We report on the magnetic and thermal properties [magnetization, specific heat, thermal expansion, magnetostriction, and magnetocaloric effect (MCE)] for the three multicomponent systems Tb _x Dy _y Gd _z Co ₂ , Tb _x Dy _y Ho _z (Co, Fe) ₂ , and Tb _x Dy _y Er _z (Co, Fe) ₂ (x + y + z = 1). We show that for Tb _x Dy _y R _z Co ₂ , the Curie temperatures, TC (which ranged from 130 to 300 K), and order of the phase transition (first or second order) could be controlled by composition. The highest MCE values (the adiabatic temperature change ΔT _ad = 2.2-2.3 K at μ ₀ ΔH = 1.8 T) were observed for the compounds exhibiting the transitions of the first order. Giant volume magnetostriction of 1500-2000 and 500-600 ppm is demonstrated at a field of μ ₀ H = 10 and 1.2 T, respectively. Structural and magnetic entropy contributions to the total isothermal entropy change are estimated for Tb _x Dy _y R _z Co ₂ . Regular recurrence and/or change of the physical properties across the varied composition of the compounds allows us to find the materials with desired magnetic characteristics, such as T _C , MCE, and magnetostriction, to use them in practice.

We report on successful synthesis under high pressure of a series of polycrystalline GdFeAsO_{1 − x} F _x high-T _c superconductors with different oxygen deficiency x = 0.12−0.16 and also with no fluorine. We have found that the high-pressure synthesis technique is crucial for obtaining the single-phase superconducting materials: by sythesizing the same compounds with no pressure in ampoules, we obtained non-superconducting materials with an admixture of incidental phases. Critical temperature for all the materials was in the range 40 to 53 K. The temperature derivative of the critical field dH _c2/dT is remarkably high, indicating potentially high value of the second critical field H _c2 ∼ 130 T.