Hypersensitive Wires Feel the (Electromagnetic) Force
The ability to pack bits of data on computer hard drives has skyrocketed more than 10,000-fold over the past 3 decades. You can now fit more than 100 Hollywood movies on the average machine. One reason has been the steady improvement in sensors used to read and write bits of data in the magnetic materials used to make the disks. Now researchers describe the most powerful such sensing material yet to work at room temperature. The discovery may open the door not just to reading out smaller data bits, but also to a wide range of improved magnetic technologies such as making cheaper touch screen displays.
At the heart of data reading and recording devices is a property called magnetoresistance (MR), in which the electrical resistance of a material changes in response to the presence of an external magnetic field. Turn on a magnetic field, and the material's ability to carry an electric current skyrockets or plummets in response. Early MR materials changed their resistance only by a few percent at room temperature. Giant magnetoresistive materials discovered in the late 1980s pushed the number up to 110%. And researchers in Japan raised it to 600% in 2002 with the discovery of materials that carry out something called tunnel magnetoresistance. But now all those numbers pale in comparison, as a paper published online today in Science reports that molecular wires are capable of a 2000% magnetoresistance change at room temperature .
Ironically, the new molecular wires aren't made with magnetic materials at all. Rather, their MR effect relies on the conductivity of nonmagnetic organic dye molecules called DXP, which the Italian automaker Ferrari once used to give their roadsters their trademark red color. Unlike conventional inorganic metals in which electrons zip through a crystalline lattice, in organics electrons must hop from one molecule to another, like pails of water being passed by a bucket brigade. To create a MR, material researchers need to switch off that bucket brigade in the presence of a magnetic field.
In organic materials researchers do this with a little help from quantum mechanics. A tenet of quantum mechanics called the Pauli Exclusion Principle states that no two fermions (particles in a family that includes electrons) can occupy the same quantum state. If two electrons with the same quantum state try to hop onto the same DXP, they can't. The bucket brigade turns off and resistance skyrockets.
But over the past several years, researchers have found that thin films of DXPs or other organic conductors have an MR well below the competition. The reason for this turned out to be another quantum mechanical property. In addition to carrying a negative electric charge, electrons also carry spin, which can point up or down like a tiny bar magnet. If two electrons have the same spin, they can't hop on the same DXP together. But if one electron's spin flips to the opposite direction, then it's no problem. The two can hop on one DXP together, and the bucket brigade continues.
In their work with films of DXPs and other organics, researchers found that two problems prevented the films from acting like good MR materials. First, thermal fluctuations at room temperature flipped electron spins. And second, even if electrons did share the same spin direction—and were thus blocked from hopping onto the same DXP—they just jumped to a neighbor that wasn't blocked. "If it's a 3D film, you can always go around the blockade," says Markus Wohlgenannt, a physicist at the University of Iowa in Iowa City, whose team was one of the first to discover organic MR materials.
To prevent this runaround, researchers led by Wilfred van der Wiel, a physicist at the University of Twente in the Netherlands sought to arrange the DXPs in straight lines. To do so, they essentially shoved them inside the narrow pores of a zeolite, a lattice-like mineral, in which the confines were so tight the organics had no choice but to line up. They then placed their zeolite atop a conductive surface with the pores facing up and used the tip of an atomic force microscope to make contact with individual DXPs at the top end of single pores. The lineup of DXPs obviously meant that electrons could no longer hop around a blockade. But they also found that even very small magnetic fields were enough to prevent thermal fluctuations from flipping electron spins. And the result was that when electrons encountered blockages, they were unable to work around them, and the resistance of the material shot upwards.
Wohlgenannt calls the new work "a groundbreaking paper." That said, he adds that it's not clear if this will lead to higher capacity disc drives. For starters, researchers must first pull off the effect without the use of atomic force microscopes, which aren't a practical addition to disk drive technology. Researchers will probably also need to figure out ways to push higher electrical currents through the molecular wires to make magnetic sensors that can compete with current technology. But even if the new materials aren't ideal for making better disk drives, Wohngenannt and van der Wiel say the powerful MR effect might still make them useful for other electronics applications such as pen-based touch screens that are responsive to a magnetic stylus or perhaps even improved magnetic sensors in smart phones that are able to pick up the Earth's magnetic field and use that for improved navigation.