Unconventional critical behavior of the magnetic refrigerant system Er_0.98□_0.02Co₂ around its ferromagnetic-paramagnetic transition

Abstract

Magnetic refrigeration is a promising energy efficient and environmentally friendly refrigeration technology. The major problem in magnetic refrigeration is to find working materials with a large magnetocaloric effect (MCE) in different temperatures regions. MCE is the reversible temperature change of a magnetic materials upon the application or removal of a magnetic field. Nanoscale magnetic materials are good candidates for magnetic refrigeration due to a presence of a large MCE in the superparamagnetic system and reduced hysteresis losses. The critical behavior of Er0.98□0.02Co2 intermetallic system has been investigated via isothermal magnetizations around its ferromagnetic-paramagnetic (FM-PM) transition at Curie temperature TC = 62 K. This study was performed through different methods; Arrott-Noakes technique, Kouvel-Fisher analysis, and critical isotherms procedure. All these models could not explain the behavior of the studied system, which suggest that it belongs to an unconventional critical behavior. The critical exponents of the magnetization β, for the temperature dependence of the spontaneous magnetization below Curie Temperature (TC), and γ for the magnetic susceptibility, and δ for the magnetic isotherm at TC were calculated. Based on Modified Arrott plots; Er0.98□0.02Co2 gives TC = 62.57 K ± 0.01 K with β = 0.68 ± 0.005 and TC = 62.59 K ± 0.01 K with γ = 0.88 ± 0.008. Using Kouvel-Fisher technique, we get TC = 62.66 K ± 0.06 K with β = 0.71 ± 0.01 and TC = 62.54 K ± 0.02 K with γ = 0.90 ± 0.06. The extracted value of δ from the critical isotherms and the Widom scaling relation are so close confirming that the obtained critical exponents are suitable and accurate within experimental error values. With 536.45 J kg−1 at 5 T of relative cooling power, Er0.98□0.02Co2 is very promising as magnetic refrigerant.

La0.6X0.1Te0.3MnO3 system with significant refrigerant capacity at low magnetic field and double magnetic entropy change peaks: effect of ball-milling time on physical and critical behaviors

“…3.6. Critical behavior around the ferro → paramagnetic transition One of the fundamental questions associated with the FM → PM transition is the universality class [58][59][60]. It depends on the values of the critical exponents defining the phase transition (FM → PM).…”