We fabricated MgO-based perpendicular magnetic tunnel junctions (p-MTJ) with Ta/CoFeB magnetic electrodes. Synthetic antiferromagnetic (SAF) pinned layers with perpendicular magnetic anisotropy (PMA) were included into the p-MTJs by using two Co/Pd multilayers (MLs) separated by a thin Ru spacer layer. The MTJs stack has the structure bottom contact/free layer CoFeB (1.0)/MgO (1)/pinned layer CoFeB (1.0)/Ta spacer layer/SAF/Ru cap layer/top contact (the units in parenthesis are in nanometers). The SAF was optimized by changing the repetition period n in one of the Co/Pd multilayers and the Ru thickness in order to obtain PMA with antiferromagnetic (AFM) coupling. The Ru spacer values were 0.7, 0.75, 0.8, 0.85, and 0.9 nm. The magnetic studies show that all magnetic films, including the Ta/CoFeB layers, are perpendicularly magnetized. The two Co/Pd MLs are AFM coupled for n > 2. Controlling the Ru thickness, the interlayer exchange coupling strength Jiec can be tailored. Jiec vs. Ru thickness exhibits a simple exponential decay. The electrical properties of the full p-MTJ with SAF show a low resistance-area (RA) product of 44.7 Ω μm2 and a tunnel magnetoresistance (TMR) ratio of 10.2%.

No abstract

Adsorption of Pb(II) from aqueous solution using MFe2O4 nanoferrites (M = Co, Ni, and Zn) was studied. Nanoferrite samples were prepared via the mechanochemical method and were characterized by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), micro-Raman, and vibrating sample magnetometry (VSM). XRD analysis confirms the formation of pure single phases of cubic ferrites with average crystallite sizes of 23.8, 19.4, and 19.2 nm for CoFe2O4, NiFe2O4, and ZnFe2O4, respectively. Only NiFe2O4 and ZnFe2O4 samples show superparamagnetic behavior at room temperature, whereas CoFe2O4 is ferromagnetic. Kinetics and isotherm adsorption studies for adsorption of Pb(II) were carried out. A pseudo-second-order kinetic describes the sorption behavior. The experimental data of the isotherms were well fitted to the Langmuir isotherm model. The maximum adsorption capacity of Pb(II) on the nanoferrites was found to be 20.58, 17.76, and 9.34 mg·g−1 for M = Co, Ni, and Zn, respectively.

We studied the effect of a thin Ta layer on the perpendicular magnetic anisotropy (PMA) of composite FM1/Ta/FM2 magnetic structures, where FM1 represents the subsystem MgO/CoFeB, and FM2 denotes a [Co/Pd]6 multilayer. The stack without Ta spacer layer shows no PMA. Once a Ta layer is inserted between the thin CoFeB layer and the [Co/Pd]6 multilayer, PMA is observed. The perpendicular magnetization loops show squareness ratios close to unity, indicating the presence of almost complete perpendicular anisotropy. These hysteresis loops also show sharp switching characteristics, indicating that the MgO/CoFeB bilayer and the [Co/Pd]6 multilayer are ferromagnetically coupled together. The coercive field Hc of the composite structure increases as Ta thickness increases. Our results show that Ta layer is essential for integrating MgO/CoFeB and [Co/Pd]6 into a composite magnetic structure with perpendicular anisotropy.

No abstract

No abstract

The magnetic behavior of an irreversible type-II superconducting slab under the action of in-plane crossed fields is investigated within both the original elliptic critical-state model and the extended one, which was recently proposed by Clem. In particular, we study the suppression of the remanent magnetization of a PbBi specimen by a sweeping external transverse magnetic field. It is found that both elliptic critical-state models reproduce the main features of available experimental magnetization curves. We also show that the average magnetizations, corresponding to diamagnetic and paramagnetic initial states at a static bias field H z , are asymmetrically reduced by the action of an oscillating transverse field H y . If the amplitude of the oscillations of H y is as large as the first penetration field H P , the resulting state becomes paramagnetic after various cycles of H y . Such a kind of paramagnetism is attributed to the anisotropy, induced by flux-line cutting effects, in the critical current density. In PbBi samples, paramagnetism is expected to be manifest in a wide range of values of the static bias field H z .

The impact of a non-magnetic Ta spacer layer on the perpendicular magnetic anisotropy (PMA) of composite magnetic structures constituted by ultra-thin Co/Pd multilayers (MLs) and MgO/CoFeB was studied. Composite structures lacking a Ta layer present in-plane magnetic anisotropy. The strong perpendicular anisotropy observed in sole Co/Pd MLs is not sufficient to pull the magnetic moment out of the film plane, not even after annealing at 300 or 350 °C. PMA with squareness values close to unity and annealing stability up to 350 °C is observed after the insertion of an ultra-thin Ta layer. Our study demonstrates that Ta layer is essential for obtaining perpendicular magnetic axis in MgO/CoFeB/Ta/[Co/Pd]6. The exchange coupling between the MgO/CoFeB bilayer and the Co/Pd MLs is ferromagnetic with sharp switching characteristics. Perpendicular composite structures with sharp magnetization reversal and annealing stability are relevant in perpendicular CoFeB-based magnetic tunnel junctions for the development of gigabit-scale nonvolatile memory.