Probing the interdependence between irreversible magnetization reversal processes by first-order reversal curves

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A procedure to probe the interdependence between irreversible magnetic processes is presented. It consists of measuring the first-order reversal curves (FORCs) without saturating the system. Depending on the variation of the reversal fields during the curves' acquirement (increasing or decreasing), it fixes the hardest or softest hysterons into their negative saturation level throughout the measurement. Differences between these FORC diagrams and the classical one, as well as variation of the end magnetization as a function of the reversal field, indicate and characterize the requirement that some irreversible processes arise from others. The procedure is described to investigate magnetic systems, but can be directly used to study any hysteretic system. (C) 2011 American Institute of Physics. [doi:10.1063/1.3538940]1097Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FQRNT (Canada)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Angular dependence of giant magnetoimpedance in an amorphous Co-Fe-Si-B ribbon

The field response of impedance is studied in a stress-annealed amorphous ribbon as a function of the angle of application of the external magnetic field in order to verify the role of induced anisotropies (and their distribution) and demagnetizing factors in the giant magnetoimpedance (GMI) phenomenon which occurs in soft magnetic materials. The experimental results are well explained by a theoretical model, based on the simultaneous solution of Maxwell equations and the Landau-Lifshitz equation of motion. Demagnetizing effects are properly taken into account in the case of ribbons or thin films. The physical parameters necessary to test the theory were obtained through complementary measurements of the ferromagnetic resonance and temperature dependence of magnetization. The results clearly indicate the enormous influence of the distribution of anisotropies on the GMI effect. Also, an experimental procedure for determining the easy-axis distribution function is proposed. [S0163-1829(99)15433-X].6096685669

An effective method to probe local magnetostatic properties in a nanometric FePd antidot array

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)A simple method to quantitatively characterize the local magnetic behaviour of a patterned nanostructure, like a ferromagnetic thin film of antidot arrays, is proposed. The first-order reversal curve (FORC) analysis, coupled with simulations using physically meaningful hysterons, allows us to obtain a quantitative and physically related description of the interaction field and each magnetization reversal process. The hysterons system is built from previously known hypotheses on the magnetic behaviour of the sample. This method was successfully applied to a highly hexagonal ordered FePd antidot array with nanometric dimensions. We achieved a complete characterization of the two different magnetization reversal mechanisms in function of the in-plane applied field angle. For a narrow range of high fields, the magnetization initiates rotating reversibly around the pores, while at lower fields, domain walls are nucleated and propagated. This in-plane magnetization reversal mechanism, partly reversible and partly irreversible, is the only angularly dependent one. While going away from the easy axis, its reversible proportion increases, as well as its switching field distribution. Finally, the results indicate that the high surface roughness between adjacent holes of the antidot thin film induces a parallel interaction field. The proposed method demonstrates its ability also to be applied to characterizing patterned nanostructures with rather complex magnetization reversal processes.13Fonds Quebecois de Recherche sur la Nature et les Technologies (FQRNT), CanadaFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Spanish Government MICINNEU [MAT2009-13108-C02-01, MAT2010-20798-C05-04]FICYT [FC-09-IB09-131]Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)John Simon Guggenheim Memorial FoundationFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)EU [MAT2009-13108-C02-01, MAT2010-20798-C05-04]FICYT [FC-09-IB09-131

Magnetic and transport properties in ordered arrays of permalloy antidots and thin films

The magnetotransport behaviors of two types of permalloy nanostructures, thin films and antidots, are presented and discussed. Antidots samples were prepared by sputtering a Ni(80)Fe(20) layer on top of a nanoporous alumina membrane. A counterpart continuous thin film grown on a continuous Si substrate was also prepared. The magnetoresistance (MR) was measured both as a function of the external applied magnetic field and of the angular orientation, and thus compared with the magnetization curves. The introduction of antidots is found to reduce the anisotropic MR and the angular dependence of the MR, simultaneously increasing the coercive field of the samples. The influence of the sample geometry on the perpendicular MR behavior is reported and discussed. (C) 2010 American Institute of Physics. [doi:10.1063/1.3383039]107

