Similar to previously demonstrated magnetocapitance but using different materials, the researchers built a quantum tunnelling junction – two metal electrodes with an insulator in the middle – to induce a large increase in the junction's capacitance.
"It gives us more parameter space to design devices," said Gang Xiao, chair of the physics department at Brown University. "Sometimes normal capacitance might be better; sometimes the inverse might be better, depending on the application. This gives us a bit more flexibility."
According to the team, magnetocapacitors could be useful in making magnetic sensors for a range of different spintronic devices, including computer hard drives and next-generation random access memory chips.
The researchers claim the junctions can work in much the same way as capacitors in standard circuits. But what makes tunnelling junctions especially interesting is that the amount of capacitance can be changed dynamically by manipulating the spins of the electrons within the two metal electrodes.
The relative spin direction between two electrodes determines how much capacitance is present at the junction.
"We used iron for one electrode and iron oxide for the other," Xiao said. "The electrical properties of the two are mirror images of each other, which is why we observed this inverse magnetocapacitance effect."
In additional experiments, using electrodes made of iron-cobalt-boron, the researchers showed that by flipping spins from anti-parallel to parallel, they could increase capacitance by 150%. Based on those results, the team developed a theory predicting that, under ideal conditions, the change in capacitance could go as high as 1000%.
