Session A40: Electric Field and Strain Control of Magnetism

Voltage control of magnetic anisotropy of a FePt/MgO(001) hetrostructure

Control of magnetic properties by an external electric field can be a potential candidate to overcome the limitation of high-power consumption for non-volatile magnetoelectric random access memory (MeRAM). We predict a huge voltage control magnetic anisotropy (VCMA) coefficient »1.77 (-1.36) pJ/(V.m) of the Fe(Pt)-interfaced FePt/MgO hetrostructure. The Fe-interfaced FePt/MgO film exhibits strain-induced magnetization reorientation instigated through second order magnetoelastic coupling along with extremely sensitive VCMA behavior.We anticipate a significant effect of the external field on the induced electric dipole that is formed in terms of charge accumulation or depletion at the metal/dielectric interface. These results are a step forward to achieve a full potential of MeRAM with write voltage below 1 V and switching bit energy below 1 fJ.   

Giant nonvolatile resistive switching in V2O3/PMN-PT heterostructures

Oxide heterostructures provide an exciting opportunity to engineer new artificial systems with unique properties that cannot be found in naturally occurring materials. Enabling the control of electronic properties of oxides that feature a metal-insulator transition (MIT) is a key requirement for developing a new class of electronics often referred to as “Mottronics” [1]. Although doping and radiation damage are effective tools to permanently change the MIT temperature in these materials, a simple method to switch the MIT properties in real-time is needed for practical applications. In this talk I will present the discovery of giant nonvolatile resistive switching (ΔR/R > 1000%) and strong modulation of the MIT temperature (ΔT_c > 30 K) in a voltage-actuated V₂O₃/PMN-PT heterostructure. The control of the V₂O₃ electronic properties is achieved using the transfer of ferroelastic strain from the PMN-PT substrate into the epitaxially-grown V₂O₃ film. Strain can reversibly promote/hinder the structural phase transition in V₂O₃, thus advancing/suppressing the associated MIT. While oxide/ferroelectric hybrids had been studied in the past using other correlated materials, such as VO₂ [2], Fe₃O₄ [3], LaNiO₃ [4], NdNiO₃ [5], etc., the reported nonvolatile resistive switching was rather modest: 1.5% < ΔR/R < 110%. More than an order of magnitude larger resistive switching in V₂O₃/PMN-PT could enable practical implementations of voltage-controlled Mott devices and provide a new platform for exploring fundamental electronic properties of V₂O₃.

[1] Ngai, J. et. al. Annu. Rev. Mater. Res. 44, 1 (2014)
[2] Nan, T. et. al. Sci. Rep. 4, 5931 (2014)
[3] Liu, M. et. al. Sci. Rep. 3, 1876 (2013)
[4] Marshall, M. et. al. Phys. Rev. Appl. 2, 051001 (2014)
[5] Heo, S. et. al. Sci. Rep. 6, 22228 (2016)   

Observation of Phase-transition-induced magnetism modulation in metal/VO2 heterostructures

High efficiency manipulation of magnetic properties is of great interest for fundamental science and applications. However, facile strategies with more modulating freedom are are still highly desired. In this work, NiFe (easy axis in-plane) and Co/Pt multilayers (easy axis out-of-plane) are combined with a strongly correlated electron systems VO₂ to fabricate newly artificial metal/oxide heterostructures. [1, 2] It endows the spintronic material with extraordinary multiple control ability of magnetism (e.g., anisotropy, magnetization, etc.) via light or temperature change based on the interfacial strain coupling. Utilizing its multiple modulation feature, a phase-transition anisotropic magnetoresistance (PTAMR) device is fabricated. The special features which distinguish from traditional materials can further benefit the emerging device applications. Our work, as an example of phase-transition spintronics, can certainly pave the way for next-generation electronics.

1) Guodong Wei, Xiaoyang Lin et al. arXiv: 1805.02453.
2) Guodong Wei, Xiaoyang Lin et al. arXiv: 1809.06999.   

