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Heating efficiency of multifunctional Ag@Fe 3 O 4 brick-like nanoparticles under alternating magnetic field was investigated by means of specific absorption rate (SAR) measurements, and compared with equivalent measurements for plain magnetite and dimer heteroparticles. The samples were synthesized by thermal decomposition reactions and present narrow size polydispersity and high degree of crystallinity. The SAR values are analyzed using the superparamagnetic theory, in which the basic morphology, size and dispersion of sizes play key roles. The results suggest that these novel brick-like nanoparticles are good candidates for hyperthermia applications, displaying heating efficiencies comparable with the most efficient plain nanoparticles.

A model describing the effect of magnetic dipolar interactions on the susceptibility of magnetic nanopillars is presented. It is an extension of a recently reported model for three-dimensional randomlike dispersions of nearly spherical nanoparticles in equilibrium [Sánchez et al., Phys. Rev. B 95, 134421 (2017)2469-995010.1103/PhysRevB.95.134421], to well-ordered arrays of nanoparticles out of equilibrium. To test it, a high-quality benchmark consisting of a two-dimensional hexagonal arrangement of quasi-identical parallel nickel nanopillars embedded in a porous alumina template was fabricated. This model is based on an effective demagnetizing tensor, which only depends on a few morphological parameters of the sample, as the nearest-neighbor distance between pillars and the volume fraction of pillars in the specimen. It allows us to obtain the nanopillar intrinsic susceptibility tensor from the magnetic response of the nanopillar ensemble. The values of the in-plane and normal-to-plane susceptibility of the sample are successfully predicted by the model. Furthermore, the model reproduces the susceptibility in the applied field direction, measured for different applied field angles. In this way, it provides a simple and accurate treatment to account for the complex magnetic effects produced by dipolar interactions., Facultad de Ciencias Exactas, Instituto de Física La Plata

Agradecimentos: This work was financially supported by the Brazilian funding agencies FAPESP, CNPq and CAPES, as well as the Chilean CONICYT. We thank the Centro Nacional de Pesquisa em Energia e Materiais (CNPEM) for the use of the microscopy facilities in the C2NANO-Brazilian Nanotechnology National Laboratory (LNNano), and the CENAPAD-SP for the use of SGI/Altix ICE and computer facilities. D. S. A. acknowledges Ms. D. Cortes-Ortuno for useful discussions Abstract: The magnetic behavior of nickel modulated nanowires embedded in porous alumina membranes is investigated. Their diameters exhibit a sharp transition between below (35 nm) and above (52 nm) the theoretical limit for transverse and vortex domain walls. Magnetic hysteresis loops and first-order reversal curves (FORCs) were measured on several ordered nanowire arrays with different wide-narrow segment lengths ratio and compared with those from homogenous nanowires. The experimental magnetic response evidences a rather complex susceptibility behavior for nanowires with modulated diameter. Micromagnetic simulations on isolated and first-neighbors arrays of nanowires show that the domain wall structure, which depends on the segment diameter, suffers a transformation while crossing the diameter modulation, but without any pinning. The experimental array magnetic behavior can be ascribed to a heterogeneous stray field induced by the diameter modulation, yielding a stronger interaction field at the wide extremity than at the narrow one. The results evidence the possibility to control the domain wall propagation and morphology by modulating the lateral aspect of the magnetic entity FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQ COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPES Aberto

This chapter will give a concise review of the fabrication methods of multiferroic and heterogeneous ferromagnetic nanowires. First, a general overview about nanowire synthesis is presented. In the context of an alumina membrane template-assisted method, we present the approaches to modulate the geometry or the composition of ferromagnetic nanowires by electrodeposition. In the same way, sol–gel methods can produce more versatile compounds as multiferroic materials (when two or more ferroic properties coexist in the same phase, namely, ferroelectricity, ferromagnetism, or ferroelasticity). They possess great potential for sensor applications as well as open several branches of fundamental science research. Finally, the fabrication of other structured nanowires composed of both ferromagnetic and multiferroic phases by combined techniques is presented.

A temperature pause introduced in a simple single-step thermal decomposition of iron, with the presence of silver seeds formed in the same reaction mixture, gives rise to novel compact heterostructures: brick-like Ag@Fe3O4 core-shell nanoparticles. This novel method is relatively easy to implement, and could contribute to overcome the challenge of obtaining a multifunctional heteroparticle in which a noble metal is surrounded by magnetite. Structural analyses of the samples show 4 nm silver nanoparticles wrapped within compact cubic external structures of Fe oxide, with curious rectangular shape. The magnetic properties indicate a near superparamagnetic like behavior with a weak hysteresis at room temperature. The value of the anisotropy involved makes these particles candidates to potential applications in nanomedicine.

The magnetic response of nickel nanowires embedded in porous alumina has been investigated in a wide temperature range, from 5 K up to 700 K. Hysteresis loops and magnetization isotherms were measured on samples of Ni nanowires with different sizes and morphologies up to the Curie temperature. At room temperature, the magnetic response shows evidences of a particle-to-wire crossover above an aspect ratio L/D=4.3. The magnetic coercivity of high aspect ratio Ni nanowires in the direction parallel to the nanowires has a maximum at T ≈ 400 K, while in the parallel direction H c decreases continuously with increasing temperature. It is explained in terms of competing anisotropies, magnetocrystalline and magnetoelastic. The expansion of the aluminium support of the membrane plays a fundamental role in the temperature dependence of the coercive field. We find also that T C progressively decreases due to a finite-size effect as the wires diameter decreases. © 2012 American Institute of Physics., This work was partly funded under grants MAT2011/ 23791 (ORBIMAT) and MAT2009/13977-C03-01 (MOLCHIP), CONSOLIDER Project INGENIO 2010, from Spanish MICINN, and DGA E34 and E98 Projects.

X-ray magnetic circular dichroism spectra have been recorded at the ${L}_{2,3}$ edges of Co, Cu, Ag, and Au, and at the $K$ edges of Co and Cu for a series of multilayer systems of partially self-assembled Co nanoparticles, both capped with alumina and with different metals (Cu, Ag, and Au). The orbital and spin moments ${m}_{L}$ and ${m}_{S}$ and their ratio ${m}_{L}/{m}_{S}$, for Co, Cu, Ag, and Au, have been determined from those measurements. The trend of increasing ${m}_{L}/{m}_{S}$ for Co is the same as found for increasing the effective anisotropy ${K}_{\text{eff}}$ alumina $l\text{Cu}l\text{Ag}l\text{Au}$ capped. It is shown that the relative increase in the surface contribution for both quantities, ${({m}_{L}/{m}_{S})}_{\text{surf}}$ and ${K}_{S}$, is directly correlated. It is argued that the increase in ${m}_{L}$ with metal capping actually reflects an increase in the anisotropy of the orbital moment for the Co surface atoms. The redistribution of the number of holes in the noble metal bands by hybridization with the Co surface atom $3d$ band and the polarization of the $nd$ band by exchange interaction with the Co bands give rise to the magnetic moments derived for the three noble metals investigated.