Atomic-Scale Control and Detection of Ferromagnetic Phase Transformation by Using Atomic-Scale Probe

Controlling and detecting ferromagnetic phase transformations at high spatial resolutions are crucial for advancing our understanding of spintronics and high-density information storage applications. Traditional methods for achieving these transformations typically involve thermal treatment or chemical agents, which can significantly alter the thermodynamic phase diagram of bulk compounds. However, these methods have a fundamental limitation for local modification, as the entire sample is subjected to the same environment. Alternative approaches, such as using light excitation, biasing, or scanning tip-based methods, have been proposed and demonstrated to manipulate thermodynamic stability at the microscale. Nevertheless, achieving control at the nanoscale remains challenging due to the intrinsic length scale constraints of these methods. In this work, we propose a unique method using a high-resolution electron beam to control and detect the transition from non-ferromagnetic to ferromagnetic phases at the atomic level. We demonstrate that an atomic probe can initiate the phase transition between the rock salt and spinel structures in NiFe₂O₄. The electron beam allows for precise control of this transition, enhancing the material's properties at high spatial resolutions. The transition between ferromagnetic and non-ferromagnetic phases can be both controlled and imaged at the atomic scale. Furthermore, the ferromagnetic signal can be detected at the nanoscale using electron magnetic circular dichroism (EMCD), enabling the manipulation and detection of ferromagnetic phases with high spatial precision. Our study also provides insights into the mechanisms behind the ferromagnetic transition. Imaging of light elements revealed that the oxygen network in the rock salt films undergoes structural distortions, and transitional metal cations migrate through various lattice sites. These movements are facilitated by the presence of cation vacancies and lead to the formation of the ferromagnetic spinel phase when the rock salt films are exposed to an electron beam. This atomic-scale engineering enables potential applications in magneto-optic-based information storage and related devices.