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We have performed detailed ferromagnetic resonance (FMR) studies of [Co80Fe20(t)/Al2O3(40 Å)]10 (t=7-13 Å) 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. [ABSTRACT FROM AUTHOR]

Dodecanethiol capped Ag nanoparticles (NPs) have been independently synthesized by the well-known Brust method under the same physical-chemical conditions. The obtained NP present similar sizes (∼2 nm) but different magnetic behaviors. The extended x-ray absorption fine structure analyses at the K-edge of Ag did not reveal any noticeable structural nor topological differences among the samples. In clear contrast with the structure provided for thiol capped ferromagnetic Au NPs, the analysis also brings out the existence of Ag-S bonds in a diffuse region surrounding a reduced Ag core where the magnetism of the Ag NPs would be located. [ABSTRACT FROM AUTHOR]

Different dodecanethiol capped Au nanoparticles (NP) with similar sizes (∼2 nm) but different ferromagnetic signals at room temperature have been investigated by means of x-ray absorption spectroscopy at the Au L3-edge. The reversion of the x-ray magnetic circular dichroism signal with the change of sign of the external applied magnetic field confirms the location of the magnetism at the Au atoms. In comparison with the Au foil, all the samples present accentuated white lines at the x-ray absorption near-edge structure (XANES) indicating generation of 5d holes in the Au atoms located at surface of the NPs as consequence of a localized charge transfer from the Au surface atoms to the S atoms of the capping agent. XANES spectra reflect differences among the electronic structure of the Au NPs which are compared with the observed different macroscopic magnetic signals. [ABSTRACT FROM AUTHOR]

Neutron diffraction patterns were acquired in situ from a melt-spun amorphous Fe74Nd5B18Cr3 alloy upon heating at constant rates of 1.7×10-2 and 1.3 K/s. The diffraction patterns revealed the following decomposition reaction with two different intermediate routes: amorphous→Fe3B/Nd2Fe23B3 (at 1.7×10-2 K/s) or Fe3B/Nd2Fe14B (at 1.3 K/s)→Fe/Fe2B/Nd1.1Fe4B4. This confirms that the changeover between the two ternary compounds in the metastable intermediate state is the direct cause of magnetic hardening induced by rapid heating. The effect of group IVA-VIA metals on the magnetic hardness of Fe77Nd5B18 was investigated and only the 3d transition-metal additives appeared to be effective in enhancing the coercivity. The heterogeneous nucleation of Nd2Fe14B on tetragonal-Fe3B precipitates promoted by the 3d transition-metal additives is proposed as a possible mechanism of magnetic hardening. [ABSTRACT FROM AUTHOR]

The field-induced magnetic anisotropy (Ku) and the hyperfine interactions in nanocrystalline Fe94-xNb6Bx (x=10, 12, and 14) alloys have been investigated with particular attention paid to the origin of Ku in the bcc-Fe/amorphous soft magnetic nanostructures. The 57Fe Mössbauer spectra from the Fe–Nb–B alloys suggest that the solute content in the residual amorphous phase remains unchanged while the volume fraction of the residual amorphous phase (Vam), increases linearly with B content. This indicates that Vam is governed by the lever rule over the partial equilibrium between the primary bcc-Fe and the residual amorphous phases. Both the saturation magnetization (Js) and Ku were confirmed to vary linearly with Vam and they are well described by taking into account the volume-weighted average of Js=2.1 T and Ku=127 J/m3 for the bcc-Fe nanocrystallites and Js=1.0 T and Ku=76 J/m3 for the residual amorphous matrix. The relatively large Ku value estimated for the bcc nanocrystallites may be attributed to a possible local tetragonal distortion induced by the interstitial B atoms. [ABSTRACT FROM AUTHOR]

