Unexpected Superparamagnetic Behavior in Nanocrystalline Niobium-Based High-Entropy Alloys

Herein, we report on the evolution of superparamagnetic behavior of Nb–Cr–Ta–V–W refractory high entropy alloy (RHEA) nanoparticles synthesized by a facile mechanical pulverization (high-energy ball milling) technique. Detailed X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) analyses have been made to evaluate the structure, morphology, compositional, and elemental distribution characteristics of RHEA nanoparticles and further compared with the bulk. XRD and SEM data coupled with EDS analyses indicate that the nano-RHEA retains the parent crystal structure and phase, but the size dramatically decreases with milling time. The average crystallite size decreases from ∼32 to ∼12 nm with an increasing milling time to 16 h. While the bulk samples exhibit diamagnetic behavior, interestingly, RHEA nanoparticles demonstrate superparamagnetic nature. Magnetic measurements, in both room temperature and cryogenic conditions, provide evidence of the temperature-independent superparamagnetic nature of the RHEA nanoparticles. Furthermore, the saturation magnetization value exponentially increased with milling time and stabilized after 8 h of pulverization, whereas the magnetic domain size followed an opposite trend. We believe drastic reduction in the magnetic domain size is responsible for this unusual and unexpected superparamagnetic pattern of the RHEA nanoparticle, which opens up a new route for low-cost magnetic applications under harsh environment conditions.