G. Pan

Graphene electrode modified with electrochemically reduced graphene oxide for label-free DNA detection

A novel printed graphene electrode modified with electrochemically reduced graphene oxide was developed for the detection of a specific oligonucleotide sequence. The graphene oxide was immobilized onto the surface of a graphene electrode via π-π bonds and electrochemical reduction of graphene oxide was achieved by cyclic voltammetry. A much higher redox current was observed from the reduced graphene oxide-graphene double-layer electrode, a 42% and 36.7% increase, respectively, in comparison with that of a bare printed graphene or reduced graphene oxide electrode. The good electron transfer activity is attributed to a combination of the large number of electroactive sites in reduced graphene oxide and the high conductivity nature of graphene. The probe ssDNA was further immobilized onto the surface of the reduced graphene oxide-graphene double-layer electrode via π-π bonds and then hybridized with its target cDNA. The change of peak current due to the hybridized dsDNA could be used for quantitative sensing of DNA concentration. It has been demonstrated that a linear range from 10(-7)M to 10(-12)M is achievable for the detection of human immunodeficiency virus 1 gene with a detection limit of 1.58 × 10(-13)M as determined by three times standard deviation of zero DNA concentration.

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.

(FeCo/Co–M)n soft magnetic multilayers with uniaxial anisotropy and very high saturation magnetization

Nanocrystalline (FeCo/CoNbZr)n and (FeCo/CoZr)n uniaxial-anisotropy soft magnetic multilayers with saturation flux density up to 22.5 kG were prepared by rf sputtering. Magnetic and microstructural properties of these films were studied before and after magnetic annealing. Cross-sectional transmission electron microscopy study showed that the soft magnetic Co–M (M=Zr, Nb, etc.) amorphous layers act as grain refiners as well as soft magnetic grain-boundary materials for the FeCo layers. The resultant multilayer films exhibit excellent uniaxial-anisotropy soft magnetic properties with small coercivity, high anisotropy field Hk, and excellent thermal stability after magnetic field annealing, which are very attractive for applications in thin-film heads for ultrahigh-density magnetic recording. The origin of the soft magnetism was discussed in the light of the random anisotropy model. The improvement of soft magnetic properties of the laminated films is closely associated with the reduction in grain size of t...

In-contact magnetic recording performance of Pt/CoCrTa thin films on glass computer disks☆

Abstract CoCrTa films deposited on fcc Pt and Au underlayers show excellent c -axis orientation. The ‘epitaxial-growth’ mechanism may explain the origin of such an effect. The perpendicular MOKE and VSM coercivity, shape of hysteresis loops and contact recording results of such media are investigated and discussed together with the film structure. A first peak D 50 of 160 kFRPI has been realized by a single-layer Pt/CoCrTa rigid disk using an in-contact MIG head.

Planar Hall effect in NiFe/NiMn bilayers

The exchange anisotropy in NiFe/NiMn bilayers was studied by using the planar Hall effect. The sputtered NiFe/NiMn films were patterned into strips of 1 mm in length and 200 μm in width and with six terminals for anisotropy magnetoresistance and planar Hall voltage measurements by a photolithographic process. It is shown that the planar Hall effect is an effective method to characterize the exchange anisotropy in ferromagnetic/antiferromagnetic (AF) systems. It can be used to accurately determine the exchange field and describe the magnetization reversal processes. The effective uniaxial anisotropy field HK eff, the effective unidirectional anisotropy field Hud, and AF domain wall energy Hw can be obtained by fitting the experimental results. We found that in the NiFe/NiMn bilayer system, the parameters HK eff, Hud, and Hw have the same values in reversible and irreversible measurements, and the domain wall energy in AF layer is larger than interfacial unidirectional anisotropy.

