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.

Gadolinium is a nearly ideal soft-magnetic material. However, one cannot take advantage of its properties at temperatures higher than the room temperature where Gd loses the ferromagnetic ordering. By using high-purity bulk samples with grains ~200 nm in size, we present proof-of-concept measurements of an increased Curie point (T_C) and spontaneous magnetization in Gd due to hydrogenation. From first-principles we explain increase of T_C in pure Gd due to the addition of hydrogen. We show that the interplay of the characteristic features in the electronic structure of the conduction band at the Fermi level in the high-temperature paramagnetic phase of Gd and “negative” pressure exerted by hydrogen are responsible for the observed effect.

High-purity Gd prepared by distillation is a structurally inhomogeneous system consisting of needle-shaped crystals of cross section 0.5–2.5 μm with near-c-axis orientation embedded in a matrix of nanosized (30–100 nm) grains. By measuring the magnetocaloric effect (MCE) directly, we find that the MCE values differ markedly for the plate-shaped samples cut out of a distillate along and perpendicular to the crystals. The effect of small controlled amounts of impurity (hydrogen) on the properties of distilled Gd is further studied. We observe opposite trends in the MCE response to hydrogen charging with respect to the crystal's orientation within the samples and discuss mechanisms interrelating the unique structural morphology with the impurity behavior. As an overall assessment, the Curie temperatures of α-GdH_x solid solutions increase from 291 K up to 294 K when increasing hydrogen concentration x from 0 to 0.15. Hydrogenation is found to broaden the ferromagnetic-to-paramagnetic phase transition. Hydrogen-containing specimens demonstrate reversibility of MCE at these temperatures.