J. S. Garitaonandia

Chemically Induced Permanent Magnetism in Au, Ag, and Cu Nanoparticles: Localization of the Magnetism by Element Selective Techniques

We report a direct observation of the intrinsic magnetization behavior of Au in thiol-capped gold nanoparticles with permanent magnetism at room temperature. Two element specific techniques have been used for this purpose: X-ray magnetic circular dichroism on the L edges of the Au and 197Au Mössbauer spectroscopy. Besides, we show that silver and copper nanoparticles synthesized by the same chemical procedure also present room-temperature permanent magnetism. The observed permanent magnetism at room temperature in Ag and Cu dodecanethiol-capped nanoparticles proves that the physical mechanisms associated to this magnetization process can be extended to more elements, opening the way to new and still not-discovered applications and to new possibilities to research basic questions of magnetism.

Structural and magnetic changes in FeNbCuSiB amorphous alloys during the crystallization process

Calorimetric and magnetic measurements, x-ray powder diffraction and Mossbauer spectroscopy have been used to study the magnetic and structural changes occurring after each of the two steps of crystallization that take place in FeNbCuSiB-type alloys. Two samples with different boron and silicon concentrations, (x=6, 9), have been studied. They give a somewhat different composition of the crystalline phases appearing after crystallization processes. The most noticeable phenomenon is the observed increase of about 50 K in the Curie temperature of the FeSi crystalline phase between the end of the first crystallization process and the end of the second one, although the composition of this phase remains unchanged. This result is discussed in terms of crystal boundary effects. Also, the Curie temperature of the remaining amorphous phase, in the crystallized samples, is greater than the expected one, due to the coupling with magnetic phases with higher Curie points and inhomogeneities in such a phase.

Room temperature ferromagnetism in nanostructured ZnO–Al system

The origin of the room-temperature ferromagnetism (RTF) in ZnO-based dilute magnetic semiconductors remains controversial. We experimentally demonstrate here that it is possible to induce RTF in ball milled (ZnO)1−x/Alx without any ferromagnetic dopant. Our work shows that RTF in (ZnO)1−x/Alx (x=0–0.5) can be realized simply by milling a mixture of high purity ZnO and Al fine powders for 8 h. The spontaneous magnetization of the milled powders is found to increase by annealing under a reduced pressure. The magnetization value depends highly on both the ratio of Al to ZnO and the annealing temperature. X-ray photoelectron spectroscopy results have revealed that the Zn2+ ions in (ZnO)1−x/Alx are partially reduced into a lower ionic state. As there are no magnetic impurities present in our samples, the origin of ferromagnetism is most likely to be due to the charge transfer between Zn and Al at the interfaces of the ball milled nanograins. Our results reinforce the significant role played by the alterations ...

Study of the enhancement of the magnetic properties of Fe70Al30 in the order-disorder transition

The order-disorder transition produced by ball milling in the intermetallic Fe70Al30 has been systematically studied by x-ray diffraction, calorimetry, and Mossbauer experiments. These techniques show a monotonous transformation that ends after 6 h of milling. In the transition, the lattice parameter increase amounts to 0.7% and there is a large enhancement of the alloy’s magnetism.

Room temperature spontaneous magnetization in calcined trioctylphosphine-ZnO nanoparticles

In this work, it is demonstrated that capping with trioctylphosphine oxide (TOPO) induces a ferromagnetic response in free-standing ZnO nanoparticles upon calcination without the necessity of metallic doping. Samples were synthesized by precipitation of zinc acetate solutions in a basic medium followed by capping with TOPO and heat treatment in static aerobic conditions. Nanoparticles show a wurtzite-type structure with an average size of 14 nm, and magnetization measurements evidence a spontaneous magnetic moment at room temperature for calcined nanoparticles, in contrast with the diamagnetic response observed in non-calcined TOPO-capped nanoparticles. Giving the absence of any magnetic impurity or metal dopant that could account for the total magnetization, it is proposed that the magnetism would be consistent with a charge transfer mechanism promoted by a phosphorous doping upon calcination of TOPO over the nanoparticles. This situation leads to a spontaneous magnetic moment by the local fulfillment of Stoner’s criterion for ferromagnetism at the nanoparticles surface.