Transverse susceptibility of nanoparticle systems: The effect of interaction, dispersion, and texture

The effect of texture, dispersion, and interaction on transverse susceptibility has been studied. We have developed a model based on the well-known Stoner-Wohlfarth model, by taking into account the texture, the anisotropy field value distribution, and the intergranular magnetic interactions. This model shows a good agreement with experimental measurements on granular FeCoV thin films and allows us to determine the mean value of the local magnetic anisotropy, intergrain interaction, and texture of these alloys.

Spin-filter specular spin valves

Both a thin free layer and high magnetoresistance (MR) ratio are required in spin valves for high magnetic density recording heads. In traditional spin valve structures, reducing the free layer normally results in a reduction in MR. We report here on a spin-filter specular spin valve with structure Ta 3.5 nm/NiFe 2 nm/IrMn 6 nm/CoFe 1.5 nm/Nol/CoFe 2 nm/Cu 2.2 nm/CoFe tF/Cu tSF/Nol2/Ta 3 nm, which is demonstrated to maintain MR ratio higher than 12% even when the CoFe free layer is reduced to 1 nm. The semiclassical Boltzmann transport equation was used to simulate MR ratio. An optimized MR ratio of ∼14.5% was obtained when tF was about 1.5 nm and tSF about 1.0 nm as a result of the balance between the increase in electron mean free path difference and current shunting through conducting layer. It is found that the Cu enhancing layer not only enhances the MR ratio but also improves soft magnetic properties of CoFe free layer due to the low atomic intermixing observed between Co and Cu. The CoFe free layer...

CoNbFe soft magnetic thin‐film backlayers for glass computer disks

Co84Nb12Fe4 films with very low coercivity down to 1 A/m, anisotropy field 1100 A/m, and saturation magnetic induction 1.1 T have been deposited by rf sputtering onto chemically strengthened glass disk substrates as a backlayer for perpendicular recording Winchester disks. Values of coercivity and anisotropy field were studied over a thickness range from 30 to 15 000 A and found to be a strong function of film thickness. Comparisons between Neel’s prediction for wall motion coercivity mechanism and the experimental data were made. The experimental coercivity fits the Neel formula when films are thicker than 400 A. The dependence of coercivity and anisotropy field on deposition conditions was also characterized. The thermal stability of the films was studied by differential scanning calorimetry (DSC) and by thermal annealing up to 500 °C. The crystallization temperature of the CoNbFe films is about 450 °C determined from DSC analysis. Thermal annealing revealed that the magnetic properties were very stable...

Spin-inversion in nanoscale graphene sheets with a Rashba spin-orbit barrier

Spin-inversion properties of an electron in nanoscale graphene sheets with a Rashba spin-orbit barrier is studied using transfer matrix method. It is found that for proper values of Rashba spin-orbit strength, perfect spin-inversion can occur in a wide range of electron incident angle near the normal incident. In this case, the graphene sheet with Rashba spin-orbit barrier can be considered as an electron spin-inverter. The efficiency of spin-inverter can increase up to a very high value by increasing the length of Rashba spin-orbit barrier. The effect of intrinsic spin-orbit interaction on electron spin inversion is then studied. It is shown that the efficiency of spin-inverter decreases slightly in the presence of intrinsic spin-orbit interaction. The present study can be used to design graphene-based spintronic devices.

Enhanced exchange anisotropy in IrMn/CoFeB systems and its correlation with uncompensated interfacial spins

Bottom pinned exchange bias systems of IrMn/CoFe and IrMn/CoFeB on CoFe seed layers were studied. Enhanced exchange anisotropy has been observed for IrMn/CoFeB samples annealed at 350 °C. The ferromagnetic and antiferromagnetic layers of both samples are polycrystalline and textured {110} for the CoFe and CoFeB, and {111} for IrMn. Results demonstrated that the enhanced exchange anisotropy in the IrMn/CoFeB system is closely associated with the increased uncompensated interfacial spins as evidenced by the enhanced Mn x-ray magnetic circular dichroism (XMCD) signal strength. A quantitative correlation between the Mn XMCD signal and the exchange anisotropy constant Jk was observed.