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%.

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

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.

In order to investigate the causes that produce some of the unwanted effects observed in the resistance versus temperature profiles, a variety of sources of error for resistance measurements in superconductors, using a standard four-probe configuration, have been studied. A piece of superconducting Y 1 Ba 2 Cu 3 O 7Ϫx ceramic material has been used as the test sample, and the resulting effects in both accuracy and precision in its temperature dependent resistance are reported here. Studied measurement error sources include thermal emf's, temperature sweep rates, Faraday currents, electrical-contact failures at the sample's surface, thermal contractions at mechanically attached instrumental wires, external electromagnetic fields, and slow sampling rates during data acquisition. Details of the experimental setup and its measurement error function are also given.

No abstract

We discuss the interfacial structure of MgO and Al–O barrier layers and influence on the magnetic properties of perpendicular magnetic tunnel junction (pMTJ) devices. The pMTJs layer structures analyzed were Si-wafer∕Pt∕Gd(FeCo)∕FeCo∕MgO (AlO)∕FeCo∕Tb(FeCo)∕Pt. The deposit of all pMTJs structures was carried out by rf and dc magnetron sputtering systems. Transmission electron microscopy (TEM) clearly showed that the interfacial structure of FeCo∕MgO or AlO∕FeCo in the pMTJs was very smooth and uniform. Hysteresis loops obtained by an alternating gradient magnetrometer (AGM) for the different oxide barrier layers of pMTJ structures showed that the Al–O layer performs better than the MgO layer. An additional discussion on the oxide layer thickness in the TEM and AGM measurements is also presented.

In the manufacture of optoelectronics devices, including solar cells, a very important parameter is the degree of chemical purity of the materials used during their fabrication process. There are two figures of merit that allow the comparison in quality of devices prepared by sputtering. The first figure of merit, the impurity concentration, directly depends on the sputtering pressure and the effective leak rate of impurities into the sputtering system. The second figure of merit, the fraction of an impurity monolayer that can be formed, depends on the time a clean surface is exposed to impurities in the growth chamber until the next layer is deposited. In this work, a study to correlate tIle sputtering pressure with the first figure of merit for Nb thin films grown on 90° oriented sapphire substrates is presented. These films were deposited by DC magnetron sputtering, using a Nb disk as the target in a high vacuum system with a base pressure of 5×10-8 torr in Ar plasma. Niobium was chosen because its electrical properties allow easy measurement and comparison. The Nb films were grown at room temperature, keeping fixed all growth parameters but the plasma pressure. Morphology, elemental composition, and structure of the films were determined by scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), and x-ray powder diffraction, respectively. Resistance versus temperature profiles in the 10-300 K range are presented, where a correlation between the plasma pressure and the electrical properties can be observed as an indication that the impurity concentration directly depends on the sputtering pressure.