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Localized reversible nanoscale phase separation in Pr_0.63Ca_0.37MnO_3 single crystal using a scanning tunneling microscope tip

We report the destabilization of the charge ordered insulating (COI) state in a localized region of Pr_0.63Ca_0.37MnO_3 single crystal by current injection using a scanning tunneling microscope tip. This leads to controlled phase separation and formation of localized metallic nanoislands in the COI matrix which have been detected by local tunneling conductance mapping. The metallic regions thus created persist even after reducing the injected current to lower values. The original conductance state can be restored by injecting a current of similar magnitude but of opposite polarity. We thus achieve reversible nanoscale phase separation that gives rise to the possibility to"write, read, and erase"nanosized conducting regions in an insulating matrix with high spatial resolution.

Rare-earth manganites having the perovskite structure R 1−x A x MnO 3 , where R is a rare earth ion and A is a divalent alkaline-earth ion, have been studied in great detail 1,2 for the rich variety of distinct phases they exhibit. Of particular interest is the charge ordered insulating (COI) phase which is found to be unstable under various external perturbations such as magnetic field 3 , electric field and current 4,5 (effects of which are also observed in manganites which are not charge ordered 6,7,8,9 ) and also by size reduction to few tens of nanometers 10 . Normally, the phase that is created by destabilization of the COI phase is the ferromagnetic metallic (FMM) ground state. Thus these materials are attractive candidates for various switching and sensing devices. The creation of FMM filaments in a bulk single crystal of a COI manganite by using a high current has been shown before 4 . In the present experiment, we demonstrate localized destabilization of the COI phase by current injection using a scanning tunneling microscope (STM) tip over a small area with size ≈ 25 − 30nm.
This phenomenon gives rise to the possibility of patterning (writing) nanosized metallic regions by controlled phase separation on an otherwise insulating background. This process can be reversed (erased) and the metallic region can be switched back to the COI state at will by passing a current of reverse polarity and comparable magnitude to the writing current.
Hence, we are able to achieve reversible and controlled phase separation over nanoscopic length scales without altering the topography or structure of the surface.
The sample used in our experiments was a single crystal of Pr 0.63 Ca 0.37 MnO 3 (PCMO) grown by float zone technique. The sample has been well characterized and shows a CO transition at T CO = 235K 4 . The variable temperature ultra-high vacuum STM used was constructed in our laboratory and we used an SPM100 controller (RHK Technology Inc., USA). The STM was pumped to a base pressure of better than 1×10 −8 Torr after which cryopumping takes over. The STM tips were mechanically formed Pt/Rh (87:13) wires. The crystal was freshly broken to create a clean surface for STM measurements.
In the COI phase, there exists a gap in the charge excitation spectra ( ). The gap collapses when the COI state is destabilized into a metallic state as has been shown in magnetic field induced destabilization 11 . We first measured through scanning tunneling spectroscopy (STS). The tunnel current was stabilized at 1nA at a bias of 0.7V. The bias was then swept between ±0.75V and the tunnel current-voltage (I − V ) spectra were recorded.
The dI/dV − V curves were evaluated from the tunneling spectra and was evaluated taking into consideration the finite temperature effect. In figure 1, we show as a function of temperature. The CO temperature can be clearly identified from the temperature dependence of the gap. Above T CO the gap is ≈ 0.05eV , as is generally observed in the polaronic insulating states of other manganites 12 . We then measured for higher tunnel currents in a step by step process by bringing the tip closer to the sample. Higher tunnel currents were stabilized at the same point for the same bias and the I − V spectra were recorded again. This process was repeated for various tunnel currents and the dI/dV − V curves were evaluated from the tunneling spectra. It was found that the gap decreases linearly with increasing tunnel current. The higher tunnel currents thus reduce the CO gap leading to destabilization of the CO state, as can be seen from the inset of figure 1 where we show the dI/dV − V curves taken at 152K for tunnel currents of 1nA and 4nA. The destabilization typically occurs around a tunnel current of 4-5nA. This information is important and is used in deciding the "write-read-erase" cycle as will be shown below.
