Epitaxial complex oxides

One of the most promising and rapidly developing branches of modern electronics is called spintronics and utilizes both the spin and charge of electric carriers. Spin valves and magnetic tunnel junctions are typical examples of spintronics devices which employ the Giant Magnetoresistance Effect in multilayered structures consisting of alternating magnetic and non-magnetic layers. In order to improve the efficiency of these spin devices and understand the physics which is responsible for their work it is important to have atomically sharp interfaces and single crystal quality of the multilayers, i.e. the whole heterostructures need to be epitaxially grown. While metals are ill-suited for this purpose, magnetic oxides present an interesting alternative, which explains the worldwide extensive research on these materials. Our goal in this project is to study epitaxial magnetic oxide (MO) heterostructures at the nanometer scale, which is the scale of actual spin devices in future electronics. The MO multilayers are obtained as part of a DOE collaboration with the University of Wisconsin, Madison.

It is expected that the low dimensionality will significantly alter the physical properties and behavior of MO heterostructures. This makes the experiments on electron transport and magnetic properties of nanometer size MO devices highly desirable. The most challenging task in this respect is to produce such nanoscale spin devices. We use e-beam lithography technique to deposit metal nano-pillows of desired shape used as a mask (e.g. circle arrays shown in the figure below) on a MO multilayer. Performing plasma etching on the samples will remove the MO material from the parts not protected by mask pillows, thus leaving MO columns whose lateral size is determined by the shape of the masks, and vertical size is determined by the etching time. Using e-beam lithography again and variable angle metal evaporation should allow us to make contacts to the top and bottom layers of the MO columns. SEM and AFM imaging is used to check the structural quality of the produced samples and to calibrate the processing parameters of each stage of the technique.

We are also trying to investigate some fundamental issues related to the CMR manganites. Physical properties of the manganites have been investigated for nearly 50 years. Still the nature of metal-insulator (MI) transition observed in heavily doped manganites is an outstanding issue. The most widely accepted explanation says that intrinsic inhomogeneities in such systems lead to phase separations into conducting and insulating domains in the same sample. The system is at its metallic (ferromagnetic) ground state at very low temperature and at higher temperatures the insulating (non-magnetic) regions are expected to emerge slowly. With such inhomogeneities, the interesting nature of the metal-insulator transition can be explained within a percolation transition model.  Our goal is to probe the temperature evolution by directly imaging the inhomogeneities by electrostatic force microscopy (EFM, which is sensitive to the local conductivity of the sample) and magnetic force microscopy (MFM, which is sensitive to the local magnetic property of the sample) simultaneously. This study is being done on epitaxially grown thin films of La_xSr_1-xMnO₃ (LSMO) with our homebuilt scanning probe microscope with a tuning fork transducer.