Hysteresis and relaxation of hard–soft nanocomposite samples

We present experimental results on the hysteresis and relaxation of Fe/Ba hexaferrite composite samples prepared by ball milling starting from precursor Fe and Ba hexaferrite particles. Our results show that, for maximum applied fields of up to 8 kOe, the measured loops show a shift of up to 85 Oe. The compositional dependence of that loop displacement presents two maxima (at approximately x=0.3 and =0.7) and a minimum at =0.5. These results are discussed in terms of the dipolar coupling between both phases

Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale

The origin of orbital magnetism recently observed in different nanostructured films and particles is discussed as a consequence of spin-orbit coupling. It is shown that contact potentials induced at the thin film surface by broken symmetries, as domain boundaries in self-assembled monolayers, lead to orbital states that in some cases are of large radius. The component of the angular momentum normal to the surface can reach very high values that decrease the total energy by decreasing spin-orbit interaction energy. Intraorbital ferromagnetic spin correlations induce orbital momenta alignment. The estimated values of the magnetic moments per atom are in good agreement with the experimental observations in thiol capped gold films and nanoparticles

Metallic magnetic nanoparticles

In this paper, we reviewed some relevant aspects of the magnetic properties of metallic nanoparticles with small size ( below 4 nm), covering the size effects in nanoparticles of magnetic materials, as well as the appearance of magnetism at the nanoscale in materials that are nonferromagnetic in bulk. These results are distributed along the text that has been organized around three important items: fundamental magnetic properties, different fabrication procedures, and characterization techniques. A general introduction and some experimental results recently obtained in Pd and Au nanoparticles have also been included. Finally, the more promising applications of magnetic nanoparticles in biomedicine are indicated. Special care was taken to complete the literature available on the subject

Two dimensional electron gas confined over a spherical surface: Magnetic moment

Magnetism of capped nanoparticles, NPs, of non- magnetic substances as Au and ZnO is briefly reviewed. The source of the magnetization is discussed on the light of recent X-ray magnetic circular dichroism experiments. As magnetic dichroism analysis has pointed out impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states. It is proposed that mesoscopic collective orbital magnetic moments induced at the surface states can account for the experimental magnetism characteristic of these nanoparticles. The total magnetic moment of the surface originated at the unfilled Fermi level can reach values as large as 10(2) or 10(3) Bohr magnetons

Spinodal decomposition of Fe-Cu nanocrystals: Control of atomic-magnetic-moment and magnetic properties

Experimental results corresponding to the saturation magnetization and coercive field during the decomposition, upon annealing, of bcc and fcc Fe_xCu_(1-x), obtained by mechanical alloying are reported. The overall behavior points out that the decomposition takes place in two steps: (i) at low temperatures a decrease of the saturation magnetic moment as well as an anomalous thermal dependence of coercive field are observed, however, no phase transformation is detected, and (ii) for further annealing temperatures a new phase appears; the magnetization tends to increase and the coercive field abruptly increases. The analysis of the results leads us to conclude that the first step corresponds to a spinodal decomposition. Fluctuations in the local composition give rise to coexistence of adjacent regions with Curie temperature varying continuously in a range of 1000 K across distances of a few nanometers, thus allowing the tailoring of the magnetic nanostructures

Spin-wave excitations in ribbon-shaped Fe nanoparticles

It has been found that in highly anisometric ribbon-shaped Fe particles with nanoscale dimensions that the magnetization decreases with temperature markedly faster than in bulk bcc Fe. This anomalous dependence, which becomes more remarkable as the length-to-cross section ratio increases, arises from the elongated shape of the particles. The analytical approximation performed on the thermal spectrum of magnons, compatible with the sample dimensions, unravels the correlated influences of shape and size on the thermal decreasing rate of magnetization

Mechanically driven alloying of immiscible elements (Comment)

In conclusion we have proven that the fact that both fcc FeCu and bcc Fe magnetization agree at 300 K is simply an accident and our data at low temperature show clearly that the Fe contribution after precipitation from the metastable phase has a deficiency in magnetization of at least 20% with respect to the Fe state in fcc FeCu metastable solid solution

Field and temperature dependence of magnetization in FeCu-based amorphous alloys

In this paper, the production of FeCu-based FeCuZr amorphous alloys by ball milling is reported. The thermal dependence of magnetization for the [Fe_(0.5)Cu_(0.5)]_85Zr_(15) (at. %) amorphous alloy has been found to show a dramatic field dependence of the kink point of the magnetization. This kink corresponds to a temperature different from the Curie temperature, above 400 K, of the ferromagnetic phase, which, according to spin waves fitting, can be induced by applying external fields. Just above 235 K, the thermoremanence increases sharply, and this feature strongly suggests an increase of the ferromagnetic ordering under zero field heating. Neutron diffraction experiments seem to confirm the enhancement of spin alignment. The thermal expansion above the compensation temperature is proposed to be the origin of the thermoremanence enhancement through the anti-Invar effect as might be explained within the framework of recent ab initio calculations [M. van Schilfgaarde et al., Nature (London) 400, 46 (1999)]

Erratum: Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale (vol 96, pg 057206, 2006)

© 2006 The American Physical Society.Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu