Tuning the magnetocaloric effect by optimizing thickness-induced three-dimensional strain states

Abstract

The effect of a three-dimensional strain state on the magnetocaloric properties of epitaxial ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3}$ (LCMO) thin films grown on two types of substrates, ${\mathrm{SrTiO}}_{3}$ (001) (STO) and ${\mathrm{LaAlO}}_{3}$ (001) (LAO), has been studied as a function of film thickness within the range of 25--300 nm. The STO substrate imposes an in-plane tensile biaxial strain, while the LAO substrate imposes an in-plane compressive biaxial strain. The in-plane biaxial strain on LCMO by the STO substrate gets relaxed more rapidly than that by the LAO substrate but both LCMO/STO and LCMO/LAO show a maximum entropy change ($\mathrm{\ensuremath{\Delta}}{S}_{max}$) of $\ensuremath{\sim}12.1 {\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{Kg}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$ $(76.33\phantom{\rule{0.16em}{0ex}}{\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}3}{\mathrm{K}}^{\ensuremath{-}1})$ and $\ensuremath{\sim}3.2 {\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{Kg}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}1}$ $(20.18\phantom{\rule{0.16em}{0ex}}{\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}3}{\mathrm{K}}^{\ensuremath{-}1})$, respectively, at a critical thickness of 75 nm (at 6 T applied magnetic field). LCMO/LAO is found to exhibit a wider transition temperature region with full width at half maxima (FWHM) $\ensuremath{\sim}40$ K of the $\frac{dM}{dT}$ vs $T$ curve compared to LCMO/STO with FWHM $\ensuremath{\sim}33$ K of that curve. This broadening of the transition region indicates that the table-like magnetocaloric effect (MCE) is attainable by changing the strain type. The maximum relative cooling power, $\ensuremath{\sim}361 {\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{Kg}}^{\ensuremath{-}1}$ $(2277\phantom{\rule{0.16em}{0ex}}{\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}3})$ of LCMO/STO and $\ensuremath{\sim}339 {\mathrm{J}\phantom{\rule{0.16em}{0ex}}\mathrm{Kg}}^{\ensuremath{-}1}$ $(2138.5\phantom{\rule{0.16em}{0ex}}{\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}}^{\ensuremath{-}3})$ of LCMO/LAO, is also observed at the thickness $\ensuremath{\sim}75\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$. The Curie temperature varies with the thickness exploring the variation of ferromagnetic interaction strength due to strain relaxation. The film thickness and substrate induced lattice strain are proved to be the significant parameters for controlling MCE. The highest MCE response at a particular thickness shows the possibility of tuning MCE in other devices by optimizing thickness.

Topics & Concepts