Magnetocaloric effect in Lao_(0.7)Ca_(0.3)MnO_3 nanotube arrays with broad working temperature span

摘要

We have studied the magnetic entropy change of highly ordered La_(0.7)Ca_(0.3)MnO_3 nanotube arrays synthesized by template assisted sol-gel method in temperatures ranging from 179 to 293 K and in magnetic fields up to 5 T. From the measurements of isothermal magnetization, we have calculated the maximum isothermal magnetic entropy change of - △S_M= 1.9 J/kg K around the Curie temperature at 236 K for a field of 5 T. The nanotubes present lower magnetic entropy change compared with their bulk counterpart (-△S_M = 4.8 J/kg K) which was prepared by the same sol-gel route. Such diminished magnetic entropy change observed in nanotubes is explained by the disordered magnetic states which are created on the surface sites of nanograins due to the larger surface to volume ratio. However, the nanotubes present an expanded magnetic transition that extends over a wide temperature range and suggest that such manganite nanotubes could be used for magnetic refrigeration with broad working temperature span.