Session Q50: Magnetic and Spintronic Devices

Computational Guiding Rules for the Design of Ultra-low Energy Spintronic Devices: Opportunities and Challenges

Voltage-induced magnetization switching can lead to a new paradigm enabling ultralow-power and high density nonvolatile MeRAM devices. Two major challenges for future MeRAM devices are to achieve large perpendicular magnetic anisotropy and high voltage-controlled magnetic anisotropy efficiency in heavy-metal/ferromagnet/insulator heterostructures.  First, I will present computational guiding rules for the design of ultralow energy spintronic devices where the heavy metal is across the 5d-transition metal series. The calculations reveal three important synergistic mechanisms to achieve low energy dissipation up to two orders of magnitude [1-4]. I will then discuss our recent work on antiferromagnetic resonance (AFMR) phenomena in metallic systems under an external electric field [5]. I will show that the AFMR linewidth can be separated into a relativistic component originating from the angular momentum transfer between the collinear AFM subsystem and the crystal through spin-orbit coupling, and an exchange component that originates from the spin exchange between the two sublattices. Both the AFMR linewidth, AFMR frequency and Néel temperature can be tuned by an external electric field. I will then present an ab initio-based theoretical framework which elucidates the origin of the bulk versus interface contributions to the spin-orbit torque (SOT) in heavy metal/ferromagnet bilayers [6-8] and reveals the microscopic mechanism for the recent experimental discovery of sign reversal of the field-like SOT due to interfacial Co oxidation. Finally I will discuss an ab-initio based Green's function approach to calculate the Dzyaloshinskii-Moriya interactions (DMI) that is computationally more efficient and accurate than the most commonly employed supercell and generalized Bloch-based approaches [9].    

Shaping terahertz emission using spintronic emitters

Terahertz (THz) falls in the gap between photonic and electronic in the electromagnetic spectrum. Conventionally, photoconductive antennas and nonlinear crystals, which employ only the electron charge, are used as standard THz sources. An alternative approach utilizes the spin degree of freedom: Exciting magnetic heterostructures with a femtosecond laser pulse results in the generation of an ultrafast spin current, which is then converted into THz transient via the inverse spin Hall effect. Here, we demonstrate the shaping of the THz spectrum from magnetic micropatterned heterostructures fabricated with optical lithography and magnetron sputtering techniques. Our results show that the properties of THz radiation can be controlled by a careful design of micropatterned geometries and dimensions. We demonstrate that the characteristic bandwidth of the THz spectrum can be efficiently controlled using microstructured stripes and the THz polarization can be tuned by forcing the magnetization into curling in chopped microdisks. Our results demonstrate control of THzproperties such as polarization and bandwidth using microfabricated spintronic emitters.   

Chirality-induced Spin Selectivity in Molecular Spin Valves: Role of the Nonmagnetic Electrode

 Chirality-induced spin selectivity (CISS), an effect in which structural chirality engenders spin polarization in the electrical current from a nonmagnetic metal (NM) electrode, has been observed in a variety of chiral molecules with various experimental probes. However, the microscopic origin and device manifestations of CISS remain controversial. Most theoretical models consider chiral molecules as a spin filter, despite the generally small spin-orbit coupling (SOC) in organic molecules. A recent theory posits that chiral molecules act as an orbital polarizer, and the SOC in nonmagnetic electrode converts the orbital polarization to spin polarization. Here, we report a comparison of CISS-induced magnetoconductance (MC) in vertical heterojunctions of (Ga,Mn)As/AHPA-L molecules/NM, between NM of Au and Al. The perpendicularly magnetized (Ga,Mn)As functions as a spin analyzer. The Au junctions show pronounced MC signals, which contain a large nonlinear-response component and a nontrivial-linear response component. In contrast, the MC of Al junctions are significantly diminished. Our observations suggest an important role for SOC in NM electrode in CISS-induced spin-valve effect.   

