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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]

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]

The magnetic and transport properties of exchange-biased perpendicular spin valves (PSVs), which associate a free (Co/Pt) multilayer with a pinned (Co/Pt)/FeMn multilayer [FeMn as the antiferromagnetic (AF) layer] with different nonmagnetic spacers (Pt, Cu, Al2O3), are investigated. The metallic PSVs with Cu spacers exhibited giant magnetoresistance but the amplitude was only of the order of 1% due to major current shunting.

Research into the magnetization reversal in polycrystalline exchange-coupled NiO/Co bilayers over 10 decades of field sweep rate dH/dt for varying NiO and Co thicknesses is presented. It was found that domain wall propagation and domain nucleation control the magnetization reversal and low and high sweep rates respectively.

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]

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]

The structural analysis by x-ray diffraction, Rutherford backscattering spectroscopy and physical investigations has shown that reactive sputtering assisted by microwave multidipolar plasma is a powerful means for tailoring the stoichiometry of transition metal nitrides. The results using the full-potential linearized augmented plane wave method have confirmed the nonferromagnetic behavior of nitrided phases and [Ni.sub.3]N is used as nonmagnetic spacer layer in spintronic devices.

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]

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]