Size dependence on the ordering process in colloidal FePt nanoparticles

An alternative method to study the effects of annealing process on colloidal FePt nanoparticles (2-4 nm) has been achieved. Annealing experiments at temperatures between 773 and 1073 K under inert atmosphere flux were performed in powder samples with excess of surfactant molecules on nanoparticle surface. Transmission electron microscopy, x-ray diffraction and magnetic measurements were performed to evidence the evolution of the chemically disordered fcc to chemically ordered face-centered tetragonal phase transformation. Magnetization measurements under zero-field-cooling and field-cooling (M-ZFC-M-FC) conditions, and hysteresis loops are extremely sensitive to the particle size distribution and were strongly affected by the annealing treatment. (c) 2007 American Institute of Physics.101

CRYSTAL-FIELD EFFECTS IN THE ELECTRON-SPIN-RESONANCE OF GD3+ AND ER3+ IN PR2CUO4

The low-temperature (T < 300 K) electron-spin-resonance (ESR) spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry. The completely resolved Gd3+ spectra allowed us to measure, at T = 2 K, the principal g values g parallel-to = 1.985(8), g perpendicular-to =2.040(8), and the crystal-field parameters [b2(0) = -399(2) X 10(-4) cm-1, b4(0) = -33.1(7) X 10(-4) cm-1, and b4(4) = 205(3) X 10(-4) cm-1]. The large broadening of the ESR lines, observed above T approximately 40 K, is due to a relaxation via the thermally populated crystal-field excited Pr levels. For Er3+ in Pr2CuO4 we observe a single ESR line corresponding to a ground-state doublet with g parallel-to = 17.94(5) and g perpendicular-to less-than-or-equal-to 0.2. The absence of any splittings of the ESR lines below the Neel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is < 45 Oe.44282682

Magnetic Modulation in Mechanical Alloyed Cr1.4fe0.6o3 Oxide

We have synthesized Cr1.4Fe0.6O3 compound through mechanical alloying of Cr2O3 and Fe2O3 powders and subsequent thermal annealing. The XRD spectrum, SEM picture and microanalysis of EDAX spectrum have been used to understand the structural evolution in the alloyed compound. The alloyed samples are matching to rhombohedral structure with R3C space group. The observation of a modulated magnetic order confirmed a systematic diffusion of Fe atoms into the Cr sites of lattice structure. A field induced magnetic behaviour is seen in the field dependence of magnetization data of the annealed samples. The behaviour is significantly different from the mechanical alloyed samples. The experimental results provided the indications of considering the present material as a potential candidate for opto-electronic applications.Comment: 8 figure

Annealing effects on 5 nm iron oxide nanoparticles

Morphological, structural and magnetic properties of 4.8 nm iron oxide nanoparticles have been investigated after annealing under inert atmosphere at different temperatures. The as-prepared iron oxide nanoparticles have been synthesized by chemical route from high temperature reaction of Fe(acac)(3) solution in presence of oleic acid and oleylamine surfactant. Annealing the particles at low temperatures (T-ann = 573 K) produces an increment of the mean size from 4.8 nm to 6.0 nm, preserving the same morphology. The coercive field of the annealed sample has a small increasing with respect to the as-prepared sample in agreement with the mean particle volume change. Annealing at higher temperature (T-ann = 823 K) leads to a bimodal size distribution of the iron oxide nanoparticles with 6.0 nm and 17 nm mean sizes respectively, where the bigger particles dominate the observed magnetic properties.793313331

Ag-Fe3O4 Dimer Colloidal Nanoparticles: Synthesis and Enhancement of Magnetic Properties

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)This paper describes a solution-phase route to prepare Ag-Fe3O4 colloidal dimer nanoparticles (NPs) with a dumbbell-like shape. Results show enhancement of the magnetic anisotropy and large coercivity that are explained through the nonzero orbital moment, m(orb), and the related strong spin orbit interaction that can be induced by a lattice distortion of the Fe-O bond lengths. In particular, distortions of the octahedral sites in magnetite result in Fe 3d-O 2p mixing and O 2p to Fe 3d charge transfer which, together with crystal field effects, lifts the electronic degeneracy, leading to a nonzero m(orb). dc magnetization and ferromagnetic resonance were employed for analyzing the magnetic response of the reference Fe3O4 NPs and Ag-Fe3O4 dimer NPs samples, showing a very good agreement with the structural characterization.114221014810152Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)John Simon Guggenheim Memorial Foundation [2009-2010]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)John Simon Guggenheim Memorial Foundation [2009-2010

Influence of substrate on the magnetic properties of Ni and permalloy sub-micrometric patterned stripes

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)The magnetic properties of sub-micrometric magnetic structures of Ni and permalloy (Ni(80)Fe(20)) patterned stripes are studied as functions of temperature. The samples are produced by combining an interferometric lithographic technique with sputtering. At room temperature, ferromagnetic resonance and hysteresis data indicate the existence of an in-plane uniaxial anisotropy in both samples. The NiFe sample has an easy axis along the stripes direction in the entire studied temperature range (2 < T < 300 K). On the other hand, an interesting change in the easy magnetization direction is observed in Ni samples, from parallel to perpendicular to the axis of the stripes, when the temperature is decreased. The results are interpreted in terms of a competition between shape and magnetoelastic anisotropies.432Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)CAPES [BR/PA03-EIII/004

Crystal-field effects in the electron-spin resonance of Gd3+ and Er3+ in Pr2CuO4.

The low-temperature (T&lt;300 K) electron-spin-resonance (ESR) spectra of Gd3+ and Er3+ in Pr2CuO4 show symmetry properties appropriate to the crystal tetragonal symmetry. The completely resolved Gd3+ spectra allowed us to measure, at T=2 K, the principal g values g?=1.985(8), g=2.040(8), and the crystal-field parameters [b20=-399(2)×10-4 cm-1, b40=-33.1(7)×10-4 cm-1, and b44=205(3)×10-4 cm-1]. The large broadening of the ESR lines, observed above T40 K, is due to a relaxation via the thermally populated crystal-field excited Pr levels. For Er3+ in Pr2CuO4 we observe a single ESR line corresponding to a ground-state doublet with g?=17.94(5) and g0.2. The absence of any splittings of the ESR lines below the Néel temperature implies that the magnetostatic dipole field at the rare-earth-ion site due to the antiferromagnetically ordered Cu moments is &lt;45 Oe. © 1991 The American Physical Society

Different Gd3+ sites associated with magnetic ordering and structural distortions in Eu2CuO4:Gd3+ observed via electron-paramagnetic-resonance measurements.

We have measured the EPR spectrum of dilute Gd3+ ions substituting for Eu3+ in Eu2CuO4. Each one of the resonance lines shows a splitting into two or more weaker lines below TN 215 K. The simultaneous appearance of a low-field microwave-absorption signal is attributed to the onset of long-range antiferromagnetic ordering with a weak ferromagnetic component. The splitting of the EPR spectrum is associated with different Gd sites in the magnetically ordered structure. A reduction of the local symmetry of the rare-earth sites below TN is described in terms of an internal magnetic field lying in the ab plane and additional crystal-field terms related to local crystallographic distortions. Dipolar and exchange contributions to the internal field are discussed. Possible random static displacements of the oxygen atoms in the CuO2 planes and their role in the development of the weak ferromagnetism and the presence of internal fields at the Gd sites are also analyzed. A strong dependence of the Gd EPR spectrum and the microwave absorption on the magnetic history of the samples has been observed, suggesting the formation of magnetic domains or a glassy state. © 1991 The American Physical Society