Tesis

Magnetocaloric Effect (MCE) is known as the adiabatic change in temperature ()  or the isothermal change in magnetic entropy () of a material when a magnetic field (H) is applied. This effect shows its maximum intensity when the temperature of the system coincides with the Curie Temperature (T_C). Within the scientific community, there is a great interest in the study of the MCE since it allows the possibility of thinking and designing new refrigeration machines that replace the current ones, which are based on the expansion/compression of gases that turn out to be harmful to the environment. In this way, with this technological change, it would be possible to have solid-state refrigerators that are more compact, more efficient, and less noxious to the environment.

This was also accompanied by a search for new materials that are optimal for solid-state cooling. Many materials that present MCE have been studied over the years, but in particular, manganites stand out among other compounds both for presenting an important MCE, but also because they have great flexibility when it comes to modifying their magnetic properties with different stimuli, thanks to the strong coupling between the different degrees of freedom (electronic, magnetic, structural).

On the other hand, the physics of systems with reduced dimensions is also of great interest within the scientific community. Many new and fascinating properties emerge working on the nanometer scale. In this thesis we propose to combine both “worlds”, studying the MCE in thin films and mixed heterostructures.

For this Thesis, thin films of La_1-xSr_xMnO₃ (LSMO) manganite were grown using pulsed laser deposition (PLD). This technique is very useful for growing mixed oxides such as LSMO, as it allows the stoichiometry of the target to be transferred to the deposited thin film. After growth, the samples obtained are characterized structurally, magnetically, and magnetocalorically.

By modifying the concentration of strontium, one can modify the properties of the bulks that we use as targets, and therefore the properties of the films are modified. In this thesis, we work with the manganites La_0.88Sr_0.12MnO₃, which has a T_C at room temperature, and with La_0.75Sr_0.25MnO₃, with which we grew films that have a T_C at room temperature.

We start by studying the MCE in LSMO thin films grown on different substrates. In this way, we compared how the MCE is modified if the growth is polycrystalline or epitaxial and the changes that occur when going from bulk material to thin films. Along with this, the effectiveness of post-deposition heat treatment on the films and how the thickness of the films modifies the MCE were discussed.

Having studied the MCE in single-layer films, we continue our study by combining the two manganites into a single two-layer system. This allowed us to extend the temperature range in which the sample presents EMC and we achieved an improvement in its magnetocaloric properties. The results obtained are compared with thin films of a single layer. The importance of the order between the layers was also discussed, finding that for the polycrystalline case it is indistinct which of the LSMO layers is on top of the other.

The next step was to work with mesoporous SiO₂ films to synthesize LSMO nanoparticles within their pores. This has two reasons of great interest: on one hand, we want to develop a technique to infiltrate oxide nanoparticles, such as LSMO, into mesoporous structures. Traditional techniques present difficulties, and PLD could be an interesting alternative to achieve this goal. On the other hand, by having a material that exhibits MCE inside the pores, one could use it to heat the mesoporous matrix by applying an external magnetic field. To study this, thin films with different deposition times were grown by PLD and were characterized structural, morphological, and magnetically to understand if the synthesis of LSMO within the nanopores was possible. In addition, we study the magnetocaloric properties of the samples.

Finally, we study possible methods to directly detect the change in temperature produced in the material by the application of a magnetic field. This turns out to be a complex and interesting challenge, given how little material we have available to measure in thin films. We started by measuring the temperature change in bulk materials to gain experience to be able to measure it in films. Using clean room microfabrication techniques, we developed platinum microthermometers that were deposited on LSMO thin films. Finally, we simulate the temperature exchange between the film and the substrate through computational calculations to better understand the complexity of measuring the EMC directly.