Influence of the Terbium film size for the magnetocaloric effect at low temperatures

Abstract

Significative enhance in magnetocaloric effect due to finite size and surface effects is reported in Terbium(Tb) thin films in the helimagnetic phase (corresponding to a temperature range from $T_C$=219 K to $T_N$=231 K), for external fields of the order of kOe. For a Tb thin film of 6 monolayers submitted to an applied field ($\Delta$H= 30 kOe, $\Delta$H= 50 kOe and $\Delta$H= 70 kOe) we report a significative change in adiabatic temperature, $\Delta$T/$\Delta$H, near the N\'eel 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 $\Delta$T/$\Delta$H=43 K/T while for thicker films, with 20 monolayers, $\Delta$T/$\Delta$H=22 K/T. Our results suggest that thin films of Tb are a promising material for magnetocaloric effect devices for applications at intermediate temperatures.