Voltage-Controlled Field-Free Switching of RKKY Coupled Multilayers

To reduce the power consumption of spintronic devices, voltage-control of magnetic order is being investigated as an alternative to current-controlled mechanisms. However, switching the magnetization of a metallic magnetic layer 180^○ with only an electric field is impractical because they do not break time-reversal symmetry and decay rapidly in metals. Magneto-ionic (MI) gating has emerged as a solution in which electric fields are used to inject molecules to modify a material’s magnetic properties. We show that MI gating of hydrogen (H) enables dynamic control of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in a solid-state magnetic multilayer device. We demonstrate that the RKKY coupling can be switched from antiferromagnetic to ferromagnetic, and vice versa, using only a small positive bias. The switching is sub 1 ms at room temperature, fully reversible, and cyclable. H loading enables both an amplitude and phase shift of the oscillatory RKKY coupling. Using an engineered soft layer and hard layer, MI gating of the RKKY interaction allows for 180^○ field-free switching of the soft layer without the need for a biasing layer. This creates new opportunities for memory-based spintronic devices since the free layer can be directly switched with just a small applied voltage.   

Spin transport in 3D Interconnected Co Nanowire Networks

Three-dimensional (3D) magnetic assemblies can host unique topological features and exotic spin textures. Here we report magnetoresistance (MR) studies of interconnected Co nanowire (NW) networks realized using multiple angle ion-tracking and electrodeposition [1]. In a large ensemble of NWs, a typical anisotropic MR behavior was observed during magnetic field sweep, with a minimum near the coercive field. However, when only a few interconnected NWs were measured, multiple kinks and local minima were found, including a significant minimum at a positive field during the positive-to-negative field sweep. Micromagnetic simulations show that this unusual feature is indicative of domain wall (DW) pinning at the NW intersections.  Interestingly, this peak position shifts when the magnitude of the applied current density is varied. This demonstrates the potential that DW motion through the network may be controlled by current to implement multistate memristors. The MR results were compared with magnetic configurations probed by magnetometry, magnetic imaging and small angle neutron scattering. Our results highlight the promise of these networks to be utilized in 3D magnetic memory and memristive devices.

[1] E. Burks et al, Nano Lett., 21, 716 (2021).   

Interlayer exchange coupling in the free layers of perpendicular magnetic tunnel junctions

Perpendicular magnetic tunnel junctions (pMTJ) are composite heterostructures that have the capacity to retain a state with zero energy consumption and therefore are examined for their capacity to be used in devices such as magnetic random access memory (MRAM). A pMTJ stack usually contains several layers of ferromagnetic and nonmagnetic materials. Free layers consisting of multiple interfaces are used in recent pMTJs to enhance thermal stability and switching speed.  We investigated the interlayer exchange coupling in CoFeB/NM/CoFeB structures with typical NM materials such as MgO, Ru, Ta, Mo and Ir. FM or AF coupling can be obtained by controlling the NM thickness and/or annealing condition. The interlayer coupling is characterized by a magneto optical Kerr effect (MOKE) system, where a photo elastic modulator and lock-in amplifier are employed to enhance the signal-to-noise ratio. Large TMR above 200% have been obtained in these pMTJs.    

Experimental and computational study of FeCrVAl and related compounds

Materials exhibiting high degree of spin polarization are in demand for spin-transport-based devices. Half-metallic Heusler compounds have attracted particular attention because they have tunable magnetic properties and exhibit high Curie temperature much above room temperature. We have synthesized one such predicted half metal, FeCrVAl, using arc melting and annealing. In addition, we also studied the effect of Mn substitution on the structural, magnetic, and electronic band properties of FeCrVAl synthesizing samples with compositions FeCr_0.5Mn_0.5VAl and FeCrV_0.5Mn_0.5Al. The room temperature, x-ray diffraction patterns indicate that FeCrVAl and FeCrV_0.5Mn_0.5Al are cubic in structure with A2 type disorder, whereas FeCr_0.5Mn_0.5VAl is more ordered as evidenced by the presence of superlattice peaks. All samples show small saturation magnetizations at room temperature and the thermomagnetic curves M(T) are similar to that of a paramagnetic material. At the same time, the M(T) of FeCr_0.5Mn_0.5VAl is different and shows a slow magnetic transition similar to that of a ferrimagnetic material at 800 K. Our first principles calculations indicate that FeCrVAl and FeCr_0.5Mn_0.5VAl exhibit nearly perfect spin polarization, which may be further enhanced by mechanical strain. Both of these alloys exhibit ferrimagnetic alignment. At the same time, FeCrV_0.5Mn_0.5Al exhibits a relatively small value of spin polarization of only 24%, making it less attractive for practical applications.   

