Search

Magnetic state of exchanged biased CoO(20nm)/Co($d_F$) bilayer ($d_F$=5-20nm) was studied by means of polarized neutron reflectometry. By spacing of CoO/Co bilayer and Al$_2$O$_3$ substrate with Nb(20nm) layer we created waveguide structure which allowed us to significantly enhance intensity of spin-flip (SF) scattering in the position of optical resonances. For the trained sample with thinnest Co(5nm) we detected strong SF scattering at the resonance position (up to 30\% of incoming intensity) speaking about high non-collinearity of the system. As $d_F$ increases, the intensity of SF scattering linearly decreases. At the same time we observed asymmetry of up-down and down-up scattering channels at the resonance positions. We attribute this asymmetry to the Zeeman splitting of neutrons energies with different initial polarization taking place in high external field. Analysis, however, shows that the applied in the PNR experiment external field is not enough to quantitatively explain the observed asymmetry for the samples with $d_F > $ 5nm and we have to postulate presence of additional magnetic field produced by sample. We attribute this additional field to the stray field produced by chiral Bloch domain walls. The chirality of the domain walls can be explained by Dzyaloshinskii-Moriya interaction arising at the CoO/Co interface. Our results can be useful for designing of spintronic devices using exchange bias effect.

The KArlsruhe TRItium Neutrino (KATRIN) experiment, which aims to make a direct and model-independent determination of the absolute neutrino mass scale, is a complex experiment with many components. More than 15 years ago, we published a technical design report (TDR) [https://publikationen.bibliothek.kit.edu/270060419] to describe the hardware design and requirements to achieve our sensitivity goal of 0.2 eV at 90% C.L. on the neutrino mass. Since then there has been considerable progress, culminating in the publication of first neutrino mass results with the entire beamline operating [arXiv:1909.06048]. In this paper, we document the current state of all completed beamline components (as of the first neutrino mass measurement campaign), demonstrate our ability to reliably and stably control them over long times, and present details on their respective commissioning campaigns., Comment: Added missing acknowledgement, corrected performance statement in chapter 4.2.5, updated author list and references

We report results of a magnetic characterization of [Cu$_{30}$Ni$_{70}$(6nm)]$_{20}$ (x=1-7nm) superlattices using Polarized Neutron Reflectometry (PNR) and SQUID magnetometry. The study has shown that the magnetic moment of the structures growths almost linearly from H = 0 to H$_{sat}$ = 1.3kOe which is an indirect evidence of antiferromagnetic (AF) coupling of the magnetic moments in neighbouring layers. PNR, however, did not detect any in-plane AF coupling. Taking into account the out-of-plane easy axis of the Cu$_{30}$Ni$_{70}$ layers, this may mean that only the out-of-plane component of the magnetic moments are AF coupled.

We investigated the upper critical magnetic field, $H_{c}$, of a superconductor-ferromagnet (S/F) bilayer of Nb/Cu$_{41}$Ni$_{59}$ and a Nb film (as reference). We obtained the dependence of $H_{c\perp}$ and $H_{c\parallel}$ (perpendicular and parallel to the film plane, respectively) on the temperature, $T$, by measurements of the resistive transitions and the dependence on the inclination angle, $\theta$, of the applied field to the film plane, by non-resonant microwave absorption. Over a wide range, $H_{c\perp}$ and $H_{c\parallel}$ show the temperature dependence predicted by the Ginzburg-Landau theory. At low temperatures and close to the critical temperature deviations are observed. While $H_{c}(\theta)$ of the Nb film follows the Tinkham prediction for thin superconducting films, the Nb/Cu$_{41}$Ni$_{59}$-bilayer data exhibit deviations when $\theta$ approaches zero. We attribute this finding to the additional anisotropy induced by the quasi-one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like state and propose a new vortex structure in S/F bilayers, adopting the segmentation approach from high-temperature superconductors., Comment: Accepted to Phys. Rev. B

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers constitute the core of a superconducting spin valve. The switching effect of the spin valve is based on interference phenomena occurring due to the proximity effect at the S/F interfaces. A remarkable effect is only expected if the core structure exhibits strong critical temperature oscillations, or most favorable, reentrant superconductivity, when the thickness of the ferromagnetic layer is increased. The core structure has to be grown on an antiferromagnetic oxide layer (or such layer to be placed on top) to pin by exchange bias the magnetization-orientation of one of the ferromagnetic layers. In the present paper we demonstrate that this is possible, keeping the superconducting behavior of the core structure undisturbed., Comment: 22 pages, 12 figures, 1 table

