V. Raposo

Giant magnetoimpedance effect enhancement by circuit matching

GMI response of amorphous wires can be greatly increased by working at resonant conditions in a LC cell. This feature of the circuit can be exploited to increase the sensibility of current GMI-based sensors while selecting the working frequency.

Universal chiral-triggered magnetization switching in confined nanodots

Spin orbit interactions are rapidly emerging as the key for enabling efficient current-controlled spintronic devices. Much work has focused on the role of spin-orbit coupling at heavy metal/ferromagnet interfaces in generating current-induced spin-orbit torques. However, the strong influence of the spin-orbit-derived Dzyaloshinskii-Moriya interaction (DMI) on spin textures in these materials is now becoming apparent. Recent reports suggest DMI-stabilized homochiral domain walls (DWs) can be driven with high efficiency by spin torque from the spin Hall effect. However, the influence of the DMI on the current-induced magnetization switching has not been explored nor is yet well-understood, due in part to the difficulty of disentangling spin torques and spin textures in nano-sized confined samples. Here we study the magnetization reversal of perpendicular magnetized ultrathin dots, and show that the switching mechanism is strongly influenced by the DMI, which promotes a universal chiral non-uniform reversal, even for small samples at the nanoscale. We show that ultrafast current-induced and field-induced magnetization switching consists on local magnetization reversal with domain wall nucleation followed by its propagation along the sample. These findings, not seen in conventional materials, provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

Study of the conductivity of a metallic tube by analysing the damped fall of a magnet

The fall of a magnet through a hollow conducting tube is described. Although this experiment is well known, a detailed treatment by means of a circuit analysis allows us to relate the conductivity of the tube to the characteristic parameters of the experiment.

Magnetic levitation by induced eddy currents in non-magnetic conductors and conductivity measurements

We report a study on magnetic levitation by induced ac currents in non-magnetic conductors at low frequencies. Our discussion, based on Faradays induction law, allows us to distinguish the two components of the current responsible for levitation and heating, respectively. The experimental evaluation of the levitation force in a copper ring revealed the accuracy of our analysis, clearly illustrating its asymptotic behaviour versus frequency, and validating it for the qualitative analysis of magnetic levitation and heating in conductors of different shapes such as tubes and discs, composed of collections of conductive loops. The analysis of the results allows precise values of its electrical conductivity to be found. With the help of a simulation technique, this work also reveals the progressive deformation undergone by magnetic induction lines due to magnetic screening when frequency increases.

Influence of Joule heating on current-induced domain wall depinning

The domain wall depinning from a notch in a Permalloy nanostrip on top of a SiO2/Si substrate is studied theoretically under application of static magnetic fields and the injection of short current pulses. The influence of Joule heating on current-induced domain wall depinning is explored self-consistently by coupling the magnetization dynamics in the ferromagnetic strip to the heat transport throughout the system. Our results indicate that Joule heating plays a remarkable role in these processes, resulting in a reduction in the critical depinning field and/or in a temporary destruction of the ferromagnetic order for typically injected current pulses. In agreement with experimental observations, similar pinning-depinning phase diagrams can be deduced for both current polarities when the Joule heating is taken into account. These observations, which are incompatible with the sole contribution of spin transfer torques, provide a deeper understanding of the physics underlying these processes and establish th...

STUDY OF THE FREQUENCY DEPENDENCE OF THE FERROMAGNETIC RESONANCE LINEWIDTHS OF NICKEL FERRITES FROM 8-60 GHZ

In this communication we present in a systematic way experimental data on ferromagnetic resonance (FMR) linewidths of polycrystalline nickel ferrites (NixFe3−xO4 with 0.8<×<1.5) from 8 to 12.4 GHz and from 26.5 to 60 GHz. Data from 26.5 to 60 GHz were taken at room temperature while measurements at X band have been carried out from 77 to 400 K. Neither in X band nor from 26.5 to 60 GHz was a clear frequency dependence found. Classical contributions to the FMR linewidth according to Sparks’ outline are analyzed in the whole range of frequencies from 8 to 60 GHz. Contributions of the porosity, anisotropy, slowly and rapidly relaxing impurities, valence exchange and eddy current mechanisms are considered.

FERROMAGNETIC RESONANCE AND MAGNETIC DISACCOMMODATION OF TI-DOPED SINGLE CRYSTAL LITHIUM FERRITES

Disaccommodation and ferromagnetic resonance measurements for single crystal titanium‐doped lithium ferrites are presented. Magnetic disaccommodation is performed at 1 kHz from 77 to 400 K while ferromagnetic resonance is carried out at 11 GHz from 77 to 300 K. Ferromagnetic resonance line shape asymmetry and anisotropy field data are also shown. A relationship between the asymmetry and the disaccommodation spectra is suggested.

Ferromagnetic resonance in low interacting permalloy nanowire arrays

Dipolar interactions on magnetic nanowire arrays have been investigated by various techniques. One of the most powerful techniques is the ferromagnetic resonance spectroscopy, because the resonance field depends directly on the anisotropy field strength and its frequency dependence. In order to evaluate the influence of magnetostatic dipolar interactions among ferromagnetic nanowire arrays, several densely packed hexagonal arrays of NiFe nanowires have been prepared by electrochemical deposition filling self-ordered nanopores of alumina membranes with different pore sizes but keeping the same interpore distance. Nanowires’ diameter was changed from 90 to 160 nm, while the lattice parameter was fixed to 300 nm, which was achieved by carefully reducing the pore diameter by means of Atomic Layer Deposition of conformal Al2O3 layers on the nanoporous alumina templates. Field and frequency dependence of ferromagnetic resonance have been studied in order to obtain the dispersion diagram which gives information ...

Domain wall dynamics along curved strips under current pulses: The influence of Joule heating

The current-induced domain wall dynamics along curved ferromagnetic strips is studied by coupling the magnetization dynamics to the heat transport. Permalloy strips with uniform and non-uniform cross section are evaluated, taking into account the influence of the electrical contacts used to inject the current pulses and the substrate on top of which the ferromagnetic strip is sited. Micromagnetic simulations indicate that the geometry and the non-ferromagnetic materials in the system play a significant role in the current-induced domain wall dynamics. Due to the natural pinning, domain walls are hardly affected by the spin-transfer torques when placed in uniform cross section strips under current pulses with reduced magnitude. On the contrary, the current-induced domain wall displacement is significantly different in strips with non-uniform cross section, where thermal gradients emerge as due to the Joule heating. It is found that these thermal gradients can assist or act against the pure spin-transfer torques, in agreement with the recent experimental observations.

Angular dependence of current-driven chiral walls

The current-driven dynamics of chiral domain walls is theoretically studied by means of realistic micromagnetic simulations. Trains of current pulses flowing through the heavy metal underneath the ferromagnetic layer are injected with different directions with respect to the ferromagnetic strip axis. The wall displacement is highly sensitive to the wall configuration and to the angle between the current and the longitudinal axis of the strip. These simulations can account for the experimental behavior at large currents, but preliminary results at lower current density point towards incompatibilities between the model and the experiment that need further experimental and theoretical efforts.