Magnetic and structural properties of fcc/hcp bi-crystalline multilayer Co nanowire arrays prepared by controlled electroplating

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)We report on the structural and magnetic properties of crystalline bi-phase Co nanowires, electrodeposited into the pores of anodized alumina membranes, as a function of their length. Co nanowires present two different coexistent crystalline structures (fcc and hcp) that can be controlled by the time of pulsed electrodeposition. The fcc crystalline phase grows at the early stage and is present at the bottom of all the nanowires, strongly influencing their magnetic behavior. Both structural and magnetic characterizations indicate that the length of the fcc phase is constant at around 260-270 nm. X-ray diffraction measurements revealed a strong preferential orientation (texture) in the (1 0-1 0) direction for the hcp phase, which increases the nanowire length as well as crystalline grain size, degree of orientation, and volume fraction of oriented material. The first-order reversal curve (FORC) method was used to infer both qualitatively and quantitatively the complex magnetization reversal of the nanowires. Under the application of a magnetic field parallel to the wires, the magnetization reversal of each region is clearly distinguishable; the fcc phase creates a high coercive contribution without an interaction field, while the hcp phase presents a smaller coercivity and undergoes a strong antiparallel interaction field from neighboring wires. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553865]1098Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Guggenheim FellowshipFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Size effect and surface tension measurements in Ni and Co nanowires

The effect of reduced size on the mechanical and elastic properties measured on 35 nm diameter Ni and Co nanowires is presented and discussed. The stresses induced in the nanowires due to the different thermal expansion constants of the metal and alumina change the magnetic properties of the nanowires, allowing one to measure the effective Young's modulus and the surface tension of the nanowires by means of simple magnetometry. The Young's modulus of the longer nanowire is higher than that of the shorter one that is comparable to its bulk value. This effect is successfully attributed to surface tension effects.762

Tailoring of magnetocaloric response in nanostructured materials: Role of anisotropy

The magnetocaloric response of an ensemble of oriented uniaxial magnetic objects, perpendicularly magnetized to their easy axes, for temperatures close to the blocking temperature is calculated with the aim of demonstrating that the control of the sample's microstructure makes up an effective way to tailor its magnetocaloric response. Coexisting positive and negative magnetocaloric effect (MCE) is found for a model material with a single magnetic phase transition. Both MCE regimes are controlled by the magnitude of the applied magnetic field. As a proof of concept, experimental results for arrays of self-assembled ferromagnetic nanowires embedded into highly ordered nanoporous anodic alumina templates are shown, suggesting the validity of the numerical calculations.771

Synthesis and Growth Mechanism of Ni Nanotubes and Nanowires

Highly ordered Ni nanotube and nanowire arrays were fabricated via electrodeposition. The Ni microstructures and the process of the formation were investigated using conventional and high-resolution transmission electron microscope. Herein, we demonstrated the systematic fabrication of Ni nanotube and nanowire arrays and proposed an original growth mechanism. With the different deposition time, nanotubes or nanowires can be obtained. Tubular nanostructures can be obtained at short time, while nanowires take longer time to form. This formation mechanism is applicable to design and synthesize other metal nanostructures and even compound nanostuctures via template-based electrodeposition

Synthesis, structural and magnetic characterization of highly ordered single crystalline BiFeO3 nanotubes

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)In this work, we report on the fabrication of highly ordered single crystalline BiFeO3 (BFO) nanotubes by a sol-gel technique using two-step anodic aluminum oxide (AAO) as template. We prepared BFO nanotubes with dimensions of 65 nm in diameter and 3 mu m in length, as confirmed by scanning electron microscopy (SEM) measurements. The obtained single crystalline nanotubes present the expected pure phase (BiFeO3) as confirmed by energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). In addition to the antiferromagnetic behavior, the magnetization curves of the BFO nanotubes also present a ferromagnetic response, which holds from 2 to 300 K. This desirable behavior is associated to the break of the antiferromagnetic helical spin ordering of the BFO nanotubes. Besides the magnetocrystalline anisotropy, the large length-to-diameter ratio induced an uniaxial shape anisotropy, attested by the applied magnetic field angle measurements. (C) 2013 Elsevier Ltd. All rights reserved.48415931597Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP