Xu Xue

Electric field induced reversible 180° magnetization switching through tuning of interfacial exchange bias along magnetic easy-axis in multiferroic laminates.

E-field control of interfacial exchange coupling and deterministic switching of magnetization have been demonstrated in two sets of ferromagnetic(FM)/antiferromagnetic(AFM)/ferroelectric(FE) multiferroic heterostructures, including NiFe/NiCoO/glass/PZN-PT (011) and NiFe/FeMn/glass/PZN-PT (011). We designed this experiment to achieve exchange bias tuning along the magnetic easy axis, which is critical for realizing reversible 180° magnetization deterministic switching at zero or small magnetic bias. Strong exchange coupling were established across AFM-FM interfaces, which plays an important role in voltage control of magnetization switching. Through the competition between the E-field induced uniaxial anisotropy in ferromagnetic layer and unidirectional anisotropy in antiferromagnetic layer, the exchange bias was significantly shifted by up to |∆Hex|/Hex = 8% in NiFe/FeMn/glass/PZN-PT (011) and 13% in NiFe/NiCoO/glass/PZN-PT (011). In addition, the square shape of the hysteresis loop, as well as a strong shape tunability of |∆Hex|/Hc = 67.5 ~ 125% in NiFe/FeMn/glass/PZN-PT and 30 ~ 38% in NiFe/NiCoO/glass/PZN-PT were achieved, which lead to a near 180° magnetization switching. Electrical tuning of interfacial exchange coupling in FM/AFM/FE systems paves a new way for realizing magnetoelectric random access memories and other memory technologies.

Review on nanomaterials synthesized by vapor transport method: growth and their related applications

Nanostructures with different dimensions, including bulk crystals, thin films, nanowires, nanobelts and nanorods, have received considerable attention due to their novel functionalities and outstanding applications in various areas, such as optics, electricity, thermoelectricity, photovoltaic fields and sensing devices. In recent years, remarkable progresses and modifications have been made upon the fabrication of nanomaterials by vapor transport method. In this review, we introduce various representative nanostructures prepared by vapor transport method and focus on the discussion of their growth, physical properties, and potential applications. Meanwhile, the essential growth mechanisms of nanostructures also have been extensively reviewed, for example, the different growth modes depending upon the specific sample growth. Finally, we conclude this review by providing our perspectives to the future vapor transport method, and indicating some key existing problems. Vapor transport process offers great opportunities for the low-cost preparation of novel single crystals with different doping level and the realization of integrating such nano/micro single crystals into spintronic and electronic devices.

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.

Thermal Driven Giant Spin Dynamics at Three-Dimensional Heteroepitaxial Interface in Ni0.5Zn0.5Fe2O4/BaTiO3-Pillar Nanocomposites

Traditional magnetostrictive/piezoelectric laminated composites rely on the two-dimensional interface that transfers stress/strain to achieve the large magnetoelectric (ME) coupling, nevertheless, they suffer from the theoretical limitation of the strain effect and of the substrate clamping effect in real ME applications. In this work, 3D NZFO/BTO-pillar nanocomposite films were grown on SrTiO3 by template-assisted pulsed laser deposition, where BaTiO3 (BTO) nanopillars appeared in an array with distinct phase transitions as the cores were covered by NiZn ferrite (NZFO) layer. The perfect 3D heteroepitaxial interface between BTO and NZFO phases can be identified without any edge dislocations, which allows effective strain transfer at the 3D interface. The 3D structure nanocomposites enable the strong two magnon scattering (TMS) effect that enhances ME coupling at the interface and reduces the clamping effect by strain relaxation. Thereby, a large FMR field shift of 1866 Oe in NZFO/BTO-pillar nanocomposite was obtained at the TMS critical angle near the BTO nanopillars phase transition of 255 K.

Voltage control of spin wave resonance in La0.5Sr0.5MnO3/PMN-PT (001) multiferroic heterostructures

The voltage modulation in spin dynamics via the spin-lattice coupling (SLC) effect has been investigated in epitaxial La0.5Sr0.5MnO3/PMN-PT multiferroic heterostructures. The critical angle for the disappearance of the first exchange (FEX) spin wave has been observed around 67° experimentally and been shifted about 4° by applying an electric field (E-field). In particular, at the critical angle, the FEX spin wave can be switched “on” and “off” by voltages, showing great potential in realizing magnonic devices. Moreover, the FEX spin wave resonance shift of 187 Oe at 173 K has been realized by the voltage driven SLC effect, which is a little larger than piezostrain-induced ferromagnetic resonance shift of 169 Oe. The experimental results can be well-explained by the modified Puszkarski spin wave theory.

