Shishun Zhao

Deterministic Switching of Perpendicular Magnetic Anisotropy by Voltage Control of Spin Reorientation Transition in (Co/Pt)3/Pb(Mg1/3Nb2/3)O3–PbTiO3 Multiferroic Heterostructures

One of the central challenges in realizing multiferroics-based magnetoelectric memories is to switch perpendicular magnetic anisotropy (PMA) with a control voltage. In this study, we demonstrate electrical flipping of magnetization between the out-of-plane and the in-plane directions in (Co/Pt)3/(011) Pb(Mg1/3Nb2/3)O3-PbTiO3 multiferroic heterostructures through a voltage-controllable spin reorientation transition (SRT). The SRT onset temperature can be dramatically suppressed at least 200 K by applying an electric field, accompanied by a giant electric-field-induced effective magnetic anisotropy field (ΔHeff) up to 1100 Oe at 100 K. In comparison with conventional strain-mediated magnetoelastic coupling that provides a ΔHeff of only 110 Oe, that enormous effective field is mainly related to the interface effect of electric field modification of spin-orbit coupling from Co/Pt interfacial hybridization via strain. Moreover, electric field control of SRT is also achieved at room temperature, resulting in a ΔHeff of nearly 550 Oe. In addition, ferroelastically nonvolatile switching of PMA has been demonstrated in this system. E-field control of PMA and SRT in multiferroic heterostructures not only provides a platform to study strain effect and interfacial effect on magnetic anisotropy of the ultrathin ferromagnetic films but also enables the realization of power efficient PMA magnetoelectric and spintronic devices.

Quantitative Determination on Ionic-Liquid-Gating Control of Interfacial Magnetism

Ionic-liquid gating on a functional thin film with a low voltage has drawn a lot of attention due to rich chemical, electronic, and magnetic phenomena at the interface. Here, a key challenge in quantitative determination of voltage-controlled magnetic anisotropy (VCMA) in Au/[DEME]+ [TFSI]- /Co field-effect transistor heterostructures is addressed. The magnetic anisotropy change as response to the gating voltage is precisely detected by in situ electron spin resonance measurements. A reversible change of magnetic anisotropy up to 219 Oe is achieved with a low gating voltage of 1.5 V at room temperature, corresponding to a record high VCMA coefficient of ≈146 Oe V-1 . Two gating effects, the electrostatic doping and electrochemical reaction, are distinguished at various gating voltage regions, as confirmed by X-ray photoelectron spectroscopy and atomic force microscopy experiments. This work shows a unique ionic-liquid-gating system for strong interfacial magnetoelectric coupling with many practical advantages, paving the way toward ion-liquid-gating spintronic/electronic devices.

The memory effect of magnetoelectric coupling in FeGaB/NiTi/PMN-PT multiferroic heterostructure

Magnetoelectric coupling effect has provided a power efficient approach in controlling the magnetic properties of ferromagnetic materials. However, one remaining issue of ferromagnetic/ferroelectric magnetoelectric bilayer composite is that the induced effective anisotropy disappears with the removal of the electric field. The introducing of the shape memory alloys may prevent such problem by taking the advantage of its shape memory effect. Additionally, the shape memory alloy can also “store” the magnetoelectric coupling before heat release, which introduces more functionality to the system. In this paper, we study a FeGaB/NiTi/PMN-PT multiferroic heterostructure, which can be operating in different states with electric field and temperature manipulation. Such phenomenon is promising for tunable multiferroic devices with multi-functionalities.

Ionic Liquid Gating Control of RKKY Interaction in FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2 Multilayers.

