D. Alba Venero

Vogel-Fulcher-Tammann freezing of a thermally fluctuating artificial spin ice probed by x-ray photon correlation spectroscopy

We report on the crossover from the thermal to the athermal regime of an artificial spin ice formed from a square array of magnetic islands whose lateral size, 30 nm × 70 nm, is small enough that they are dynamic at room temperature. We used resonant magnetic soft x-ray photon correlation spectroscopy as a method to observe the time-time correlations of the fluctuating magnetic configurations of spin ice during cooling, which are found to slow abruptly as a freezing temperature of T0=178±5 K is approached. This slowing is well described by a Vogel-Fulcher-Tammann law, implying that the frozen state is glassy, with the freezing temperature being commensurate with the strength of magnetostatic interaction energies in the array. The activation temperature, TA=40±10 K, is much less than that expected from a Stoner-Wohlfarth coherent rotation model. Zero-field-cooled/field-cooled magnetometry reveals a freeing up of fluctuations of states within islands above this temperature, caused by variation in the local anisotropy axes at the oxidised edges. This Vogel-Fulcher-Tammann behavior implies that the system enters a glassy state upon freezing, which is unexpected for a system with a well-defined ground state.

Configuration of the magnetosome chain: a natural magnetic nanoarchitecture

Magnetospirillum gryphiswaldense is a microorganism with the ability to biomineralize magnetite nanoparticles, called magnetosomes, and arrange them into a chain that behaves like a magnetic compass. Rather than straight lines, magnetosome chains are slightly bent, as evidenced by electron cryotomography. Our experimental and theoretical results suggest that due to the competition between the magnetocrystalline and shape anisotropies, the effective magnetic moment of individual magnetosomes is tilted out of the [111] crystallographic easy axis of magnetite. This tilt does not affect the direction of the chain net magnetic moment, which remains along the [111] axis, but explains the arrangement of magnetosomes in helical-like shaped chains. Indeed, we demonstrate that the chain shape can be reproduced by considering an interplay between the magnetic dipolar interactions between magnetosomes, ruled by the orientation of the magnetosome magnetic moment, and a lipid/protein-based mechanism, modeled as an elastic recovery force exerted on the magnetosomes.

Magnetic nanoscopic correlations in the crossover between a superspin glass and a superferromagnet

Collective behaviors in which the magnetic response depends not only on the individual constituents but also on their interactions are an area of active research. We have produced a paradigmatic system where DC magnetron sputtered FexAg100–x (xu2009=u200915, 35) nanogranular films exhibit a crossover between a superspin glass (SSG) state and a superferromagnetism (SFM), where direct exchange interactions overcome the frustration. The systems have been studied by non-linear susceptibility (NLS) and small angle neutron scattering (SANS). The NLS measurements were carried out between 2 and 300u2009K, in the absence of a biasing magnetic field, with frequencies spanning two decades. These measurements shed light on the complex nature of the interactions and the intricate relationship between direct exchange and long range magnetic interactions. The use of SANS allows us to estimate qualitatively the lengthscale of the magnetic correlations, and therefore identify a clear difference between the collective “supermagnetic” ...

Magnetic disorder in nanostructured Fe7Au93 films and Fe14Au86 powders

Thin films and powders of dilute Fe-Au alloys have been produced by DC-magnetron sputtering and high-energy milling, respectively. Energy disperse X-ray spectroscopy gives Fe7Au93 for the films and Fe14Au86 for the powders. The film, with a thickness below 200 nm measured by atomic force microscopy, was deposited onto a Si(100) substrate. X-ray diffraction reveals a major presence of fcc-Au peaks masking the bcc-Fe phase. The (1 0-300 K) DC-susceptibility (H = 100-1000 Oe) shows a clear cusp in the films in contrast to the powders, with a reentrant spin glass-like behavior.