Elvira Paz

Femtosecond control of electric currents in metallic ferromagnetic heterostructures

The idea to use not only the charge but also the spin of electrons in the operation of electronic devices has led to the development of spintronics, causing a revolution in how information is stored and processed. A novel advancement would be to develop ultrafast spintronics using femtosecond laser pulses. Employing terahertz (10(12) Hz) emission spectroscopy and exploiting the spin-orbit interaction, we demonstrate the optical generation of electric photocurrents in metallic ferromagnetic heterostructures at the femtosecond timescale. The direction of the photocurrent is controlled by the helicity of the circularly polarized light. These results open up new opportunities for realizing spintronics in the unprecedented terahertz regime and provide new insights in all-optical control of magnetism.

Large Area and Low Aspect Ratio Linear Magnetic Tunnel Junctions With a Soft-Pinned Sensing Layer

Large area MTJs with linear response and very larger sensitivities are needed to fulfill the requirements for the detection of pT/Hz^0.5 magnetic fields at low frequency and room temperature. MTJ stacks with soft-pinned sensing layers have been developed and the stack was optimized, providing a tunable soft-pinning field over a large range of values. Sensitivities corresponding to resistance changes of up to 7%/Oe are obtained in patterned MTJs with areas of 20 × 20 μm² starting from films with high sensitivity soft-pinned layers made from 3.0Co₄₀Fe₄₀B₂₀/0.21Ta/8.0Ni₈₁Fe₁₉/8.0Ir₂₀Mn₈₀.

MgO-based magnetic tunnel junction sensors array for non-destructive testing applications

A MgO-based magnetic tunnel junction (MTJ) sensor including 72 MTJs in series with 50 × 50 μm2 was successfully microfabricated. Due to a two-step annealing strategy, a linear transfer curve was obtained. The tunneling magnetoresistance (TMR) value is as high as 159% and the sensitivity reaches 2.9%/Oe. The field detectivity exhibits the lowest value at 1 V bias current, attaining 1.76 nT/Hz0.5 and 170 pT/Hz0.5 for 10 Hz and 1 kHz, respectively. The results show that the sensor could be applied in non-destructive testing systems which are used for detecting small defects inside conductive materials.

Room temperature direct detection of low frequency magnetic fields in the 100 pT/Hz0.5 range using large arrays of magnetic tunnel junctions

We developed a highly sensitive magnetic tunnel junction (MTJ) based magnetic field sensor device with a magnetic field sensitivity of 1.29 Ω/μT. For this, more than 1000 microstructured linear MgO based MTJ pillars were fabricated and connected in series. By current biasing the MTJ sensor device under small AC external magnetic fields (μ0HAC), a large AC output voltage proportional to μ0HAC is induced. Carrying out noise spectral density measurements at such conditions, we demonstrate that our MTJ sensor device is able to detect magnetic fields down to 115 pT/Hz0.5 at frequencies fAC < 200 Hz in an unshielded environment without amplification of the MTJ AC output voltage.

Exploring the Potential of Starch/Polycaprolactone Aligned Magnetic Responsive Scaffolds for Tendon Regeneration

The application of magnetic nanoparticles (MNPs) in tissue engineering (TE) approaches opens several new research possibilities in this field, enabling a new generation of multifunctional constructs for tissue regeneration. This study describes the development of sophisticated magnetic polymer scaffolds with aligned structural features aimed at applications in tendon tissue engineering (TTE). Tissue engineering magnetic scaffolds are prepared by incorporating iron oxide MNPs into a 3D structure of aligned SPCL (starch and polycaprolactone) fibers fabricated by rapid prototyping (RP) technology. The 3D architecture, composition, and magnetic properties are characterized. Furthermore, the effect of an externally applied magnetic field is investigated on the tenogenic differentiation of adipose stem cells (ASCs) cultured onto the developed magnetic scaffolds, demonstrating that ASCs undergo tenogenic differentiation synthesizing a Tenascin C and Collagen type I rich matrix under magneto-stimulation conditions. Finally, the developed magnetic scaffolds were implanted in an ectopic rat model, evidencing good biocompatibility and integration within the surrounding tissues. Together, these results suggest that the effect of the magnetic aligned scaffolds structure combined with magnetic stimulation has a significant potential to impact the field of tendon tissue engineering toward the development of more efficient regeneration therapies.