We fabricated a nanolayered hybrid superconductor-ferromagnet spin-valve structure, the resistive state of which depends on the preceding magnetic field polarity. The effect is based on a strong exchange bias (about -2 kOe) on a diluted ferromagnetic copper-nickel alloy and generation of a long range odd in frequency triplet pairing component. The difference of high and low resistance states at zero magnetic field is 90% of the normal state resistance for a transport current of 250 {\mu}A and still around 42% for 10 {\mu}A. Both logic states of the structure do not require biasing fields or currents in the idle mode., Comment: 9 pages, 4 figures, Accepted to Applied Physics Letters

The upper critical magnetic field H_{c2} in thin-film FSF trilayer spin-valve cores is studied experimentally and theoretically in geometries perpendicular and parallel to the heterostructure surface. The series of samples with variable thicknesses of the bottom and of the top Cu_{41}Ni_{59} F-layers are prepared in a single run, utilizing a wedge deposition technique. The critical field H_{c2} is measured in the temperature range $0.4-8$ K and for magnetic fields up to 9 Tesla. A transition from oscillatory to reentrant behavior of the superconducting transition temperature versus F-layers thickness, induced by an external magnetic field, has been observed for the first time. In order to properly interpret the experimental data, we develop a quasiclassical theory, enabling one to evaluate the temperature dependence of the critical field and the superconducting transition temperature for an arbitrary set of the system parameters. A fairly good agreement between our experimental data and theoretical predictions is demonstrated for all samples, using a single set of fit parameters. This confirms adequacy of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) physics in determining the unusual superconducting properties of the studied Cu_{41}Ni_{59}/Nb/Cu_{41}Ni_{59} spin-valve core trilayers., Comment: 16 pages, 7 figures; published version

The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu41Ni59/nc-Nb/Co/CoOx spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu41Ni59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature Tc for NCA is lower than the fixed measuring temperature. The existence of a minimum Tc, at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation., Comment: 7 pages, 4 fgures, Submitted to Physical Review Letters

Ferromagnet/Superconductor/Ferromagnet (F/S/F) trilayers, in which the establishing of a Fulde-Ferrell Larkin-Ovchinnikov (FFLO) like state leads to interference effects of the superconducting pairing wave function, form the core of the superconducting spin valve. The realization of strong critical temperature oscillations in such trilayers, as a function of the ferromagnetic layer thicknesses or, even more efficient, reentrant superconductivity, are the key condition to obtain a large spin valve effect, i.e. a large shift in the critical temperature. Both phenomena have been realized experimentally in the Cu 41 Ni 59 /Nb/Cu 41 Ni 59 trilayers investigated in the present work., Comment: 22 pages, 6 figures

We studied the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state establishing due to the proximity effect in superconducting Nb/Cu41Ni59 bilayers. Using a special wedge-type deposition technique, series of 20-35 samples could be fabricated by magnetron sputtering during one run. The layer thickness of only a few nanometers, the composition of the alloy, and the quality of interfaces were controlled by Rutherford backscattering spectrometry, high resolution transmission electron microscopy, and Auger spectroscopy. The magnetic properties of the ferromagnetic alloy layer were characterized with superconducting quantum interference device (SQUID) magnetometry. These studies yield precise information about the thickness, and demonstrate the homogeneity of the alloy composition and magnetic properties along the sample series. The dependencies of the critical temperature on the Nb and Cu41Ni59 layer thickness, Tc(dS) and Tc(dF), were investigated for constant thickness dF of the magnetic alloy layer and dS of the superconducting layer, respectively. All types of non-monotonic behaviors of Tc versus dF predicted by the theory could be realized experimentally: from reentrant superconducting behavior with a broad extinction region to a slight suppression of superconductivity with a shallow minimum. Even a double extinction of superconductivity was observed, giving evidence for the multiple reentrant behavior predicted by theory. All critical temperature curves were fitted with suitable sets of parameters. Then, Tc(dF) diagrams of a hypothetical F/S/F spin-switch core structure were calculated using these parameters. Finally, superconducting spin-switch fabrication issues are discussed in detail in view of the achieved results., Comment: 34 pages, 9 figures

