Gerda Fischer

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.

Superconductivity of a Crystalline Ga84-Cluster Compound

We report on magnetization and resistivity measurements of a cluster compound in which negatively charged Ga84 entities are strongly bonded to organic ligands and confined in an ionic crystal structure. The macroscopic single crystals have a resistivity in the range of 100 Ωcm at room temperature with a semiconducting-like temperature dependence. They reveal a superconducting transition at Tc≈7.2 K and an upper critical field of Bc2=13.8 T. The presence of the superconducting state in the regularly arranged Ga84 clusters implies an electronic coupling between the individual clusters.

Structural and magnetic properties of TbMn2Hx hydrides

Abstract Powder samples of TbMn 2 H x hydrides, with 0≤ x ≤4.3, are characterised by X-ray analysis and SQUID magnetometry for temperatures ranging between 2 and 375 K. We observed structural phase transformations as a function of the hydrogen concentration and temperature. In particular a spinodal decomposition for x

Structural and magnetic transformations in the GdMn2Hx hydrides

Powder samples of GdMn2Hx hydrides, with 0pxp4.3, have been characterized by X-ray analysis and SQUID magnetometry for temperatures ranging between 4 and 375 K.The observed phase transformations as a function of temperature and hydrogen concentration are discussed and explained.The correlated changes of magnetization and susceptibility have been analyzed and the temperature dependent development of magnetic ordering of Mn and Gd atoms as a function of temperature and hydrogen content has been discussed.The structural and magnetic phase diagrams have been proposed. r 2002 Elsevier Science B.V. All rights reserved.

Spin-polarized quasiparticle transport in exchange-split superconducting aluminum on europium sulfide

We report on nonlocal spin transport in mesoscopic superconducting aluminum wires in contact with the ferromagnetic insulator europium sulfide. We find spin injection and long-range spin transport in the regime of the exchange splitting induced by europium sulfide. Our results demonstrate that spin transport in superconductors can be manipulated by ferromagnetic insulators, and opens a path to control spin currents in superconductors.

Cu-doped GaN grown by molecular beam epitaxy

Cu-doped GaN is a promising candidate for a nitride-based diluted magnetic semiconductor. Theoretical predictions show the possibility of ferromagnetism and high spin-polarization for certain arrangements of Cu atoms in the GaN lattice. Initial experimental results have already indicated ferromagnetism. However, the influence of structural defects on the ferromagnetic order in Cu-doped nitrides is not clear. Hence, the origin of the ferromagnetism is still under debate. We have used density functional theory (DFT) to verify previous theoretical predictions and to investigate the effects of the position of Cu atoms on the ferromagnetic properties. Our DFT calculations show high degrees of spin-polarization, independent of the arrangement of Cu atoms. Additionally, we have investigated the growth of Cu-doped GaN by molecular beam epitaxy. The influence of parameters, such as Cu to Ga ratio and growth temperature, on the structural and magnetic properties will be discussed.

Low-temperature proton spin-lattice relaxation maxima of lanthanum hydrides doped with paramagnetic rare earth ions

The low-temperature proton spin–lattice relaxation is analysed for lanthanum hydrides LaHx intentionally doped with Gd or Ce. These paramagnetic impurities were also characterized by static magnetic susceptibility and electron spin resonance measurements. A quantitative description of the proton relaxation rate maxima, as well as of the electron spin relaxation data, is presented. These analyses indicate that the density of states at the rare earth site disappears only for .

Switching of a large anomalous Hall effect between metamagnetic phases of a non-collinear antiferromagnet

The anomalous Hall effect (AHE), which in long-range ordered ferromagnets appears as a voltage transverse to the current and usually is proportional to the magnetization, often is believed to be of negligible size in antiferromagnets due to their low uniform magnetization. However, recent experiments and theory have demonstrated that certain antiferromagnets with a non-collinear arrangement of magnetic moments exhibit a sizeable spontaneous AHE at zero field due to a non-vanishing Berry curvature arising from the quantum mechanical phase of the electron’s wave functions. Here we show that antiferromagnetic Mn5Si3 single crystals exibit a large AHE which is strongly anisotropic and shows multiple transitions with sign changes at different magnetic fields due to field-induced rearrangements of the magnetic structure despite only tiny variations of the total magnetization. The presence of multiple non-collinear magnetic phases offers the unique possiblity to explore the details of the AHE and the sensitivity of the Hall effect on the details of the magnetic texture.

Thermoelectric effects in superconductor-ferromagnet tunnel junctions on europium sulfide

We report on large thermoelectric effects in superconductor-ferromagnet tunnel junctions in proximity contact with the ferromagnetic insulator europium sulfide. The combination of a spin-splitting field and spin-polarized tunnel conductance in these systems breaks the electron-hole symmetry and leads to spin-dependent thermoelectric currents. We show that the exchange splitting induced by the europium sulfide boosts the thermoelectric effect in small applied fields and can therefore eliminate the need to apply large magnetic fields, which might otherwise impede applications in thermometry or cooling.

Anion-rotation slow down on approach to the Peierls transition of a 12:7 pyrene hexafluoroantimonate radical cation salt

Abstract Nuclear magnetic resonance of 19F nuclei in the radical cation salt (pyrene)12(SbF6)7 is analysed in dependence of temperature and frequency. Linewidth and spin-lattice relaxation reflect the rotational motion of the SbF6 octahedra that is frozen-in on approaching the Peierls transition. The relevance of this mode of molecular motions for the conduction electrons is demonstrated via Overhauser-shift detected pyrene-1H spin-lattice relaxation.