Zhuhua Cai

Synthesis of ordered arrays of multiferroic NiFe2O4-Pb(Zr0.52Ti0.48)O3 core-shell nanowires

A synthesis method was developed for producing core-shell nanowire arrays, which involved a combination of a modified sol-gel process, electrochemical deposition, and subsequent oxidization in anodized nanoporous alumina membranes. This method was applied to generate ordered arrays of one dimensional multiferroic NiFe2O4 core and Pb(Zr0.52Ti0.48)O3 (PZT) shell nanostructures. Extensive microstructural, magnetic, and ferroelectric characterizations confirmed that the regular arrays of core-shell multiferroic nanostructures were composed of a spinel NiFe2O4 core and perovskite PZT shell. This synthesis method can be readily extended to prepare different core-shell nanowire arrays and is expected to pave the way for one dimensional core-shell nanowire arrays.

Soft magnetism, magnetostriction and microwave properties of FeGaB thin films

A series of (Fe100−yGay)1−xBx (x=0–21 and y=9–17) films were deposited; their microstructure, soft magnetism, magnetostrictive behavior, and microwave properties were investigated. The addition of B changes the FeGaB films from polycrystalline to amorphous phase and leads to excellent magnetic softness with coercivity <1Oe, high 4πMs, self-biased ferromagnetic resonance (FMR) frequency of 1.85GHz, narrow FMR linewidth (X band) of 16–20Oe, and a high saturation magnetostriction constant of 70ppm. The combination of these properties makes the FeGaB films potential candidates for tunable magnetoelectric microwave devices and other rf/microwave magnetic device applications.

Electrical tuning of magnetism in Fe3O4/PZN–PT multiferroic heterostructures derived by reactive magnetron sputtering

Strong magnetoelectric (ME) coupling was demonstrated in Fe3O4/PZN–PT (lead zinc niobate–lead titanate) multiferroic heterostructures obtained through a sputter deposition process. The dependence of the magnetic anisotropy on the electric field (E-field) is theoretically predicted and experimentally observed by ferromagnetic resonance spectroscopy. A large tunable in-plane magnetic anisotropy of up to 600 Oe, and tunable out-of-plane anisotropy of up to 400 Oe were observed in the Fe3O4/PZN–PT multiferroic heterostructures, corresponding to a large ME coefficient of 100 Oe cm/kV in plane and 68 Oe cm/kV out of plane, which match well with predicted results. In addition, the electric field manipulation of magnetic anisotropy is also demonstrated by the electric fields dependence of magnetic hysteresis loops, showing a large squareness ratio change of 44%. These Fe3O4/PZN–PT multiferroic heterostructures exhibiting large E-field tunable magnetic properties provide great opportunities for novel electrostatic...

Strong magnetoelectric coupling in ferrite/ferroelectric multiferroic heterostructures derived by low temperature spin-spray deposition

Strong magnetoelectric (ME) interaction was demonstrated at both dc and microwave frequencies in a novel Zn0.1Fe2.9O4/PMN‐PT (lead magnesium niobate‐lead titanate) multiferroic heterostructure, which was prepared by spin-spray depositing a Zn0.1Fe2.9O4 film on a single-crystal PMN‐PT substrate at a low temperature of 90 ◦ C. A large electric-field induced ferromagnetic resonance field shift up to 140Oe was observed, corresponding to an ME coefficient of 23OecmkV −1 . In addition, a large electrostatic field tuning of the magnetic hysteresis loops was observed with a large squareness ratio change of 18%. The spin-spray deposited ferrite/piezoelectric multiferroic heterostructures exhibiting strong ME interactions at both dc and microwave frequencies provide great opportunities for novel electrostatically tunable microwave magnetic devices synthesized at a low temperature. (Some figures in this article are in colour only in the electronic version)

Epitaxial growth of M-type Ba-hexaferrite films on MgO (111)‖SiC (0001) with low ferromagnetic resonance linewidths

Barium hexaferrite (BaM) films were deposited on 10nm MgO (111) films on 6H silicon carbide (0001) substrates by pulsed laser deposition from a homogeneous BaFe12O19 target. The MgO layer, deposited by molecular beam epitaxy, alleviated lattice mismatch and interdiffusion between film and substrate. X-ray diffraction showed strong crystallographic alignment while pole figures exhibited reflections consistent with epitaxial growth. After optimized annealing, these BaM films have a perpendicular magnetic anisotropy field of 16900Oe, a magnetization (as 4πMs) of 4.4kG, and a ferromagnetic resonance peak-to-peak derivative linewidth at 53GHz of 96Oe, thus demonstrating sufficient properties for microwave device applications.

