Magnetoresistance of Fe3O4/Au/Fe3O4 and Fe3O4/Au/Fe spin-valve structures

A detailed study of the in-plane magnetotransport properties of spin valves with one and two Fe₃O₄ electrodes is presented. Fe₃O₄/Au/Fe₃O₄ spin valves exhibit a clear anisotropic magnetoresistance in small magnetic fields but no giant magnetoresistance (GMR). The absence of GMR in these structures is due to simultaneous magnetization reversal in the two Fe₃O₄ layers. By contrast, a negative GMR effect is measured on Fe₃O₄/Au/Fe spin valves. The negative GMR is attributed to an electron spin scattering asymmetry at the Fe₃O₄/Au interface or an induced spin scattering asymmetry in the Au interfacial layers.     

Magnetoresistance in point contacts of the Heusler alloy Co2Cr0.6Fe0.4Al

Following a recent report predicting half-metallicity of Co₂Cr_0.6Fe_0.4Al, we have examined point contacts of this material. Large magnetoresistances (up to 80%) have been observed at room temperature in small fields (10 mT). Magnetostriction in these fields is negligible. A spin polarisation of 81% is inferred. Mössbauer spectra show that 90% of the iron is ordered. The magnetisation of the alloy is 3.65μ_B per formula unit, and its Curie temperature is 665 K.     

Simultaneous sensing of L-tyrosine and epinephrine using a glassy carbon electrode modified with nafion and CeO2 nanoparticles

An electrochemical sensor was developed and tested for detection of L-tyrosine in the presence of epinephrine by surface modification of a glassy carbon electrode (GCE) with Nafion and cerium dioxide nanoparticles. Fabrication parameters of a surfactant-assisted precipitation method were optimized to produce 2–3 nm CeO₂ nanoparticles with very high surface-to-volume ratio. The resulting nanocrystals were characterized structurally and morphologically by X-ray diffractometery (XRD), scanning and high resolution transmission electron microscopy (SEM and HR-TEM). The nanopowder is sonochemically dispersed in a Nafion solution which is then used to modify the surface of a GCE electrode. The electrochemical activity of L-tyrosine and epinephrine was investigated using both a Nafion-CeO₂ coated and a bare GCE. The modified electrode exhibits a significant electrochemical oxidation effect of L-tyrosine in a 0.2 M Britton-Robinson (B-R) buffer solution of pH 2. The electro-oxidation peak current increases linearly with the L-tyrosine concentration in the molar concentration range of 2 to 160 μM. By employing differential pulse voltammetry (DPV) for simultaneous measurements, we detected two reproducible peaks for L-tyrosine and epinephrine in the same solution with a peak separation of about 443 mV. The detection limit of the sensor (signal to noise ratio of 3) for L-tyrosine is ~90 nM and the sensitivity is 0.20 μA μM^?1, while for epinephrine these values are ~60 nM and 0.19 μA μM^?1. The sensor exhibited excellent selectivity, sensitivity, reproducibility and stability as well as a very good recovery time in real human blood serum samples.

Simultaneous electrochemical determination of L-tyrosine and epinephrine in blood plasma with Nafion-CeO₂/GCE modified electrode showing a 443 mV peak-to-peak potential difference between species oxidation peak currents.     

Magnetic structures determined by neutron diffraction in the EuBϵ−xCx system

Magnetic structures of pure and carbon-doped europium hexaboride EuB_6?xC_x, were determined by neutron diffraction on powders prepared from ¹¹B and ¹⁵³Eu. EuB₆ is a simple ferromagnet, whereas the x = 0.20 compound has an incommensurate spiral structure with propagation vector τ = (0.28, 0, 0). Data for a magnetically inhomogeneous intermediate composition, x = 0.05, indicate a mixture of ferromagnetic and helimagnetic domains with τ = (0, 0, 0.104). Helimagnetism in EuB_6?xC_x arises from a competition between ferromagnetic near-neighbour exchange and antiferromagnetic interactions due to conduction electrons.     

Mössbauer studies of interstitial intermetallics

After an introduction to interstitial incorporation of 2p elements in iron-based alloys, the main physical effects of these interstitials on the intrinsic properties of rare-earth intermetallics are outlined. Then follows a survey of the results obtained using the⁵⁷Fe and several rare-earth resonances for rare-earth intermetallics with the Th₂Zn₁₇, Th₂Ni₁₇, ThMn₁₂ and BaCd₁₁ structures in which nitrogen or carbon has been interstitially incorporated. The results of Mössbauer studies in this area are discussed, and future prospects are assessed.     

119Sn Mössbauer spectra of RMnSn2 intermetallics (R = La,Ce, Pr,Nd, Sm)

The ternary stannide series of RMnSn₂ compounds crystallize in the defect orthorhombic CeNiSi₂-type structure. They order magnetically close to room temperature. Isomer shifts are approximately 1.9 and 2.8 mm/s at the two tin sites, and there are transferred hyperfine fields of 3–6 T at 15 K, which depend on the rare-earth partner, especially at Sn₂ sites. The magnitude of the transferred hyperfine field per manganese neighbour is 4T.     

Recent advances in scanning tunneling microscopy involving magnetic probes and samples

We report on recent developments in the field of STM performed with magnetic probes and samples. The choice of appropriate magnetic sensors and their in situ preparation will be described. We further focus on the information obtained in spin-polarized scanning tunneling microscopy (SPSTM) as well as on possible modes of operation for simultaneous acquisition of topographic and magnetic data. The prospects for SPSTM and related magnetic sensitive SXM techniques will be discussed.Permanent address: Forschungszentrum Jülich, ISI, W-5170 Jülich 1, Fed. Rep. Germany