Non-trivial spin arrangements in magnetic materials give rise to the topological Hall effect observed in compounds with a non-centrosymmetric cubic structure hosting a skyrmion lattice, in double-exchange ferromagnets and magnetically frustrated systems. The topological Hall effect has been proposed to appear also in presence of non-coplanar spin configurations and thus might occur in an antiferromagnetic material with a highly noncollinear and non-coplanar spin structure. Particularly interesting is a material where the non-collinearity develops not immediately at the onset of antiferromagnetic order but deep in the antiferromagnetic phase. This unusual situation arises in non-cubic antiferromagnetic Mn 5 Si 3 . Here we show that a large topological Hall effect develops well below the Néel temperature as soon as the spin arrangement changes from collinear to non-collinear with decreasing temperature. We further demonstrate that the effect is not observed when the material is turned ferromagnetic by carbon doping without changing its crystal structure.

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We report measurements of the electric field vs current density ͓E(J)͔ characteristics in the mixed state of amorphous Nb 0.7 Ge 0.3 microbridges. Close to the transition temperature T c the Larkin-Ovchinnikov theory of nonlinear flux flow and the related instability describes the data quantitatively up to ϳ70% of the upper critical magnetic field B c2 and over a wide electric-field range. At lower temperatures the nonlinearities of E(J) can be described by electron heating which reduces B c2 and leads to a second type of flux flow instability, as shown by a scaling analysis of the high-dissipation data.

We present a study of the Andreev reflections in superconductor/ferromagnet nanostructured point contacts. The experimental data are analyzed in the frame of a model with two spin-dependent transmission coefficients for the majority and minority charge carriers in the ferromagnet. This model consistently describes the whole set of conductance measurements as a function of voltage, temperature, and magnetic field. The ensemble of our results shows that the degree of spin polarization of the current can be unambiguously determined using Andreev physics.The field of spintronics is largely based on the ability of ferromagnetic materials to conduct spin-polarized currents. 1 Thus, the experimental determination of the degree of current polarization has become a key issue. Recently the analysis of Andreev reflections in superconductor/ferromagnet ͑S/F͒ point contacts has been used to extract this spin polarization in a great variety of materials. 2-7 The underlying idea is the sensitivity of the Andreev process to the spin of the carriers, which in a spin-polarized situation is manifested in a reduction of its probability. 8 The theoretical analysis of these S/F point-contact experiments has been mainly carried out following the ideas of the Blonder-Tinkham-Klapwijk ͑BTK͒ theory. 9 Different generalizations of this model to spin-polarized systems have been proposed, in which with an additional phenomenological parameter P, the spin polarization of the ferromagnet, excellent fits to the experimental data have been obtained. 2-7 However, a microscopic justification of these models is lacking. 10-12 Recently, Xia et al. 13 have combined ab initio methods with the scattering formalism to analyze the Andreev reflection in spin-polarized systems. Their main conclusion is that, in spite of the success in fitting the experiments, these modified BTK models do not correctly describe the transport through S/F interfaces. Therefore, at this stage several basic questions arise: what is the minimal model that describes on a microscopic footing the Andreev reflection in spin-polarized systems? More importantly, can the current polarization be experimentally determined using Andreev physics?In this Rapid Communication we address these questions both experimentally and theoretically. We present measurements of the differential resistance of nanostructured Al/Co point contacts as a function of voltage, temperature, and magnetic field. To analyze the experimental data we have developed a model based on quasiclassical Green functions, the main ingredients of which are two transmission coefficients accounting for the majority-and minority-spin bands in the ferromagnet. We show that this model consistently describes the whole set of data, which unambiguously demonstrates that the spin polarization of current in a ferromagnet can indeed be determined employing Andreev reflection.We have fabricated Al/Co point contacts following the process described in Ref. 14. Briefly, a bowl-shaped hole is drilled through a 50 nm thick silicon nitride (Si ...

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