Magnetism

The magnetization of ferromagnetic nanoparticles commonly exhibit thermally-induced fluctuations known as superparamagnetism which attracts increasing interest in connection with a wider use of spintronic devices consisting of nanoscale magnetic components. Although the best way to study superparamagnetism is by exploring the superparamagnetic behavior of an individual nanoparticle, so far due to technical challenges the study of superparamagnetism has been mainly performed with ensembles of magnetic nanoparticles where the fluctuations of the individual nanoparticles are not observed directly but inferred from the field and temperature dependence of the average magnetization of the ensemble. In this project BINA researchers monitored superparamagnetic fluctuations in nanostructures of SrRuO₃ and were able to demonstrate for the first the applicability of the fundamental Langevin equation which describes the effect of an applied magnetic field on a superparamagnetic particle to the behavior of an individual fluctuating volume. 

Among the wide range of magnetic sensors, those based on magnetoresistance (MR) effects are particularly attractive as they combine low cost, small size, and relatively high resolution at room temperature. To date, within the group of MR sensors, anisotropic magnetoresistance (AMR) sensors – at 200 pT at 1 Hz – hold the best resolution. Planar Hall effect (PHE) sensors have important intrinsic advantages compared to AMR sensors. Nevertheless, so far the reported resolution of PHE sensors is lower than that of AMR sensors. In this project, BINA researchers have shown that PHE sensors can approach the resolution of AMR sensors at 1 Hz and surpass it at frequencies below 0.2 Hz. Furthermore, they point out routes that may further improve the resolution which can make these sensors useful in a wide range of applications. 

In this project a Superconducting Quantum Interference Device (SQUID) was used to image the current flow at an engineered conducting interface (LaAlO3/SrTiO3). The SQUID data revealed that the current does not flow uniformly in a 2D sheet, but flows with higher current density in a network of narrow paths that are related to the underlying crystal structure. This discovery affects our understanding of conductance at complex oxide interfaces and may be useful for engineering exotic superconductivity and conductivity.