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R. Ferreira

Publications and source records attributed to R. Ferreira.

43 records · Page 3Linked to original sources
arXiv2006

Short-range effects and magnetization reversal in Co$_{80}$Fe$_{20}$ thin films: a MOKE magnetometry/domain imaging and AMR study

A MOKE magnetometry unit simultaneously sensitive to both in-plane magnetization components, based on an intensity differential detection method, allows us to observe the uniaxial anisotropy impressed during CoFe-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axes. Our MOKE imaging unit, using a CCD camera for Kerr effect domain visualization provides direct evidence on the dominant M-processes, namely domain wall motion and moment rotation. Further magnetic information was obtained by AMR measurements due to the dependence of the electrical resistivity on the short-range spin disorder and also on the angle between the electrical current direction (I) and the spontaneous magnetization ($\emph{\textbf{M}}_{S}$).

cond-mat.mtrl-sci↗
arXiv2006

Dielectric breakdown in underoxidized magnetic tunnel junctions: Dependence on oxidation time and area

Magnetic tunnel junctions (MTJs) with partially oxidized 9 ÅAlO$_x$-barriers were recently shown to have the necessary characteristics to be used as magnetoresistive sensors in high-density storage devices. Here we study dielectric breakdown in such underoxidized magnetic tunnel junctions, focusing on its dependence on tunnel junction area and oxidation time. A clear relation between breakdown mechanism and junction area is observed for the MTJs with the highest studied oxidation time: samples with large areas fail usually due to extrinsic causes (characterized by a smooth resistance decrease at dielectric breakdown). Small area junctions fail mainly through an intrinsic mechanism (sharp resistance decrease at breakdown). However, this dependence changes for lower oxidation times, with extrinsic breakdown becoming dominant. In fact, in the extremely underoxidized magnetic tunnel junctions, failure is exclusively related with extrinsic causes, independently of MTJ-area. These results are related with the presence of defects in the barrier (weak spots that lead to intrinsic breakdown) and of metallic unoxidized Al nanoconstrictions (leading to extrinsic breakdown).

cond-mat.mtrl-sci↗
arXiv2006

Domain imaging, MOKE and magnetoresistance studies of CoFeB films for MRAM applications

We present a detailed study on domain imaging, Kerr effect magnetometry (MOKE) and magnetoresistance (MR), for a series of 20 nm Co$_{73.8}$Fe$_{16.2}$B$_{10}$ thin films, both as-deposited (amorphous) and annealed (crystalline). By considering the two different (orthogonal) in-plane magnetization components, obtained by MOKE measurements, we were able to study the uniaxial anisotropy induced during CoFeB-deposition and to discriminate the magnetization processes under a magnetic field parallel and perpendicular to such axis. MOKE magnetic imaging enabled us to observe the dominant magnetization processes, namely domain wall motion and moment rotation. These processes were correlated with the behavior of the magnetoresistance, which depends both on short-range spin disorder electron scattering and on the angle between the electrical current and the spontaneous magnetization ($\emph{\textbf{M}}_{S}$). A simple numerical treatment based on Stoner-Wolfarth model enables us to satisfactorily predict the magnetization behaviour observed in these films. A comparison between the results in Co$_{73.8}$Fe$_{16.2}$B$_{10}$ films and the previous ones obtained in annealed Co$_{80}$Fe$_{20}$ films, show that the introduction of boron in CoFe reduces significatively the coercive and saturation fields along the easy axis (e.g. $H_{c}$ from $\sim$ 2 down to $\sim$ 0.5 kAm$^{-1}$). Also, the magnetization along the hard axis saturates at lower fields. We conclude that amorphous and nanocrystalline CoFeB films show low coercive fields and abrupt switching, as well as absence of short range spin disorder effects after switching when compared with Co$_{80}$Fe$_{20}$.

cond-mat.mtrl-sci↗
arXiv2005

Fast exciton spin relaxation in single quantum dots

Exciton spin relaxation is investigated in single epitaxially grown semiconductor quantum dots in order to test the expected spin relaxation quenching in this system. We study the polarization anisotropy of the photoluminescence signal emitted by isolated quantum dots under steady-state or pulsed non-resonant excitation. We find that the longitudinal exciton spin relaxation time is strikingly short ($\leq$100 ps) even at low temperature. This result breaks down the picture of a frozen exciton spin in quantum dots.

cond-mat.other↗
arXiv2004

Giant optical anisotropy in a single InAs quantum dot in a very dilute quantum-dot ensemble

We present the experimental evidence of giant optical anisotropy in single InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals a linear polarization ratio with huge fluctuations, from one quantum dot to another, in sign and in magnitude with absolute values up to 82%. Systematic measurements on hundreds of quantum dots coming from two different laboratories demonstrate that the giant optical anisotropy is an intrinsic feature of dilute quantum-dot arrays.

cond-mat.other↗
arXiv2004

Bound-to-bound and bound-to-continuum optical transitions in combined quantum dot - superlattice systems

By combining band gap engineering with the self-organized growth of quantum dots, we present a scheme of adjusting the mid-infrared absorption properties to desired energy transitions in quantum dot based photodetectors. Embedding the self organized InAs quantum dots into an AlAs/GaAs superlattice enables us to tune the optical transition energy by changing the superlattice period as well as by changing the growth conditions of the dots. Using a one band envelope function framework we are able, in a fully three dimensional calculation, to predict the photocurrent spectra of these devices as well as their polarization properties. The calculations further predict a strong impact of the dots on the superlattices minibands. The impact of vertical dot alignment or misalignment on the absorption properties of this dot/superlattice structure is investigated. The observed photocurrent spectra of vertically coupled quantum dot stacks show very good agreement with the calculations.In these experiments, vertically coupled quantum dot stacks show the best performance in the desired photodetector application.

cond-mat.other↗