Electrical Transport Studies of (111)-Oriented BTO/LSMO-Based Heterostructures
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Strain-induced magnetoelectric coupling in thin film heterostructures is a populartopic in the emerging field of multiferroic materials. Normally such heterostructuresare grown with a (001) orientation, but (111)-oriented structures may exhibit in-creased coupling at the interfaces. In this study, the electrical transport propertiesof (111)-oriented ferroelectric-ferromagnetic BTO/LSMO heterostructures were in-vestigated in order to explore the possible effects of magnetoelectric coupling.An experimental technique was established that enabled important transportproperties to be measured, such as resistivity, magnetoresistance, carrier densityand mobility. Van der Pauw s method formed the basis for the measurements,but a simplified version was employed which utilized data from a single bondingconfiguration. Because of additional simplifications and other potential errors, thetechnique was considered to be best suited for qualitative investigations.A one-band model was adopted when analyzing the Hall measurements, butthe process was complicated by the presence of the anomalous Hall effect. Tocircumvent the anomalous contributions, the slope in the linear region of the Hallresistance was used as a measure of the ordinary Hall effect. Overall, the Hallmeasurements were found to be most accurate in the region 100 275K. At highertemperatures, the complexity of the Hall effect prevented qualitative results frombeing obtained by the use of simple models. Below 100K instrument limitationswas the main issue.To improve the accuracy in future work, the following suggestions were pro-posed: A complete implementation of van der Pauw s method, usage of metalmasks for gold contact formation, and an automated method of optimizing curvefits and instrument parameters.Between 50 400K, the resistivities ranged from 0.3 mohm · cm to 80 mohm · cm,which is comparable to known values for (001)-oriented LSMO. A reference sampleexhibited a magnetoresistance close to -45% at 3T, which surpasses the findings insimilar studies. Between 100 275K, the carrier densities were calculated as 1 2holes/unit cell, which is in agreement with reports on (001)-oriented LSMO. An upto 10-fold increase of the resistivities was observed for samples with LSMO grownon top of BTO rather than directly on the STO substrates. Furthermore, the metal-insulator transition temperatures and magnetoresistance peak temperatures wereup to 70K lower for these samples. This was believed to be caused by lower LSMOfilm quality due to non-ideal epitaxial growth of the BTO layers. Even though thetransport properties differed significantly between the heterostructures, no clearsigns of strain-mediated magnetoelectric coupling were observed.