A.M.H. de Andrade

Magnetoimpedance effect in structured multilayered amorphous thin films

In this work, we report magnetoimpedance (MI) results for frequencies of up to 1.8 GHz in a series of as deposited nano-structured [F/Cu/F] and [F/i/Cu/i/F] films where F is a Fe-rich alloy/Cu multilayer and i is a SiO2 insulating layer. A high MI ratio (205%) was obtained at 180 MHz for the [F/i/Cu/i/F] structure. The results are analysed in terms of the SiO2 insulating layer geometry and of the widths of the ferromagnetic and conductive layers.

Tunable asymmetric magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films

We investigate the magnetization dynamics through the magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films. We observe that the magnetoimpedance response is dependent on the thickness of the non-magnetic Cu spacer material. We verify asymmetric magnetoimpedance in films with biphase magnetic behavior and explore the possibility of tuning the linear region of the magnetoimpedance curves around zero magnetic field by varying the thickness of the spacer and probe current frequency. We discuss the experimental results in terms of the different mechanisms governing the magnetization dynamics at distinct frequency ranges, quasi-static magnetic properties, thickness of the spacer, and the kind of the magnetic interaction between the ferromagnetic layers. The results place films with biphase magnetic behavior exhibiting asymmetric magnetoimpedance effect as very attractive candidates for application as probe element in the development of auto-biased linear magnetic field sensors.

Magnetoimpedance of single and multilayered FeCuNbSiB films in frequencies up to 1.8GHz

In this work, very high magnetoimpedance measured at frequencies up to 1.8GHz in nanostructured Fe73.5Cu1Nb3Si13.5B9∕Cu films is reported. Magnetoimpedance ratios of 280% and 50% were observed in multilayered and sandwiched samples, respectively. For the last one, the magnetoimpedance ratio exhibited a nearly constant value in the frequency range of 10–200MHz. The results are discussed in terms of the anisotropies present in all samples and by considering the connection of magnetoimpedance effects with ferromagnetic resonance phenomena.

Magnetization dynamics in nanostructures with weak/strong anisotropy

We investigate the high-frequency response of magnetization dynamics through magnetoimpedance (MI) effect in Permalloy-based multilayered thin films produced with two different non-magnetic metallic spacers: Cu and Ag. Due to the nature of the spacer materials, we are able to play with magnetic properties and to study both systems with weak/strong magnetic anisotropy. We verify very rich features in the magnetoimpedance behavior and high magnetoimpedance ratios, with values above 200%. We compare the MI results obtained in multilayered thin films with distinct spacers and number of bilayers, and discuss them in terms of the different mechanisms that govern the MI changes observed at distinct frequency ranges, intensity of the magnetic anisotropy, alignment between dc magnetic field and anisotropy direction. Besides, by considering a theoretical approach that takes into account two single models together and calculate the transverse magnetic permeability and the MI effect, we support our interpretation via...

Limiting thermomagnetic avalanches in superconducting films by stop-holes

It is demonstrated that circular holes in superconducting films of Nb can arrest the propagation of thermomagnetic avalanches. The effect was found over a range of temperatures where the material is susceptible to this instability. For other hole shapes, like square and triangular, the sharp corners provoke secondary avalanches, thus extending the breakdown. Making use of circular stop-holes can become a practical way to limit thermomagnetic breakdown in superconducting films.

Cascade dynamics of thermomagnetic avalanches in superconducting films with holes

Vestgarden, Jorn Inge; Colauto, Fabiano; de Andrade, AMH; Oliveira, AAM; Ortiz, W. A.; Johansen, Tom Henning. Cascade dynamics of thermomagnetic avalanches in superconducting films with holes. Physical Review B. Condensed Matter and Materials Physics 2015 ;Volum 92.(14) s. -

Domain structure in Joule-heated CoFeSiB glass-covered amorphous microwires probed by magnetoimpedance and ferromagnetic resonance

Impedance spectra (100 kHz⩽f⩽1.8 GHz) were measured at different magnetic fields, and magnetoimpedance and magnetization measurements were performed in as-produced and Joule-heated Co70.4Fe4.6Si10B15 glass-covered amorphous microwires. From the magnetization curves and ferromagnetic resonance features obtained from the impedance spectra, it was possible to propose a domain structure in the as-produced microwires, as well as to follow its evolution with the annealing current. The inner core and outer shell domain structure were observed. The outer shell domains evolve from a circumferential, in the as-cast sample, to a helical structure in the case of the annealed samples. On the other hand, the inner core evolves from a domain structure typical of a radial anisotropy to a longitudinal one. This domain structure evolution is explained in terms of the combined effects of the stress, promoted by the annealing temperature, and the magnetic field caused by the annealing current.

Trapping Flux Avalanches in Nb Films by Circular Stop-Holes of Different Size

Dendritic flux avalanches triggered by thermomagnetic instabilities in Nb superconducting films have been limited by inclusion of circular holes of different diameters produced by optical lithography. We have observed a compromise between dendrite and hole sizes, for which avalanches are effectively arrested. For holes much smaller than the dendrites, only individual branches are stopped. Large holes are not a good solution, since they change the film geometry. A noteworthy trapping is observed at the holes with diameter between the width of an individual branch and the size of a whole dendrite. The present work shows the trend to optimize stop-holes for limiting thermomagnetic avalanches in superconducting films.

Strongly enhanced exchange bias of top-pinned Co/IrMn films with Py spacers

This work investigates the effects of inserting an ultrathin ferromagnetic Py or Co spacer layer (SL) at the interface of ferromagnet/antiferromagnet films, namely Co/SL(Py)/IrMn and Py/SL(Co)/IrMn. The exchange bias field, HEB, of the two series of films changes with the spacer thickness in very different manners. Whereas HEB of the Py/SL(Co)/IrMn series decreases monotonously with the Co layer thickness, a fourfold initial bias enhancement is obtained for Py spacers with thicknesses up to 1 nm. Moreover, the Co/SL(Py)/IrMn series shows a maximum HEB more than 50% higher than that of the Py/IrMn film which, in turn, is almost three times higher than the HEB of the control Co/IrMn film. Simulations of experimental magnetization curves through a polycrystalline model for exchange bias showed that these variations, which could yield improvement in the performance of modern spintronic devices, are predominantly governed by modifications of the interfacial exchange interactions. Several mechanisms that could be responsible for the observed behavior are discussed.

Barkhausen noise measurements in Finemet type materials

In this paper, Barkhausen noise studies of Finemet-type materials with several compositions are presented. The samples were subjected to a series of annealings under temperatures which allowed to establish different stress levels and microstructures.