Xuemeng Han

Demonstration of magnetoelectric memory cell in (110) [Pb(Mg1/2Nb2/3)O3]0.68-[PbTiO3]0.32/Ru/FeCo heterostructures

An electric-field pulses driven magnetoelectric memory cell in a single layered ferromagnetic thin film was fabricated by direct-current magnetron sputtering Ru/Fe65Co35 on ferroelectric (110) [Pb(Mg1/2 Nb2/3)O3]0.68-[PbTiO3]0.32 (PMN-PT) substrates. The magnetization in the orthogonal directions can be reset by the positive/negative electric fields pulse in PMN-PT/Ru/FeCo heterostructures due to the strain mediated converse magnetoelectric effect. The high (low) resistance state was realized under the negative (positive) electric fields pulse due to the anisotropy magnetoresistance of FeCo films. Then, a non-volatile magnetic memory cell with resistance and electric field, respectively, as the media and writing field was realized.

Tunable in-plane uniaxial anisotropy and the magnetization reversal mechanism of patterned high-frequency soft magnetic FeTa strips

FeTa films with thickness of 110 nm are fabricated on glass substrates by magnetron sputtering, and then a series of strips is designed on the FeTa films by conventional optical lithography and the ion beam etching method. Patterned FeTa strips show a tunable in-plane uniaxial magnetic anisotropy property in contrast with the magnetic isotropic property of as-deposited FeTa thin films. The magnetization reversal mechanism of the patterned FeTa strips is investigated via the in-plane angular dependences of magnetization and coercivity. The angular dependence of coercivity (ADC) is explained well in terms of the two-phase model, giving good quantitative agreement with the experimentally measured M-shaped ADC curve. The domain structure and spatial resolution magneto-optical Kerr effect measurement indicate that the smaller the strip width, the stronger will be the anisotropy field. Regarding the dynamic magnetic properties, a transformation from Debye dispersion spectrum for strips with weak anisotropy to natural resonance spectrum for strips with strong anisotropy is finally obtained. The tunable in-plane anisotropy fields of the FeTa strips result in tunable high-frequency soft magnetic properties by altering the strip width, indicating that patterned FeTa strips have great potential in high-frequency soft magnetic application fields.

Electric field induced magnetic anisotropy transition from fourfold to twofold symmetry in (001) 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3/Fe0.86Si0.14 epitaxial heterostructures

The epitaxial growth of FeSi film on (001) 0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 (PMN-0.32PT) was fabricated by sputtering and confirmed by high-resolution transmission electron microscopy. A fourfold symmetric angular remanent magnetization curve of as-deposited FeSi thin film is well fitted theoretically by considering the cubic magnetocrystalline anisotropy. We found that the fourfold anisotropy decreases slightly when an electric field (E) is applied on the Pt/PMN-0.32PT/FeSi/Ta heterostructures with Pt layer as the positive electrode. However, a magnetic anisotropy transition from fourfold anisotropy to twofold anisotropy occurs under negative E. The strain-electric field curve suggests that the observed different variation trend of magnetic anisotropy results from the asymmetric strain response on the polarity of E. Moreover, once the transition happens, it was irreversible unless the heterostructures are heated above the phase transition temperature of PMN-0.32PT.

Reduction of magnetic damping constant of FeCo films by rare-earth Gd doping

Magnetic damping constant (α) is one of the key parameters to determine the critical current density of spin-transfer-torque devices and the switching time of magnetization for ultra-high-frequency devices. In this work, Gd doped FeCo films were fabricated to investigate α based on the ferromagnetic resonance technique. Gd doping not only can efficiently decrease the magnetic inhomogeneity and the extrinsic part of α but also the Lande g-factor and intrinsic part of α. The obtained α was roughly proportional to (g-2)2 and the magnetic anisotropic constant, indicating that the decreased spin-orbit interaction decreases α by Gd doping.

In-plane Uniaxial Anisotropy and Magnetization Reversal Mechanism of FeCo Films by Strip Pattern

Strip-like FeCo films were patterned by a traditional lithograph process from intrinsically isotropic continuous FeCo films. The strip-patterned FeCo film shows a strong in-plane uniaxial magnetic anisotropy with easy axis along the length direction of the strip. The angular dependences of remanence ratio, switching field, and coercivity indicate that the magnetization reversal mechanism of the strip-patterned FeCo film is coherent rotation and domain wall depinning when the applied field is near the hard axis and easy axis, respectively. The consistency of the experimental hysteresis loops of the strip-patterned FeCo film and calculated hysteresis loops with a simple in-plane uniaxial anisotropy model indicates that the strip-patterned FeCo film behaves as a single domain. The absence of the domain wall and the strong in-plane anisotropy field make the strip-patterned FeCo films have much potential for high-frequency application.

Enhanced field emission of amorphous Alq3 submicrometre thorns

Amorphous tris-(8-hydroxyquinoline) aluminum (Alq(3)) layers, which first form Alq(3) islands and then grow into submicrometre thorns, with different nominal thicknesses are investigated. The field emission characteristics of Alq(3) submicrometre thorns with nominal thicknesses of 20 nm, 50 nm and 100 nm include low turn-on fields of 3.2V mu m(-1), 6.8V mu m(-1) and 9.0V mu m(-1) at 10 mu Acm(-2), and low threshold fields of 5.1V mu m(-1), 10.0V mu m(-1) and 13.0V mu m(-1) at 1mAcm(-2), respectively. The enhanced field emission properties are governed by the morphology of Alq(3) submicrometre thorns, which can be controlled by the evaporation rate and the layer thickness. A significant hysteresis in the cycle testing of the current density with a rise and fall electric field process, which is an undesirable property for practical applications, is observed in the case of the considered samples. This hysteresis can be eliminated via an increase in the nominal thicknesses and tests, and this is important for practical applications. The microstructure and adsorption/desorption effect are responsible for this hysteresis phenomenon. Amorphous Alq(3) submicrometre thorns with good field emission properties are promising candidates for application as field emitters.

Large electric tunable remanent magnetization and in-plane anisotropy field in (110) PMN-0.32PT/Cu/Fe65Co35 multiferroic heterostructures

Large electric field tunable remanent magnetization and in-plane anisotropy in a Fe65Co35 film was observed in sputtered ferromagnetic/ferroelectric heterostructures. Two stable magnetization directions were established through a 90° transition of the in-plane uniaxial anisotropy of FeCo film under a positive electric field, and large nonvolatile remanence ratio tunability between 0.50 and 0.12 was realized using electric field pulses. With the increase of negative electric field, the anisotropy field increases at a rate of around 31.1 Oe/(kV cm−1). The large tunable anisotropy field and remnant magnetization may have potential applications in electrically tunable microwave magnetic devices and information storage memories.