Characteristic Features of the Anomalous Magnetic Properties of some Mixed Terbium-Yttrium Ferrite Garnets at Low Temperatures

This article is devoted to the experimental investigation of the occurrence of the so-called “low-temperature Belov point” $T_{B}$ predicted previously for the mixed rare earth-yttrium iron garnet compounds RE $_{x}Y_{\mathrm {3 - {}}x}$ Fe5O12 possessing or not a magnetic compensation point $T_{\mathrm {comp}}$ . Isothermal magnetizations $M_{T}(H)$ of two single crystals of Tb $_{x}Y_{3-x}$ Fe5O12 with rare-earth (RE) = Tb, and $x = 1.98$ , 0.37, were measured in the 4.2–300 K range, using dc magnetic field $H$ up to 100 kOe applied in one or more of the three main crystallographic directions. Anomalies are observed in the region of $T_{B}^{\mathrm {TbIG}} \approx 58$ K, the previous estimated value for TbIG ( $x = 3$ ), in the temperature dependences of the paraprocess susceptibility $\chi _{p}$ , the parameter $\vert b\vert $ of the bH 2 term of the quadratic expansion of $M_{T}(H)$ curves, and other pertinent magnetic characteristics. For $x = 1.98$ with $T_{\mathrm {comp}} = 137$ K and when $H$ is applied along $\langle 110\rangle $ , the $\chi _{p}$ - and $\vert b\vert $ -maxima take place close to $T_{B}^{\mathrm {TbIG}}$ . The anisotropy of the $\chi _{p}$ -data gives evidence for a spontaneous noncollinear structure around the easy axis of magnetization $\langle 111\rangle $ at 4.2 K. For $x = 0.37$ which has no $T_{\mathrm {comp}}$ point and when $H$ is applied along $\langle 100\rangle $ , the $\chi _{p}$ - and $\vert b\vert $ -maxima are clearly disturbed by the sign anomalies caused by the noncollinear structure instability mechanism due to the crossing of the Tb3+ levels at the critical point $T^{\ast } = 16$ K and by the lower continuous change from the easy axis to a low-symmetry angular phase $\langle $ uuw $\rangle $ due to the first spontaneous spin reorientation phase transition at $T_{\mathrm {SR1}} = 40$ K. When $x$ decreases from 3 to 0.37, it is found that despite different anisotropy effects of the Tb3+ ion, its magnetic moment $m_{\mathrm {Tb}}$ close to $T_{B}^{\mathrm {TbIG}}$ is practically constant (~4.99 in $\mu _{B}$ per Tb3+ ion).