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There exists a controversy in the literature concerning the values of coercive and bias fields in antidots magnetic structures formed by a hexagonal network of nanoholes. The coercive fields (H C) and the exchange bias fields (∣ H E X C ∣) for antidots (deposited on ultrathin anodic aluminum oxide, namely, AAO) are either increased or diminished by comparison with the same magnetic nanostructures grown on continuous substrates (namely, CML). We propose to elucidate these debates by showing the importance of the easy axis of the magnetization, the direction of the applied magnetic field, the thicknesses of the layers, and the 3D-topology of nanoholes, as well as the magnetic and thermal history of the magnetic measurements. Here, biased Ta(5 nm)/Pt(5 nm)/Co(0.6 nm)/Fe 50 Mn 50 (X)/Ta(5 nm) antidots are investigated by extraordinary Hall effect measurements at 5 K, where X varies in the (0–5.5) nm range. The substrate consists in a hexagonal array of holes, described by the pair of (p , d) values, respectively, the period as the distance from center to center of two consecutive holes and the hole diameter. The dimensions of antidots are (p ≈ 100 and d ≈ 40 nm) for X = (2–5.5) nm, (p ≈ 150 and d ≈ 60 nm) for X = 3.5 nm, and (p ≈ 100 and d ≈ 60 nm) for X = 0. A continuous stack using Si/SiO 2 (100 nm) is used for comparison. H C and ∣ H E X C ∣ gradually increase when X is enhanced for both substrates, with nevertheless a weak decrease at high X for the continuous system. Perpendicular magnetic anisotropy is only observed for both unbiased samples, the X = 2 nm continuous sample, and both X = 5 nm samples that have undergone field cooling treatment from 500 to 5 K under − 2 T. Usually, H C (AAO) > --> H C (CML), ∣ H E X C (AAO) ∣ > -->∣ H E X C (CML) ∣ , and ∣ H A (AAO) ∣ <∣ H A (CML) ∣ (H A designating the anisotropy field). However, for certain conditions, as, for instance, for FC-procedures starting from high temperatures and/or strong magnetic field, other situations might be observed. A discussion pertaining to the amplitudes of H C , ∣ H E X C ∣ and the anisotropy field (∣ H A ∣) of continuous and discontinuous samples is given for our experimental results as well as for published data in the literature, in the light of structural characteristics (wedge-to-wedge distance, porosity, or coverage ratio). Such biased perpendicular antidots might be particularly used in specific nanomaterials devoted to spintronics. [ABSTRACT FROM AUTHOR]

In this work, we used the soft X-ray resonant magnetic reflectivity to study the depth-resolved out-of-plane (oop) magnetization profile of a CoFeB/MgO sample with W/Ta cap layer after annealing at 400°C. It is a powerful technique to probe buried magnetic interfaces of ultra-thin films by combining the depth-resolved information of X-ray reflectivity with the species selectivity of X-ray magnetic circular dichroism. It allowed us to resolve the oop magnetization within a 1.36 nm thick CoFeB layer by the measurement of angle-dependent specular reflectivity at large scattering angles (up to 80°). We determined a graded magnetic distribution for both Fe and Co with a 20% increase at the interface with MgO, decreasing slightly over a thickness of 0.7 nm from MgO before it rapidly decreases to 50% at the interface with W. After applying a non-saturating magnetic field in the plane of the sample, we also quantified a similar magnetization profile with an inclined moment configuration. This indicates that the magnetization gradient is a robust property of the CoFeB layer in the studied sample. [ABSTRACT FROM AUTHOR]

Magnetotransport properties of La0.5−xHoxCa0.5MnO3 (x = 0.05 and 0.1) polycrystalline samples were investigated in order to study the effect of magnetic Ho3+ ions on the stability of the charge ordering (CO) and magnetic phase coexistence in pristine La0.5Ca0.5MnO3. Our samples were synthesized by using solgel 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. [ABSTRACT FROM AUTHOR]

We report on the evolution of the structure and composition of a Pt(3 nm)/Co(0.6 nm)/AlOx(2 nm) trilayer sputtered on Si/SiO2 under oxidation and annealing processes by combined x-ray reflectivity and x-ray absorption studies. We describe the progressive and inhomogeneous oxidation of the layers by increasing the oxidation time. Before annealing, the layers have lower density than bulk samples and noticeable roughness. After thermal annealing, a significant improvement of the quality of the alumina layer goes along with the formation of a CoPt alloy that reduces the number of Co-O bonds. These structural outcomes clarify the evolution of the magnetic and transport properties reported at room temperature in these samples. [ABSTRACT FROM AUTHOR]

We 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. [ABSTRACT FROM AUTHOR]