The theoretical description of the Fulde-Ferrell-Larkin-Ovchinnikov like state establishing in nanostructered bilayers of ferromagnetic (F) and superconducting (S) material leads to critical temperature oscillations and reentrant superconductivity as the F-layer thickness gradually increases. The experimental realization of these phenomena is an important prerequisite for the fabrication of the Ferromagnet/Superconductor/Ferromagnet core structure of the superconducting spin-valve. A switching of the spin-valve is only expected if such non-monotonic critical temperature behavior is observed in F/S bilayers as well as in the S/F bilayers, a combination of which the spin-valve core structure can be regarded to consist of. In our former investigations we could demonstrate the required non-monotonic behavior of the critical temperature in S/F bilayers. In this study we succeeded in the preparation of F/S bilayers, where the superconducting material is now grown on top of the ferromagnetic metal, which show deep critical temperature oscillations as a function of the ferromagnetic layer thickness as well as an extinction and recovery, i.e. a reentrant behavior, of superconductivity. Especially, the latter is necessary to obtain a spin-valve with a large critical temperature shift between the parallel and antiparallel configurations of magnetizations in the F layers., Comment: 18 pages, 5 figures

We studied the crystal structure of TiOCl up to pressures of $p$=25~GPa at room temperature by x-ray powder diffraction measurements. Two pressure-induced structural phase transitions are observed: At $p_{c1}$$\approx$15~GPa emerges an 2$a$$\times$2$b$$\times$$c$ superstructure with $b$-axis unique monoclinic symmetry (space group P2$_1$/$m$). At $p_{c2}$$\approx$22~GPa all lattice parameters of the monoclinic phase show a pronounced anomaly. A fraction of the sample persists in the ambient orthorhombic phase (space group $Pmmn$) over the whole pressure range., Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev. B

We have studied the electronic structure of the two-dimensional Heisenberg antiferromagnet VOCl using photoemission spectroscopy and density functional theory including local Coulomb repulsion. From calculated exchange integrals and the observed energy dispersions we argue that the degree of one-dimensionality regarding both the magnetic and electronic properties is noticeably reduced compared to the isostructural compounds TiOCl and TiOBr. Also, our analysis provides conclusive justification to classify VOCl as a multi-orbital Mott insulator. In contrast to the titanium based compounds density functional theory here gives a better description of the electronic structure. However, a quantitative account of the low-energy features and detailed line shapes calls for further investigations including dynamical and spatial correlations., Comment: 7 pages, 6 figures

The nature of the gapped ground state in the quasi-one-dimensional compound eta-Na9V14O35 cannot easily be understood, if one takes into account the odd number of spins on each structural element. Combining the results of specific heat, susceptibility and electron spin resonance measurements we show that eta-Na9V14O35 exhibits a novel ground state where multi-spin objects build up a linear chain. These objects - pentamers - consist of five antiferromagnetically arranged spins with effective spin 1/2. Their spatial extent results in an exchange constant along the chain direction comparable to the one in the high-temperature state., Comment: 6 pages, 5 figures

Pressure-dependent transmittance and reflectance spectra of TiOBr and TiOCl single crystals at room temperature suggest the closure of the Mott-Hubbard gap, i.e., the gap is filled with additional electronic states extending down to the far-infrared range. According to pressure-dependent x-ray powder diffraction data the gap closure coincides with a structural phase transition. The transition in TiOBr occurs at slightly lower pressure ($p$=14 GPa) compared to TiOCl ($p$=16 GPa) under hydrostatic conditions, which is discussed in terms of the chemical pressure effect. The results of pressure-dependent transmittance measurements on TiOBr at low temperatures reveal similar effects at 23 K, where the compound is in the spin-Peierls phase at ambient pressure., Comment: 11 pages, 12 figures; to appear in Phys. Rev. B

We investigated the pressure-dependent optical response of the low-dimensional Mott-Hubbard insulator TiOBr by transmittance and reflectance measurements in the infrared and visible frequency range. A suppression of the transmittance above a critical pressure and a concomitant increase of the reflectance are observed, suggesting a pressure-induced metallization of TiOBr. The metallic phase of TiOBr at high pressure is confirmed by the presence of additional excitations extending down to the far-infrared range. The pressure-induced metallization coincides with a structural phase transition, according to the results of x-ray powder diffraction experiments under pressure., Comment: 4 pages, 3 figures

We report muon spin relaxation (muSR) and magnetic susceptibility investigations of two Ti3+ chain compounds which each exhibit a spin gap at low temperature, NaTiSi2O6 and TiOCl. From these we conclude that the spin gap in NaTiSi2O6 is temperature independent, with a value of 2*Delta=660(50)K, arising from orbital ordering at Too = 210K; the associated structural fluctuations activate the muon spin relaxation rate up to temperatures above 270K. In TiOCl we find thermally activated spin fluctuations corresponding to a spin gap 2*Delta=420(40)K below Tc1=67K. We also compare the methods used to extract the spin gap and the concentration of free spins within the samples from muSR and magnetic susceptibility data., Comment: 4 pages, 3 figures

