Crystallographic, magnetic and magnetocaloric properties in novel intermetallic materials R3CoNi (R = Tb, Dy, Ho, Er, Tm, Lu)

A family of novel intermetallic R3CoNi with heavy rare earth ions has been synthesized (R = Tb, Dy, Ho, Er, Tm, Lu) and a study of the crystal structure of these phases performed. All the compounds adopt the rhombohedral Er3Ni2-type structure [Pearson's symbol hR45; space group R-3h (N. 148)]. A thorough investigation of their magnetic and magnetocaloric properties has been undertaken. Magnetization and ac-susceptibility measurements as a function of temperature show that the samples with Tb, Dy, Ho, Er, and Tm present a paramagnetic to ferromagnetic (PM-FM) transition at temperatures in the range 96–6 K; different reorientation transitions take place below the respective TC, in most cases with thermomagnetic irreversibility. Thermal and magnetic measurements have been used to retrieve the set of critical exponents (α, β, γ, δ) for the PM-FM transition to assign a universality class to Tb3CoNi, Dy3CoNi, and Ho3CoNi, with the result that, in the first case, it is close to the Mean Field model, in the second one it is between the Chiral Heisenberg and the XY-Chiral, and in the third one it is close to the XY-Chiral model. Therefore, in Tb3CoNi the transition is governed by long-range order interactions, whereas in Dy3CoNi and Ho3CoNi there must be some kind of frustrated non-collinear ferromagnetism. The magnetocaloric measurements in five members of the family indicate that all of them present highly competitive magnetocaloric properties in their respective temperature ranges, with high magnetic entropy changes (from 12.8 to 18.5 J/Kg.K at μ0ΔH = 5 T) and refrigerant capacities (from 412 to 699 J/Kg at μ0ΔH = 5 T). These results assess and highlight the applicative interest of these compounds, besides suggesting the possibility of tuning the range of the operating temperature by modifying the rare earth ion. Finally, universal curves for the magnetocaloric properties have been found for Tb3CoNi, Dy3CoNi, and Ho3CoNi; the scaling of the magnetocaloric variables confirms the validity of the assigned universality classes.