From structural analysis by x-ray diffraction, Rutherford backscattering spectroscopy, and physical investigations (magnetization and resistance), reactive sputtering assisted by microwave multidipolar plasma appears to be a powerful means for tailoring the stoichiometry of transition metal nitrides. Reactive sputtered nitride Ni layers were elaborated while keeping the Ar partial pressure constant (∼1.5 μbar) and varying the N2 partial pressure in the 0.1–0.5 μbar range. A mixing of Ni2N and Ni3N and a single-phase Ni3N are stabilized in thin layers for high and low p(N2), respectively. For the first time, an optimization of the crystallization of Ni3N pure layers is obtained for intermediate p(N2) (0.23 μbar). Both nitrided phases Ni2N and Ni3N, elaborated with appropriate deposition parameters, do not present ferromagnetic behavior in contrast to experimental results in the literature. Theoretical results using the full-potential linearized augmented plane wave method confirm the nonferromagnetic behavior of nitrided phases. Ni3N could be used as nonmagnetic spacer layer in spintronic devices. [ABSTRACT FROM AUTHOR]

Bottom and top spin-valves comprising NiO as pinning layer were prepared by depositing the NiO layer either at normal or oblique incidence onto Si/SiO[sub 2] substrates. When the NiO layer is deposited at oblique incidence, a strong uniaxial anisotropy is observed in the hysteresis loop of the Co layer which is exchange coupled to this layer. The easy axis is perpendicular to the plane of incidence. In contrast, when the NiO layer is deposited at normal incidence, the hysteresis loops of the exchange coupled Co layer show an in-plane isotropy. The uniaxial anisotropy observed after oblique incidence deposition is ascribed to growth-induced uniaxial strain and magnetoelastic effects in the NiO antiferromagnetic layer. It is observed that after annealing under a magnetic field, Co/NiO bilayers in which the NiO has been deposited at oblique incidence exhibit symmetric hysteresis loops whereas similar bilayers in which the NiO has been deposited at normal incidence exhibit shifted hysteresis loops. This effect is related to a difference in grain size between these systems. Furthermore, bottom and top spin-valves in both of which the NiO has been deposited at oblique incidence are compared. In contrast to the bottom spin-valves, the top spin-valves exhibit shifted hysteresis loops after annealing from the NiO Ne´el temperature (520 K) under a magnetic field. It is shown that this difference is due to structural defects in the bulk of the NiO layer for bottom spin-valves which do not exist in top spin-valves. These defects tend to disrupt the exchange interactions within the NiO thickness thus reducing the pinning energy of this layer. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have prepared spin valves exhibiting perpendicular magnetic anisotropy [perpendicular spin valves (PSVs)] by sputtering. These PSVs associate a "free" (Co/Pt) multilayer with a "pinned" (Co/Pt)/FeMn multilayer separated by various spacer materials ((Pt, Cu, Al[SUB2]O[SUB3]). We carried out a comprehensive study of the magnetic and magnetotransport properties of the biased multilayers and of the complete spin valves. When the number of repeats in the (Co/Pt) exchange-biased multilayer is larger than 3, the samples present 100% remnant magnetization in the perpendicular configuration. The major hysteresis cycles exhibit two well-separated loops associated with the free and the exchange-biased (Pt/Co) multilayers. When optimized, the exchange-bias field can be larger than the coercivity of the pinned layer. Metallic PSVs with Cu spacers exhibit giant magnetoresistance but the amplitude is only of the order of 1% due to significant current shunting. In contrast, perpendicularly magnetized tunnel junctions are very promising. [ABSTRACT FROM AUTHOR]

We performed a detailed study of the magnetization reversal in polycrystalline exchange-coupled NiO/Co bilayers over 10 decades of field sweep rate dH/dt for different NiO and Co thicknesses. For all sweep rates and thicknesses, the symmetry of the hysteresis loops shows that an identical pinning strength has to be overcome in both directions of the reversal. At low dH/dt the reversal is governed by domain wall displacement while domain nucleation is dominant at higher ones. The dH/dt at which the transition between the two regimes takes place depends on the relative thickness of the NiO and Co layers. It increases (decreases) when the Co (NiO) thickness is increased. Experimentally, it was found that the energy barrier varies linearly with the square root of the area corresponding to the activation (Barkhausen) volume which is consistent with a random walk model of the coupling between antiferromagnetic and ferromagnetic layers. The results can be explained in terms of! a thermally activated switching of the NiO magnetization dragged by the Co reversal. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper examines the effect of ion-beam sputtering conditions on the nucleation of Co nanofilms on Si(100). The argon ion energy is shown to play a key role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at Ar ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the cobalt atoms have a finite penetration depth in the silicon substrate, where they give up energy which facilitates the formation of a continuous layer in the initial stage of the process. [ABSTRACT FROM AUTHOR]

We have studied the surface morphology, interfaces, and structure of individual Co and TiO2 films 2 to 10 nm in thickness and those of Co/TiO2 multilayers up to 100 nm in thickness. Auger depth profiling and transmission electron microscopy results show that [Co(2 nm)/TiO2(2 nm)]15, [Co(2 nm)/TiO2(4 nm)]15, and [Co(4 nm)/TiO2(4 nm)]12 structures are composed of continuous layers well adherent to one another, with sharp interfaces. The conclusion is made that such multilayer structures can be used as model systems in designing magneto-optical and spintronic devices. [ABSTRACT FROM AUTHOR]

