Interaction effects and energy barrier distribution on the magnetic relaxation of nanocrystalline hexagonal ferrites

The static and dynamic magnetic properties of nanocrystalline BaFe10.4Co0.8Ti0.8O19 M-type doped barium ferrite were studied in detail to clarify the effect of interactions on the magnetic relaxation of an assembly of small particles. The logarithmic approximation was unable to account for the magnetic relaxation of the sample. Interaction effects were analyzed from the low-field susceptibility, DM plots and the time dependence of thermoremanence, indicating that demagnetizing interactions led to an enhancement of both the relaxation rate at low temperatures and the amount of the lowest energy barriers. It is thus suggested that care should be taken when analyzing thermoremanent data at low temperature, in order not to confuse these experimental findings with the signature of macroscopic quantum tunneling

Aggregation state and magnetic properties of magnetite nanoparticles controlled by an optimized silica coating

The control of magnetic interactions is becoming essential to expand/improve the applicability of magnetic nanoparticles (NPs). Here, we show that an optimized microemulsion method can be used to obtain homogenous silica coatings on even single magnetic nuclei of highly crystalline Fe3-xO4 NPs (7 and 16nm) derived from a high-temperature method. We show that the thickness of this coating is controlled almost at will allowing much higher average separation among particles as compared to the oleic acid coating present on pristine NPs. Magnetic susceptibility studies show that the thickness of the silica coating allows the control of magnetic interactions. Specifically, as this effect is better displayed for the smallest particles, we show that dipole-dipole interparticle interactions can be tuned progressively for the 7 nm NPs, from almost non-interacting to strongly interacting particles at room temperature. The quantitative analysis of the magnetic properties unambiguously suggests that dipolar interactions significantly broaden the effective distribution of energy barriers by spreading the distribution of activation magnetic volumes. Published by AIP Publishing

Geometric frustration in a hexagonal lattice of plasmonic nanoelements

We introduce the concept of geometric frustration in plasmonic arrays of nanoelements. In particular, we present the case of a hexagonal lattice of Au nanoasterisks arranged so that the gaps between neighboring elements are small and lead to a strong near-field dipolar coupling. Besides, far-field interactions yield higher-order collective modes around the visible region that follow the translational symmetry of the lattice. However, dipolar excitations of the gaps in the hexagonal array are geometrically frustrated for interactions beyond nearest neighbors, yielding the destabilization of the low energy modes in the near infrared. This in turn results in a slow dynamics of the optical response and a complex interplay between localized and collective modes, a behavior that shares features with geometrically frustrated magnetic systems

Deviation from bulk in the pressure-temperature phase diagram of V2O3 thin films

We found atypical pressure dependence in the transport measurements of the metal to insulator transition (MIT) in epitaxial thin films of vanadium sesquioxide (V2O3). Three different crystallographic orientations and four thicknesses, ranging from 40 to 500 nm, were examined under hydrostatic pressures (P-h) of up to 1.5 GPa. All of the films at transition exhibited a four order of magnitude resistance change, with transition temperatures ranging from 140 to 165 K, depending on the orientation. This allowed us to build pressure-temperature phase diagrams of several orientations and film thicknesses. Interestingly, for pressures below 500 MPa, all samples deviate from bulk behavior and show a weak transition temperature (T-c) pressure dependence (dT(c)/dP(h) = 1.2 x 10(-2) +/- 0.3 x 10(-2) K/MPa), which recovers to bulklike behavior (3.9 x 10(-2) +/- 0.3 x 10(-2) K/MPa) at higher pressures. Furthermore, we found that pressurization leads to morphological but not structural changes in the films. This indicates that the difference in the thin film and bulk pressure-temperature phase diagrams is most probably due to pressure-induced grain boundary relaxation, as well as both plastic and elastic deformations in the film microstructure. These results highlight the difference between bulk and thin films behaviors

Universality of the electrical transport in granular metals

The universality of the ac electrical transport in granular metals has been scarcely studied and the actual mechanisms involved in the scaling laws are not well understood. Previous works have reported on the scaling of capacitance and dielectric loss at different temperatures in Co-ZrO2 granular metals. However, the characteristic frequency used to scale the conductivity spectra has not been discussed, yet. This report provides unambiguous evidence of the universal relaxation behavior of Pd-ZrO2 granular thin films over wide frequency (11 Hz-2 MHz) and temperature ranges (40-180 K) by means of Impedance Spectroscopy. The frequency dependence of the imaginary parts of both the impedance Z″ and electrical modulus M″ exhibit respective peaks at frequencies ωmax that follow a thermal activation law, ωmax ∝ exp(T1/2). Moreover, the real part of electrical conductivity σ′ follows the Jonscher's universal power law, while the onset of the conductivity dispersion also corresponds to ωmax. Interestingly enough, ωmax can be used as the scaling parameter for Z″, M″ and σ′, such that the corresponding spectra collapse onto single master curves. All in all, these facts show that the Time-Temperature Superposition Principle holds for the ac conductance of granular metals, in which both electron tunneling and capacitive paths among particles compete, exhibiting a well-characterized universal behavior

