Voltage-controlled magnetism in 3d ferromagnets

It is of fundamental importance to investigate the coupling of electric and magnetic order parameters in a solid state system. Many such studies have previously focused on multiferroic materials and magnetic semiconductors where a large electric field can be established within the solids. In metals, the penetration depth of electric fields is typically less than a few angstroms, therefore magnetic properties normally are not influenced by electric fields. However, with better understandings of magnetism in thin films and the advancement of various fabrication techniques, it is now possible to control the magnetic properties of metallic thin films by electric fields.

Two type of voltage effects on 3d ferromagnetic metals (FM) are under investigation. The first effect is the voltage controlled anisotropy (VAC) where the magnetic anisotropy field can be substantially modify by an external electric field, while the change of saturation magnetization is small. This is also refereed as voltage controlled magnetic anisotropy (VCMA) effect. Theoretically, VCMA/VCA can be described by the redistribution of charge density among different d orbitals of FMs due to the applied electric field, or through voltage-controlled Rashba spin-orbit coupling and Dzyaloshinskii-Moriya Interaction. Since only motion of electrons is involved, VCA if fast (sub-nanosecond), but without nonvolatility. The second effect is the voltage controlled magnetism (VCM), where both anisotropy field and saturation magnetization can be fully controlled by changing the oxidation states of FMs through voltage-induced ionic motion of oxygen vacancies from the gate oxide. Since the motion of O- ions is involved, VCM can be nonvolatile, but its speed is accordingly slower in the current stage of investigation.

An example of VCA/VCMA: In a bilayer structure consisted by CoFeB and MgO with appropriate physical and chemical properties, an interfacial perpendicular magnetic anisotropy can be realized due to the hybridization between the 3d orbitals of Fe/Co and the 2p orbital of oxygen. Remarkably, the magnetic properties of the metallic ferromagnetic layer such as coercivity and anisotropy field can be controlled by an electric field, via a voltage applied to the oxide layer. As shown in the figure below, the coercivity of a metallic CoFeB nanomagnet could be dramatically changed by more than 20 times through the VCA effect.

An example of VCM: The nominal structure of the samples is Si/SiO2/Pt(4nm)/ Co(0.7nm)/Gd2O3(80nm)/Ta(5nm)/Ru(100nm). Gadolinium oxide films were fabricated by reactive sputtering in a UHV deposition system. X-ray diffraction shows that the as-grown films are of cubic Gd2O3 phase. The Co films have a saturation magnetization of 1200 emu/cm³ and an anisotropy field of 12.5 kOe They can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. Unlike the VCA effect, here both the saturation magnetization and anisotropy field of the Co layers can be simultaneously controlled by voltage in a non-volatile fashion, resulting in a large change of magnetic anisotropy energy up to 0.73 erg/cm² with a small electric field of 625 kV/cm. Through a combination of structural, magnetic, transport and spectroscopic studies, it has been demonstrated that this giant effect is achieved by voltage-induced reversible oxidation of the Co layer, which can be understood by a large interfacial EF and the high O^2- ion mobility in Gd2O3.

Relevant publications:

• "Reversible control of Co magnetism by voltage induced oxidation", Phys. Rev. Lett., 113, 267202 (2014)

• "Metal Based Nonvolatile Field-effect Transistors", Advanced Functional Materials, 26, 3490 (2016).

• "Electrical Control of Metallic Heavy-Metal-Ferromagnet Interfacial States", Phys. Rev. Appl., 8, 034003 (2017)