Magnetocaloric Gadolinium thick films for energy harvesting applications

Abstract

Magnetization, Curie temperature and isothermal entropy change have been measured in Gadolinium 3 μm thick films. We present the comparison between the structural and magnetic properties of samples deposited either at room temperature or at 550°C onto silicon substrate (100) with a thermally oxidized layer. The results are discussed using as a reference, data from bulk Gd single crystals. As compared to bulk values, the room temperature grown film shows lower Curie temperature ( ∼ 280 K) and weaker entropy change ( ∼ 6.8 Jkg−1K−1) under application of magnetic field ramp from 0 to 5 Tesla. On the contrary the 550○C deposited film shows a Curie temperature ( ∼ 293 K) and a maximum isothermal entropy change ( ∼ 8.9 Jkg−1K−1) both in good agreement with values expected for bulk samples. The behavior of the room temperature deposited samples are briefly discussed and compared with similar effects observed in bulk samples with different degree of purity and homogeneity.

Magnetocaloric Effect in Flexible, Free-Standing Gadolinium Thick Films for Energy Conversion Applications

“…The maximum magnetization at 10 K under an applied field H = 5 T is M = 1951 kA/m in good agreement with the bulk value reported in the literature [26] and with our measurement on thick films grown on Si/SiO 2 substrates [14].…”

“…The film looks dense and continuous, without porosity. The apparent columnar structure has been previously reported in Gd thick films [14], and can be related to the high deposition rate, and to similar structures observed in the cross-section of the tantalum layer (not shown here).…”

“…The magnetization bump appearing on M (T) under weak applied field below 25 K has been reported and discussed in the literature as well as in our previous study [14].…”

“…[11]). This T c corresponds to about one third the value we found on 3 μm Gd films grown on silicon substrate with tungsten capping layers [14]. The narrower free-standing film T c distribution 064045-6 suggests a reduced role of surface defects possibly due to the buffer, the capping layer, and to the increased thickness.…”

“…Gadolinium, with its 7.6 μ B per atom, and with a close to room-temperature Curie point T c ≈ 294 K [10,11], has been recognized as a promising single-element magnetic material for low-temperature high-magnetic-moment applications [12,13]. Moreover, bulk Gd has been the benchmark material for MC properties [7], and Gd films will arguably follow a similar path in the domain of microscale and nanoscale energy-conversion applications [14,15]. Getting thick (in the micrometer range), freestanding Gd films can be particularly relevant for energytransduction applications where achieving high conversion powers, and avoiding the reduction in caloric properties associated with the buffer heat capacity are the key objectives.…”