Large converse magnetoelectric coupling in FeCoV/lead zinc niobate-lead titanate heterostructure

Multiferroic behavior was directly verified in a laminated ferroelectric-ferromagnetic heterostructure consisting of a FeCoV thick film (70 μm) and lead zinc niobate-lead titanate (PZN-PT) single crystal. This unique heterostructure demonstrates a significant converse magnetoelectric (CME) effect corresponding to a CME coupling constant of 31 Oe/kV cm−1. It derives from the soft magnetic and magnetostrictive properties (λ=60 ppm) of FeCoV alloy and the superior electromechanical properties (d32=−2800 pC/N) of PZN-PT crystal. The electric field controlled magnetic hysteresis is discussed in terms of a stress-induced anisotropy field model. The theoretical calculation is within 7% of the measured induced field of 240 Oe.

Enhanced Néel temperature in Mn ferrite nanoparticles linked to growth-rate-induced cation inversion

Mn ferrite (MnFe(2)O(4)) nanoparticles, having diameters from 4 to 50 nm, were synthesized using a modified co-precipitation technique in which mixed metal chloride solutions were added to different concentrations of boiling NaOH solutions to control particle growth rate. Thermomagnetization measurements indicated an increase in Néel temperature corresponding to increased particle growth rate and particle size. The Néel temperature is also found to increase inversely proportionally to the cation inversion parameter, delta, appearing in the formula (Mn(1-delta)Fe(delta))(tet)[Mn(delta)Fe(2-delta)](oct)O(4). These results contradict previously published reports of trends between Néel temperature and particle size, and demonstrate the dominance of cation inversion in determining the strength of superexchange interactions and subsequently Néel temperature in ferrite systems. The particle surface chemistry, structure, and magnetic spin configuration play secondary roles.

Epitaxial MgO as an alternative gate dielectric for SiC transistor applications

Power transistor applications require alternative gate dielectrics on SiC that can operate at high fields without breaking down, as well as provide a high quality interface in order to minimize mobility degradation due to interface roughness. We have grown epitaxial MgO (111) crystalline layers on 6H-SiC (0001) substrates and characterized their structural and electrical properties. Measurements of gate leakage, breakdown fields, and dielectric properties make epitaxial MgO a potential candidate gate dielectric for SiC-based transistors.

Spin-spray deposited multiferroic composite Ni0.23Fe2.77O4∕Pb(Zr,Ti)O3 with strong interface adhesion

Ni0.23Fe2.77O4 (NFO)/Pb(Zr,Ti)O3 (PZT) multiferroic composites were synthesized by spin-spray deposition of NFO film onto PZT at 90°C. Strong interface adhesion between NFO and PZT was observed, which was verified by high resolution transmission electron microscopy indicating excellent wetting between the NFO and PZT, and by the strong magnetoelectric coupling in the NFO/PZT multiferroic composite showing an electric field induced remnant magnetization change of 10%. This strong interface adhesion and low-temperature spin-spray synthesis of multiferroic materials provide an alternative route for novel integrated multiferroic materials and devices.

Low temperature growth of crystalline magnesium oxide on hexagonal silicon carbide (0001) by molecular beam epitaxy

Magnesium oxide (111) was grown epitaxially on hexagonal silicon carbide (6H-SiC) (0001) substrates at low temperatures by molecular beam epitaxy and a remote oxygen plasma source. The films were characterized by reflection high-energy electron diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy. Crystal structure, morphology, and growth rate of the magnesium oxide (MgO) films were found to be dependent on the magnesium flux, indicating a magnesium adsorption controlled growth mechanism. The single crystalline MgO thin films had an epitaxial relationship where MgO (111)‖6H-SiC (0001) and were stable in both air and 10−9Torr up to 1023K.