Guolin Yu

Magnetic properties and x-ray photoelectron spectroscopy study of NiO/NiFe films prepared by magnetron sputtering

Ta/NiOx/Ni81Fe19/Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange-coupling field (Hex) and the coercivity (Hc) of NiOx/Ni81Fe19 as a function of the ratio of Ar to O2 during the deposition process were studied. The composition and chemical states at the interface region of NiOx/NiFe were also investigated using the x-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that the ratio of Ar to O2 has a great effect on the nickel chemical states in NiOx film. When the ratio of Ar to O2 is equal to 7 and the argon sputtering pressure is 0.57 Pa, the x value is approximately 1 and the valence of nickel is +2. At this point, NiOx is antiferromagnetic NiO and the corresponding Hex is the largest. As the ratio of Ar/O2 deviates from 7, the exchange-coupling field (Hex) will decrease due to the presence of magnetic defects such as Ni+3 or metallic Ni at the interface region of NiOx/NiFe, while the coercivity (Hc) will increase due to the meta...

A specular spin valve with discontinuous nano-oxide layers

Microstructures of the specular spin valve with two nano-oxide layers (NOL1 and NOL2) have been studied at the atomic level. When the NOLs are incorporated in a bottom-pinned spin valve, a significant enhancement in magnetoresistance ratio with greatly decreased sense-layer thickness is achieved. Cross-sectional high-resolution electron microscopy (HREM) studies show that the NOL1 introduced from oxidation of the original bottom-pinned CoFe layer is actually a mixture of oxides and ferromagnetic metals. No CoFe oxides but Ta2O5 is found over the oxidation-treated CoFe sense layer by HREM and x-ray photoelectron spectroscopy study. The Ta2O5 layer acting as the NOL2 can be interpreted as being formed through a solid-state oxidation reaction between the oxidized CoFe sense layer and the Ta capping layers.

Graphene in proximity to magnetic insulating LaMnO3

Proximity to functional substrates may enhance the coupling between the quantum degrees of freedom and thus develop nontrivial quantum effects in graphene. Here, we demonstrate the successful fabrication of graphene in proximity to atomically flat magnetic insulating LaMnO3 films. The insulating nature of the LaMnO3 films not only ensures the electronic transport only occur in the graphene layers but also allow them to serve as dielectric layers for gating. Transport measurements reveal anomalous behaviors, including asymmetrical longitudinal magnetoresistivity and nonlinear Hall effect. This work may pave a way toward the realization of intriguing quantum phases in graphene.

Weak antilocalization effect in high-mobility two-dimensional electron gas in an inversion layer on p-type HgCdTe

Magnetoconductance of a gated two-dimensional electron gas (2DEG) in an inversion layer on a p-type HgCdTe film is investigated. At strong magnetic fields, characteristic features such as the quantum Hall effect of a 2DEG with single subband occupation are observed. At weak magnetic fields, the weak antilocalization effect in the ballistic regime is observed. Phase coherence time and zero-field spin-splitting are extracted following Golub’s model [L. E. Golub, Phys. Rev. B 71, 235310 (2005)]. The temperature dependence of the dephasing rate is consistent with the Nyquist mechanism.

Long phase coherence length and anisotropic magnetoresistance in MgZnO thin film

We comprehensively investigate magnetotransport properties of MgZnO thin film grown on ZnO substrate by molecular-beam epitaxy. We measure the weak localization effect and extract the electron phase coherence length by fitting to a three-dimensional weak localization theory and by analyzing the different changing rate of the magnetoresistance, results of which are in good agreement with each other. The phase coherence length ranges from 38.4±1u200au200au200au200anm at 50u2009K to 99.8±3.6u2009nm at 1.4u200au200aK, almost the same as that of ZnO nanoplates and In-doped ZnO nanowires, and its temperature dependence is found to scale as T−3/4. Meanwhile, we study the anisotropic magnetoresistance resulting from the geometric effect as well as the Lorentz force induced path-length effect, which will be enhanced in higher magnetic fields.

Strained HgTe plates grown on SrTiO3 investigated by micro-Raman mapping

HgTe plates have been grown by vapor phase epitaxy on (111) SrTiO3 substrates with a preferred orientation in the (111) crystalline direction, as indicated by x-ray diffraction. Examination of the plates using the micro-Raman mapping shows that the HgTe plates exhibit unusual strain patterns: the Raman peaks from the transverse-optical and longitudinal-optical phonons for the thicker (central) parts of the HgTe plates are at the same frequency as that of the bulk HgTe, while the Raman peaks for the thinner parts of the HgTe plates, which surround the thicker parts and can hardly be seen in a scanning electron microscope, are significantly larger in frequency. The full width at half maximum is smaller in the thinner areas than in the thicker parts. Theoretical analysis shows that the HgTe plates on SrTiO3 substrates suffer from compressive stress, and this may be sufficient to induce the three-dimensional topological insulator behavior in HgTe.

