Ferrimagnetic order in the mixed rare-earth iron garnet {Y 1-x Gd x } 3 Fe 5 O 12 system has been reinvestigated to cover the temperature range 5.0 to 700 K. The magnetization versus temperature exhibits a systematic variation with changing concentration x. The ferric-ion exchange coupling is strong enough to determine the Curie temperature ≈ 559 K for all the values x. The compensation temperature at which the magnetization crosses zero (shows the minimum) demonstrates the applicability of the three-sublattice model.
The behaviour of the coercive force, H c , of ferrimagnets near the compensation temperature, T comp , for polycrystalline rare earth iron garnets R 3 Fe 5 O 12 (R ¼ Gd, Tb, Dy, Ho and Er) has been studied in detail. The present experimental results have verified the double peak in H c near T comp for these rare earth iron garnets R 3 Fe 5 O 12 . For Dy 3 Fe 5 O 12 , the experimental values are T comp ¼ 224 K, H c (max) ¼ 600 Oe at T comp AE K and % 10 K. The two peaks of H c arise at 214 and 232 K. The appearance of the double peak of H c is an intrinsic and characteristic property for R 3 Fe 5 O 12 . The results can be understood on the basis of a simple model. The hysteresis loop in the magnetization is caused by rotation of single domain under the influence of crystalline anisotropy rather than domain wall displacements of the multi-domain structure. The rare earth ion behaves as superimposed paraprocess under the influence of an internal field produced by iron sublattices and a weak exchange magnetic field.
Vanadium garnets NaPb 2 Co 2 V 3 O 12 and NaPb 2 Ni 2 V 3 O 12 have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group Ia -3d (O 10 h ) with the lattice constant of 12.742 Å (NaPb 2 Co 2 V 3 O 12 ) and 12.666 Å (NaPb 2 Ni 2 V 3 O 12 ), respectively. The magnetic susceptibility of NaPb 2 Ni 2 V 3 O 12 shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment µ eff of NaPb 2 Ni 2 V 3 O 12 is 3.14 µ B due to Ni 2+ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb 2 Co 2 V 3 O 12 , the simple Curie-Weiss law can not be applicable. The ground state is the spin doublet 2 E(t 2 6 e) and the first excited state is spin quartet 4 T 1 (t 2 5 e 2 ), according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb 2 Co 2 V 3 O 12 .
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