We have performed a comparative study of the electronic structures of the spin-Peierls systems TiOCl and TiOBr by means of photoemission spectroscopy and density functional calculations. While the overall electronic structure of these isostructural compounds is qualitatively similar, the bromide appears to be less one-dimensional. We present a quantitative analysis of the experimental dispersions in terms of exchange constant J and hopping integral t as well as a discussion of the qualitative spectral features. From that we conclude that despite the one-dimensional physics triggering the ground state in both compounds a proper description of the normal state electronic structure has to take into account the anisotropic frustrated interchain interactions on the underlying triangular lattice., Comment: 5 pages, 4 figures

We studied the transmittance and reflectance of the low-dimensional Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a function of pressure. The strong suppression of the transmittance and the abrupt increase of the near-infrared reflectance above 12 GPa suggest a pressure-induced insulator-to-metal transition. The pressure-dependent frequency shifts of the orbital excitations, as well as the pressure dependences of the charge gap and the spectral weight of the optical conductivity above the phase transition are presented., Comment: 4 pages, 6 figures

We present detailed ESR investigations on single crystals of the low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates a stable orbital configuration below room temperature, and allows to estimate the energy of the first excited state as 0.3(1) eV ruling out a possible degeneracy of the orbital ground state. Moreover, we discuss the possible spin relaxation mechanisms in TiOCl and analyze the angular and temperature dependence of the linewidth up to 250 K in terms of anisotropic exchange interactions. Towards higher temperatures an exponential increase of the linewidth is observed, indicating an additional relaxation mechanism., Comment: 10 pages, 8 figures; accepted for publication in Phys. Rev. B

We report on the first observation of a pronounced re-entrant superconductivity phenomenon in superconductor/ferromagnetic layered systems. The results were obtained using a superconductor/ferromagnetic-alloy bilayer of Nb/Cu(1-x)Ni(x). The superconducting transition temperature T_{c} drops sharply with increasing thickness d_{CuNi} of the ferromagnetic layer, until complete suppression of superconductivity is observed at d_{CuNi}= 4 nm. Increasing the Cu(1-x)Ni(x) layer thickness further, superconductivity reappears at d_{CuNi}=13 nm. Our experiments give evidence for the pairing function oscillations associated with a realization of the quasi-one dimensional Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) like state in the ferromagnetic layer., Comment: 3 pages, 3 figures, REVTEX4/twocolumn

We report on a detailed single crystal x-ray diffraction study of the unconventional spin-Peierls compound TiOCl. The intermediate phase of TiOCl is characterized by an incommensurate modulation which is virtually identical to that recently found in the homologue compound TiOBr. The first order phase transition between the spin-Peierls ground state and the incommensurate phase reveals the same kind of thermal hysteresis in both, its crystal structure and magnetic susceptibility. A weak, but significant magnetic field effect is found for this phase transition with a field induced shift of the transition temperature of Delta Tc1= -0.13 K for an applied field of B=10 T along the chain direction. The field induced changes of the incommensurate crystal structure are compatible with a scenario of competing intra- and inter-chain interactions., Comment: 4 pages, 3 figures

The strong line broadening observed in electron spin resonance on NaV$_2$O$_5$ is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost two orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange, only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K., Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Letters

The temperature dependence of the local structure of V_2O_3 in the vicinity of the metal to insulator transition (MIT) has been investigated using hard X-ray absorption spectroscopy. It is shown that the vanadium pair distance along the hexagonal c-axis changes abruptly at the MIT as expected. However, a continuous increase of the tilt of these pairs sets in already at higher temperatures and reaches its maximum value at the onset of the electronic and magnetic transition. These findings confirm recent theoretical results which claim that electron-lattice coupling is important for the MIT in V_2O_3. Our results suggest that interactions in the basal plane play a decisive role for the MIT and orbital degrees of freedom drive the MIT via changes in hybridization., Comment: 6 pages, 5 figures, 2 tables

Using surface acoustic waves, the elastomagnetic coupling could be studied in thin single crystalline plates of the ferrimagnetic semiconductor FeCr2S4 by measuring the attenuation and the frequency tracking in the temperature range 4.2 K to 200 K. The data clearly display the anomalies found in low-field magnetization measurements., Comment: 15 pages, 3 figures. To appear in J. Appl. Phys., 99 (2006)