Magnetic domains and annealing-induced magnetic anisotropy in nanocrystalline soft magnetic materials

The magnetic domains of nanocrystalline Fe84Nb6B10 annealed under static and rotating magnetic fields have been investigated by means of magneto-optical Kerr effect (MOKE) microscopy in order to clarify the origin of the dramatic magnetic softening brought about by rotating field annealing. The coercivity (Hc) values after static- and rotating-magnetic field annealings are 5.9 and 3.0A∕m, respectively. The MOKE image after static field annealing implies a highly coherent uniaxial anisotropy (Ku) in the sample whereas no sign of such a strong Ku is evident in the domain configuration after rotating field annealing. Our analytical solution of the random anisotropy model with additional Ku predicts that the fluctuating amplitude of the effective anisotropy (δK) in nanocrystalline Fe84Nb6B10 decreases from 20to11J∕m3 by removing Ku. The observed reduction of Hc may be attributed to this decrease in δK induced by rotating field annealing.

Influence of disorder on the magnetic properties of FeAl alloys: theory

Abstract Self-consistent electronic calculations are performed by means of the tight binding linear muffin tin orbital (TB-LMTO) method to study the influence of disorder on the magnetic properties of ordered Fe50Al50 and Fe75Al25 alloys. The calculations indicate that in these alloys the disordering of the structure increases the mean magnetic moment of the alloys, which is larger in the case of the Fe50Al50 alloy. The disordering of the alloy also causes an increase of the lattice parameter in relation to the ordered structures.

High saturation magnetization and soft magnetic properties of nanocrystalline (Fe,Co)90Zr7B3 alloys annealed under a rotating magnetic field

The effects of magnetic-field annealing on the soft magnetic properties of nanocrystalline (Fe1−xCox)90Zr7B3 (x=0, 0.1, 0.2, and 0.3) have been investigated. The annealing-induced uniaxial anisotropy (Ku) in these alloys was found to follow an x2 dependence and Ku appears to reflect the population density of Co–Co atomic pairs. The annealing-induced anisotropy is suppressed by applying a rotating field during annealing and the coercivity is reduced by up to 60%. Nanocrystalline (Fe0.8Co0.2)90Zr7B3 annealed under a rotating field of 640kA∕m exhibits a low coercivity of 9.1A∕m and high permeability values (under 0.4A∕m) of 4500 at 1 kHz and 3100 at 100 kHz along with a high saturation magnetization of 1.74 T. These permeability values are comparable to those of Fe-based amorphous alloys while the saturation magnetization is clearly higher than those of amorphous or nanocrystalline soft magnetic alloy ribbons developed to date.

Crystal structure and magnetic behaviour of nanocrystalline Fe-Nb-Cu-Si-B alloys studied by means of in situ neutron diffraction

Two Fe-Nb-Cu-Si-B alloys, (B9) and (B6), prepared with the isotope, have been analysed using data obtained by means of in situ neutron diffraction. This technique allows one to scrutinize crystallographic phases during thermal treatments, avoiding problems due to sample handling. The B9 sample develops Fe(Si) nanometric crystals (10 nm) with 19 at.% Si in the phase when it is annealed at for one hour. An increase to favours the growth of Fe(Si) grains and the crystallization of other phases, mostly Fe borides. A Rietveld analysis of these phases results in a good reproduction of the nominal composition of the alloy. It also elucidates the crystallographic structure of the Fe(Si) phase. This is similar to the structure, but with some of the Fe atoms occupying some (45%) of the Si 4a sites. The compositions and amounts of the phases derived are in agreement with Mossbauer spectroscopy results for the same sample. Knowledge of the Fe(Si) composition enables one to compare the different magnetic behaviours observed for bulk and nanocrystalline alloys. By contrast, B6 alloy does not show the presence of a Fe(Si) structure, presumably due to the lower amount of Si in the Fe(Si) phase. The thermal expansion of the phases that appear is fairly linear and the corresponding thermal expansion coefficients for the different phases have been extracted. The magnetic structure of the Fe(Si) phase is ferromagnetic collinear, without any trace of antiferromagnetic ordering. The thermal variation of the (1, 1, 1) magnetic peak intensity of the Fe(Si) phase matches well with reported DC magnetization results.

The role of dipolar interactions in magnetic nanoparticles : Ferromagnetic resonance in discontinuous magnetic multilayers

We have performed detailed ferromagnetic resonance (FMR) studies of [Co80Fe20(t)∕Al2O3(40 A)]10 (t=7−13 A) discontinuous multilayers, with measurements taken as a function of the angle of the applied static magnetic field with respect to the sample at room temperature. We outline an approach to FMR in granular systems by using the magnetic dipolar interaction between magnetic nanoparticles. This can be easily incorporated into the free energy of the system and provides a method for evaluating the average interaction strength in particulate media. Angular dependent measurements show how FMR can be used to evaluate interparticle interactions. Our measurements indicate that there is a change of coupling regime, going from superferromagnetic, for thicker samples (with larger average particle sizes and smaller separations), to superparamagnetic in thinner samples.