To overcome the fundamental challenge of the weak natural response of antiferromagnetic materials under a magnetic field, voltage manipulation of antiferromagnetic interaction is developed to realize ultrafast, high-density, and power efficient antiferromagnetic spintronics. Here, we report a low voltage modulation of Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction via ionic liquid gating in synthetic antiferromagnetic multilayers of FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2. At room temperature, the distinct voltage control of transition between antiferromagnetic and ferromagnetic ordering is realized and up to 80% of perpendicular magnetic moments manage to switch with a small-applied voltage bias of 2.5 V. We related this ionic liquid gating-induced RKKY interaction modification to the disturbance of itinerant electrons inside synthetic antiferromagnetic heterostructure and the corresponding change of its Fermi level. Voltage tuning of RKKY interaction may enable the next generation of switchable spintronics between antiferromagnetic and ferromagnetic modes with both fundamental and practical perspectives.Energy efficient control of magnetization in antiferromagnetic materials is one of the key ingredients for antiferromagnetic spintronics. Here the authors demonstrate voltage control of RKKY interaction and magnetization switching in synthetic antiferromagnetic multilayers by ionic liquid gating.

Ionic Liquid Gating Control of Spin Reorientation Transition and Switching of Perpendicular Magnetic Anisotropy

Electric field (E-field) modulation of perpendicular magnetic anisotropy (PMA) switching, in an energy-efficient manner, is of great potential to realize magnetoelectric (ME) memories and other ME devices. Voltage control of the spin-reorientation transition (SRT) that allows the magnetic moment rotating between the out-of-plane and the in-plane direction is thereby crucial. In this work, a remarkable magnetic anisotropy field change up to 1572 Oe is achieved under a small operation voltage of 4 V through ionic liquid (IL) gating control of SRT in Au/[DEME]+ [TFSI]- /Pt/(Co/Pt)2 /Ta capacitor heterostructures at room temperature, corresponding to a large ME coefficient of 378 Oe V-1 . As revealed by both ferromagnetic resonance measurements and magnetic domain evolution observation, the magnetization can be switched stably and reversibly between the out-of-plane and in-plane directions via IL gating. The key mechanism, revealed by the first-principles calculation, is that the IL gating process influences the interfacial spin-orbital coupling as well as net Rashba magnetic field between the Co and Pt layers, resulting in the modulation of the SRT and in-plane/out-of-plane magnetization switching. This work demonstrates a unique IL-gated PMA with large ME tunability and paves a way toward IL gating spintronic/electronic devices such as voltage tunable PMA memories.

Discovery of Enhanced Magnetoelectric Coupling through Electric Field Control of Two-Magnon Scattering within Distorted Nanostructures

Electric field control of dynamic spin interactions is promising to break through the limitation of the magnetostatic interaction based magnetoelectric (ME) effect. In this work, electric field control of the two-magnon scattering (TMS) effect excited by in-plane lattice rotation has been demonstrated in a La0.7Sr0.3MnO3 (LSMO)/Pb(Mn2/3Nb1/3)-PbTiO3 (PMN-PT) (011) multiferroic heterostructure. Compared with the conventional strain-mediated ME effect, a giant enhancement of ME effect up to 950% at the TMS critical angle is precisely determined by angular resolution of the ferromagnetic resonance (FMR) measurement. Particularly, a large electric field modulation of magnetic anisotropy (464 Oe) and FMR line width (401 Oe) is achieved at 173 K. The electric-field-controllable TMS effect and its correlated ME effect have been explained by electric field modulation of the planar spin interactions triggered by spin-lattice coupling. The enhancement of the ME effect at various temperatures and spin dynamics control are promising paradigms for next-generation voltage-tunable spintronic devices.

Spin-orbital coupling induced four-fold anisotropy distribution during spin reorientation in ultrathin Co/Pt multilayers

In this work, we synthesized (Co(t)/Pt)3 multilayers and quantitatively determined thickness and temperature dependence of spin reorientation transition (SRT) and perpendicular magnetic anisotropy (PMA) using ferromagnetic resonance measurement. The critical thickness for PMA switching as well as tremendous magnetic anisotropy change up to 645 Oe once the temperature varies from 25 °C to −153 °C are demonstrated. More interestingly, a four-fold symmetry of magnetic anisotropy was found to be prominent during the SRT. By conducting magnetic simulation with involving high order energy term, we highly related this phenomenon to the strong spin-orbital coupling, which is considered to be the major energy term to tip the balance between the surface anisotropy and shape anisotropy. These results provide an opportunity for better understanding the transition behaviors which is essential for PMA structure preparation and their related devices.

