D. Halley

Ordering intermetallic alloys by ion irradiation: a new way to tailor magnetic media

In this paper, we describe ordering mechanism under irradiation by combining experiments and Kinetic Lattice Monte Carlo (KLMC) simulations on the FePt and FePd equiatomic systems. Both experiments and simulations confirm that our irradiation conditions (small energy transfer, low collision cross section, irradiation temperature under 550K) specifically affect DSRO.

Distance dependence of the energy transfer rate from a single semiconductor nanostructure to graphene.

The near-field Coulomb interaction between a nanoemitter and a graphene monolayer results in strong Förster-type resonant energy transfer and subsequent fluorescence quenching. Here, we investigate the distance dependence of the energy transfer rate from individual, (i) zero-dimensional CdSe/CdS nanocrystals and (ii) two-dimensional CdSe/CdS/ZnS nanoplatelets to a graphene monolayer. For increasing distances d, the energy transfer rate from individual nanocrystals to graphene decays as 1/d(4). In contrast, the distance dependence of the energy transfer rate from a two-dimensional nanoplatelet to graphene deviates from a simple power law but is well described by a theoretical model, which considers a thermal distribution of free excitons in a two-dimensional quantum well. Our results show that accurate distance measurements can be performed at the single particle level using graphene-based molecular rulers and that energy transfer allows probing dimensionality effects at the nanoscale.

Voltage-controlled inversion of tunnel magnetoresistance in epitaxial nickel/graphene/MgO/cobalt junctions

We report on the fabrication and characterization of vertical spin-valve structures using a thick epitaxial MgO barrier as spacer layer and a graphene-passivated Ni film as bottom ferromagnetic electrode. The devices show robust and scalable tunnel magnetoresistance, with several changes of sign upon varying the applied bias voltage. These findings are explained by a model of phonon-assisted transport mechanisms that relies on the peculiarity of the band structure and spin density of states at the hybrid graphene|Ni interface.

Domain wall pinning on strain relaxation defects in FePt(001)/Pt thin films

Thin FePt (001) films, grown by molecular-beam epitaxy on Pt(001), exhibit a very large perpendicular magnetic anisotropy (Ku=5×106 J m−3) and a 100% magnetic remanence in perpendicular field. The lattice misfit between FePt and Pt (1.5%) relaxes through the pileup of a/6 〈112〉 partial dislocations along {111} planes, leading to the formation of microtwins. Atomic force microscopy images demonstrate that this process induces a spontaneous rectangular nanostructuration of the sample, while magnetic force microscopy shows that the microtwins act as pinning sites for the magnetic walls. This leads to square magnetic domains and explains the large coercivity associated with the domain wall propagation.

Chemical ordering at low temperatures in FePd films

We demonstrate that, if a high degree of short range order is present in FePd disordered films, a high value of the long range order parameter S can be obtained by using postgrowth ion irradiation at very low processing temperatures. FePd films deposited monolayer by monolayer at room temperature on MgO(001) substrates exhibit a very low degree of long range order (S∼0.1) but a high degree of short range order as demonstrated by extended x-ray absorption fine structure measurements. Irradiation with 130 keV He+ ions at low fluences (2.0×1016 ions/cm2) leads to a large increase in the long range order parameter and to a large increase in perpendicular anisotropy for irradiation at substrate temperatures lower than 200 °C. This could have a great impact on the current race toward high magnetic recording density media.

Electrical switching in Fe∕Cr∕MgO∕Fe magnetic tunnel junctions

Hysteretic resistance switching is observed in epitaxial Fe∕Cr∕MgO∕Fe magnetic tunnel junctions under bias voltage cycling between negative and positive values of about 1V. The junctions switch back and forth between high- and low-resistance states, both of which depend on the device bias history. A linear dependence is found between the magnitude of the tunnel magnetoresistance and the crafted resistance of the junctions. To explain these results, a model is proposed that considers electron transport both by elastic tunneling and by defect-assisted transmission.

Magneto-optical spectroscopy of (Ga,Mn)N epilayers

We report on the magneto-optical spectroscopy and cathodoluminescence of a set of wurtzite (Ga,Mn)N epilayers with a low Mn content, grown by molecular beam epitaxy. The sharpness of the absorption lines associated to the Mn

Localized states in advanced dielectrics from the vantage of spin- and symmetry-polarized tunnelling across MgO

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Size-induced enhanced magnetoelectric effect and multiferroicity in chromium oxide nanoclusters

internal transitions allows a precise study of its Zeeman effect in both Faraday and Voigt configurations. We obtain a good agreement if we assume a dynamical Jahn-Teller effect in the 3d

L10 ordering at different stages of Fe0.5Pd0.5 epitaxial growth

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