J. de la Venta

Sources of experimental errors in the observation of nanoscale magnetism

It has been recently reported that some nonmagnetic materials in bulk state exhibit magnetic behavior at the nanoscale due to surface and size effects. The experimental observation of these effects is based on the measurement of very small magnetic signals. Thus, some spurious effects that are not critical for bulk materials with large magnetic signals may become important when measuring small signals (typically below 10−4emu). Here, we summarize some sources of these small magnetic signals that should be considered when studying this new nanomagnetism.

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.

Upper limit to magnetism in LaAlO3/SrTiO3 heterostructures.

Using polarized neutron reflectometry we measured the neutron spin-dependent reflectivity from four LaAlO(3)/SrTiO(3) superlattices. Our results imply that the upper limit for the magnetization averaged over the lateral dimensions of the sample induced by an 11 T magnetic field at 1.7 K is less than 2 G. SQUID magnetometry of the neutron superlattice samples sporadically finds an enhanced moment, possibly due to experimental artifacts. These observations set important restrictions on theories which imply a strongly enhanced magnetism at the interface between LaAlO(3) and SrTiO(3).

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.

Room-temperature ferromagnetism in the mixtures of the TiO2 and Co3O4 powders

We report here the observation of ferromagnetism (FM) at 300 K in mixtures of TiO₂ and Co₃O₄ powders despite the antiferromagnetic and diamagnetic characters of both oxides, respectively. The ferromagnetic behavior is found in the early stages of reaction and only for TiO₂ in anatase structure; no FM is found for identical samples prepared with rutile-TiO². Optical spectroscopy and x-ray absorption spectra confirm a surface reduction of octahedral Co^(+3) -> Co^(+2) in the mixtures which is in the origin of the observed magnetism.

Surface magnetism in ZnO/Co3O4 mixtures

We recently reported the observation of room temperature ferromagnetism in mixtures of ZnO and Co3O4 despite the diamagnetic and antiferromagnetic character of these oxides, respectively. Here, we present a detailed study on the electronic structure of this material in order to account for the unexpected ferromagnetism. Electrostatic interactions between both oxides lead to a dispersion of Co3O4 particles over the surface of ZnO larger ones. As a consequence, the reduction Co+3→Co2+ at the particle surface takes place as evidenced by x-ray absorption spectroscopy measurements and optical spectroscopy. This reduction allows explaining the observed ferromagnetic signal within the well established theories of magnetism in oxides.

Control of magnetism across metal to insulator transitions

Magnetic properties (coercivity and magnetization) of ferromagnetic films are strongly affected by the proximity to materials that undergo a metal to insulator transition. Here, we show that stress associated with structural changes across the metal-insulator phase transition in VO2 and V2O3 produces a magnetoelastic anisotropy in ferromagnetic films (Co and Ni) deposited on top of the oxides. The changes in coercivity are as large as 168% and occur in a very narrow temperature range. This effect can be controlled and inverted by the thickness and the deposition temperature of the ferromagnetic films, which is very flexible for important technological applications.

Coercivity enhancement in V2O3/Ni bilayers driven by nanoscale phase coexistence

We studied the temperature dependence of coercivity and magnetization of V2O3/Ni bilayers across the Structural Phase Transition in V2O3. We found a coercivity peak that coincides with the V2O3 phase transition on top of an overall increase of the coercivity with decreasing temperature. We propose that this sharp increase arises from a length scale competition between magnetic domains of Ni and phase coexistence during the V2O3 phase transition. This model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by a proximal first order phase transition.

Exchange bias induced by the Fe3O4 Verwey transition

We present a study of exchange bias in different configurations of V

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

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