Abstract:
Since high entropy alloys (HEA) were proposed and fabricated in the early twenty-first century, design on multicomponent materials has captured considerable attention worldwide. [1] Fabrication and development of high entropy alloys (HEAs) with exceptional functionalities is a rapidly expanding field with many degrees of freedom. When considering the role of entropy in HEA, extrinsic factors such as the existence of grains and different phases should be separated from intrinsic configurations of the atomic lattice. [2] Here, we fabricate CoCrFeNi2Al0.5 HEA/muscovite heterostructures, with some prepared as epitaxial bilayers and others as an amorphous system. In this study, we uncover the structure order role in correlation to the structural, electronic, and magnetic properties of the HEA using a combination of energy-dispersive X-ray spectrometry, X-ray diffraction, transmission electron microscopy, magneto-transport, ac magnetometry, and X-ray absorption spectroscopy with magnetic circular dichroism. The atomic site disorder state shows a fully metallic behavior. In contrast, the structural disorder state displays metallic with intense magnetic saturation, Kondo-like behavior under 50K, and a low-temperature coefficient of resistivity of ~64 ppm/℃. The difference between the saturation magnetic moment and the electron relaxation behavior in the atomic site disorder and structural disorder states results from the existence of structure order affecting the atomic distance and periodicity to modify the exchange interaction [3] and tune the electron-phonon interaction for scattering. X-ray absorption and magnetic circular dichroism prob the ferromagnetic behavior contributed by Co, Fe, and Ni atoms to understand the magnetic interaction in the atomic site disorder and structural disorder states.
After understanding the role of entropy in epitaxial alloy systems, we further expand our research to high-entropy oxides (HEO) on this basis. Studying these new material systems forms a new playground due to the potential of developing materials with outstanding performance. [4] In this study, the heteroepitaxy of high entropy (Fe, Co, Ni, Cr, Mn)3O4 films with varying strain states have been investigated in magnetic performance. It is discovered that the high entropy oxide thin film with compressive strain exhibits an effect of crystalline magnetic anisotropy. Diverse analyses provide a detailed understanding of high entropy magnetic oxide systems, including x-ray diffraction, reciprocal space mapping, macroscopic magnetic characterization, x-ray absorption spectroscopy (XAS), etc. Notably, the element-specific XAS technique proves effective in uncovering the origin of the crystalline magnetic anisotropy. Due to the substrate-induced epitaxial strain, the eg orbitals of Mn3+ form different energy levels, leading to different preferred electron occupancy. The exploration of magnetic properties in epitaxial high entropy oxide film is then raveled, providing a new design rule for further studies in multicomponent materials.
In summary, we simplify and establish the impact of entropy on the physical properties in HEA systems through epitaxial growth. In epitaxial growth of HEO systems, even complex multi-element systems, specific elements' physical behavior can be analyzed with the assistance of a synchrotron light source. In the future, regardless of how complex the composition of multi-element materials may be, systematic research can be conducted based on this foundation.