The growth of high quality thin films is essential for the success of our projects. We grow our thin films using pulsed laser deposition (PLD). Thin film growth of materials such as (La1−yPry)1−xCaxMnO3 is complicated by the structural phase separation which accompanies the electronic phase separation at low temperatures. Thus, thin films which show excellent structural properties at room temperature may not show the expected transport and magnetic properties at low temperature due to the presence of the substrate. We have grown atomically flat and epitaxial (La1−yPry)1−xCaxMnO3 thin films on nearly lattice matched (110) NdGaO3 (NGO) substrates with optimized magnetic and transport properties using fine control of oxygen pressure, substrate temperature, and growth rate and without any post-annealing. This optimization was confirmed by various physical property measurements, viz. surface morphology, transport, structural, and magnetic measurements. In addition, we observed that disorder in the LPCMO thin films can be further reduced by using thermally treated NGO substrates which show atomic steps. We have been able to obtain such thin films from thicknesses of 20 nm to 120 nm. The quality of our as-grown thin films is the reason we were able to observe some of the effects such as single domain to multi-domain transition and dielectrophoresis.
The thin films are characterized using resistivity, and magnetization measurements to confirm the insulator to metal transition and growth of the FMM phase. In particular, we check for the sharpness of the resistivity transition at the insulator to metal transition temperature (TIM), the width of the thermal hysteresis, saturation magnetization, and magnetic anisotropy. The combination of the excellent structural, electronic, and magnetic properties make our thin films unique.
Scanning probe microscopy is used in our group to check the surface quality (atomic force microscopy, AFM), surface termination (lateral force microscopy, LFM), magnetic domain structure (magnetic force microscopy, MFM), and surface conductance (conducting atomic force microscopy, cAFM). We have also obtained direct evidence of phase coexistence using low temperature conducting atomic force microscopy (LTcAFM). The LTcAFM measures the surface conductivity using a conducting AFM tip and therefore has better spatial resolution to distinguish between the ferromagnetic (and metallic) and non-magnetic (and insulating) phases. Hence, to observe the phase separated state and the fluidity of the magnetic regions with higher spatial resolution we will use LTcAFM which has revealed features of about 1 nm in our LPCMO thin films.
In-plane electric fields show much larger effects on the transport behavior of phase separated manganites compared to field effect transistor (FET) configurations. The as-grown thin films are usually about 5 × 5 mm2 and while these samples show clear evidence of colossal electroresistance (CER), the time dependence of resistance is not well pronounced due to a smaller fractional resistance change when the dynamic percolation occurs in the thin film. Hence, to observe the electric field effects, we need to reduce the sample size. We were able to clearly observe the effects of dielectrophoresis when the sample size was reduced to the scale of about 100 × 100 µm2. To study the effect of spatial confinement on the phase separated state, we fabricate structures of sizes down to about a micrometer using photolithography and focussed ion beam (FIB) techniques. We then measure R(t) data for these structures and quantify the effect of sample size on dielectrophoresis. We are currently studying effect of sample size on the electric field effect as the sample size approaches the scale of phase separation where the transport is dominated by a few percolating paths. These measurements will be coupled with magnetization measurements in the presence of an electric field to form a clearer picture of the dielectrophoresis behavior using the bulk measurements.