3D magnetization profile and multi-axes exchange bias in Co antidot arrays

Cu/Co/Cu trilayers have been deposited on nanoporous alumina membranes. Magnetic properties of the resulting Co antidot arrays are investigated using SQUID magnetometry. Hysteresis loops of these arrays show two-step magnetization reversal. In addition, exchange bias is observed, whether the cooling field is applied within or perpendicular to the surface plane. In the former case, the exchange bias changes sign close to the blocking temperature, and becomes positive. We attribute these effects to the local, crescent shape of the Co films, induced by the surface morphology of the alumina membranes. This morphology leads to a three-dimensional magnetization distribution at the nanoscale.Comment: 3 pages, 3 figure

Crossover from Kondo assisted suppression to co-tunneling enhancement of tunneling magnetoresistance via ferromagnetic nanodots in MgO tunnel barriers

Recently, it has been shown that magnetic tunnel junctions with thin MgO tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR) values at room temperature1, 2. However, the physics of spin dependent tunneling through MgO barriers is only beginning to be unravelled. Using planar magnetic tunnel junctions in which ultra-thin layers of magnetic metals are deposited in the middle of a MgO tunnel barrier here we demonstrate that the TMR is strongly modified when these layers are discontinuous and composed of small pancake shaped nanodots. At low temperatures, in the Coulomb blockade regime, for layers less than ~1 nm thick, the conductance of the junction is increased at low bias consistent with Kondo assisted tunneling. In the same regime we observe a suppression of the TMR. For slightly thicker layers, and correspondingly larger nanodots, the TMR is enhanced at low bias, consistent with co-tunneling.Comment: Nano Letters (in press

Enhancement of the magnetic anisotropy of nanometer-sized Co clusters: influence of the surface and of the inter-particle interactions

We study the magnetic properties of spherical Co clusters with diameters between 0.8 nm and 5.4 nm (25 to 7500$ atoms) prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data and it is compared to previous structural characterizations. The distribution of activation energies was independently obtained from a scaling plot of the ac susceptibility. Combining these two distributions we have accurately determined the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and that it is dominated by a strong anisotropy induced at the surface of the clusters. Interactions between the magnetic moments of adjacent layers are shown to increase the effective activation energy barrier for the reversal of the magnetic moments. Finally, this reversal is shown to proceed classically down to the lowest temperature investigated (1.8 K).Comment: 13 figures submitted to Phys. Rev.

Granular systems in the Coulomb blockade regime

Disordered granular systems, at temperatures where charging effects are important, are studied, by means of an effective medium approximation. The intragrain charging energy leads to insulating behavior at low temperatures, with a well defined Coulomb gap. Non equilibrium effects can give rise to a zero temperature transition between a metallic, gapless phase, and an insulating phase

Stability of charge ordering in La0.5−xHoxCa0.5MnO3 polycrystalline manganites

International audienceMagnetotransport properties of La0.5-xHoxCa0.5MnO3 (x=0.05 and 0.1) polycrystalline samples were investigated in order to study the effect of magnetic Ho 3+ ions on the stability of the charge ordering (CO) and magnetic phase coexistence in pristine La0.5Ca0.5MnO3. Our samples were synthesized by using sol-gel method. Temperature dependence of resistivity shows an insulating behavior with high resistivity values, confirming the presence of long-range CO. The application of 7 T magnetic field slightly reduces resistivity values and demonstrates relatively small magnetoresistance (MR) values (-MR(T) did not exceed 14 % under 7 T applied field). This observation indicates that the CO in the Ho-based specimens is strong, which indicates that Ho-substitution for La in La0.5Ca0.5MnO3 gives more strength to the CO

Magnetotransport mechanisms and magnetoresistive properties in La0.75Dy0.05Sr0.2MnO3 polycrystalline manganite

International audienceWe have studied in this work the electrical and magnetoresistive properties of polycrystalline La0.75Dy0.05Sr0.2MnO3. Magnetic study has shown the presence of a paramagnetic–ferromagnetic transition at Curie temperature TC = 225 K. Temperature-dependent resistivity shows two resistivity peaks. The magnetotransport in the metallic phase can be described by Zener’s polynomial law, while small polaron hopping mechanism is more suitable to describe resistivity evolution in the insulating phase. The resistivity upturn observed around 75 K was ascribed to electron–electron scattering process at low temperature. We have recorded important values of negative magnetoresistance (MR) around TC (MR (T) ~ 22% and MR (H) ~ 19%) for only 1 T applied magnetic field. This suggests the possibility of using our sample for magnetic field sensing and spintronics

Magnetic Properties of Cobalt Films at the Initial Stage of Ion-Beam Deposition

International audienceVariations in the magnetic properties of ion-beam-deposited cobalt (Co) films from the onset of nucleation until the passage to a bulk-like state have been studied using ferromagnetic resonance (FMR) measurements at 9.55 GHz and SQUID magnetometry. Depending on the Co film thickness, the FMR line width ΔH exhibits a sharp transition from large values (0.24 kOe 0.54 kOe at 10 K), while thicker films are char acterized by HC < 0.16 kOe in the entire range of temperatures up to 300 K. Large values of ΔH and HC at the nucleation and initial growth stages are related to the contribution from a transition Co/Si layer formed under the action of self irradiation with a high-energy component of the deposited flux, which is inherent in the ion-beam sputtering in high vacuum. This fraction of high-energy Co atoms does not exceed 10% of their total flux and is characterized by a mean projected range of 0.8 nm in the growing Co layer and 1.2 nm in the Si substrate. Conditions of using Co films with intermediate thicknesses within 0.8 nm < t ≤ 2 nm for the injection of a spin-polarized current into silicon at room temperature are discussed

Magnetic characterization of granular Co/Al2O3 multilayers

We study the magnetic properties of Co clusters with diameters between 0.8 and 3 nm prepared by sequential sputtering of Co and Al2O3. The particle size distribution has been determined from the equilibrium susceptibility and magnetization data. Comparing this to the distribution of activation energies, obtained from the AC susceptibility, we have determined accurately the effective anisotropy constant Keff. We find that Keff is enhanced with respect to the bulk value and it increases as the average particle size decreases.This work has been partly funded by Spanish Grant MAT 99/1142 and the European network MASSDOTS.Peer reviewe

Temperature Variation of Magnetic Anisotropy in Pt / Co / AlO x Trilayers

International audienceHysteresis-loop measurements using an extraordinary Hall effect are carried out at different temperatures (4.2–300 K) in Pt=Co=AlO x trilayers. The AlO x layer is prepared by plasma oxidation of the Al capping layer during various oxidation times. The samples are subsequently annealed at different temperatures ð300 °C–450 °CÞ. Most of the samples exhibit perpendicular anisotropy of increasingly high amplitude as the measurement temperature decreases. In addition, some samples exhibit unusual phenomena such as a temperature-induced reorientation of anisotropy from in plane to out of plane or a large increase of anisotropy at low temperature associated with the onset of exchange bias. The perpendicular magnetic anisotropy, coercivity, thermally induced anisotropy reorientation, and exchange-bias effects are explained by the influence of different chemical bonds, namely, Co─Al, Co─O, and Co─Pt, which appear and/or evolve as a function of oxidation time and annealing temperature. They are linked to the formation of new phases such as CoO or CoPt alloy and to the evolution of the Co=AlO x , Co=Pt, and Co=CoO interfaces modified by the oxidation time and the interdiffusion between species taking place during the anneals