arXiv subjects

M. A. Correa

Publications and source records attributed to M. A. Correa.

9 recordsLinked to original sources

arXiv2022

Fundamental inequalities in the Stoner-Wohlfarth model

We report two fundamental inequalities in the Stoner-Wohlfarth model. Specifically, we investigate the theoretical limit for the initial magnetic susceptibility in a system described by the Stoner-Wohlfarth approach. We also find analytical solutions for the magnetization in the low-field regime and obtain the borderline value for the uniaxial anisotropy constant in such an ideal Stoner-Wohlfarth system. We go beyond and introduce a general mean-field theory for interacting Stoner-Wohlfarth-like systems, thus estimating how the initial magnetic susceptibility is affected by the dipolar and exchange interactions inside the system. By means of a simple insight from a fundamental inequality for the magnetic susceptibility of an ideal Stoner-Wohlfarth system, we show its violation is a signature of the existence of exchange interactions between nanoparticles in an interacting Stoner-Wohlfarth-like system.

cond-mat.soft↗

arXiv2021

Unambiguous signature of exchange interactions between nanoparticles in a collective system

We report a simple, efficient manner to assess magnetic interactions, more specifically the exchange interaction, in systems as blocked magnetic nanoparticles. Specifically, we investigate what is the theoretical limit for the external magnetic susceptibility in a system described by the plain old Stoner-Wohlfarth model. We go beyond and introduce a general mean field theory for interacting systems, thus estimating how the magnetic susceptibility is affected due to the dipolar and exchange interactions inside the system. We disclose a fundamental inequality for the magnetic susceptibility and show its violation is an unambiguous signature of the existence of exchange interactions between nanoparticles in a collective system. To test the robustness of our theoretical achievements, we examine magnetization measurements and Henkel plots for blocked magnetic nanopowders. The agreement between experiment and theory provides evidence to confirm the validity of our findings.

cond-mat.mes-hall↗

arXiv2021

Specific loss power of magnetic nanoparticles (fluid) hyperthermia in non-adiabatic conditions

We investigate the magnetic nanoparticles (fluid) hyperthermia in non-adiabatic conditions through the calorimetric method. Specifically, we propose a theoretical approach to magnetic hyperthermia from a thermodynamic point of view. To test the robustness of the approach, we perform hyperthermia experiments and analyze the thermal behavior of magnetite and magnesium ferrite magnetic nanoparticles dispersed in water submitted to an alternating magnetic field. From our findings, besides estimating the specific loss power value from a non-adiabatic process, thus enhancing the accuracy in the determination of this quantity, we provide physical meaning to parameters found in literature that still remained not fully understood, and bring to light how they can be obtained experimentally.

cond-mat.other↗

arXiv2019

Tailoring anomalous Nernst effect in stressed magnetostrictive film grown onto flexible substrate

The anomalous Nernst effect in nanostructured magnetic materials is a key phenomenon to optimally control and employ the internal energy dissipated in electronic devices, being dependent on for instance the magnetic anisotropy of the active element. Thereby, here we report a theoretical and experimental investigation of the magnetic properties and anomalous Nernst effect in a flexible magnetostrictive film with induced uniaxial magnetic anisotropy and under external stress. Specifically, we calculate the magnetization behavior and the thermoelectric voltage response from a theoretical approach for a planar geometry and with a magnetic free energy density which takes into account the induced uniaxial and magnetoelastic anisotropy contributions. Experimentally, we verify modifications of the effective magnetic anisotropy and thermoelectric voltage with the stress and explore the possibility of tailoring the anomalous Nernst effect in a flexible magnetostrictive film by modifying both, the magnetic field and external stress. We find quantitative agreement between experiment and numerical calculations, thus elucidating the magnetic and thermoelectric voltage behaviors, as well as providing evidence to confirm the validity of the theoretical approach to describe the magnetic properties and anomalous Nernst effect in ferromagnetic magnetostrictive films having uniaxial magnetic anisotropy and submitted to external stress. Hence, the results place flexible magnetostrictive systems as a promising candidate for active elements in functionalized touch electronic devices.

cond-mat.mtrl-sci↗

arXiv2016

ZrO$_2$ tapes as an alternative low-cost flexible substrate to artificially nanostructured materials

