Christoph Sürgers

Large topological Hall effect in the non-collinear phase of an antiferromagnet

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 non-collinear 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 Mn5Si3. 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.

Determining the current polarization in Al/Co nanostructured point contacts

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. PACS numbers: 74.45.+c, 72.25.-b, 74.78.Na 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/F) point contacts has been used to extract this spin polarization in a great variety of materials [2, 3, 4, 5, 6, 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 (BTK) 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, 3, 4, 5, 6, 7]. However, a microscopic justification of these models is lacking [10, 11, 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? And, more importantly, can the current polarization be experimentally determined using Andreev physics? In this paper 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

Electrical spin injection in multiwall carbon nanotubes with transparent ferromagnetic contacts

We report on electrical spin injection measurements on multiwall carbon nanotubes(MWNTs). We use a ferromagnetic alloy Pd 1 − x Ni x with x ≈ 0.7 which allows us to obtain devices with resistances as low as 5.6 k Ω at 300 K. The yield of device resistances below 100 k Ω , at 300 K, is around 50%. We measure at 2 K a hysteretic magneto-resistance due to the magnetization reversal of the ferromagnetic leads. The relative difference between the resistance in the antiparallel ( A P ) orientation and the parallel ( P ) orientation is about 2%.

Magnetic order by C-ion implantation into Mn5Si3 and Mn5Ge3 and its lateral modification

Ferromagnetic Mn5Si3C0.8 and Mn5Ge3C0.8 films with Curie temperatures TC well above room temperature are obtained by C+12-ion implantation in antiferromagnetic Mn5Si3 or ferromagnetic Mn5Ge3. Patterning of the films with a gold mesh serving as a stencil mask during implantation allows a lateral modification of magnetic order creating ferromagnetic regions of Mn5Si3C0.8 which are embedded in antiferromagnetic Mn5Si3. This provides a procedure for the fabrication of magnetoelectronic hybrid devices comprised of different magnetic phases.

Effect of oxygen segregation on the surface structure of single-crystalline niobium films on sapphire

Single-crystalline Nb films are grown on (1120) oriented sapphire substrates by electron-beam evaporation in ultra-high vacuum. The films are studied in-situ by RHEED and Auger analysis. At a substrate temperature TS=750° C the RHEED pattern shows a smooth growth of bcc-Nb in the [110] direction. In addition to the fundamental streaks, we observe superlattice streaks of fractional order in several azimuthal directions. The reciprocal lattice of the surface is determined. The basic vectors of the superlattice in real space are given by b1=2a1, b2=−a1+3a2 where a1 and a2 are the basic vectors of the Nb (110) surface. Auger analysis shows that the surface of these films is contaminated with oxygen. Therefore, the superstructure is attributed to a modified surface structure due to segregated oxygen, possibly having diffused from the sapphire to the film surface. The superstructure dissappears during further evaporation of Nb at TS<450° C with a concomitant decrease of the oxygen signal. Nb films on sapphire with a clean, oxygen-free surface can only be prepared at lower temperatures in an island-growth mode.

Optical absorption in silicon layers in the presence of charge inversion/accumulation or ion implantation

We determine the optical losses in gate-induced charge accumulation/inversion layers at a Si/SiO2 interface. Comparison between gate-induced charge layers and ion-implanted thin silicon films having an identical sheet resistance shows that optical losses can be significantly lower for gate-induced layers. For a given sheet resistance, holes produce higher optical loss than electrons. Measurements have been performed at λ = 1550 nm.

Size Dependence of Current Spin Polarization through Superconductor/Ferromagnet Nanocontacts

The spin polarization P of the transport current through the interface between superconducting Al and ferromagnetic Fe is determined by means of Andreev reflection at nanostructured point contacts. We observe a systematic decrease of P with decreasing contact resistance. Our data provide evidence for the reduction of P by spin-orbit scattering and thus establish a link between density of states and transport spin polarizations.

Ferromagnetism in carbon-doped Mn5Si3 films

The effect of carbon doping on the structural and magnetic properties of Mn5Si3Cx films is investigated for different substrate temperatures TS and concentrations x. Samples with x≈0.75 prepared at TS=650–750 K exhibit ferromagnetic order with enhanced ordering temperatures TC well above room temperature in contrast to the undoped antiferromagnetic Mn5Si3 compound. Structural analysis shows that C is incorporated interstitially in the hexagonal Mn5Si3-type structure with a lattice expansion with respect to the undoped compound. The enhanced ferromagnetic order is presumably not simply due to a change of the interatomic Mn–Mn distances but due to a change of the electronic structure and/or exchange interactions.

Fabrication and superconducting properties of nanostructured SFS contacts

Superconducting hybrid structures of submicrometer size utilizing high-melting transition metals such as Nb or Ta can be fabricated in ultra-high vacuum by means of a non-organic evaporation mask (Si3N4) of high thermal and mechanical stability. We report on the magnetic and superconducting properties of mesoscopic superconductor/ferromagnet/superconductor (SFS) junctions realized in a Nb/Cu/Co/Cu/Nb multilayer (ML). Below the superconducting transition temperature, the magnetic hysteresis loop shows a contribution from the strongly pinned magnetic flux of the superconducting Nb layers. Electrical transport measurements perpendicular to the layered structure clearly demonstrate a Josephson coupling between the Nb layers through the 5-nm thick ferromagnetic Co film

Anomalous Hall effect in the noncollinear antiferromagnet Mn5Si3

Metallic antiferromagnets with noncollinear orientation of magnetic moments provide a playground for investigating spin-dependent transport properties by analysis of the anomalous Hall effect. The intermetallic compound Mn5Si3 is an intinerant antiferromagnet with collinear and noncollinear magnetic structures due to Mn atoms on two inequivalent lattice sites. Here, magnetotransport measurements on polycrstalline thin films and a single crystal are reported. In all samples, an additional contribution to the anomalous Hall effect attributed to the noncollinear arrangment of magnetic moments is observed. Furthermore, an additional magnetic phase between the noncollinear and collinear regimes above a metamagnetic transition is resolved in the single crystal by the anomalous Hall effect.