The above observation enables us to use an STM tip to selectively destabilize a given CO region thus controlling spatially the region that one would like to drive metallic in a COI matrix. We refer to this as the "writing" process. We demonstrate this in fig 2 where we have carried out local tunneling conductance mapping (LCMAP) over a given area at T = 152K (T < T CO ). The LCMAP technique generates a two-dimensional map of the tunneling conductance across the surface for a fixed bias and at a fixed average tunnel current. This is achieved by applying a small ac bias modulation over the dc sample bias and using a lock-in amplifier to measure dI/dV directly. One can simultaneously record topography as 3 well as the LCMAP of the same area. Fig 2(a) shows the LCMAP of a 100nm×100nm area of the surface of PCMO, taken at a tunnel current of 1nA. We then zoomed into an area marked by the arrow and scanned an area of 25nm×25nm using a tunnel current of 5nA. This was the "writing" cycle. We then zoomed out to the original 100nm×100nm scan area and recorded the LCMAP again with a tunnel current of 1nA. This was the "read" process that established whether the chosen region has become more conducting. As can be seen from fig2(b), there is a square area of about 30nm×30nm which has significantly higher tunnel conductance than the surrounding region. In our color scheme, the higher tunneling conductance region appears brighter. The brighter region has a conductance that is typically ≈ 3 times higher than the surrounding darker region. (The surrounding region is not uniformly dark and has small bright pockets that already existed in the pristine single crystal.) To "erase" the conducting region created in the above process, we injected a current of comparable magnitude but of opposite polarity. We zoomed back into the 25nm×25nm spot that was written before and scanned the same area again using -5nA tunnel current.
When we recorded the zoomed out image at 1nA again, we found that we had managed to erase the metallic region almost completely thus reverting it back to a region of low tunneling conductance, as can be seen in fig 2(c). This is a very important observation as this phenomenon can be used to selectively write, read and erase nanosized metallic domains (regions of high tunnel conductance) in an insulating background (regions of lower tunneling conductance) using an STM tip. The small region of high conductance marked with an arrow in figure 2(a) was used as a marker, so that when the erase process was done one could see that the region had almost reduced to the original size. The conductance of the starting marker patch was about 5.9nA/V (fig 2(a)) and after the erasing the conductance of the marker patch is 6.4nA/V. (fig 2(c)). We note that in this process the topography had not changed.
We then carried out STS measurements to establish that the nano-island created was a metallic region with no CO gap ( → 0). After the write process, we took tunneling spectra at different points on the surface that included the written region as well as the pristine regions. We found two distinct I − V curves as shown in figure 3. The inset shows the corresponding dI/dV − V curves. One set of I − V curves taken on the "written" region shows metallic behavior with no gap ( ≈ 0) while the other set taken on the virgin regions The coexisting phases were created after injecting a large tunnel current (5nA). The inset shows the corresponding dI/dV − V curves.
shows a finite gap ( = 0) opening up in the tunneling spectra. This shows that we have two distinct phases coexisting, the COI phase with a finite gap and the metallic region where a high tunnel current had been injected.
To investigate whether we can create controlled phase separation over a larger area, we carried out LCMAP measurements over a 75nm×75nm area. To start with, we scanned the entire area at a tunnel current of 1nA and bias of 200mV and recorded the LCMAP (fig 4(a)). We then increased the tunnel current to 5nA and scanned the entire area at this higher tunnel current. We then reduced the tunnel current back to 1nA and recorded the LCMAP taken over the same area, shown in figure 4(b). We find that, in this case, the LCMAP is brighter, that is, some regions have become more conducting upon passing It should be noted that the underlying topography of the single crystal surface makes this process somewhat difficult to observe over larger length scales as most of the contrast in the LCMAP can sometimes be masked due to the inhomogeneity of the surface. The experiment in principle can be better done on an epitaxial film. However, the strain in such films affects the charge ordering and can initiate phase separation.
We note that the write-read-erase cycle can be done repeatedly and also one can reverse the polarity to achieve the same cycle; that is, one can destabilize the charge ordering with a large negative tunnel current and subsequently restore it using a tunnel current of the opposite polarity but having roughly the same magnitude. This establishes that the direction of the tunnel current is not important in this process.
One particular issue that this experiment can address is whether the phenomenon is driven by current injection or is due to field effect. We argue that it is current injection driven. Firstly, since the tunnel junction has much higher resistance (≈ 100MΩ or more) than the sample, most of the voltage drop occurs across the tunneling gap and there is no significant voltage drop within the sample that can give rise to the observed phenomenon.
Secondly, if the phenomenon is due to a field effect, it would have depended on the polarity, since this would decide whether the concentration of holes or electrons was increasing. It is known that charge ordering is strongest in the x = 0.5 composition. Thus, in our sample where x = 0.37, a negative bias would imply injection of holes which should drive the system towards the x = 0.5 range, making the charge ordering stronger. Thus destabilization of the COI state would not have been possible. A positive bias would lead to the opposite effect.
However, we find that regardless of the sign, a higher tunnel current always destabilizes the charge ordering leading to formation of the metallic region. In conclusion, we establish that current injection through an STM tip can give rise to controlled phase separation thus creating metallic nanoislands in a COI. This observation raises the possibility of information storage in COI materials by tunnel current injection.
S.K. thanks CSIR, Government of India, for a fellowship. A.K.R. thanks DST, Government of India, for a sponsored project. 7