X. F. Han

The study of interaction between graphene and metals by Raman spectroscopy

Different metal films (Co, Ni, Au, and Ag) were deposited on graphene and the interactions between these metals and graphene were studied by Raman spectroscopy. The Raman peaks were shifted after the deposition of metal films. The electron doping of graphene with cobalt contacts and the hole doping with the nickel contacts are the main reasons for Raman peak shift. However, for gold contacts and silver contacts with graphene, strain effect dominates Raman peak shift instead of charge transfer.

80% tunneling magnetoresistance at room temperature for thin Al–O barrier magnetic tunnel junction with CoFeB as free and reference layers

Magnetic tunnel junctions (MTJs) with structures of Ta(5)∕Cu(10)∕Ta(5)∕Ir21Mn79(10)∕Co75Fe25(2)∕Ru(0.75)∕Co40Fe40B20(3)∕Al(0.6)–O∕Co40Fe40B20(2.5)∕Ta(3)∕Ru(7) (units in nanometers) were deposited via ultrahigh vacuum magnetron sputtering (ULVAC). Microscale ring-type magnetic tunnel junctions (RMTJs) with an outer radius of 2μm and an inner radius of 1μm were patterned using standard UV lithography combined with ion milling. Both reference and free layers were Co40Fe40B20 and a very thin Al–O (0.6nm) barrier layer was used. Tunneling magnetoresistances (TMRs) of up to 81% at room temperature and 107% at 4.2K were observed. These RMTJs with high TMR and low coercivity, of about 26Oe, combined with the ring-type geometry, which greatly reduces stray magnetic field, are ideal for certain magnetic field sensor applications.

Chemical diffusion: Another factor affecting the magnetoresistance ratio in Ta/CoFeB/MgO/CoFeB/Ta magnetic tunnel junction

This letter investigates the microstructure and mean inner potential (MIP) profile of Ta/CoFeB/MgO/CoFeB/Ta magnetic tunnel junctions (MTJs) by high resolution transmission electron microscopy (HRTEM) and electron holography, respectively. The inconspicuous crystallization of MgO barrier is confirmed by HRTEM in the post-annealed sample at 250u2009°C. An obvious MIP difference is displayed in the Ta layers between the top and bottom of the MTJ, and elemental content difference of them is confirmed by energy dispersive spectroscopy. These results imply that the chemical diffusion can also give rise to a lower tunnel magnetoresistance ratio besides the inconspicuous crystallization of MgO barrier.

Synthesis and magnetic characterization of Co-NiO-Ni core-shell nanotube arrays

Well-aligned Ni and Co-NiO-Ni core-shell nanotube arrays with an average outer diameter of about 200 nm have been synthesized in a porous anodic aluminum oxide (AAO) template by the direct electro-deposition method. The result shows that the nanotubes were polycrystalline phase. Angular dependent coercivity Hc(θ) indicates that the magnetization reversal mechanisms are mainly dominated by the “curling mode” for Ni nanotubes, while a transition from curling to transverse is observed in Co-NiO-Ni core-shell nanotubes at a large angle. A sharp increase in saturated magnetization (Ms) of Ni and Co-NiO-Ni core-shell nanotubes at 5 K was observed, due to dominance of the surface effect. Besides, the existed superparamagnetic nanoparticles also resulted in the increase of Ms for both cases. These structures have potential applications in the novel spintronics device, ultrasmall magnetic media, and other nanodevices.

Effect of annealing on the magnetic tunnel junction with Co/Pt perpendicular anisotropy ferromagnetic multilayers

Perpendicular magnetic tunnel junctions (pMTJs) with tunneling magnetoresistance (TMR) as high as 14.7% at room temperature were fabricated. The continuous film and pMTJs with Co/Pt multilayer magnetic electrodes and AlOx tunnel barrier were annealed at different temperatures and the effect of annealing on their properties was investigated. The hysteresis loops and X-ray reflectivity measurement show that the interdiffusion of Co and Pt atoms is slight when annealed below 523 K. However, the patterned magnetic tunnel junction gets TMR ratio from 12.3% to the maximum value of 14.7% after annealing at 483 K for 1 h.