The effect of process conditions on the properties of cobalt films grown on silicon by ion-beam sputtering is analyzed from the nucleation stage to film thicknesses corresponding to the properties of bulk material. The argon ion energy is shown to play a central role in determining the sputtering process. Sputtering a cobalt target with argon ions less than 0.8 keV in energy produces granular layers. The cobalt layers grown at argon ion energies above 1.2 keV are continuous even in the nucleation stage. The layers 1.2 to 2 nm in thickness have high resistivity and are comparable in magnetic properties to bulk material. The high-energy component of the total flux of cobalt atoms ejected from the target plays an important role in the initial stages of deposition, especially at argon ion energies from 1.2 to 2.2 keV. In the nucleation stage, the energy deposited by cobalt atoms in the silicon substrate facilitates the formation of a continuous layer in the initial stage of the process. [ABSTRACT FROM AUTHOR]

X-ray reflectivity (XRR) is used to determine the oxidation front at the nanometer scale in sputtered perpendicular semi tunnel junctions, as the form Pt/Co/AlOx, by varying the oxidation time tox of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of t0x. For low t0x values (< 40 s), a simple Pt/Co/Al/AlOx multilayer is appearing whereas a Pt/Co/CoO/AlOx architecture takes place for higher t0x The oxygen-induced magnetic properties obtained by extraordinary Hall effects measurements are explained by the structural results. The increase of Co-O bondings with t0x is at the origin of the appearing of the perpendicular magnetic anisotropy (PMA). [ABSTRACT FROM AUTHOR]

Variations 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 Δ H exhibits a sharp transition from large values (0.24 kOe < Δ H < 0.33 kOe) at the initial deposition stage to slightly varying values of Δ H < 0.16 kOe for film thicknesses above 1 nm. Similarly, Co films with thicknesses below 1 nm exhibit a significant coercivity ( H C > 0.54 kOe at 10 K), while thicker films are characterized by H C < 0.16 kOe in the entire range of temperatures up to 300 K. Large values of Δ H and H C 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. [ABSTRACT FROM AUTHOR]

Magnetic properties of Cu/Co/Cu trilayers deposited on nanoporous alumina membranes are investigated using a superconducting quantum interference device. Hysteresis loops of the resulting Co antidot arrays show two-step magnetization reversal. Due to oxidation when samples are exposed to air, CoO forms and 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 of Co/CoO. We attribute these effects to the local, crescent shape of the Co films induced by the surface morphology of the membranes, which leads to a three-dimensional magnetization distribution. [ABSTRACT FROM AUTHOR]

Two key parameters were analyzed in Si/SiO/Pt/Co/AlOx: the oxidation time of the Al layer resulting in AlOx, and the ex situ annealing temperatures varied in the 15 and 55 s and 20, 300, and 450 °C ranges, respectively. For intermediate annealing temperatures (∼300 °C), the quantitative analysis of specular reflectometry data shows that the progressive oxidation of layers by increasing the oxidation time goes along with an improvement of the homogeneity of the alumina layer. This outcome casts new light on the temperature dependence of magnetic properties of the samples. The remarkable temperature variation of the coercive field, extracted from extraordinary Hall effects in the 5-300 K range, is associated with structural change due to Co-oxygen bondings, which leads to strong pinning of Co spins in the low temperature regime. [ABSTRACT FROM AUTHOR]

By measuring the extraordinary Hall effect on a series of naturally oxidized Pt3 nm/Co[sub 90]Fe[sub 10] 0.6 nm/Al t[sub Al] samples with 0 < t[sub Al] < 1.2 nm, a first crossover from in-plane to perpendicular magnetic anisotropy is observed when t[sub Al] is varied from 0 to 0.2 nm. The CoFe magnetization remains out of plane for 0.2< t[sub Al] < 0.5 nm. When the Al thickness is further increased, a second crossover back to in plane takes place. In a series of samples in which the Al thickness is kept constant, the same behavior is observed as the time of exposure to an oxygen plasma is varied. The results clearly indicate that the degree of oxidation of Al at the CoFe/AlO[sub χ] interface has a dramatic effect on the magnetic anisotropy of the transition-metal layer. These sharp crossovers of anisotropy provide a very accurate and convenient way to monitor the oxidation of the tunnel barrier in magnetic tunnel junctions. This technique is also applied to characterize the oxidation kinetics of various ultrathin metallic layers as well as the aging effect in alumina barriers. [ABSTRACT FROM AUTHOR]

By sequential deposition of thin layers of cobalt and alumina by sputtering, we have fabricated granular multilayers consisting of successive planes of nanosized cobalt clusters separated by alumina along the growth direction. Combining grazing-incidence small-angle x-ray scattering and transmission electron microscopy experiments, we show that, in a given range of thickness, the vertical arrangement of clusters from plane to plane is not random but shows a topology-induced self-organization. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]