A. Canizo-Cabrera

Perpendicular magnetic tunnel junctions with synthetic antiferromagnetic pinned layers based on [Co/Pd] multilayers

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 si...

Effect of Ta thickness on the perpendicular magnetic anisotropy in MgO/CoFeB/Ta/[Co/Pd]n structures

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.

Selected error sources in resistance measurements on superconductors

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 Y1Ba2Cu3O7−x 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.

Comparison of the interfacial structure between MgO and Al–O oxidation layers for perpendicular magnetic tunnel junction

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.

Spin-polarized current-induced magnetization precession and switching for perpendicular anisotropic ferrimagnetic thin films

We present here a theoretical calculation on magnetization precession and switching induced by spin-polarized current spin-transfer torque using perpendicular anisotropic magnetic multilayered thin films which contain two strongly coupled antiparallel magnetic subnetworks. We develop a modified Landau-Lifshitz-Gilbert equation for this material structure to obtain the final equation governing the dynamics of the net magnetization. We present results of the time evolution of the net magnetization under the influence of a spin current as well as the estimation of the critical current for practical application in magnetic random access memory writing. A relation between magnetization switching time and current is also given.

Current-induced switching study of the canting model for ferrimagnet thin films

We present a study on the influence of the canting angle between the magnetization vectors of a two-subnetwork system on the magnetization switching exerted by a spin-polarized induced current. Numerical calculations are performed on current-induced magnetization switching in a magnetic system with a canting state. The modified Landau-Lifshitz-Gilbert equation is used to produce quantitative predictions for the critical current as a function of the exchange coupling constant (which characterizes the canting strength) at several applied magnetic fields. The results indicate that the critical current reduces its value to one third when the coupling constant is varied.

Tunneling magnetoresistance of magnetic tunneling junction cell measured by conducting atomic force microscopy with ramping dc bias voltage rate

Magnetoresistance (MR) ratio in a magnetic tunneling junction cell with a structure of SiO2 (20nm)∕Ta (5nm)∕Cu (20nm)∕Ta (5nm)∕NiFe (2nm)∕Cu (5nm)∕MnIr (10nm)∕CoFe (4nm)∕Al–O (1.5nm)∕CoFe (4nm)∕NiFe (20nm)∕Ta (50nm) was measured by conducting atomic force microscopy to obtain I‐V curves. Tunnel magnetoresistance was characterized from these nonlinear I‐V curves. MR values of 33.9%, 30.5%, 30.3%, and 28% were obtained when applying magnetic fields of ±150Oe at various dc bias voltage ramping rates. Several ramping rate values were 0.498, 4.65, 9.3, and 27.9Hz, respectively.

Ramping effects on dielectric tunnel capacitance of magnetic random access memory cell

To characterize the dielectric tunnel capacitance of the thin insulating layer in magnetic random access memory cell under the various ramping rates, the examination of current images and current-voltage (I–V) characteristics of the magnetic tunnel junctions were carried out by conductive atomic force microscopy. It is confirmed that in all three contrasted regions, dielectric tunnel capacitance in the very low leaky state decreases with higher ramping rate. With more induced polarization charges in the dielectrics under slower ramping rate, the dielectric tunnel capacitance will increase due to a higher reversed polarized field formed within the dielectrics.