Magnetism and Nanocrystals Promise Denser Storage, New Devices
When complex materials are reduced to the nanoscopic scale, never before observed electronic transport phenomena can be seen. Researchers at Oak Ridge National Laboratory are striving to gain a fuller understanding of the mechanisms at play, which could lead to new device applications in spin valves, MRAM or photovoltaics.
Alexander E. Braun, Senior Editor -- Semiconductor International, 10/30/2008 6:36:00 AM
An Oak Ridge National Laboratory (ORNL, Oak Ridge, Tenn.) research team is working on creating a new class of nanomaterials. Simple materials such as gold and silicon are widely studied at the nanoscopic scale and can behave differently in this regime where quantum complexity begins to dominate their properties.
Complex materials such as transition metal oxides and the cuprates rely on a fine balance of energies within their structures to create desirable macroscopic properties such as colossal magnetoresistance (CMR) and high-temperature superconductivity. By reducing complex materials to nanoscopic scales, researchers are finding never before observed electronic transport phenomena while gaining a fuller understanding of the mechanisms at play. These findings could lead to new device applications with integration into current semiconducting systems, such as spin valves, MRAM or photovoltaics.
The research, funded by the Department of Energy (DoE, Washington, D.C.) and the National Science Foundation (NSF, Arlington, Va.), is looking at how nanoscale changes can result in tremendous effects on a complex material’s properties. Now, for the first time, researchers have a method to see and possibly even predict those effects. The current work is focused on certain CMR single-crystal materials that have enormous — seemingly disproportionate — changes in resistance under an applied magnetic field. “That doesn’t sound too interesting until you remember that your computer hard drive relies on giant magnetoresistance (GMR),” said Zac Ward, a member of the Materials Science and Technology Division at ORNL.
By applying the concept of complexity to the study of materials at the nanoscale level, scientists hope to be able to see the interrelations between base components and be able to tune existing and new materials to create previously unseen properties. “If we can start to unravel interactions within complex materials, we’ll be able to better engineer devices from materials that are based on complexity,” Ward said.
The Oak Ridge group is working with complex oxides, such as (La1-yPry)5/8Ca3/8MnO3 (LPCMO) manganites. “This material has an interesting electronic phase separation — it’s a single-crystal disordered system,” Ward said. “However, inside you have large-scale, micron-sized domains of insulating and metallic regions, which is fascinating since it is atomically disordered. It isn’t because we have one doping of an insulating material coexisting with one doping of a metallic material. This type of strange behavior is what makes complex materials so interesting.”
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| Spatial confinement allows transport probing of complex materials. This figure illustrates a transport lane being forced to pass through a region of high resistance. (Source: Oak Ridge National Laboratory) |
“If you can sufficiently confine such a structure, you can take advantage of these inherent regions — these intrinsic regions,” Ward said. “When the electrons are going through, you’re now probing both regions of the material. So, specifically for LPCMO, what happens when observing the CMR effect may make it possible to figure out exactly how these phase transitions and interactions actually feed and create the observed property.” The importance of this relates to the use of regular GMR in hard drives and other similar devices. A material that is orders of magnitude more strongly affected by a magnetic field, insofar as its resistance is concerned, could have significant large-scale storage and device implications once it is better understood.
Among other possibilities, this might allow some materials to be fine-tuned for different applications. “This is what we are looking for,” Ward said. “The materials with which we are currently working, where these interesting transitions happen, are down at ~100 K; these are low-temperature effects taking place. However, with other phase-separated materials like La1-xSrxMnO3 (LSMO), which also has this strong response, these transitions happen at room temperature.”
Ward added that in the latter case, the domain sizes are much smaller, and that one of his goals is to continue shrinking device sizes sufficiently to take advantage of just about any phase-separated material, regardless of domain size. “At some point, we’ll be working at an atomic level; right now we’re dealing with structures in the micron range — very large-scale domains. That’s what is so interesting about LPCMO — it has huge domains in it. But whenever you go to LSMO or LCMO, those have domain sizes between tens and hundreds of nanometers.” It is at this range where the researchers will begin to use focused ion-beam (FIB) etching and some plasma etching techniques. They are beginning to design from the top down, but also taking advantage of the materials’ intrinsic properties.
Ward emphasized that this is an ongoing work on basic physics. The researchers are trying to understand the physics involved in electronic phase interactions, what happens at separation. They recently obtained, for the first time ever, the timings of individual phase transitions as they happen. “We’re still trying to understand all of the implications of these new observations,” Ward said. “However, we are already beginning to look at how to engineer new devices to take advantage of what we have found by, for example, integrating complex materials with semiconducting substrates and determining how this might be implemented into the computing industry or in other applications.”
The Oak Ridge researchers will continue their search for new materials and new ways of getting these devices smaller and smaller. As Ward put it, “I am pretty confident that spatial confinement of complex materials will continue to lead to the discovery of new and interesting electronic behaviors. We are just now taking the first step, but already we can see from these studies that there could be some very exciting future applications on the horizon.”
