For years, ruthenium dioxide (RuO\u2082) has kept physicists confused. Depending on which experiment they looked at, the metallic oxide either appeared magnetic or not magnetic at all.\u00a0<\/p>\n
This uncertainty has made it difficult to determine whether RuO\u2082 belongs<\/a> to an unusual new class of materials known as altermagnets, which could one day power faster and more energy-efficient electronics.<\/p>\n Now, in a new study, researchers have shown that the missing piece may not be the material itself\u2014but how tightly it is stretched. By growing an ultrathin RuO\u2082 film just two nanometers thick and keeping its crystal lattice under strain, they detected clear signs of magnetic order that had remained hidden in thicker, relaxed samples.\u00a0<\/p>\n The findings suggest that magnetism in RuO\u2082 is not a fixed property but something scientists may be able to switch on simply by engineering the material\u2019s atomic structure. \u201cRuO2 has attracted renewed interest due to expectations of prominent altermagnetic spin splitting,\u201d the researchers said<\/a>.<\/p>\n The debate over an unusual magnetic material<\/p>\n RuO\u2082 is already widely used as an industrial catalyst<\/a> and is valued for its excellent electrical conductivity.\u00a0<\/p>\n However, over the past few years, it has become the focus of an intense debate because some experiments suggested it might be an altermagnet\u2014a recently proposed type of magnetic material that combines features of conventional magnets and nonmagnetic materials.<\/p>\n Unlike an ordinary bar magnet, an altermagnet<\/a> has almost no overall magnetization even though its electrons remain magnetically ordered. Previous studies, however, produced conflicting results.\u00a0<\/p>\n Some reported magnetic order, unusual Hall effects, and efficient spin-charge conversion, while others found no evidence of magnetism in bulk crystals or relaxed thin films. A recent study<\/a> even concluded that RuO\u2082 showed no altermagnetic behavior down to films 5 nanometers thick.\u00a0<\/p>\n However, no one had directly examined fully strained films thinner than about four nanometers using techniques capable of measuring both the motion and spin of electrons at the same time.<\/p>\n \u201cThe spin structure of RuO2 remains largely unexplored in the ultrathin limit,\u201d the study authors note.<\/p>\n Stretching atoms to uncover hidden magnetism<\/p>\n To explore that missing regime, the researchers grew atomically smooth RuO\u2082 films only two nanometers thick on titanium dioxide-based substrates. Since the substrate\u2019s atomic spacing differs slightly from that of RuO\u2082, the ultrathin film is forced to stretch to match it\u2014a condition known as epitaxial strain.<\/p>\n You can think of it like laying a small wire mesh over a frame with slightly different spacing. The mesh has to stretch to fit, subtly changing the positions of every junction. In a crystal, those tiny shifts alter how electrons move and interact.<\/p>\n The team then used spin-resolved angle-resolved photoemission spectroscopy (spin-ARPES<\/a>), which shines light on the material to eject electrons. By measuring each electron\u2019s energy, momentum, and spin, scientists can map the material\u2019s electronic structure in remarkable detail.<\/p>\n To ensure the results were genuine, they repeated the measurements in two different experimental geometries to separate real magnetic signals from possible measurement artifacts. Additional X-ray and optical experiments confirmed that the films remained fully strained and revealed their crystal symmetry.<\/p>\n Hidden magnetic signals emerge<\/p>\n The measurements uncovered an unusual momentum-dependent spin texture, meaning the direction of an electron\u2019s spin changed systematically depending on how it moved through the crystal.<\/p>\n After performing a detailed symmetry analysis, the researchers ruled out nonmagnetic explanations, including effects caused by the crystal\u2019s polar structure or by the measurement process itself.<\/p>\n As the study authors note<\/a>, \u201cA comprehensive symmetry analysis rules out nonmagnetic origins of this spin texture. These findings suggest an emergent nonrelativistic spin structure enabled by epitaxial strain in the ultrathin limit, marking a distinct departure from the behavior of relaxed or bulk RuO\u2082.\u201d<\/p>\n