Effect of external magnetic field on magnetic properties of Co–Pt nanotubes and nanowires

Well-ordered Co90Pt10 nanotubes and nanowires have been fabricated into anodic aluminum oxide (AAO) templates by using a dc electrodeposition method. The effects of an external magnetic field on the magnetic properties of Co90Pt10 nanotubes and nanowires are investigated. The results show that the external magnetic field during electrodeposition has a significant effect on Co90Pt10 nanowires, as well as nanotubes. The external magnetic field induces good texture and large grain size in Co90Pt10 nanowires, causing a reduction of coercivity. An obvious uniaxial anisotropy is induced in Co90Pt10 nanotubes, causing the crossover of the easy axis along the directions of an external magnetic field.

Electrical spin injection into GaAs based light emitting diodes using perpendicular magnetic tunnel junction-type spin injector

Remanent electrical spin injection into an InGaAs/GaAs based quantum well light emitting diode is realized by using a perpendicularly magnetized MgO/CoFeB/Ta/CoFeB/MgO spin injector. We demonstrate that the Ta interlayer plays an important role to establish the perpendicular magnetic anisotropy and the thickness of Ta interlayer determines the type of exchange coupling between the two adjacent CoFeB layers. They are ferromagnetically or antiferromagnetically coupled for a Ta thickness of 0.5u2009nm or 0.75u2009nm, respectively. A circular polarized electroluminescence (Pc) of about 10% is obtained at low temperature and at zero magnetic field. The direction of the electrically injected spins is determined only by the orientation of the magnetization of the bottom CoFeB layer which is adjacent to the MgO/GaAs interface. This work proves the critical role of the bottom CoFeB/MgO interface on the spin-injection and paves the way for the electrical control of spin injection via magnetic tunnel junction-type spin injector.

Characterization of stearic acid adsorption on Ni(111) surface by experimental and first-principles study approach

Long-chain alkanoic acids usually form close-packed monolayer films with alkyl chains highly oriented on substrates. Previous studies have reported the adsorption of stearic acid on gold, aluminum, copper, silver, and aluminum oxide. However, there are no reports of stearic acid adsorption on magnetic metals. In this work, the characterization of stearic acid adsorbed on Ni(111) surface has been studied experimentally and with first-principles calculation. The results suggest that the stearic acid is chemically adsorbed on the Ni(111) surface via a bidentate interaction with a distance of about 1.8 A. Besides this, we have also obtained results for the charge transfer and magnetic proximity effect.

Patterned magnetic tunnel junctions with Al conduction layers: Fabrication and reduction of pinhole effect

Magnetic tunnel junctions (MTJs), Ta(5u2002nm)/Al(20u2002nm)/Ni79Fe21 (5u2002nm)/Ir22Mn78 (10u2002nm)/Co75Fe25 (4u2002nm)/Al(x nm)-oxide/Co75Fe25 (4u2002nm)/Ni79Fe21 (20 nm)/Ta (5 nm) [x=0.8 and 0.9 nm], with 20 nm thick Al conduction layers were fabricated using magnetron sputtering deposition. Due to the surface roughness of the Al conduction layer, metallic pinholes could easily occur at the barrier layer. Remedies for such a pinhole issue can be solved by a critical underoxidation condition for the Al-oxide tunneling barriers and a postfabrication annealing process of the MTJs at 300u200a°C. A high magnetoresistance ratio of 45%, low resistance-area product of around 10u2009kΩu200aμm2, and sensing layer coercivity of approximately 20 Oe were obtained at room temperature for the annealed MTJs. The results suggest that pinhole defects could be eliminated or significantly reduced with a critical Al-oxide layer fabrication condition together with the annealing process. The MTJ structure after further structural and patterning modifications ...

Field sensing in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer

Linear response and low frequency noise have been investigated in MgO double barrier magnetic tunnel junctions with a superparamagnetic Co50Fe50 free layer. Linear and hysteresis-free switching was observed for the Co50Fe50 thickness tu2009≤u20091u2009nm. A tunneling magnetoresistance ratio of up to 108% and large magnetic field sensitivity value of 61%/mT were obtained at room temperature when tu2009=u20091.0u2009nm. The angular dependence of magnetoresistance suggests that weak coupling between superparamagnetic islands in a 1.0u2009nm free layer permits continuous rotation of magnetization, whereas the islands in a 0.8u2009nm layer switch rather independently. The frequency dependence of noise power spectrum density and field dependence of Hooge parameter (α) also behave differently for junctions with 0.8 and 1.0u2009nm free layers. The noise sensitivity of 1.0u2009nm free layer junctions is independent of bias, and it is estimated to reach 400 pT/Hz0.5 at 500u2009kHz.