An international interdisciplinary research team led by Prof. Richard GU Hongri, Assistant Professor of the Division of Integrative Systems and Design at The Hong Kong University of Science and Technology (HKUST), has made a groundbreaking discovery that challenges a centuries-old understanding of friction. For over 300 years, scientists have adhered to Amontons' law, which posits that friction increases monotonically with the load pressing two surfaces together. However, this new study reveals that friction can manifest even without physical contact, opening avenues for the development of wear-free technologies and reshaping our comprehension of this fundamental rule that governs everyday activities from walking to braking a car. The study titled "Nonmonotonic Magnetic Friction from Collective Rotor Dynamics" was recently published in the leading international journal Nature Materials.
In collaboration with scholars from the Universität Innsbruck and the University of Konstanz, the findings demonstrate that friction can arise entirely without mechanical contact, driven instead by collective magnetic dynamics. Even more strikingly, the friction does not increase steadily with load-it reaches a peak at a specific distance where magnetic interactions become frustrated and hysteretic.
Prof. Gu stated, "This study shows that friction is not solely a mechanical phenomenon. It can originate entirely from internal reconfigurations-specifically, collective magnetic reorientations-even when two surfaces never make contact."
Watching Friction Emerge from Magnetism Alone
To uncover this effect, the research team built a controlled experimental system consisting of two parallel magnetic layers: a two-dimensional array of tiny, rotatable magnets on the top and a fixed magnetic substrate with a matching lattice pattern at the bottom. As the upper layer slides laterally, the magnets interact solely through their magnetic fields, without any physical contact. By adjusting the vertical separation between the two layers, the researchers could precisely tune the balance between competing magnetic interactions. This system yields distinct behaviors: at large separations, the magnets adopted an orderly antiferromagnetic pattern; at small separations, they aligned ferromagnetically. These tendencies competed at an intermediate distance.
"It is precisely in this frustrated regime that friction reaches its peak," explained Prof. Gu. "As the system slides, the magnetic moments repeatedly switch their collective orientation, leading to hysteresis and energy dissipation."
This setup provides direct visualization of each individual magnet, allowing the team to track magnetic configurations in real time and unambiguously correlate them with measured friction forces-a feat not achievable in conventional atomic-scale experiments.
Challenging a 300-year-old Law of Friction
This discovery marks the first clear experimental demonstration that friction can be generated solely by collective magnetic reordering, without any physical contact or surface wear. It challenges the presumed universality of Amontons' law and reframes friction as a phenomenon intrinsically linked to internal degrees of freedom, such as magnetic or structural order.
Prof. Gu said, "Traditionally, friction is associated with surface roughness and mechanical deformation. Our results show that energy dissipation can instead be governed by how an internal system reorganizes itself during motion."
Implications for Nanoscale Magnetism and Tunable, Wear-Free Interfaces
The scale-free nature of the underlying physics indicates that the significance of these findings extends far beyond the macroscopic model system experimentally demonstrated. Comparable effects could manifest in atomically thin magnetic materials, where even very small mechanical displacements may disrupt magnetic order. This breakthrough paves the way for probing and controlling magnetism through frictional measurements.
Looking ahead, the work also heralds to a new class of tunable frictional interfaces that operate without wear. By harnessing magnetic hysteresis, friction could be adjusted remotely and reversibly, enabling technologies such as frictional metamaterials, adaptive dampers, and contactless control elements. Potential applications are vast, spanning micro- and nanoelectromechanical systems, magnetic bearings, vibration isolation platforms, and ultrathin magnetic devices where motion is intimately coupled with internal magnetic states.
More broadly, the study proposes magnetic friction as a novel mechanical means to access collective spin dynamics, forging a fresh bridge between tribology and magnetism.