Ultralow Voltage Control of Magnetism

The key to integrating the concepts of spintronics into conventional nanoelectronics lies with the ability to control the magnetic order in nanoscale devices. With the continuous shrinkage of integrated circuits, the energy efficiency required to control these tiny magnets as power dissipation becomes more and more important. Over the past decades, the oxide community has been exploring the materials that can provide the opportunities to control magnetism. Among the large investigated materials, multiferroics might be one of the most promising material family. Multiferroics are the materials which possess at least two order of parameters, particularly, the coexistence of ferroelectricity(P) and magnetism(M), and exhibit coupling from one to another. In this talk, I will demonstrate an ultra-low-voltage (<500 mV) and non-volatile manipulation of ferromagnetism at room temperature via the heterostructure of spin valves on a multiferroic layer, BiFeO₃(BFO). Finally, I will conclude this talk with a summary of current challenges and future direction of multiferroics, especially BFO, toward the low-power electronics.   

Electric field manipulation of exchange bias in antiferromagtic Cr2O3 thin films

Cr₂O₃ is an interesting insulating antiferromagnetic material, which has been widely investigated recently, including in the studies of the spin Seebeck effect and the long spacing distance spin transport. Cr₂O₃ is also a well-known magneto-electric material, whose sublattice spins can be switched by an electric field. In this study, we first grow a 5 nm thick Pt layer on the substrate of (0001)-oriented Al₂O₃ using magnetron sputtering, and then the Cr₂O₃ thin films using high vacuum pulsed laser deposition. 1 nm Co is grown on top of Cr₂O₃ by molecular beam epitaxy or magnetron sputtering, followed by a 2 nm thick Pt capping layer deposited in-situ to prevent the oxidation of the ferromagnetic Co. The Co layer shows a very strong perpendicular magnetic anisotropy with exchange bias by anisotropy magnetoresistance measurements. The exchange bias can be reversibly manipulated by the gating electric field in Cr₂O₃ between the bottom and top Pt layers. The magneto-electric manipulation of Cr₂O₃ spins offers potential to switch the magnetization in high-density memory devices incorporating antiferromagnets with low energy consumption.   

Control over the ISHE in a platinum film by ionic gating

We report reversible modulation of the inverse spin Hall effect (ISHE) in Pt achieved through the careful control of Pt thickness and an ionic gating technique. The ISHE converts spin current into charge current, which allows easy detection of spin current and integration of spintronics and electronics. The effect is governed by spin-orbit interaction and is particularly strong in Pt, which has been widely used as the ISHE-based spin current detector. However, the gate tuning of the spin-orbit interaction in metals was considered out of reach because of high carrier density.

We fabricated Pt films, 1.5 nm to 20 nm thick, on top of the YIG ferrimagnetic layer and used spin pumping to inject spin current in the Pt layer. Using the the ionic gel (DEME-TFSI) top gate, the resistance of the Pt film was tuned up to 280%. Such control over the resistance and carrier density in Pt allowed us to gate tune the amplitude of the ISHE over two orders of magnitude—a result that can be used in spin-torque and other spintronics devices that use spin-charge conversion [1].

[1] S. Dushenko, M. Hokazono, M. Shiraishi et al., Tunable inverse spin Hall effect in nanometer-thick platinum films by ionic gating, Nat. Commun. 9, 3118 (2018).   