Integration of TMR Sensors in Silicon Microneedles for Magnetic Measurements of Neurons

In this work an alternative neuroscience tool for electromagnetic measurements of neurons at the level of individual cells is developed. To perform such measurements we propose the integration of an array of magnetoresistive sensors on micro-machined Si probes capable of being inserted within the brain without further damage. Si-etch based micromachining process for neural probes is demonstrated in the manufacture of a probe with 15 magnetoresistive sensors in the tip of each shaft. Magnetic tunnel junction sensors with dimensions of 30 μm × 2 μm, sensitivities of 3.32 V/T and detectivity of ≈13 nT/Sqrt (Hz) are placed in the end of the sharply defined probe tips. In order to measure the small signals coming from the neurons, a homemade signal amplifying system was used with a noise level of 240 n VRMS for the system bandwidth. The full system noise is 2772 n VRMS.

MgO Magnetic Tunnel Junction Electrical Current Sensor With Integrated Ru Thermal Sensor

Full Wheatstone bridge electrical current sensor incorporating 114 MgO-based magnetic tunnel junction elements (3 × 30 μm2) connected in series was produced for improved electrical robustness. To that end, magnetic tunnel junctions with R × A ~7 KΩ μm2 tunneling magnetoresistance of 200%, were produced. The sensor was designed with an integrated Ru thin film resistive thermal detector (RTD) for temperature drift monitoring and compensation. In order to achieve a full bridge signal, a U-shaped copper trace was placed under a printed circuit board (PCB) specifically designed for this type of device. The resulting device exhibit sensitivities of 63.9 V/Oe/A in a 75 Oe linear range biased with 1 mA current, providing a significantly advantageous alternative to AMR and GMR based bridges.

Magnetic tunnel junction based eddy current testing probe for detection of surface defects

In recent years, magnetoresistive sensors have been applied to a large spectrum of applications from biomedical devices to industrial devices. Their high sensitivity and high spatial resolution are of special interest for eddy current based non-destructive testing. In this particular application, giant magnetoresistive sensors have been recently used for detecting surface and buried defects. Nevertheless, although very promising, magnetic tunnel junctions (MTJs) are still barely used in this application. In this work, two sensors with 6 and 10 MTJs in series were successfully fabricated, characterized, and tested on an aluminum mock-up including defects 100 μm wide and with a depth ranging from 0.2 to 1 mm. The sensors including 6 MTJ in series showed sensitivities of 50.8 mV/mT, while the sensor with 10 MTJ in series showed a sensitivity of 84.5 mV/mT. Due to its high sensitivity the latter was able to detect the smallest defect with a signal to noise ratio of 50, which seems promising for more challengi...

2-Axis Magnetometers Based on Full Wheatstone Bridges Incorporating Magnetic Tunnel Junctions Connected in Series

Full Wheatstone Bridge incorporating a serially connected ensemble of Magnetic tunnel junctions was produced, targeting an application as a magnetic field compass. To that end, MTJs with RxA ~ 10kΩ μm2 TMR~150-200%, Hf = 5 Oe and Hf = 5 Oe were produced. In order to achieve a full bridge signal, two stacks with an asymmetric SAF reference structure where used to produce MTJs with opposite dR/dH upon annealing in the same substrate. The resulting Bridges exhibit sensitivities between 13.5-32 mV/V/Oe depending on the field range and provide a significantly advantageous alternative to AMR and GMR based bridges.

Switching Field Variation in MgO Magnetic Tunnel Junction Nanopillars: Experimental Results and Micromagnetic Simulations

The switching field dependence on the size of nanometric magnetic tunnel junctions was studied. CoFe/Ru/CoFeB/MgO/CoFeB nanopillars were fabricated down to 150 × 300 nm2 and characterized, revealing a squared transfer curve with a sharp transition between magnetic states. A micromagnetic finite element tool was then used to simulate the magnetic behavior of the studied nanopillar. The simulations indicated a single-domain like state at remanence, also displaying a sharp transition between parallel/antiparallel free-layer configurations. Overall, the experimentally measured switching fields (Hsw) were smaller than those obtained from simulations. Such trend was consistent with the presence of a particular free layer profile, signature of the two angle etching step used for pillar definition. Further decrease of experimental Hsw was attributed to local defects and thermal activated processes. This study was able to validate this particular simulation tool for the control of the nanofabrication process.