The advantages of planar Hall effect (PHE) sensors-their thermal stability, very low detection limits, and high sensitivities-have supported a wide range of advanced applications such as nano-Tesla (nT) magnetometers, current sensing, or low magnetic moment detection in lab-on-a-chip devices. In this review we outline the background and implications of these PHE sensors, starting from fundamental physics through their technological evolution over the past few decades. Key parameters affecting the performance of these sensors, including noise from different sources, thermal stability, and magnetoresistance magnitudes are discussed. The progression of sensor geometries and junctions from disk, cross-to-bridge, ring, and ellipse configuration is also reviewed. The logical sequence of these structures from single magnetoresistive layers to bi-, tri-layers, and spin-valves is also covered. Research contributions to the development of these sensors are highlighted with a focus on microfluidics and flexible sensorics. This review serves as a comprehensive resource for scientists who wish to use PHE for fundamental research or to develop new applications and devices. The conclusions from this report will benefit the development, production, and performance evaluation of PHE-based devices and microfluidics, as well as set the stage for future advances.

Spin-dependent charge transport in magnetic tunnel junctions (MTJs) can be manipulated by a temperature gradient, which can be utilized for spintronic and spin caloritronic applications. Evaluation of the thermally induced phenomena requires knowledge of the temperature differences across the oxide tunnel barrier adjacent to the ferromagnetic (FM) leads. However, it is challenging to accurately measure thermal properties of an oxide tunnel barrier consisting of only a few atomic layers. In this work, we experimentally interrogate the temperature evolutions in Ru/oxide/FM/seed/MgO (oxide=MgO, MgAl2O4; FM=Co, CoFeB; seed=Pt, Ta) structures having perpendicular magnetic anisotropy using ultrafast thermometry. The Ru layer is optically thick and heated by ultrafast laser pulses; the subsequent temperature changes are monitored using thermoreflectance of Ru and magneto-optic Kerr effect (MOKE) of the FM layers. We independently measure the response times of Co and CoFeB magnetism using quadratic MOKE and obtain {\tau}em=0.2 ps for Co and 2 ps for CoFeB. These time scales are much shorter than the time scale of heat transport through the oxide tunnel barrier, which occurs at 10-3000 ps. We determine effective thermal conductivities of MgO and MgAl2O4 tunnel barriers in the range of 0.4-0.6 W m-1 K-1, comparable to an estimate of the series conductance of the Ru/oxide and oxide/FM interfaces and an order of magnitude smaller than the thermal conductivity of MgO thin films. We find that the electron-phonon thermal conductance near the tunnel barrier is only a factor of 5-12 larger than the thermal conductance of the oxide tunnel barrier. Therefore, the drop in the electronic temperature is approximately 20-30% larger than the drop in the phonon temperature across the tunnel barrier.

Magnetite (Fe3O4) is an eligible candidate for magnetic tunnel junctions (MTJs) since it shows a high spin polarization at the Fermi level as well as a high Curie temperature of 585°C. In this study, Fe3O4/MgO/Co-Fe-B MTJs were manufactured. A sign change in the TMR is observed after annealing the MTJs at temperatures between 200°C and 280°C. Our findings suggest an Mg interdiffusion from the MgO barrier into the Fe3O4 as the reason for the change of the TMR. Additionally, different treatments of the magnetite interface (argon bombardment, annealing at 200°C in oxygen atmosphere) during the preparation of the MTJs have been studied regarding their effect on the performance of the MTJs. A maximum TMR of up to -12% could be observed using both argon bombardment and annealing in oxygen atmosphere, despite exposing the magnetite surface to atmospheric conditions before the deposition of the MgO barrier.