Search

CoPt and FePt nanowire arrays were successfully prepared by electrodeposition into nanochannels of porous alumina membranes. The as-deposited CoPt alloy has a face centred cubic structure and displays soft magnetic properties. Heat treatment at 700??C for different durations, under vacuum condition was carried out in order to obtain the ordered face centred tetragonal phase L10, exhibiting hard magnetic properties. After annealing, arrays of nanowires actually show hard magnetic properties with coercive fields up to 1.1?T at room temperature. Phase transformation, structural and magnetic properties were analysed and necessary conditions to obtain optimum magnetic properties are concluded.The first results obtained with FePt nanowires denote a more complicated system since the as-deposited material shows no magnetization. Magnetism appears only after annealing at 700??C minimum. Coercivity up to 1.1?T has been obtained at room temperature but with inhomogeneous phase composition.

International audience; In this work, exchange bias was used as a probe to characterise the temperature profile induced by the inelastic relaxation of electrons tunnelling across a MgO barrier. Thermally assisted magnetic random access memory (TA-MRAM) cells comprising a magnetic tunnel junction (MTJ) with a reference pinned layer and a FeMn exchange biased storage layer were used. The pinning direction of the ferromagnetic storage layer is reversed when heated above the blocking temperature of the antiferromagnetic layer (FeMn). The power density required to reach this blocking temperature in the FeMn layer depends on the current polarity, indicating that the heat source term associated with the current flowing through the barrier depends itself on the current direction in contrast to simple Joule heating. This effect is due to the mechanism of energy dissipation in tunnelling. The tunnelling itself is ballistic i.e., without dissipation. However, after tunnelling, the hot electrons very quickly relax to the Fermi energy thereby loosing their excess energy in the receiving electrode. Therefore, the heat is essentially generated on one side of the barrier so that the whole profile of temperature throughout the pillar depends on the current direction. Full 3D thermal simulations also confirmed the temperature profile asymmetry. The proper choice of heating current direction (i.e., voltage polarity applied to the MTJ) can yield a reduction of about 10% in the heating power density required to enable writing in thermally assisted MRAM cells.

International audience; The coherence of the precessional switching was compared in planar spin-valves comprising either an additional simple perpendicular polarizer or a synthetic antiferromagnet perpendicular polarizer. A significant improvement in the precession coherence was observed experimentally in the second type of samples. Micromagnetic simulations were performed to study the effect of the stray field from the perpendicular polarizer. They provide an explanation for the gradual loss of coherence of the precession in terms of vortex formation, which occurs much faster when a simple perpendicular polarizer is used.

In magnetic tunnel junctions (MTJ), synthetic antiferromagnets (SAF) are usually used as reference layer to minimize dipolar interactions induced between this layer and the free layer (FL). We show here that the use of SAF allows us to reduce the asymmetry of the FL reversal due to stray fields in nanosized MTJs with perpendicular magnetic anisotropy.

International audience; We have investigated CoPt and FePt alloys with the face centered tetragonal phase L10, which present very large magnetocrystalline anisotropy. Equiatomic CoPt nanowires exhibiting large coercive fields up to 1.1 T have been successfully prepared by electrodeposition into nanopores of commercial and home-made alumina membranes from a very simple electrolyte. The as-deposited material has the FCC structure with soft magnetic properties. An annealing treatment at 700 °C is crucial to transform this phase into the L1 0 phase, which presents hard magnetic properties. Nanowires of annealed samples consist of small grains around 20 nm, with their c axes randomly distributed. The coercivity does not depend on the morphology and porosity of the two types of membranes but only on the deposited material elaborated with the appropriate thermal annealing process. Our preliminary results with FePt alloy indicate a more complicated system since the as-deposited material shows no magnetization. Magnetism appears only after annealing at 700-750 °C. Coercivity up to 0.85 T has been obtained at room temperature but with inhomogeneous phase composition. To achieve a single hard phase L1 0, it is essential to get for the as-deposited sample the equiatomic composition and then to employ the suitable annealing parameters (temperature and time) to change the whole FCC phase into the FCT ordered L1 0 phase.

We have determined the write power density dependence on heating pulse width and antiferromagnet (AF) thickness using magnetic tunnel junctions whose storage layer is exchange biased with IrMn or FeMn. An increase in write power density needed to write the storage layer has been observed for both AF as pulse width is decreased from 0.1 ms to 10 ns. Quasistatic blocking temperatures (Tb) were measured on both full sheet and patterned samples, showing a reduction in Tb for patterned samples which we link to process induced damages. Power-temperature relationship was established based on a correspondence between writing power for long pulse duration and quasistatic Tb measurement. Both write power density and associated temperature results show a dependence of the Tb on AF thickness at pulse width as short as 10 ns. Particularly for IrMn, the relationship between write power and pulse width becomes weakly dependent on the AF thickness above a certain AF thickness. The write power density is significantly low...

The evolution of the perpendicular magnetic anisotropy of Pt/Co/AlOx structures has been followed by extraordinary Hall Effect measurements as a function of both Al thickness and annealing treatment. A nonmonotonous evolution of the magnetic anisotropy is observed with increasing aluminum thickness, with a maximum around 1.4 nm attributed to the formation of quantum well states in the remaining metallic Al layer. This maximum gradually disappears after annealing. High resolution electron microscopy images indicate that the vanishing of this maximum is associated with homogenization of oxygen throughout the whole Al layer.