We have studied the electronic structure of the spin-1/2 quantum magnet TiOCl by polarization-dependent momentum-resolved photoelectron spectroscopy. From that, we confirm the quasi-one-dimensional nature of the electronic structure along the crystallographic b-axis and find no evidence for sizable phonon-induced orbital fluctuations as origin for the non-canonical phenomenology of the spin-Peierls transition in this compound. A comparison of the experimental data to our own LDA+U and Hubbard model calculations reveals a striking lack of understanding regarding the quasi-one-dimensional electron dispersions in the normal state of this compound., Comment: 7 pages, 8 figures

Thin V2O3 films were deposited on a piezoelectric substrate by electron-beam evaporation. Surface acoustic waves (SAW) were generated by interdigital-transducers (IDTs). The attenuation and sound velocity was investigated from 260K to 10K, providing an insight into the temperature dependent electrical, dielectrical and elastic properties of V2O3 at the metal to insulator transition., Comment: 7 pages, 7 figures

Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at T_{c2} = 90 K corresponds to a lowering of the lattice symmetry. Below T_{c1} = 66 K a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure 1D chains of atoms. The first-order character of the transition at T_{c1} is explained by the competition between the structurally deformed state below T_{c2} and the spin-Peierls state below T_{c1}., Comment: Phys. Rev. B (Rapid Communications) in press

Measurements of the heat capacity C(T,H) of the one-dimensional quantum magnet TiOCl are presented for temperatures 2K < T < 300K and magnetic fields up to 5T. Distinct anomalies at 91K and 67K signal two subsequent phase transitions. The lower of these transitions clearly is of first order and seems to be related to the spin degrees of freedom. The transition at 92K probably involves the lattice and/or orbital moments. A detailed analysis of the data reveals that the entropy change through both transitions is surprisingly small (~ 0.1R), pointing to the existence strong fluctuations well into the non-ordered high-temperature phase. No significant magnetic field dependence was detected., Comment: 4 pages, 2 figures

We have investigated the electronic structure of nano-sized iron oxide by scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by photoelectron spectroscopy. Nano particles were produced by thermal treatment of Ferritin molecules containing a self-assembled core of iron oxide. Depending on the thermal treatment we were able to prepare different phases of iron oxide nanoparticles resembling gamma-Fe2O3, alpha-Fe2O3, and a phase which apparently contains both gamma-Fe2O3 and alpha-Fe2O3. Changes to the electronic structure of these materials were studied under reducing conditions. We show that the surface band gap of the electronic excitation spectrum can differ from that of bulk material and is dominated by surface effects., Comment: REVTeX, 6 pages, 10 figures, submitted to PRB

Low-temperature heat-capacity investigations on the spinel FeCr2S4 with ferrimagnetic spin order and orbitally degenerated Jahn-Teller active Fe2+ ions in a tetrahedral crystal field, provide experimental evidence of an orbital liquid state above 10 K. We demonstrate that the low-temperature transition at 10 K arises from orbital order and is very sensitive to fine tuning of the stoichiometry in polycrystals. In single crystals the orbital order is fully suppressed resulting in an orbital glass state with the heat capacity following a strict T^2 dependence as temperature approaches zero., Comment: 4 pages, 3 figures

The origin of the resistive transition broadening for MgB2 thin films was investigated. Thermally activated flux flow is found to be responsible for the resistivity contribution in the vicinity of Tc. The origin of the observed extraordinary strong magnetic field dependence of the activation energy of the flux motion is discussed., Comment: 10 pages, 5 figures

The pressure dependence of phase transitions in the quasi one-dimensional vanadium oxide $\beta$-Na$_{1/3}$V$_2$O$_5$ has been studied by magnetic susceptibility and electrical resistivity measurements. The pressure dependence of the various transition temperatures is quite differently. The transition at T=240 K, previously reported and attributed to ordering on Na sites, and a second transition at $T \approx 222$ K, reported here for the first time and attributed to a further increase of order on Na sites, are almost independent of pressure. On the other hand, the metal-insulator (MI) transition at $T_{MI}=130$ K shifts to lower temperatures, while the magnetic transition at $T_N=24$ K shifts to higher temperatures with increasing pressure. We discuss the different pressure dependencies of $T_{MI}$ and $T_N$ in terms of increasing interchain coupling and the MI transition to be of Peierls type., Comment: 5 pages, 5 figures