Abstract
We report a theoretical study of the magnetocaloric effect of Terbium (Tb) thin films due to finite size and surface effects in the helimagnetic phase, corresponding to a temperature range from TC=219K to TN=231K, for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field (ΔH=30kOe, ΔH=50kOe and ΔH = 70kOe) we report a significative change in adiabatic temperature, ΔT/ΔH, near the Néel temperature, of the order ten times higher than that observed for Tb bulk. On the other hand, for small values of the magnetic field, large thickness effects are found. For external field strength around few kOe, we have found that the thermal caloric efficiency increases remarkably for ultrathin films. For an ultrathin film with 6 monolayers, we have found ΔT/ΔH = 43K/T while for thicker films, with 20 monolayers, ΔT/ΔH = 22K/T. Our results suggest that thin films of Tb are a promising material for magnetocaloric effect devices for applications at intermediate temperatures.
Highlights
The magnetocaloric effect (MCE) is a phenomenon that enables the temperature of a material to be altered by the application of external fields
Our studies revealed that the efficiency of the MCE in nanofilms of Tb has a strong dependence with its thickness
We have chosen a small value of H because the thin film effects on the MCE properties are more visible for small values of the external field
Summary
The magnetocaloric effect (MCE) is a phenomenon that enables the temperature of a material to be altered by the application of external fields. In thin films, when the thickness is comparable to the periodicity of the ordered structure, it is expected that even the magnetic arrangement itself can be strongly modified Rare earth helimagnets such as Ho, Dy, and Tb represent the best candidates to put into evidence such finite-size effects. The MCE of the Gd5(Si, Ge)4 [17,18] have been extensively studied due to their potential use for magnetic refrigeration applications near room temperature, as we have pointed above These compounds undergo a first-order phase transition and exhibit a large MCE. In this paper we are interested to investigate the thickness influence on MCE of Terbium thin films, in the frontier between nanometric 6– 20 monolayers) and bulk structures, in the temperature range from 220 K to 230 K.
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