Magnetocaloric properties and unconventional critical behavior in (Gd,Tb)6(Fe,Mn)Bi2 intermetallics

Abstract

The magnetic and magnetocaloric properties of the intermetallic family (Gd,Tb)6(Fe,Mn)Bi2 have been studied from 2 K to temperatures above the respective Curie temperatures TC. The substitution of Gd by Tb (Gd6FeBi2, Gd3Tb3FeBi2, Tb6FeBi2) tunes TC in the range 350-250 K and favors the apparition of a metamagnetic transition at very low temperature (below 10 K) from a complex magnetic state to a ferromagnetic one, as well as a spin reorientation transition below Tm = 72 K. As a consequence, an important inverse magnetocaloric effect (IMCE) appears below 20 K and an interesting direct magnetocaloric effect (DMCE) appears over a wide temperature span between TC and Tm with maxima at those temperatures. The partial substitution of Fe by Mn in Tb6Fe0.5Mn0.5Bi2 shifts these effects upwards in temperature while expanding the region of the direct magnetocaloric effect between 70 and 400 K. The combination of adjoint IMCE and DMCE as well as the wide span of the latter shows that tuning this family allows to locate the magnetocaloric effect in different regions of interest. The critical behavior of the PM-FM transitions has been studied obtaining the critical exponents α, β, γ, δ and checking that the respective magnetocaloric effects also scale with the critical parameters n and δ. The transition in Gd6FeBi2 belongs to the Heisenberg universality class with deviations due to magnetocrystalline anisotropies; the critical exponents for Gd3Tb3FeBi2 (in agreement with the Mean Field model) suggest the presence of long range order magnetic interactions, while Tb6FeBi2 and Tb6Fe0.5Mn0.5Bi2 present an unconventional critical behavior aligned with long range order interactions.