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.

Voltage Control of Two-Magnon Scattering and Induced Anomalous Magnetoelectric Coupling in Ni–Zn Ferrite

Controlling spin dynamics through modulation of spin interactions in a fast, compact, and energy-efficient way is compelling for its abundant physical phenomena and great application potential in next-generation voltage controllable spintronic devices. In this work, we report electric field manipulation of spin dynamics-the two-magnon scattering (TMS) effect in Ni0.5Zn0.5Fe2O4 (NZFO)/Pb(Mg2/3Nb1/3)-PbTiO3 (PMN-PT) multiferroic heterostructures, which breaks the bottleneck of magnetostatic interaction-based magnetoelectric (ME) coupling in multiferroics. An alternative approach allowing spin-wave damping to be controlled by external electric field accompanied by a significant enhancement of the ME effect has been demonstrated. A two-way modulation of the TMS effect with a large magnetic anisotropy change up to 688 Oe has been obtained, referring to a 24 times ME effect enhancement at the TMS critical angle at room temperature. Furthermore, the anisotropic spin-freezing behaviors of NZFO were first determined via identifying the spatial magnetic anisotropy fluctuations. A large spin-freezing temperature change of 160 K induced by the external electric field was precisely determined by electron spin resonance.

Non-volatile switching of magnetism for reconfigurable microwave devices

The central challenge in tunable magnetic microwave devices lies in finding an energy efficient way to perform wide range ferromagnetic resonance (FMR) voltage tuning in a reversible and reproducible manner, rather than with a current-driven electromagnet.1 Multiferroic heterostructures, exhibiting a strong strain-mediated magnetoelectric (ME) coupling between distinct ferromagnetic and ferroelectric phases, have shown great promise for frequency agile microwave applications. In these materials, a single control parameter of in situ voltage-induced piezo-strain, arising from ferroelectrics, is used to shift FMR frequency in elastically-coupled ferromagnetic phases via magnetoelastic effects.2, 3 Therefore, devices based upon such materials are, in principle, lightweight, fast, and energy efficient, overcoming some of the intrinsic limitations in conventional microwave components, while providing new functionality. However, in most prototype ME microwave devices, tuning of FMR frequency has been achieved through the use of a linear piezo response.4, 5 Upon removing the electric field, the FMR decays to the initial state. While these devices point towards a unique pathway for enhancing FMR tunability, reversible and non-volatile tuning of FMR using strain has remained relatively unexplored, and this is indispensable from a device application point of view. In this presentation, we will demonstrate three approaches to realizing non-volatile tuning FMR in microwave magnetoelectric composites. They are including 1) Ferroelastic domain dynamic switching in (011) oriented PMN-PT (0.71Pb(Mg1/3Nb2/3) O3-0.29PbTiO3) single crystal, that allows polarization vectors to rotate from an out-of-plane to a purely in-plane direction, thereby producing two distinct, stable and reversible lattice strain states. Voltage-impulse-induced non-volatile tuning of FMR can be realized in FeCoB/PMN-PT (011) through dynamic switching between these two distinct strain states as shown in Fig. 1. 2) Voltage induced 109° ferroelastic polarization switching in (001) oriented PZN-PT (0.93Pb(Zn1/3Nb2/3) O3-0.07PbTiO3) single crystal, that enables distinct lattice strain states in the in-plane diagonal directions ([110] or [1-10]), thereby results in the modulation of FMR in a stable and reproducible manner in FeGaB/PZN-PT (001) heterostructures.6 3) Voltage-induced hysteretic phase transition in (011) oriented PZN-PT single crystal enables two reversible rhombohedral and orthorhombic stain states. Switching between these two states stimulated by voltage impulse, the FMR can be tuned non-volatilely in FeGaB/PZN-PT (011) heterostructures.5 These results point to opportunities for electrical tuning of strain sensitive properties in all materials and provide a framework for realizing reconfigurable, frequency agile, non-volatile and energy efficient electronics and microwave devices.