Voltage control of magnetism in metal oxide/metal nanoislands and nanostripes

Voltage control of magnetism by ionic approaches presents a promising pathway to low-power magnetic devices. Up to now, magneto-ionic manipulation has been reported mainly for ultrathin films and nanoporous structures.[1-3] Since the mechanism is based on interfacial charge transfer, the morphology may be key to the magneto-ionic efficiency.
We investigate the influence of morphology on magneto-ionic changes during electrolytic gating of FeO_x/Fe nanostructures. In FeO_x/Fe nanoislands [4], the magneto-ionic changes are enhanced compared to continuous films and close to ON/OFF switching of magnetization is achieved.[5] The results are explained mainly by the higher surface/volume ratio of the nanoislands. Further, we investigate the transfer of magneto-ionic effects from extended thin film to nanostripe geometry. We present initial results on voltage-controlled magnetoresistance in FeO_x/Fe stripes probed via in situ magneto-transport measurements.
[1] Gilbert et al., Nat. Commun., 2016, 7, 12264
[2] Quintana et al., Small, 2018, 14, 1704396
[3] Duschek et al., APL Mater.,2016, 4, 032301
[4] Leistner et al., Nanoscale, 2017, 9, 5315
[5] Duschek et al., J. Mater. Chem. C, 2018, 6, 8411   

The Effect of Material Defects on Magneto-Elastic Switching

Past experiments have shown that magneto-elastic switching phenomenon is extremely energy-efficient but error-prone. We have studied the effect of material defects (voids, thickness variations, etc.) on the switching probability of elliptical magnetostrictive nanomagnets subjected to uniaxial strain using micromagnetic simulations in the presence of thermal noise. Defects drastically increase the switching error rate. Curiously, there is a critical value of stress that results in the minimum error rate for both defective and defect-free nanomagnets. The critical stress is much higher for defective nanomagnets. Another interesting observation was that if the nanomagnet’s thickness is different in two different halves, then stable magnetization states are spawned along the nanomagnet’s hard axis. Above a certain thickness variation, the magnetization tends to get pinned along the hard axis when the nanomagnet is stressed and remains stuck there even after stress removal. These unexpected observations reveal some of the complexities associated with magneto-elastic switching, with important ramifications for practical applications of straintronics.   

Ferroelectric control of magnetism in Pt/BaTiO3 thin films

The quest for artificial multiferroic heterostructures consisting of ferroelectric (FE) and ferromagnetic (FM) layers are of increasing interest. Because the optimizing interfacial magnetoelectric coupling between the magnetic and electric polarizations allows novel means of controlling magnetization or polarization in view of efficient, low-power spintronic devices. Using first principles calculations we predict the emergence of magnetism in ultrathin nonmagnetic Pt films induced by the ferroelectric polarization of BaTiO₃ films, where the induced Pt magnetization depends on the polarization direction. More importantly, the calculations reveal that the Pt/BatiO₃ bilayer undergoes an in-plane to out-of-plane spin reorientation via ferroelectric polarization switching of the BTO, with a giant change of magnetic anisotropy from 1.15 to -0.49 erg/cm², indicating a huge magnetoelectric effect. These findings open interesting prospects for exploiting higher electric field efficiency of magnetic anisotropy for the next generation of magnetoelectric random access memory devices.   

Strain and magnetic properties of antiferromagnetic NaNiF3 thin films

Fluoro-perovskites have been proposed as an alternative candidate to the oxide perovskites for magneto-electric applications.^{1 ,2} It is therefore desirable to create thin film samples to integrate into devices. NaNiF3 (NNF) is an antiferromagnet which has a distorted perovskite crystal structure that sustains weak ferromagnetism with a bulk T_N = 156 K and a Pnma space group symmetry.³ We have grown NNF via molecular beam epitaxy on SrTiO3 (100) to produce high quality epitaxial films in the thickness range of 5- 50 nm. The films had the expected the Pnma structure and were predominantly oriented along the [101] direction. The SQUID magnetometry displayed a temperature-dependent magnetization consistent with antiferromagnetic ordering and a weak ferromagnetic moment. We observe a thickness-dependent transition temperature, where the change is ΔT_N = -7 K for decreasing thickness between 40 and 10 nm, which correlates with the increased tensile strain measured using x-ray diffraction.
[1] A. KC et al., Appl. Phys. Lett. 110, 92901 (2017).
[2] M. Yang et al., Sci. Rep. 7 7182 (2017).
[3] V. M. Judin and A. B. Sherman, Phys. Stat. Sol. 20 759 (1967).