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Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100-nm room-temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of two and ten, respectively. We thus establish a platform for investigating functional sub-50-nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

The influence of the location of nonmagnetic impurities (Au) inserted in the NiO layer of NiO/Co bilayers on the exchange bias is investigated. This interpretation is supported by numerical simulations, which also predict the formation and annihilation of bubble-like domain walls.

Sputtered [(Pt-Co).sub.n]-FeMn multilayers exhibit both perpendicular magnetic anisotropy and exchange bias at room temperature. Their magnetic properties after in-plane and perpendicular-to-plane magnetic annealing have been studied by means of extraordinary Hall effect and magnetic force microscopy or polar Kerr microscopy. A crossover from in-plane to perpendicular anisotropy was observed as a function of number of repeats n. The dynamics of magnetization switching has been investigated on both sides of the hysteresis loops. A clear asymmetry exists in the dynamics depending whether the field is parallel or antiparallel to the exchange bias field. Similarly, strong differences in the domain patterns at the coercive field were observed. Exchange-biased (Pt-Co)-Pt-(Pt-Co)-FeMn spin-valves were also investigated. Well-separated hysteresis loops were associated with the 'soft' and pinned components of the stack. These structures were used to study the interlayer coupling through the Pt spacer. Index Terms--Exchange bias, interlayer coupling, magnetic switching dynamics, perpendicular magnetic anisotropy.

The magnetorefractive effect (MRE) has become an established technique for the contactless measurement of giant and colossal magnetoresistance and has also been extended to the study of thick granular tunnelling films. Here, the MRE is used to study the behaviour of a MgO(100)/Mg0(20 nm)/Fe(20 nm)/MgO(1.5 nm)/Fe(5 nm)/Co(3 nm)/Au(3 nm) magnetic tunnel junction patterned into square pillars of lateral dimension: 50/[micro]m and 5 [micro]m. A significant MRE is observed, enhanced by the presence of an absorption edge associated with MgO, although the exact origin of the effect is unclear. Index Terms--Infrared, magnetoresistance, tunnelling.

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 study of spin-dependent single electron tunneling requires the preparation of particles with a nanometric size. These particles need not be ferromagnetic if the electrodes are. In order to fulfill the requirements of single electron tunneling effect, the preparation conditions of nanoparticles were studied, mainly for Fe and Au particles. The structure of these particles was then studied by scanning probe microscopy and transmission electron microscopy, and compared to the results of transport measurements. Particles with a diameter of the order of a nanometer were prepared and their lateral arrangement was controlled by self-organization phenomena. [ABSTRACT FROM AUTHOR]

Spin-dependent single electron tunneling was investigated in a magnetic double tunnel junction including Au nanoparticles as a center electrode. Tunnel magnetoresistance (TMR) clearly emerged with increasing spin-polarized current injected into Au nanoparticles and reached a maximum value of about 12% at 4.2 K. The observation indicates that spin accumulation occurs in Au nanoparticles and causes TMR. The spin relaxation time in Au nanoparticles, as estimated from the critical current for the appearance of TMR, is of the order of 10 ns, which is much longer than that in the bulk state. [ABSTRACT FROM AUTHOR]

Nanometric metallic particles were prepared on top of a thin epitaxial oxide layer. Samples with the following structure: Fe electrode/MgO/Fe particles were fabricated and the arrangement of the Fe particles could be tuned from random to self-aligned by simply varying the thickness of the Fe electrode. Under appropriate deposition conditions, the particles were found to be self-aligned along the <110> directions of the underlying Fe electrode. Scanning tunneling microscope (STM) showed that their mean diameter and size distribution were then significantly reduced compared to randomly organized particles. Transmission electron microscope (TEM) images indicated that the self-alignment process originates from the strain relaxation of the Fe electrode which favors faceting of its surface and the formation of pyramidal structures. These self-aligned particles may be straightly used for applications based on a thin oxide tunnel barrier such as single-electron tunneling devices. [ABSTRACT FROM AUTHOR]

The CoFeB/MgO interface is important not only because of its ability to act as a good spin filter, but also because it exhibits significant magnetic interface anisotropy. However, there is experimental evidence that the mineral kotoite (Mg3B2O6) forms at this interface, possibly as a result of B diffusion into the oxide. Recent calculations have demonstrated that kotoite is a surprisingly good spin filter material, despite the lack of cubic symmetry. It is not a priori clear how the perpendicular anisotropy is affected if kotoite forms at the interface. Hereby, we have performed first-principles density functional calculations to investigate the magnetic anisotropy of CoFe/kotoite interfaces. We find that the Co/Fe atoms bind strongly to O atoms of kotoite, and the CoFe/kotoite interfaces. We find that the Co/Fe atoms bind strongly to O atoms of kotoite, and the CoFe/kotoite interfaces exhibit strong perpendicular magnetic anisotropy (PMA) comparable to that of CoFefMgO interfaces. Analysis of the electronic structure shows that CoFe states having dxz and dyz character are strongly affected by the magnetization direction and play an important role in determining the magnetic anisotropy of both CoFe/kotoite and CoFe/MgO interfaces. Our observations are consistent with previous experimental observations of PMA. We demonstrate that the interface properties are only weakly affected by the formation of kotoite compared to MgO. [ABSTRACT FROM AUTHOR]

Spin injection and accumulation are key phenomena supporting a variety of concepts for spin-electronic devices. These phenomena are expected to be enhanced in nanoparticles over bulk structures due to their discrete energy levels and large charging energies. In this article, precise magnetotransport measurements in the single-electron tunnelling regime are performed by preparing appropriate microfabricated devices containing cobalt nanoparticles. Here we provide experimental evidence for characteristic features of spin accumulation in magnetic nanoparticles, such as oscillations of the magnetoresistance with a periodical sign change as a function of bias voltage. Theoretical analysis of the magnetoresistance behaviour clearly shows that the spin-relaxation time in nanoparticles is highly enhanced in comparison with that in the bulk.

An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.