  The effect of 3-dimensional strain state on the magnetocaloric properties of
epitaxial La0.8Ca0.2MnO3 (LCMO) thin films grown on two types of substrates,
SrTiO3 (001) (STO) and LaAlO3 (001) (LAO) has been studied as a function of
film thickness within the range of 25 to 300 nm. The STO substrate imposes an
in-plane tensile biaxial strain while LAO substrate imposes an in-plane
compressive biaxial strain. The in-plane biaxial strain on LCMO by STO
substrate gets relaxed more rapidly than that by LAO substrate but both
LCMO/STO and LCMO/LAO show a maximum entropy change of 12.1 J/Kg-K and 3.2
J/Kg-K, 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) 40 K of the dM/dT vs T curve compared to
LCMO/STO with FWHM 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, 361 J/Kg of
LCMO/STO and 339 J/Kg of LCMO/LAO is also observed at the thickness 75 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.
