Magnetic and magnetocaloric properties of iron nanoparticle and iron thin films embedded or sandwiched between titanium nitride thin films*

We have studied the magnetocaloric effect (MCE) in iron (Fe) nanoparticles embedded in titanium nitride thin film matrix in a multilayer structure and Fe thin films sandwiched between titanium nitride thin films. The total volume of Fe is kept same in both structures. This study has allowed a better understanding of the effect of dimensionality on the MCE in simple metallic systems assuming Fe particles to behave as zero-dimensional and Fe thin films as two-dimensional geometries.  Fe nanoparticles of different sizes were embedded in a titanium nitride thin film matrix by varying the number of laser pulses during pulsed laser deposition (PLD). Similarly, Fe thin films were sandwiched between two layer of TiN films using PLD.  Isomagnetic magnetization (M) versus temperature (T) measurements carried on nano particulate structures have shown the absence of any separation between zero field cooled (ZFC) and field cooled (FC) MT curves above the blocking temperature suggesting a negligible thermal hysteresis loss in the samples. Quantitative information about the isothermal entropy change (ΔS) in the Fe-TiN heterostructure system has been obtained by applying Maxwell relation to the FC MT data at various fields. The Fe-TiN heterostructure systems show a sizable ΔS over a broad range of temperatures (T_B < T < 300 K). With the dynamic magnetic hysteresis absent above the blocking temperature, the negative ΔS as high as 1.6×10³ J/Km³ is obtained for 0.2 T at 300 K. Finally, we report that Fe nanoparticle samples exhibit higher refrigerant capacity (RC) in comparison to Fe thin film multilayer sample and the RC increases with decreasing Fe particle size. The high RC value in Fe-TiN nanoparticle heterostructure is brought about by the weak temperature dependence of the isothermal entropy change. The broad range of usable entropy change and easy accessibility makes the Fe-TiN system interesting for next-generation solid-state cooling.