Ana V. Silva

Field Detection in Spin Valve Sensors Using CoFeB/Ru Synthetic-Antiferromagnetic Multilayers as Magnetic Flux Concentrators

This work compares the performance of spin valve sensors comprising magnetic flux concentrators (MFCs) composed of Co₉₃Zr₃Nb₄ (CZN) or (Co₇₀Fe₃₀)₈₀B₂₀-based synthetic-antiferromagnet (SAF) multilayer stacks. In addition, the influence of a tapered MFC tip is also studied. When compared to CZN films, SAFs have the disadvantage of lower magnetic susceptibility (χ = 196 for SAF vs. χ = 753 for CZN), which affects negatively the field gain in gap (10 for SAF vs. 43 for CZN). However, from the overall noise spectrum, one can conclude that the magnetic field detections for sensors incorporating CoFeB/Ru multilayers as MFCs are close to the ones obtained with CZN, being mainly determined by the sensor intrinsic properties instead. For low frequencies, field detection levels at 10 Hz improved from ~ 61 nT/Hz^0.5 for single spin valve sensors down to ~ 1.8 nT/Hz^0.5 when CZN concentrators with a steep-profile are used.

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.

Nanoscale Magnetic Tunnel Junction Sensing Devices With Soft Pinned Sensing Layer and Low Aspect Ratio

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high-accuracy imaging of isolated magnetic nanoparticles or mapping of magnetic fields. Here, we fabricated nanosensor devices based on MgO-magnetic tunnel junctions with soft pinned sensing layer. The exchange interaction at the free-layer is tuned to yield distinct linear operation ranges for the nanosensors. Circular (diameter D = 120-500 nm) and elliptical pillars with low aspect ratio (120 nm × 130 nm- 120 nm × 200 nm) displaying a linear non-hysteretic transfer curves with tunnel magnetoresistance values up to 143% were obtained. A noticeable improvement in the sensitivity for circular structures from an average value of ~1%/mT up to ~2%/mT is observed with the use of a CoFe/CoFeB/Ta/NiFe/MnIr free-layer. The sensitivity values are almost independent on the size for circular devices, consistent with a linear operation range dominated by the exchange field strength. For elliptical devices, a high sensitivity is also observed, although displaying a dependence on the size, due to a competition with the demagnetizing field. The low-frequency noise features were also addressed revealing a detectivity in the tens of μT/√Hz with Hooge parameters within 1-3 × 10-9 μm2 in the linear range. Nevertheless, such high sensitivity values are a major improvement in comparison with those reported previously for nanometric sensors, and extremely competitive with values reported for micrometric spin-valve sensors, with the advantage of providing a reduced device footprint suitable for highly resolved measurements.

Linear nanometric tunnel junction sensors with exchange pinned sensing layer

Highly sensitive nanosensors with high spatial resolution provide the necessary features for high accuracy imaging of isolated magnetic nanoparticles. In this work, we report the fabrication and characterization of MgO-barrier magnetic tunnel junction nanosensors, with two exchange-pinned electrodes. The perpendicular magnetization configuration for field sensing is set using a two-step annealing process, where the second annealing temperature was optimized to yield patterned sensors responses with improved linearity. The optimized circular nanosensors show sensitivities up to 0.1%/Oe, larger than previously reported for nanometric sensors and comparable to micrometric spin-valves. Our strategy avoids the use of external permanent biasing or demagnetizing fields (large for smaller structures) to achieve a linear response, enabling the control of the linear operation range using only the stack and thus providing a small footprint device.

Magnetoresistive nanosensors: controlling magnetism at the nanoscale.

The ability to detect the magnetic fields that surround us has promoted vast technological advances in sensing techniques. Among those, magnetoresistive sensors display an unpaired spatial resolution. Here, we successfully control the linear range of nanometric sensors using an interfacial exchange bias sensing layer coupling. An effective matching of material properties and sensor geometry improves the nanosensor performance, with top sensitivities of 3.7% mT(-1). The experimental results are well supported by 3D micromagnetic and magneto-transport simulations.

Optimization of exposure parameters for lift-off process of sub-100 features using a negative tone electron beam resist

A thorough study of exposure parameters for electron beam lithography using AR7520 negative tone electron beam resist is here presented. We optimized the beam voltage, apertures diameter and resist thickness in order to achieve the smaller dimensions possible for each resist thicknesses. Monte Carlo simulations of the electrons scattering process correlated the experimental results indicating a less efficient energy deposition into the resist layer for larger beam energies and resist thicknesses, thus resulting in larger doses required to expose a selected dot size. Furthermore, for the particular exposure conditions used we determined a forward scattered electrons range between 50 nm and 170 nm, depending on the dot nominal size. On the other hand, a reduced backscattering electrons range was observed showing a constant value of ~ 560 nm, being therefore more significant when larger dimensions are exposed in a point-by-point exposure, and thus supporting the smaller doses observed for larger sizes. Finally, a baking step is used to further improve the etch resistance of the resist, which allied to the optimized exposure parameters, opens a pathway to achieve sub-100nm critical dimensions for the reproducible fabrication of nanometric devices using a simple lift-off method.

Magneto-transport behavior of double exchange magnetic tunnel junction sensors

Magnetic tunnel junctions with exchange biased sensing-layer provide a promising solution for nanoscale sensors with improved spatial resolution [1]. In order to design optimized sensors a complete micromagnetic model for a double exchange MTJ stack with coupled magneto-transport was established.