influenced by Ka thus indicating an inadequacy of the approach based on Kx, K2 and K3 only. From the single Fe-+ ion anisotropy results we propose a two-center model which implies a new mechanism of the spin reorientation in YIG: Si. The bulk onset (77) and completion (T2) temperatures are linked to the relative number (a) of the near Fe2+ sites. Spin orientation diagrams in the plane a -T are established for the total Fe2+-Fe3+ system for a wide range of the parameters involved. The two-center model accounts well for the bulk experimental temperatures T[ and T2 in YIG: Si and for the absence of spin reorientation in samples having Si-content less than 0.18. The total magnetization M is predicted not to rotate smoothly between the [100] and [111] direction. There exists an upper critical angle beyond which M switches sharply to [111] as the temperature approaches T2. Comparison with the previous model which requires a nonuniform distribution of Fe2+ ions is also discussed.