Spin-flip-driven anisotropic magnetoresistance in antiferromagnetic spin-valve-like structure

A spin valve is a prototype of spin-based electronic devices found on ferromagnets, in which an antiferromagnet plays a supporting role. Recent findings in antiferromagnetic spintronics show that an antiferromagnetic order in single-phase materials solely governs dynamic transport, and antiferromagnets are considered promising candidates for spintronic technology. In this study, antiferromagnet-based spintronic functionality on single crystals of Ca_0.9Sr_0.1Co₂As₂ was demonstrated by integrating nanoscale spin-valve-type structure and anisotropic magnetic properties driven by spin-flips. Multiple stacks of 1 nm thick spin-valve-like unit are intrinsically embedded in the antiferromagnetic spin structure. The switching operation between low and high resistance states was observed in the presence of a rotating magnetic field. This leads to anisotropic magnetoresistance, which is maximized in the vicinity of the flip transition. Phenomenological calculations based on an easy-axis anisotropic spin model reproduce observed magnetic and magnetotransport properties and suggest an essential role of magnetocrystalline anisotropy in the observed spintronic functionality. Our results observed in a natural antiferromagnet offer the potential of utilizing spin flip/flop transitions in extensive spintronic applications.   

Predictive Model for Tunneling Magnetoresistance in 2D Material Magnetic Tunnel Junctions Utilizing Bulk Properties

Recently, magnetic tunnel junctions (MTJs) have been formed using two-dimensional materials (2DM) heterostructures, leading to increased attention regarding the spin transport of these layered systems. The large number of possible MTJs that can be formed using this expanding family of 2D magnetic materials poses an exhaustive challenge for both experimental and computational efforts. Hence, alternative search methods are required for the realization of 2DM MTJs with exceptional tunneling magnetoresistance (TMR) values. In this work, we developed a model based on the readily obtained physical properties of bulk constituents to estimate the TMR in the full junctions as well as identify materials that yield positive response. We characterize the properties of multilayer junctions formed with transition metal chalcogenides and halides using density functional theory and ballistic transport. Our model shows agreement with sample structures and further predicts multiple MTJs with TMR values greater than 1,000%. From the comparison against full quantum transport calculations, we discuss virtues and limitations of our model.  This work serves to create a data driven design of complex 2DM MTJs which will guide experimental studies as to realize their use for applications in spintronics.   

Magnetotransport Properties in Fe/MgO/Fe Magnetic Tunnel Junction: DFT+JunPy+LLG Calculation

Our self-developed “JunPy” package has successfully combined the self-consistent Hamiltonian by using the first-principles calculation with the non- equilibrium Green’s function (NEGF) method to calculate the noncollinear spin torque effect in the nm-scale magnetic heterojunctions [1]. The divide-and-conquer (DC) method was first applied to reveal the oscillatory decay of layer-resolved spin torques away from the MgO/Fe interface, and suggests a very thin Fe layer thickness below 2 nm to preserve the efficient current-driven magnetization switching [2]. On the other hand, we include the spin-orbit coupling (SOC) in the first-principles calculation to study the MgO/Fe interfacial magnetic anisotropy (IMA), and obtain the spin torque caused by the anisotropy field. By employing the Landau-Lifshitz-Gilbert (LLG) equation, we study the macroscope magnetization switching in Fe/MgO/Fe magnetic tunnel junction including the influences of current-driven spin torque and interfacial anisotropy field. The calculated resistance as a function of magnetic field (R-H loop) and of bias (R-V loop) are thus obtained and well comparable with experimental results.

[1] Y. -H. Tang and B. -H. Huang, Phys. Rev. Research, 3, 033264 (2021). 

[2] B. -H. Huang et al., AIP Advances 11, 015036 (2021).