Magnetocaloric effect and thermal transport management in lanthanum manganites

This thesis investigates two challenges associated with the use of manganites for magnetocaloric applications. The first challenge is associated with methods to engineer the thermal conductivity, K. The second challenge is to understand the limits of the entropy change achievable in magnetocaloric manganites. Thermal management has been achieved via different microstructuring routes and their influence on thermal transport properties such as K, resistivity and thermopower, have been studied. A factor of two increase in K is demonstrated by using density and grain size optimization, while three-fold and six-fold increases are seen by employing the introduction of a second highly conductive phase via: (1) silver impregnation and silver particle coating and (2) copper electroplating, respectively. Understanding the magnetocaloric effect (MCE) characteristics in manganites has been achieved by bringing together magnetisation, magneto-structural, magneto-Seebeck, and neutron diffraction independent measurements. We first show that the temperature T* up to which a spontaneous magnetisation is observed in the inverse magnetic susceptibility of La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 above Tc, is related to the transition temperature of the low temperature (high-magnetic field and high-magnetisation) magnetic phase. In the most widely studied La(1-x)CaxMnO3 (x = 0.2, 0.25, 0.3), we then conclude that unlike between the degree of static Jahn-Teller distortion and the interval [T*-Tc]/Tc where we show that there exists a close relationship, there is no apparent correlation between the magnitude of the MCE and [T*-Tc]/Tc . We then unravel the competing strength of the various degrees of freedom and show that the inhibition of a large magnetocaloric response is due to the strong correlations that underpin the collosal magnetoresistance effect: both clustering of magnetic Mn atoms due to polaron formation and the insulator to metal transition. Finally we discuss prospects to improve material properties for application in light of these findings.Open Acces