Low voltage induced reversible magnetoelectric coupling in Fe3O4 thin films for voltage tunable spintronic devices

The ongoing demand for efficient and low-energy consumption spintronic devices has motivated the idea of manipulating magnetism by ionic liquid (IL) electrolyte gating at low voltages. Although magnetoelectric (ME) coupling has already been realized in some field-effect-transistor (FET) structures, some vital parameters such as giant ME coupling coefficient, excellent reversibility and low gating voltage seldom come at the same time, which greatly suppresses industrialization. Here we demonstrate a large 552 Oe spin dynamics modulation of Fe3O4 thin films induced at a gating voltage of Vg = +1.5 V in an IL-gated Au/[DEME]+[TFSI]−/Fe3O4/MgO heterostructure with good reversibility up to 80 cycles, giving rise to a high ME coefficient of 368 Oe V−1. Such a large ME tunability under low Vg could be attributed to the electric field (E-field) induced ionic transformation between Fe2+ and Fe3+ at the interface. The tiny thickness change (∼2 angstrom) and roughness change of Fe3O4 films under Vg = +1.5 V illustrated by in situ X-ray reflection (XRR) give a reasonable explanation of the outstanding reversible property. Interestingly, it is found that the Verwey transition temperature of Fe3O4 has a strong dependence on Vg, revealing the potential of IL gating control of the intrinsic spin ordering inside magnetic films. This work drives forward the low-voltage induced reversible ME coupling to high-performance spintronic devices.

Low-Voltage Control of (Co/Pt)x Perpendicular Magnetic Anisotropy Heterostructure for Flexible Spintronics

The trend of mobile Internet requires portable and wearable devices as bio-device interfaces. Electric field control of magnetism is a promising approach to achieve compact, light-weight, and energy-efficient wearable devices. Within a flexible sandwich heterostructure, perpendicular magnetic anisotropy switching was achieved via low-voltage gating control of an ionic gel in mica/Ta/(Pt/Co) x/Pt/ionic gel/Pt, where (Pt/Co) x acted as a functional layer. By conducting in situ VSM, EPR, and MOKE measurements, a 1098 Oe magnetic anisotropy field change was determined at the bending state with tensile strain, corresponding to a magnetic anisotropy energy change of 3.16 × 105 J/m3 and a giant voltage tunability coefficient of 0.79 × 105 J/m3·V. The low voltage and strain dual control of magnetism on mica substrates enables tunable flexible spintronic devices with an increased degree of manipulation.

Ferroelectric Phase Transition Induced a Large FMR Tuning in Self-Assembled BaTiO3:Y3Fe5O12 Multiferroic Composites

Yttrium iron garnet (YIG) is of great importance in RF/microwave devices for its low loss, low intrinsic damping, and high permeability. Nevertheless, tuning of YIG-based multiferroics is still a challenge due to its near-zero magnetostriction and the difficulty of building epitaxial interface between ferromagnetic garnet and ferroelectric perovskite phases. In this work, the vertically aligned heterostructure of YIG:BTO/STO(001) with local epitaxial interface between BTO and YIG is well-constructed, where the single crystal BTO pillars are embedded in YIG matrix. A large magnetoelectric coupling effect that drives YIGs FMR shift up to 512 and 333 Oe (1-2 order greater than those of all state-of-the-art progresses) is obtained through BTO ferroelectric phase changes induced by temperature variation at 295 and 193 K, correspondingly. This record high magnetoelectric tunability of YIG paves a way toward thermal/electrical tunable YIG devices.