Mingmin Zhu

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.

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.

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.

Voltage Tuning of Ferromagnetic Resonance and Linewidth in Spinel Ferrite/Ferroelectric Multiferroic Heterostructures

An energy efficient approach is demonstrated to remarkably shift the ferromagnetic resonance (FMR) field by applying an electric field to various spinel ferriteferroelectric multiferroic heterostructures prepared by the low temperature spin-spray technique. The electric field-induced magnetic anisotropy changes, as well as the magnetoelectric (ME) coupling coefficient, were quantitatively determined in all multiferroic heterostructures. The broadness of FMR linewidth upon applying an electric field indicates that inhomogeneous ME coupling takes place, which arises from the ferrroelastic domain switching and polarization elongation. These results provide a framework for realizing compact, light-weight, and ultralow power electronics and microwave devices.

Voltage control of ferromagnetic resonance

Voltage control of magnetism in multiferroics, where the ferromagnetism and ferroelectricity are simultaneously exhibiting, is of great importance to achieve compact, fast and energy efficient voltage controllable magnetic/microwave devices. Particularly, these devices are widely used in radar, aircraft, cell phones and satellites, where volume, response time and energy consumption is critical. Researchers realized electric field tuning of magnetic properties like magnetization, magnetic anisotropy and permeability in varied multiferroic heterostructures such as bulk, thin films and nanostructure by different magnetoelectric (ME) coupling mechanism: strain/stress, interfacial charge, spin–electromagnetic (EM) coupling and exchange coupling, etc. In this review, we focus on voltage control of ferromagnetic resonance (FMR) in multiferroics. ME coupling-induced FMR change is critical in microwave devices, where the electric field tuning of magnetic effective anisotropic field determines the tunability of the performance of microwave devices. Experimentally, FMR measurement technique is also an important method to determine the small effective magnetic field change in small amount of magnetic material precisely due to its high sensitivity and to reveal the deep science of multiferroics, especially, voltage control of magnetism in novel mechanisms like interfacial charge, spin–EM coupling and exchange coupling.

Structural and magnetic properties of La0.7Sr0.3MnO3 ferromagnetic thin film grown on PMN-PT by sol–gel method

We report the preparation of epitaxial La0.7Sr0.3MnO3 thin films grown on (001)-oriented 0.72Pb(Mg1∕3Nb2∕3)O3-0.28PbTiO3 substrates by the sol–gel technique. The phase structure, magnetic properties and magnetoresistance of the samples are investigated by using high solution X-ray diffraction, atomic force microscopy, physical property measurement system, respectively. The La0.7Sr0.3MnO3 thin films display a well-defined hysteresis loop and typical ferromagnetism behavior at lower temperature. High magnetoresistance at 5T of 42% appears at 227K for La0.7Sr0.3MnO3 thin film.

Advances in Magnetics Epitaxial Multiferroic Heterostructures and Applications

The ever increasing demand for ultralow power electronics has propelled the exploration of novel multiferroics for realizing voltage control of magnetism in an energy efficient manner. Epitaxial multiferroic heterostructures, possessing a strong lattice-coupled mechanical interface and allowing an electric field (E-field) significantly modulating spin, charge, lattice, and orbital order parameters, have drawn much attention for creating novel magnetoelectric (ME) coupling effects. In this paper, we review the recent progress on control of ME properties in epitaxial multiferroic heterostructures, including their novel physical properties, various strong interfacial ME interactions, and potential applications. Meanwhile, the essential interfacial couplings, allowing cross-tuning of lattice and charge-coupled parameters, have been extensively reviewed. Electric tuning of magnetism in epitaxial multiferroic heterostructures provides a great platform to develop lightweight, compact, and energy-efficient spintronic or electronic devices.

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.

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.