Voltage control of magnetism (VCM) is attracting increasing interest and exciting significant research activity driven by its profound physics and enormous potential for application. This review article aims to provide a comprehensive review of recent progress in VCM in different thin films. We first present a brief summary of the modulation of magnetism by electric fields and describe its discovery, development, classification, mechanism, and potential applications. In the second part, we focus on the classification of VCM from the viewpoint of materials, where both the magnetic medium and dielectric gating materials, and their influences on magnetic modulation efficiency are systematically described. In the third part, the nature of VCM is discussed in detail, including the conventional mechanisms of charge, strain, and exchange coupling at the interfaces of heterostructures, as well as the emergent models of orbital reconstruction and electrochemical effect. The fourth part mainly illustrates the typical performance characteristics of VCM, and discusses, in particular, * Corresponding author. Tel: +86-10-62781275; fax: +86-10-62771160 E-mail address: songcheng@mail.tsinghua.edu.cn † Present at Max-Planck Institute of Microstructure Physics, Weinberg 2, D-06120Halle, Germany 2 its promising application for reducing power consumption and realizing high-density memory in several device configurations. The present review concludes with a discussion of the challenges and future prospects of VCM, which will inspire more in-depth research and advance the practical applications of this field.

We investigate the current-induced switching of the Néel order in NiO(001)/Pt heterostructures, which is manifested electrically via the spin Hall magnetoresistance. Significant reversible changes in the longitudinal and transverse resistances are found at room temperature for a current threshold lying in the range of 10^{7}  A/cm^{2}. The order-parameter switching is ascribed to the antiferromagnetic dynamics triggered by the (current-induced) antidamping torque, which orients the Néel order towards the direction of the writing current. This is in stark contrast to the case of antiferromagnets such as Mn_{2}Au and CuMnAs, where fieldlike torques induced by the Edelstein effect drive the Néel switching, therefore resulting in an orthogonal alignment between the Néel order and the writing current. Our findings can be readily generalized to other biaxial antiferromagnets, providing broad opportunities for all-electrical writing and readout in antiferromagnetic spintronics.

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.

metal (Pt, Ta, etc.) and/or the Rashba effect from the inversion asymmetry of HM/FM and FM/oxide interfaces both contribute to the spin-orbit torque and the effective switching fi eld. [16][17][18][19] Half a decade has already passed since the fi rst report of SOT in perpendicularly magnetized heterostructures. [ 20 ] Efforts are increasingly shifting not only to explore SOT in different systems, but also to its manipulation in practical devices. [21][22][23][24] At the same time, electric fi eld control of magnetization is of great interest in the area of spintronics for its rich physics and potential use in information technology due to its low energy consumption and compatibility with classic electronics. [25][26][27] Following the effective manipulation of magnetic order by electrical means in different systems, Liu et al. [ 28 ] proposed the concept of electrical control of SOT in Pt/Co/Al 2 O 3 heterostructures, but the efficiency is in a quite limited scale. Very recently, Fan et al. [ 29 ] utilized the Cr-doped topological insulator and successfully modulated the SOT by a factor of four using a gate voltage ( V G ) of 10 V at 1.9 K taking the ultralow Curie temperature ( T C ) of the magnetic topological insulator into account. Now the research interest is whether an elegant approach could significantly modulate the SOT in a nonvolatile manner at a practical temperature ( T ). We experimentally demonstrated the strong electrical control of SOT in perpendicularly magnetized Pt/Co/ HfO x heterostructures. Positive and negative V G enhance and reduce the critical current for magnetization switching respectively with the assistance of ionic liquid gating mainly through the modulation of damping-like spin-orbit effective fi elds and the concomitant effective spin Hall angle of Pt/Co. Results and DiscussionIn our Ta(2)/Pt(4)/Co(0.3)/HfO x (4) (unit in nanometers) heterostructures ( Figure 1 a), the bottom Ta layer acts as a seed layer to form a better Pt(111) orientation, and consequently induces a strong perpendicular magnetic anisotropy (PMA). Despite its high spin Hall angle, [ 9,30 ] the contribution of spin current from Ta to Co is negligible due to the short spin diffusion length of Pt and opposite sign of the spin Hall angle of Ta and Pt. [ 8,31 ] The capping HfO x layer, with a large permittivity, also enhances the PMA and protects the metal layers from erosion by ionic Electrical control of current-induced spin-orbit effects in magnets is supposed to reduce the power consumption in high-density memories to the utmost extent, but the effi cient control in metallic magnets at a practical temperature remains elusive. Here, the electrical manipulation of spin-orbit torque is investigated in perpendicularly magnetized Pt/Co/HfO x heterostructures in a nonvolatile manner using an ionic liquid gate. The switching current of magnetization can be reversibly tuned by a factor of two within a small gate voltage range of 1.5 V. The modulation of effective spin Hall angle and the corresponding damping-like torque...

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