In order to clarify the potential of bulk amorphous alloys as a new class of structural materials with high dimensional stability, the thermal expansion and magnetic properties of amorphous Fe56Co7Ni7Zr10B20 doped with Cr have been investigated. Both the magnetic moment and the Curie point of amorphous Fe56Co7Ni7Zr10B20 were suppressed by Cr addition. The effect of Cr on the magnetic moment was well described by assuming a reduction of 4μB/Cr atom. It was found that the Curie temperature can be controlled by Cr content over a wide range of 200 K in the vicinity of room temperature. Although Invar characteristics were absent from the thermal expansion curves, amorphous Fe56Co7Ni7Zr10B20 doped with 1 at. % Cr was found to exhibit a small coefficient of thermal expansion comparable to Kovar alloys, indicating that bulk amorphous alloys may have a potential for commercial low expansion materials. [ABSTRACT FROM AUTHOR]

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 3.0 A/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 20 to 11 J/m3 by removing Ku. The observed reduction of Hc may be attributed to this decrease in δK induced by rotating field annealing. [ABSTRACT FROM AUTHOR]

While suitable texture has been developed in Nd2Fe14B/α-Fe nanocomposites via thermomechanical processing methods such as die upsetting by incorporating low melting point eutectic Nd-Cu additives, significant grain coarsening occurs during this process due to the high temperature and long timescales involved, resulting in a loss of exchange coupling. Equal channel angular pressing (ECAP) is a severe plastic deformation technique which has been successfully used to produce a suitable texture in single-phase Nd2Fe14B at temperatures on the order of 500°C while preserving grain sizes on the order of 20-30nm. We investigate the development of texture in a commercial Nd2Fe14B/α-Fe nanocomposite alloy with added Nd90Cu10 produced via ECAP and then characterise it using texture x-ray diffraction and magnetic measurements. It is found that initial texture can be developed in this nanocomposite system at T = 520°C via ECAP. The average grain size of Nd2Fe14B as measured via X-ray diffraction after ECAP remains below 50nm with a developed texture. The effect of varying the amount of Nd90Cu10 additive is also investigated. It is found that with decreasing Nd90Cu10, the degree of texture is reduced while the volume fraction of α-Fe increases. This work demonstrates the development of texture in nanocomposite Nd2Fe14B/α-Fe with Nd-Cu additives whilst maintaining a grain size of approximately 50nm. [ABSTRACT FROM AUTHOR]

Fe–Al alloys around the concentration of 30 at. % Al show very interesting magnetic properties. Alloys with this concentration present magnetic inhomogeneities and their evolution with temperature is also interesting because they show a reentrant spin-glass behavior at low temperatures. Small-angle neutron-scattering measurements were performed in Fe69.5Al30.5,Fe70Al30, and Fe70.5Al29.5 alloys in the temperature range of 4–300 K. Even though the change in Fe content is very small, the response variation is quite large. The results show that the samples have magnetic inhomogeneities that evolve with temperature. Only in the Fe-richest sample the random-field model can be applied. [ABSTRACT FROM AUTHOR]

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 640 kA/m exhibits a low coercivity of 9.1 A/m and high permeability values (under 0.4 A/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. [ABSTRACT FROM AUTHOR]

The order-disorder transition produced by ball milling in the intermetallic Fe[SUB70]Al[SUB30] has been systematically studied by x-ray diffraction, calorimetry, and Mössbauer 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. [ABSTRACT FROM AUTHOR]

The influence of defects and local stresses on the magnetic properties and martensitic transformation in Ni50Mn35Sn15 is studied at macroscopic and atomic scale levels. We show that both the structural and magnetic properties of the alloy are very sensitive to slight microstructural distortions. Even though no atomic disorder is induced by milling, the antiphase boundaries linked to dislocations promote the antiferromagnetic coupling of Mn, resulting in a significant decrease in the saturation magnetization. On the other hand, the temperature range of the transformation is considerably affected by the mechanically induced local stresses, which in turn does not affect the equilibrium temperature between the austenitic and martensitic phases. Finally, we demonstrate that the recovery of the martensitic transformation is directly related to the intensity of the non-magnetic component revealed by 119Sn Mössbauer spectroscopy. This result opens the possibility of quantifying the whole contribution of defects and the local stresses on the martensitic transformation in Ni-Mn-Sn alloys. [ABSTRACT FROM AUTHOR]