Ultra-low energy magnetization switching

Among many challenges facing the future generations of transistors, wasting of energy due to leakage current posts a serious problem when the size of the transistors is reduced to only a few nanometers. In fact, at such small dimension the energy wasted due to charge leakage will suppress the energy required to switch the transistors between the 0 and 1 states. One approach to this challenge is to employ a nonvolatile scheme to eliminate the steady-state power consumption. In this regard, spintronics offers a very promising route to solve this problem, especially for memory applications. There will be zero energy waste in the steady-state if the information is stored by spins in a nanomagnet. The challenge for spintronics is that at present the most well-known methods to switch magnetization, either by a magnetic field or by a spin-polarized current, still consumes more than 100 times of energy compared to conventional CMOS transistors. The focus of our study is to explore new physics, new materials and new device concepts that could lead to ultra-low energy magnetization switching. Historically, power consumption has been reduced dramatically through the transition of current-based bipolar junction transistor to voltage-based field effect transistors. It is believed the same benefit will occur for spintronic devices. We investigate the effects of electric fields on anisotropy, magnetization, spin-orbit interaction and magnetoelastic coupling. Understanding the physics of these effects will help us greatly reducing the switching energy in spintronic structures.

Voltage-induced precessional switching

In many spintronic applications, a magnetic tunnel junction (MTJ) structure is required. Normally, either a magnetic field or a spin-polarized current is required to switch a MTJ. The switching by a spin-polarized current, through the spin transfer torque (STT) effect, is generally more energy efficient than a magnetic field, and is site-specific to a particular nanomagnet. However, even for a state-of-the-art MTJ with perpendicular magnetic anisotropy (p-MTJ), the STT critical switching current density is still in the order of MA/cm², which corresponds to a large switching energy of ~100 fJ. Thanks to the large voltage controlled anisotropy in CoFeB/MgO perpendicular MTJs (pMTJs). The switching can now be accomplished by a voltage.

In essence, when a voltage with the right polarity is applied to a pMTJs, the easy axis of the free ferromagnetic layer can be changed to an in-plane orientation, e.g. the applied voltage can generate an effective in-plane field. The free layer magnetization will precess under the influence of this effective in-plane field. When the pulse duration of the voltage is controlled to be half of the precession period, the magnetization thus the pMTJ can be switched to a different resistance state. So far this method gives the lowest switching energy in pMTJs. We demonstrated sub-10 fJ switching in pMTJs with more than 140% TMR.

Electric-field-assisted magnetization switching

In this method both VCMA and STT are utilized to switch a MTJ. The energy required to change a magnetic tunnel junction between P and AP states is directly related to the anisotropy energy barrier of the free FM layer. A nanomagnet with uniaxial anisotropy has two energetically equivalent states with the magnetization pointing in opposite directions, in which the two energy wells are separated by an energy barrier. Note that, in both the H-field and STT switching scenarios, the energy barrier remains largely unchanged during magnetization reversal. One approach is to momentarily reduce barrier height during switching through the voltage controlled anisotropy effect. As a result, a large reduction of switching current could possibly be achieved.

The resistance states of a MTJ can be reversibly controlled by a special unipolar switching process under a small, constant biasing magnetic field.The optimal PMA for the MTJs is achieved through rapid thermal annealing after the fabrication of the samples. The STT effect occurring in MTJs with submicron sizes is greatly facilitated by the reduced PMA at negative voltages, which can be used to complement the electric field to achieve reversible switching. This unipolar switching process is schematically shown in the following figure. The zero bias hysteresis loop of the top CoFeB is in blue color with a large coercivity (the magnetization of the bottom CoFeB layer is kept pointing down). The position of biasing magnetic field is represented by the vertical dotted line. The initial state of the magnetization of top CoFeB layer is pointing down. When a negative voltage V₁ is applied, the hysteresis loop dramatically reduces its width (red curve) due to the electric field. Simultaneously, the STT switching occurs at the current density of -1.2 x 10⁴ A/cm². Therefore the magnetization is very efficiently brought to the up direction by V₁ and stays pointing up after V₁ is removed. The switching back is achieved by applying a more negative voltage V₂. Now the loop becomes the one in black color because |V₂| > |V₁|. Under thebiasing magnetic field, the only stable state is pointing down. The magnetization of top CoFeB is switched back by V₂ and stays so when V₂ is removed, as for V = 0 (blue curve) both up and down are stable states. The average switching current density is only -2.4 x 10⁴ A/cm² for the quasistatic pulses with width of 200ms, clearly demonstrating its merit as a very energy efficient switching process.

Related publications:

• "Electric field assisted switching in magnetic tunnel junctions", Nature Materials 11, 64 (2012)

• "Voltage-induced switching in magnetic tunnel junctions with perpendicular magnetic anisotropy", J. Phys. D: Appl. Phys.46, 074004 (2013).

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.

Related 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)