M. A. García

Giant magnetic anisotropy at the nanoscale: Overcoming the superparamagnetic limit

It has been observed for palladium and gold nanoparticles that the magnetic moment at a constant applied field does not change with temperature over the range comprised between 5 and 300 K. These samples, with sizes smaller than 2.5 nm, exhibit remanent magnetization up to room temperature. The existence of permanent magnetism up to so high temperatures has been explained as due to the blocking of a local magnetic moment by giant magnetic anisotropies. In this Brief Report we show, by analyzing the anisotropy of thiol capped gold films, that the orbital momentum induced at the surface conduction electrons is crucial to understand the observed giant anisotropy. The orbital motion is driven by a localized charge and/or spin through spin- orbit interaction, which reaches extremely high values at the surfaces. The induced orbital moment gives rise to an effective field of the order of 10(3) T that is responsible for the giant anisotropy.

Ferromagnetism in bulk Co-Zn-O

The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.The origin of ferromagnetism in diluted magnetic semiconductors is still an open question, yielding a great deal of research across the world. This work focuses on the Co-Zn-O system. Room-temperature ferromagnetism is observed after a partial reaction of Co3O4 and ZnO, which can be ascribed neither to carrier mediation nor segregated cobalt metallic clusters. Another mechanism is yielding room-temperature ferromagnetism. This mechanism is associated with a partial reaction of ZnO and Co3O4 grains, and always appears when the starting phases (Co3O4 and ZnO) are present in the sample, suggesting that interfaces are involved in the origin of the observed ferromagnetism.

Structural and magnetic characterization of oleic acid and oleylamine-capped gold nanoparticles

In this work the study of oleic acid and oleylamine-capped gold nanoparticles is presented. The structural characterization of the sample shows 6.7 nm gold nanoparticles with a narrow size distribution. The experimental optical absorption spectrum has a maximum at 2.35 eV. The calculated optical absorption spectrum is shifted and narrower than the experimental one, indicating that the oleic acid and oleylamine do not merely passivate the metallic nanoparticles but modify its electronic structure. These gold nanoparticles show in addition a kind of magnetic order similar to other organic passivated gold nanoparticles as thiol-capped gold nanoparticles. Although the magnetic interactions seem to be weaker than in thiol-capped ones, the magnetic behavior looks similar to that, i.e., an invariant temperature dependence of the magnetization from 5 to 300 K and a noticeable coercive field. We analyze the influence of the organic layer bonding the nanoparticles on the magnetic behavior.

sp magnetism in clusters of gold thiolates

Using first-principles calculations, we consider the bond between thiolate and small Au clusters, with particular emphasis on the resulting magnetic moment. The moment of pure gold clusters is 1µB for clusters with an odd number of Au atoms and zero for those with an even number. The addition of the thiolate, having an odd number of electrons itself, shifts the phase of the odd-even oscillations so that particles with an even number of Au atoms now have unit moment. Surprisingly, gold thiolate exhibits a dramatic and non- intuitive distribution of charge and spin moment. Our results show that the S-Au bond is such that sulfur does not get charge and an electron is transferred to the Au cluster. This extra electron is mainly sp in character and resides in an electronic shell below the Au surface. The calculations suggest that any thiolate- induced magnetism occurs in the gold nanoparticle and not the thiolate, and can be controlled by modifying the thiolate coverage.

Effect of photodiode angular response on surface plasmon resonance measurements in the Kretschmann-Raether configuration.

We study the effect of photodiode angular response on the measurement of surface plasmon resonance (SPR) in metallic thin films using the Kretschmann-Raether configuration. The photodiode signal depends not only on the light intensity but also on the incidence angle. This implies that the photodiode sensitivity changes along the SPR curve. Consequently, the measured SPR spectrum is distorted, thus affecting fits and numerical analyses of SPR curves. We analyze the magnitude of this change, determine when it is significant, and develop a calibration method of the experimental setup which corrects for this type of spectral shape distortions.

XAS study of Mn, Fe and Cu as indicators of historical glass decay

We present here a study of historic glass decay by means of X-ray absorption spectroscopy (XAS). Transition metal cations incorporated in the glass as chromophores exhibit modifications of their oxidising state and chemical environment as the glass suffers a decay process. These modifications can be monitored by measuring X-ray absorption near edge structure, XANES, and extended X-ray absorption fine structure, EXAFS, spectra around a selected atomic species. We apply the technique here to glasses from different periods ranging from 1st century BC to 18th century, demonstrating that XAS provides an advanced tool for qualitative analysis of glass decay. In particular we have found that it is possible to establish a relationship between the oxidation state of Fe and Cu cations and the decay suffered by the glass. In contrast, our results indicate that the Mn oxidizing state is not directly involved in the glass decay of the studied samples.

Plasmonic nanodevice with magnetic funcionalities: fabrication and characterization

We have designed and fabricated a nanodevice exhibiting simultaneously ferromagnetic properties of nanostructures with plasmonic properties of continuous films. Our device consists of an array of nanomagnets on top of a continuous plasmonic film. The patterned nanomagnets magnetic state is single domain and well-defined shape anisotropy. Despite the presence of the patterned media on top of the Au film, the system exhibits surface plasmon resonance characteristics of a continuous film, i.e., propagating surface plasmon-polaritons.

Local characterization of the optical properties of annealed Au films on glass substrates

We present scanning near field microscopy and local spectroscopic characterisation of gold nanoparticles fabricated on glass sodalime cover slides. The nanoislands are fabricated by the thermal annealing of gold thin films. Results are presented for samples annealed at 300 °C, 400 °C, and 500 °C. We study the spectral dependence of the transmittance at the nanoscale level with respect to the nanoislands size, shape, and interparticle distance employing a Scanning Near-field Optical Microscopy.

Temperature dependence of the magnetic properties in LaMnO3+δ

Data are presented on the thermal dependence of the hysteretic properties of cationic vacancies including manganite samples of composition LaMnO3+δ (δ=0.05 and 0.12). Our results evidence the presence in both samples of two magnetic phases having ferro- and antiferromagnetic orders, respectively. The temperature dependence of the coercivity and relaxational properties of the samples is closely linked to the connectivity of the magnetic moment bearing Mn3+–Mn4+ ferromagnetic clusters that demagnetize independently in the case of the δ=0.05 sample and collectively in that of the δ=0.12 one, as evidenced from the activation volume results (δ=0.05) which yielded a size of the same order magnitude as that obtained in previous works for the Mn3+–Mn4+ ferromagnetic cluster size.

Two state coercivity driven by phase coexistence in vanadium sesquioxide/nickel bulk hybrid material

We developed a bulk hybrid material consisting of a vanadium sesquioxide (V2O3) matrix with nickel (Ni) rich inclusions that exhibit a switchable two-state magnetic coercivity. The V2O3 matrix magnetoelastically couples with the Ni-rich inclusions and its structural phase transition causes two possible magnetic coercivity states. Differences of up to 13% in the temperature window of 14 K are observed, depending whether the transition occurs from rhombohedral to monoclinic or vice versa. These findings provide a pathway for the development of bulk switchable coercivity materials. We present routes to further enhance the magnetoelastic effect by increasing the oxide/ferromagnetic material coupling.