Rice University researchers have discovered that gently compressing an iron sulphide crystal can modify two of its unusual properties at the same time: its small magnetic signal and the way electricity moves through it. The result gives scientists a clearer view of how a recently recognised class of magnetic materials works and suggests a simple way to control their behaviour.
The material belongs to a group known as altermagnets. These materials are attracting growing interest because they combine useful features of two familiar types of magnets. Like antiferromagnets, their internal magnetic moments largely cancel each other out, so they do not produce the strong external magnetic field of an ordinary magnet. At the same time, they can still affect moving electrons in ways that could be useful for future electronic devices.
The Rice team studied a hexagonal form of iron sulphide, or FeS. Although most of its magnetism cancels out, the material has a very small residual magnetic moment. It also produces an unusual electrical signal known as the anomalous Hall effect: when current flows through the material, a small voltage appears perpendicular to it even when no external magnetic field is applied.
To conduct the experiment, the researchers built a device that gently compresses the crystal in one direction. As pressure increased, the small magnetic moment weakened, as did the unusual perpendicular voltage. The much larger underlying magnetic order, however, remained essentially unchanged. The team then used neutron beams at Oak Ridge National Laboratory to look inside the material and determine what the pressure was doing to its magnetic arrangement. Neutron measurements showed that the basic magnetic structure remains the same, but compression changes which magnetic orientations are most common within the crystal.
The findings also address an important question about why the material produces its unusual electrical signal. A common explanation involves the way electrons move through the crystal's electronic structure, an effect described by physicists using the term "Berry curvature". Rice experiments do not rule out that explanation. But they show that the electrical signal changes in line with the material's small magnetic moment, suggesting the two are linked to the same underlying physics. According to officials, the ability to control such effects with small mechanical pressure could eventually be useful in spintronics, a field that seeks to use the magnetic properties of electrons to store or process information. Devices based on this idea could potentially operate with less unwanted magnetic interference and lower power consumption than some conventional technologies.
The study was published in Advanced Materials. The work combined crystal growth, electrical measurements, magnetisation measurements and neutron scattering experiments, in collaboration with groups from Qimiao Si and Emilia Morosan at Rice's Emerging Magnetic Materials Laboratory.