F. Carpinteiro

Versatile, high sensitivity, and automatized angular dependent vectorial Kerr magnetometer for the analysis of nanostructured materials

Magneto-optical Kerr effect (MOKE) magnetometry is an indispensable, reliable, and one of the most widely used techniques for the characterization of nanostructured magnetic materials. Information, such as the magnitude of coercive fields or anisotropy strengths, can be readily obtained from MOKE measurements. We present a description of our state-of-the-art vectorial MOKE magnetometer, being an extremely versatile, accurate, and sensitivity unit with a low cost and comparatively simple setup. The unit includes focusing lenses and an automatized stepper motor stage for angular dependent measurements. The performance of the magnetometer is demonstrated by hysteresis loops of Co thin films displaying uniaxial anisotropy induced on growth, MnIr/CoFe structures exhibiting the so called exchange bias effect, spin valves, and microfabricated flux guides produced by optical lithography.

The effect of pinhole formation/growth on the tunnel magnetoresistance of MgO-based magnetic tunnel junctions

In this study, we focus on how the formation and enlargement of metallic pinholes in MgO barriers (induced by large electrical currents) affect the tunnel magnetoresistance (TMR) of low and high resistance (R) magnetic tunnel junctions. The junctions were deposited by physical vapor deposition with barrier thicknesses of either 0.75 or 1.35 nm. For the parallel state, temperature-dependent R(T) measurements readily revealed a metallic conductance in the low-R sample, indicating that pinholes are already present in its thin barrier; a slight R(T) decrease with increasing temperature is observed for the high-R junction. After applying large current pulses to the low-R sample, we observe that the initially small R-decrease (∼6%) is accompanied by a significant TMR increase (∼20% at 20 K). Higher applied electrical currents continue to decrease R, leading to a gradual but steady TMR decrease. In contrast, the high-R sample exhibits a sharp and immediate decrease in TMR as soon as the first pinhole is formed. ...

Heat Generation in Tunnel Junctions for Current-Written Pinned Layer Switching

To commute between the different resistance states of a magnetic tunnel junction (TJ) one can use a thermally-induced pinned layer switching mechanism. When a sufficiently high electrical current flows through the insulating barrier, local temperatures inside the tunnel junction can increase above the blocking temperature of the antiferromagnetic layer used to pin the magnetization of the adjacent ferromagnet. Then, it is possible to switch the magnetization of the pinned layer with a small magnetic field H and thus revert the magnetic state of the TJ. Here we demonstrate thermally-induced pinned layer switching in thin magnetic tunnel junctions. We further present numerical results that suggest that heating is small when one takes into consideration the uniform current density flowing through the tunnel junction and that one must conclude that nanoconstrictions concentrate most of the current, increasing local current densities and temperature. Simulation of heating and cooling times demonstrates that current-induced pinned layer switching is a competitive mechanism for actual technological applications.

Short-Range Effects and Magnetization Reversal in Co80 Fe20 Thin Films: A MOKE Magnetometry/ Domain Imaging and AMR Study

A MOKE magnetometry unit simultaneously sensitive to both in-plane magnetization components, based on an intensity differential detection method, allows us to observe the uniaxial anisotropy impressed during CoFe-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axes. Our MOKE imaging unit, using a CCD camera for Kerr effect domain visualization provides direct evidence on the dominant M-processes, namely domain wall motion and moment rotation. Further magnetic information was obtained by AMR measurements due to the dependence of the electrical resistivity on the short-range spin disorder and also on the angle between the electrical current direction (I) and the spontaneous magnetization (MS).