Xiaobin Guo

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.

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.

Voltage-Driven In-Plane Magnetization Easy Axis Switching in FeNi/Piezoelectric Actuator Hybrid Structure

FeNi thin films with the in-plane uniaxial anisotropy were fabricated by direct current magnetron sputtering. An FeNi/piezoelectric actuator hybrid structure was used to investigate the voltage-controlled magnetization switching. A theoretical calculation was performed to provide a simplified vision of the magnetoelastic contribution to the magnetic anisotropy. Magneto-optical Kerr effect measurements were performed and the rotation of the magnetization easy axis in the FeNi film upon application of a voltage with or without the application of a magnetic field was demonstrated. A film-thickness dependent in-plane magnetization easy axis rotation angle was observed and explained by the variation of the magnetostriction.

Effect of inserting a non-metal C layer on the spin-orbit torque induced magnetization switching in Pt/Co/Ta structures with perpendicular magnetic anisotropy

Magnetization switching via charge current induced spin-orbit torques (SOTs) in heavy metal/ferromagnetic metal/heavy metal heterostructures has become an important issue due to its potential applications in high stability and low energy dissipation spintronic devices. In this work, based on a Pt/Co/Ta structure with perpendicular magnetic anisotropy (PMA), we report the effect of inserting a non-metal C interlayer between Co and Ta on the current-induced magnetization switching. A series of measurements based on the extraordinary Hall effect were carried out to investigate the difference of the anisotropy field, switching field, and damping-like and field-like SOT-induced effective fields as well as the current-induced spin Hall effect (SHE) torque after C decoration. The results show that PMA can be reduced by C decoration and the ratio of the effective SHE torque per unit current density and anisotropy field plays an essential role in the switching efficiency. In addition, the obtained switching curren...

Electrical field control of non-volatile 90° magnetization switching in epitaxial FeSi films on (001) 0.7[Pb(Mg1/3Nb2/3)O3]-0.3[PbTi0.3O3]

The epitaxial FeSi thin films on (001) 0.7[Pb(Mg1/3Nb2/3)O3]-0.3[PbTi0.3O3] (PMN-0.3PT) substrates were fabricated by radio frequency magnetron sputtering. The observed asymmetric strain-electric field curve suggests a tensile strain switching between two perpendicular directions in PMN-0.3PT with the variation of polarity of electric fields. A simple theoretical simulation of the free energy landscape shows that the stable magnetization easy axis (MEA) of FeSi with the inherent cubic magnetocrystalline anisotropy depends on the strength and direction of an extra strain induced uniaxial anisotropy. A reversible and non-volatile 90° switching of MEA by ±6 kV/cm pulses was confirmed by the angular dependence of remanent magnetization and Kerr hysteresis loops in Ta/FeSi/PMN-0.3PT/Pt heterostructures.

Current-induced magnetization switching in Pt/Co/Ta with interfacial decoration by insertion of Cr to enhance perpendicular magnetic anisotropy and spin–orbit torques

Perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) are studied in a perpendicularly magnetized ultrathin Co film sandwiched between heavy metal Pt and Ta with an interfacial decoration performed by inserting a 3d transition metal Cr layer with a thickness of 2 nm between the Co and Ta. A significant enhancement of the perpendicular anisotropy field ( Oe) is established and estimated by measuring the extraordinary Hall resistance due to the Cr interfacial decoration. The improved antidamping-like field of ~18.59 × 10−6 Oe/(Acm−2) reveals a large effective spin Hall angle of Pt/Co/Cr/Ta ~0.41, which is caused by employing the dissimilar metals, Pt and Ta, with spin Hall angles of opposite sign and an additional interfacial spin–orbit coupling effect related to the Cr insertion. Thus, SOT-induced magnetization switching was achieved under a relatively small critical current density of the order of 106 A/cm2 owing to the improved SOTs in the Pt/Co/Cr/Ta system. Our results indicate that the PMA and SOTs in Pt/Co/Cr/Ta structures can be enhanced via a Cr interfacial decoration.

High temperature and full-in-plane-direction workable high-frequency soft magnetic epitaxial FeSi thin films on MgO(0 0 1)

The epitaxial FeSi(0 0 1)[1 1 0]//MgO(0 0 1)[1 0 0] films were fabricated by sputtering and post annealing at 800 degrees C. A four-fold symmetric angular dependence of remanence ratios and coercivities of FeSi films were observed and well fitted by theoretical models considering the cubic anisotropy. The experimental ferromagnetic resonance frequency (f(r)) of epitaxial FeSi films reaches to 8.0 GHz, which is in agreement with the theoretical value derived from Landau-Lifshitz-Gilbert equation at room temperature. Moreover, the resonance phenomenon can be observed in any in-plane directions in contrast with the absence of resonance phenomenon in some specific directions for in-plane uniaxial soft magnetic Fe2Co films. Although the saturation magnetization, cubic anisotropy constant and fr all decrease with increasing temperature, fr still can keep as high as 3.2 GHz at 800 K, indicating that the epitaxial FeSi films with high Curie temperature have potential application in full angle workable microwave devices at relatively high temperature.

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.

Roles of Joule heating and spin-orbit torques in the direct current induced magnetization reversal

Current-induced magnetization reversal via spin-orbit torques (SOTs) has been intensively studied in heavy-metal/ferromagnetic-metal/oxide heterostructures due to its promising application in low-energy consumption logic and memory devices. Here, we systematically study the function of Joule heating and SOTs in the current-induced magnetization reversal using Pt/Co/SmOx and Pt/Co/AlOx structures with different perpendicular magnetic anisotropies (PMAs). The SOT-induced effective fields, anisotropy field, switching field and switching current density (Jc) are characterized using electric transport measurements based on the anomalous Hall effect and polar magneto-optical Kerr effect (MOKE). The results show that the current-generated Joule heating plays an assisted role in the reversal process by reducing switching field and enhancing SOT efficiency. The out-of-plane component of the damping-like-SOT effective field is responsible for the magnetization reversal. The obtained Jc for Pt/Co/SmOx and Pt/Co/AlOx structures with similar spin Hall angles and different PMAs remains roughly constant, revealing that the coherent switching model cannot fully explain the current-induced magnetization reversal. In contrast, by observing the domain wall nucleation and expansion using MOKE and comparing the damping-like-SOT effective field and switching field, we conclude that the current-induced magnetization reversal is dominated by the depinning model and Jc also immensely relies on the depinning field.