Role of oxygen ion migration in the electrical control of magnetism in Pt/Co/Ni/HfO₂ films

We investigate the electrical control of magnetization and exchange bias in Pt/Co/Ni/HfO₂ films gated by ionic liquid. Results show that saturated magnetization of Co/Ni can be significantly manipulated under finite voltages at room temperature. Unlike the conventional electrostatic effect, the electric gating here exhibits dynamic and nonvolatile features clearly. Hence, an electrochemical mechanism is proposed naturally and the analysis of chemical states of Ni and Co confirms the reversible oxygen ion migration across the interface between HfO₂ and Co/Ni under gate voltages. Furthermore, robust exchange bias is observed below 200 K and both the bias field and coercive field can be prominently modulated in Pt/Co/Ni/HfO₂ under electric field, resulting from gate voltage dependent content of antiferromagnetic oxidation products of Ni and Co. The demonstration of oxygen ion migration in ferromagnetic metals/oxides heterostructures followed by magnetization modulation at room temperature might pave the way for the magnetoionic memory with low power consumption.