When water freezes into ice or boils into vapour, its properties change dramatically at specific temperatures. These so-called phase transitions are fundamental to understanding materials. But how do such transitions behave in nanomaterials? In Nature Communications, a team of scientists led by TU Delft (The Netherlands) presents new insights into the complex nature of phase transitions in magnetic nanomaterials. Their findings reveal the coupling between magnetic and mechanical properties, paving the way for ultra-sensitive sensors.
The scientists from TU Delft, together with colleagues from the University of Valencia and the National University of Singapore, studied the 2D nanomaterial FePS₃, which is just a few atoms thick. For the first time, they developed a method to gain deeper insights into the highly complex phase transitions of such materials. By using tiny, suspended membranes of FePS₃, the team vibrated the material at high amplitudes while sweeping the temperature. This revealed how the material's vibrations change near its phase transition temperature and, with that, its magnetic properties.
"Imagine a drum with a magnetic structure, where the laser light acts as the drumstick—continuously making it vibrate while its rhythm subtly shifts with changing temperature," explains Farbod Alijani, associate professor at the TU Delft Faculty of Mechanical Engineering. "While warm, this magnetic drum is loose, and its magnetic spins, which are natural turns in particles that make them act like small magnets, are in a disordered phase. But once cold, the drum tightens up, with the spins snapping into an orderly pattern. Now, imagine that while drumming, you slowly change the temperature from warm to cold. As you do, you notice not only when the drum starts to feel different but also that this change isn't smooth (linear)—it unfolds in an intricate and irregular (nonlinear) manner, affecting its mechanical properties."
Phase transition temperature
The researchers essentially measured this nonlinear change during the phase transition. By using a nanoscale drum, they could detect the temperature at which this sudden transformation occurs and study how the drum's mechanical behaviour changes in detail. "We pinpointed the phase transition temperature at around -160°C," says Makars Šiškins, whose PhD work inspired this study. "Additionally, we found that the changes in the mechanical response driven by the temperature shifts are directly coupled to the material's magnetic and elastic properties."
Ultra-sensitive sensors
These membranes are exceptionally sensitive to both internal and external forces. Šiškins adds: "This sensitivity positions them as ideal candidates for sensors capable of detecting even very small environmental changes or internal stresses in the material itself."
The team plans to apply this methodology to unveil the secrets of phase transitions in other nanomaterials. Co-author Herre van der Zant: "In our lab, we will investigate whether we can detect so-called spin waves with the nanodrum. You can think of spin waves as carriers of information in a magnetic material, much like electrons are for conductive materials." Alijani will focus on translating these findings into practical applications, such as improving sensor performance. "Understanding these nonlinear processes lays the basis for innovative nanomechanical devices, including ultra-sensitive sensors," he notes.
