Large refrigerant capacity in rare earth free nanostructures of matrix embedded Fe nanoparticles

A magnetocaloric effect with sizable isothermal entropy change (△S) maintained over a broad range of temperature (10K ˂T˂ 300K) is reported for a rare earth free nanoparticle heterostructure. The nanostructure is composed of environment friendly and non-toxic iron (Fe) nanoparticles confined in layers of a titanium nitride (TiN) thin-film matrix. The magnetocaloric Fe particles of uniform shape and narrow size distribution are embedded in a TiN/Fe/TiN multilayered pattern using a pulsed laser deposition (PLD) method. Crystallographic studies carried out using x-ray diffraction have indicated the epitaxial nature of TiN film. M  vs.T data at various fields, allows application of the Maxwell relation which provides quantitative information about the isothermal entropy change in the Fe nanoparticulate system. Our data show a magnetocaloric effect (MCE) with a |△S^max| of 4.18×10³  J/K m³ in the magnetic field range of 0.1 T ˂ μ₀H ˂ 3 T for the TiN/Fe/TiN sample having Fe particles size of ~15 nm. At lower applied fields (0.0025T ˂ μ₀H ˂ 0.075T) the change in △S vs T peaks in the vicinity of the blocking temperature (T_B∼60K). For T>T_B, dynamic hysteresis is absent and the magnetocaloric effect becomes potentially useful for near room temperature cooling applications. The weak temperature dependence of △S over a wide range of temperature brings about a maximum refrigerant capacity value of 7.40×10⁵ J/m³ (94 J/kg) at 3 T that is comparable with other attractive rare-earth-free nanostructured transition metal magnetocaloric materials.