The effective g-factor in In0.53Ga0.47As/In0.52Al0.48As quantum well investigated by magnetotransport measurement

The magneotransport properties of a high carrier concentration and high mobility 20-nm thick In0.53Ga0.47As/In0.52Al0.48As quantum well (QW) are investigated by tilt angle dependent Shubnikov-de Haas oscillations and by weak antilocalization (WAL) in an in-plane magnetic field. The effective g-factor g* and zero field spin splitting Δ0 are extracted from tilt angle dependent beating pattern. We found that g* shows a dramatic reduction with increasing carrier density due to the increased effective band gap. Furthermore, an anomalously rapid suppression of the WAL effect with increasing in-plane magnetic field B|| is observed. This reveals that the total dephasing rate is not solely contributed by Zeeman splitting. The microroughness scattering in the QW is proposed to be another factor to cause the dephasing and thus responsible for this effect.

Optical Manipulation of Rashba Spin–Orbit Coupling at SrTiO3-Based Oxide Interfaces

Spin-orbit coupling (SOC) plays a crucial role for spintronics applications. Here we present the first demonstration that the Rashba SOC at the SrTiO3-based interfaces is highly tunable by photoinduced charge doping, that is, optical gating. Such optical manipulation is nonvolatile after the removal of the illumination in contrast to conventional electrostatic gating and also erasable via a warming-cooling cycle. Moreover, the SOC evolutions tuned by illuminations with different wavelengths at various gate voltages coincide with each other in different doping regions and collectively form an upward-downward trend curve: In response to the increase of conductivity, the SOC strength first increases and then decreases, which can be attributed to the orbital hybridization of Ti 3d subbands. More strikingly, the optical manipulation is effective enough to tune the interferences of Bloch wave functions from constructive to destructive and therefore to realize a transition from weak localization to weak antilocalization. The present findings pave a way toward the exploration of photoinduced nontrivial quantum states and the design of optically controlled spintronic devices.

Zeeman splitting and spin-orbit interaction in Hg1−xCdxTe inversion layers

We studied the suppression of the weak antilocalization (WAL) effect and the dependence of spin dynamics for a two-dimensional electron gas in the inversion layers of two different Hg 1− x Cd x Te samples in the presence of in-plane magnetic field . The WAL magnetoconductance is fitted by the Golub model to acquire the variations of phase coherence time with increasing . The effective g -factors in the form of ( is the relative effective mass) at zero magnetic field and high magnetic field are obtained by investigating the electron dephasing with varying and measuring the spin splitting of the Shubnikov-de Hass oscillations, respectively. As the obtained g -factors are in accordance with the reported results, the suppression of the WAL effect can be attributed to the competition between Zeeman splitting and spin-orbit interaction rather than to the microroughness scattering.

Quenching and temperature dependence of perpendicular magnetic anisotropy of Pt/Co multilayers

Magnetic metallic multilayers separated by nonmagnetic metal films are of great importance in magnetoelectronics and spintronics, due to their capacity of giving rise to giant magneto-resistance as well as the electric field control of ferromagnetism. Co/Pt multilayers are one of the typical platforms that own perpendicular magnetic anisotropy which can be tuned in various ways. Since previous investigations focus on the anomalous Hall(transverse) resistivity which characterizes the magnetization of the multilayers, much less attention has been paid to the longitudinal resistivity. In this work, we find that the longitudinal resistivity also gives rich phenomena that need further theoretical treatment. We have grown two Co/Pt multilayer structures that have different spacings between neighboring ferromagnetic layers. The one with smaller spacing shows a superparamagnetic behavior in its Hall resistivity even at a temperature as low as 1.5 K, but the longitudinal resistivity shows a well established hysteresis. The other sample shows square hysteresis in the Hall resistivity at all available temperatures up to 300 K, while the longitudinal resistivity gives no significant signals because they are mostly engulfed in the noises. The corresponding temperature dependence of the coercive field are also different. While the former gives an approximately exponential function of the temperature T, the latter can be divided to two zones, each of which can be characterized by a lnTs dependence, where s is not necessarily an integer. Such distinct features may be deeply related to the microstructures as well as the magnon scattering, which require further investigations.