H-T phase diagrams in the region of the magnetic compensation point of Tb1.98Y1.02Fe5O12 and mean field analysis at high temperature

Magnetization investigations have been performed on single crystal of the substituted terbium-yttrium iron garnet, Tb _1.98Y _1.02Fe ₅O ₁₂ in strong dc magnetic fields up to 200 kOe applied along the easy 〈1 1 1〉, the intermediate 〈1 1 0〉, and the hard 〈1 0 0〉 axes of magnetization. When the internal magnetic field H is applied along the two difficult directions 〈1 1 0〉 and 〈1 0 0〉, different field-induced phase transitions between the associated collinear and the so-called canted magnetic phases are observed in the isothermal magnetizations M _T(H). They are chiefly manifested by a sudden change for a critical field H _C2 with a variation in slope in the temperature region within 60 K around the magnetic compensation point, T _{c omp} = 137.0 ± 0.1 K. The magnetic structure becomes collinear for H applied along the 〈1 0 0〉 direction at a first critical field H _C1 ≤ 10 kOe after a substantially curved part where the slope varies when H is increased. Isofield magnetizations M _Hap(T) are recorded along the 〈1 1 1〉 direction at fixed external field H _ap = 20.09, 60.24, and 70.14 kOe while T varies by steps in the range of 119–151 K around T _comp. The magnetic transitions between one canted phase and two inverse collinear phases on each side of T _comp are observed by magnetization anomalies at a critical temperature T _C2 well-defined with a magnetization jump in the curves of the associated derivatives dM _Hap(T)/dT. Based on the (H _C2,T _C2)-points, the experimental magnetic phase diagrams (MPDs) are precisely determined in the H-T plane with a more accurate values of the critical point C* (H _C* = 188 ± 1.3 kOe; T _C* = 152 ± 2 K) above which, ferri-and ferromagnetic collinear phases exist for all the directions. Other particularity of the MPDs, is the first-order phase transition I-T _comp line separating two inverse collinear phases along the axis 〈1 1 1〉 with (H _I = 20.09 kOe;T _I = 136.33 K), being the coordinates of the critical point I, in agreement with the prediction for weakly anisotropic uniaxial ferrimagnets in a low-field parallel to the easy axis. Based on the mean field approximation applied to a two-sublattice model at high temperature, molecular field coefficients of the Fe-Tb interaction and within the Tb sublattice are obtained and compared to those found previously for TbIG.