Selective Control over the Morphology and the Oxidation State of Iron Oxide Nanoparticles

Iron oxide nanoparticles (NPs) have been extensively used for both health and technological applications. The control over their morphology, crystal microstructure, and oxidation state is of great importance to optimize their final use. However, while mature in understanding, it is still far from complete. Here we report on the effect of the amount of 1,2-hexadecanediol and/or 1-octadecene in the reaction mixture on the thermal decomposition of iron(III) acetylacetonate in oleic acid for two series of iron oxide NPs with sizes ranging from 6 to 48 nm. We show that a low amount of either compound leads to both large, mixed-phase NPs composed of magnetite (Fe3O4) and wüstite (FeO) and high reaction yields. In contrast, a higher amount of either 1,2-hexadecanediol or 1-octadecene gives rise to smaller, single-phase NPs with moderate reaction yields. By infrared spectroscopy, we have elucidated the role of 1,2-hexadecanediol, which mediates the particle nucleation and growth. Finally, we have correlated the magnetic response and the structural features of the NPs for the two series of samples

Temperature dependence of the magnetization processes in Co/Al oxide/Permalloy trilayers

The magnetization process of Co/Al oxide/Py trilayers and its evolution with the temperature have been analyzed. The particular behavior of the Co layers, including the shift of the hysteresis loops and a coercivity increase with the decrease of temperature, is related with the apparition of a CoO layer at the Co/Al-oxide interface

Tunable circular dichroism through absorption in coupled optical modes of twisted triskelia nanostructures

We present a system consisting of two stacked chiral plasmonic nanoelements, so-called triskelia, that exhibits a high degree of circular dichroism. The optical modes arising from the interactions between the two elements are the main responsible for the dichroic signal. Their excitation in the absorption cross section is favored when the circular polarization of the light is opposite to the helicity of the system, so that an intense near-feld distribution with 3D character is excited between the two triskelia, which in turn causes the dichroic response. Therefore, the stacking, in itself, provides a simple way to tune both the value of the circular dichroism, up to 60%, and its spectral distribution in the visible and near infrared range. We show how these interaction-driven modes can be controlled by fnely tuning the distance and the relative twist angle between the triskelia, yielding maximum values of the dichroism at 20° and 100° for left- and right-handed circularly polarized light, respectively. Despite the three-fold symmetry of the elements, these two situations are not completely equivalent since the interplay between the handedness of the stack and the chirality of each single element breaks the symmetry between clockwise and anticlockwise rotation angles around 0°. This reveals the occurrence of clear helicity-dependent resonances. The proposed structure can be thus fnely tuned to tailor the dichroic signal for applications at will, such as highly efcient helicity-sensitive surface spectroscopies or single-photon polarization detectors, among others

Scaling of the entropy change at the magnetoelastic transition in Gd5(SixGe1-x)4

Differential scanning calorimetry under a magnetic field H has been used to measure the entropy change DS at the magnetoelastic transition in Gd5(SixGe12x)4 alloys, for x<0.5. We show that DS scales with the transition temperature, Tt , which is tuned by x and H, from 70 to 310 K. Such a scaling demonstrates that Tt is the relevant parameter in determining the giant magnetocaloric effect in these alloys, and proves that the magnetovolume effects due to H are of the same nature as the volume effects caused by substitutio

Probing the variability in oxidation states of magnetite nanoparticles by single-particle spectroscopy

We have studied the electronic and chemical properties of a variety of ensembles of size-and shape-selected Fe3O4 nanoparticles with single-particle sensitivity by means of synchrotron-based X-ray photoemission electron microscopy. The local X-ray absorption spectra reveal that the oxidation states and the amount and type of cations within the individual nanoparticles can show a striking local variability even when the average structural and magnetic parameters of the monodisperse ensembles appear to be compatible with those of conventional homogeneous magnetite nanoparticles. Our results show the key role played by oleic acid concentration in the reaction mixture on the formation and compositional homogeneity within individual nanoparticles. When the concentration of oleic acid is high enough, the nanoparticles are composed of a Fe3O4 core surrounded by a thin gamma-Fe2O3 shell. However, at a low concentration of the fatty acid, the Fe3O4 nanoparticles are likely inhomogeneous with small inclusions of FeO and Fe phases, as a result of an uncontrolled reduction of Fe3+ cations. All the foregoing underlines the importance of combining both advanced synthesis techniques and complementary single-particle investigations performed on a statistically significant number of particles so as to improve the understanding and control over electronic and magnetic phenomena at the nanoscale