We associate tape casting and magnetron sputtering techniques to engineer flexible nanostructures using ZrO$_2$ green tape as substrate. We systematically investigate the structural, magnetic and electrical properties of NiFe/Cr/NiFe trilayer nanostructures, with variable thickness values of the NiFe and Cr layers, grown onto rigid glass and flexible ZrO$_2$ tape substrates. We verify the mirroring of these properties in the trilayer nanostructures, irrespective on the kind of employed substrate. The fact that the trilayer nanostructures can be reproduced in distinct substrates corresponds to an important advance for their applicability. The results place the ZnO$_2$ green tape as an attractive candidate for application as low-cost flexible substrate in the development of electrical and magnetic sensor elements with high sensitivity to mechanic stress.

cond-mat.mtrl-sci↗

arXiv2016

Quantitative scaling of magnetic avalanches

We provide the first quantitative comparison between Barkhausen-noise experiments and recent predictions from the theory of avalanches for pinned interfaces, both in and beyond mean-field. We study different classes of soft magnetic materials: polycrystals and amorphous samples, characterized by long-range and short-range elasticity, respectively; both for thick and thin samples, i.e. with and without eddy currents. The temporal avalanche shape at fixed size, and observables related to the joint distribution of sizes and durations are analyzed in detail. Both long-range and short-range samples with no eddy currents are fitted extremely well by the theoretical predictions. In particular, the short-range samples provide the first reliable test of the theory beyond mean field. The thick samples show systematic deviations from the scaling theory, providing unambiguous signatures for the presence of eddy currents.

cond-mat.mtrl-sci↗

arXiv2016

Handling magnetic anisotropy and dynamic magnetic response of ferromagnetic multilayers in flexible substrates

We investigate the dynamic magnetic response of ferromagnetic flexible NiFe/Ta and FeCuNbSiB/Ta multilayers under external stress. We explore the possibility of handling magnetic anisotropy and dynamic magnetic response of flexible magnetostrictive nanostructures. We quantify the sensitivity of the multilayers under external stress by calculating the ratio $|Δ\textrm{MI}|/|Δ\barσ|$ and show that considerable values can be reached by tuning the magnetic field, frequency, magnetostriction constant, and external stress. The results reveal possibilities of application of flexible magnetostrictive multilayers under external stress and place them as very attractive candidates as element sensor for the development of sensitive smart touch sensors.

cond-mat.mtrl-sci↗

arXiv2012

Anomalous Magnetoresistance in Fibonacci Multilayers

The present paper theoretically investigates magnetoresistance curves in quasiperiodic magnetic multilayers for two different growth directions, namely [110] and [100]. We considered identical ferromagnetic layers separated by non-magnetic layers with two different thicknesses chosen based on the Fibonacci sequence. Using parameters for Fe/Cr multilayers, four terms were included in our description of the magnetic energy: Zeeman, cubic anisotropy, bilinear and biquadratic couplings. The minimum energy was determined by the gradient method and the equilibrium magnetization directions found were used to calculate magnetoresistance curves. By choosing spacers with a thickness such that biquadratic coupling is stronger than bilinear coupling, unusual behaviors for the magnetoresistance were observed: (i) for the [110] case there is a different behavior for structures based on even and odd Fibonacci generations; and more interesting, (ii) for the [100] case we found magnetic field ranges for which the magnetoresistance increases with magnetic field.

cond-mat.mes-hall↗

arXiv2011

High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered ferromagnetic thin films

We studied the dynamics of magnetization through an investigation of the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered thin films grown by magnetron sputtering. Impedance measurements were analyzed in terms of the mechanisms responsible for their variations at different frequency intervals and the magnetic and structural properties of the multilayers. Analysis of the mechanisms responsible for magnetoimpedance according to frequency and external magnetic field showed that for the CoFeB/Cu multilayer, ferromagnetic resonance (FMR) contributes significantly to the magnetoimpedance effect at frequencies close to 470 MHz. This frequency is low when compared to the results obtained for CoFeB/Ta and CoFeB/Ag multilayers and is a result of the anisotropy distribution and non-formation of regular bilayers in this sample. The MImax values occurred at different frequencies according to the used non-magnetic metal. Variations between 25% and 30% were seen for a localized frequency band, as in the case of CoFeB/Ta and CoFeB/Ag, as well as for a wide frequency range, in the case of CoFeB/Cu.

cond-mat.mtrl-sci↗