Search

Albino ferrets contrary to their pigmented conspecifics show no optokinetic nystagmus. Therefore, in this study motion perception was compared between pigmented and albino ferrets (Mustela putorius furo) trained to discriminate between coherently moving random dot patterns and dynamic noise stimuli in a two-alternative forced choice task. Fully coherently versus incoherently moving patterns could be distinguished by ferrets of both phenotypes. Motion coherence thresholds, however, were significantly higher in albinos. These results indicate that albino ferrets are not motion blind as could be expected from their total lack of optokinetic reactions. However, they are severely impaired in global motion perception.

Motion perception was tested in pigmented Long-Evans and albino Wistar rats (Rattus norvegicus sp.) using moving random dot patterns. Pigmented as well as albino rats could distinguish a fully coherently moving pattern from dynamic noise. However, motion coherence thresholds were significantly lower in pigmented compared to albino rats (12% and 30% coherence, respectively). These results indicate that pigmented rats have well developed motion coherence perception, whereas albino rats are severely impaired but not motion blind.

Proximity effects in an [Er20|Tb5] superlattice lead to the formation of new magnetic phases. Modulated magnetic order, expected for pure Er, and ferromagnetic order, expected for pure Tb, coexist at low temperatures. Employing X-ray resonance exchange scattering, we could probe directly the respective spin polarisations of the conduction band electrons, providing a mechanism for the interlayer coupling. The different anisotropies of Er and Tb compete with this tendency to long-range magnetic order, leading to substantial thermal-hysteresis effects.

It is shown experimentally that the temperature dependence of the magnetic order parameter of real Heisenberg magnets, ms(T), can be described in the whole range 0

We have employed a new experimental configuration to measure ferromagnetic X-ray resonance exchange scattering (XRES) from a EuS single crystal. Using polarisation analysis we could achieve a magnetic-scattering intensity stronger than charge scattering resulting in an asymmetry ratio as large as Ra = 0.67 at the EuLII edge. By a combined refinement of the dependencies of the scattered intensity on the energy and the applied magnetic field we could uniquely determine spectroscopic information such as the 5d conduction band exchange splitting = 0.27(1) eV.

We report on an investigation of the magnetic properties of Gd x Eu 1 - x S mixed crystals with compositions in the range of 0.6 < x < 1. For the two samples Gd 0.8 Eu 0.2 S and Gd 0.73 Eu 0.27 S a long-range antiferromagnetic order was observed at low temperatures. Element-specific measurements exhibited a different temperature dependence of the reduced sublattice magnetisation of the two magnetic species. A model calculation and Monte Carlo simulations revealed that the different temperature dependence is due to frustration effects. These frustration effects lead to a breakdown of the long-range order for higher europium contents. For the Gd 0.67 Eu 0.33 S-sample we were able to observe a short-range antiferromagnetic order with correlation lengths of a few 10A with X-ray resonance exchange scattering.

We report on the determination of element-specific magnetic order for a Gd0.8Eu0.2S mixed crystal by means of X-ray resonance exchange scattering. For the first time, a different temperature dependence of the sublattice magnetization of two lanthanide ions was observed. With model calculations we were able to reproduce the measured temperature dependence of the reduced sublattice magnetization. We can show that competing magnetic interactions lead to the observed effects and can reveal the frustration mechanism.

We report on investigations of fourth-order exchange interactions, i.e. biquadratic, three-spin and four-spin interactions in Gd x Eu 1− x S using measurements of the cubic susceptibility χ 3 and the magnetic specific heat c p . In the paramagnetic phase of GdS these interactions are ferromagnetic since χ 3 is negative (the magnetization increases stronger than linearly with magnetic field). At the Neel temperature of GdS of 59 K the onset of a strong field dependence of the linear susceptibility χ 1 has early been observed. This feature is attributed here to a quasi-ferromagnetic ordering process induced by the fourth-order exchange interactions. Evidence for this is provided by a divergence of the cubic susceptibility χ 3 at 59 K. Two further magnetic phase transformations were observed in GdS: (a) at 47 K the magnetic susceptibility and specific heat exhibit small but definite anomalies and, (b) below 27 K a second critical field curve reaching only 2.6 T for T →0 is observed. Also these phase transitions are attributed to the fourth-order interactions which apparently are not constant but change from ferromagnetic at high temperature to antiferromagnetic at low temperature. The critical magnetic behavior of the magnetic specific heat, c mag , at T N =59 K is as complicated in GdS as in EuTe because two ordering phenomena superpone. A description by power laws with Heisenberg critical exponents is inadequate. On substituting Gd by Eu fourth-order interactions change, as usual, rapidly as function of composition. One sample with composition Gd 0.8 Eu 0.2 was investigated in detail. In this sample the total fourth-order interaction is antiferromagnetic at the Neel temperature of T N =55 K. As a consequence, the ferromagnetic component observed in GdS is absent and a second critical field curve reaching only 5.5 T occurs below 55 K.

Results of high-energy magnetic X-ray diraction for photon energies up to 500 keV are presented and the form of the magnetic scattering cross-section for relativistic energies is discussed. At 300 keV we experimentally proved that the scattering from the spin component perpendicular to the scattering plane is dominant. Within our statistical and systematic errors, the magnetic scattering amplitude for this component does not exhibit any variation with energy between 100 and 500 keV. Introduction. { Magnetic X-ray diraction with high-energy photons exhibits certain advantages with respect to neutron and medium-energy X-ray diraction which are discussed in detail in (1), (2). These include true bulk sensitivity, volume-enhancement of the magnetic signal, high momentum space resolution and a simple form of the magnetic scattering cross- section. For this reason, magnetic scattering with hard X-rays at an energy of 80 keV has been developed during the last years (1), (2). At this energy, the rst-order approximation in h!=mc 2 for the magnetic scattering amplitude is valid in very good approximation. For magnetic diraction at higher energies, when the photon energy approaches the electron rest mass mc 2 = 511 keV, a complete quantum-relativistic treatment is necessary, taking into account all eects arising from the coupling of the quantized photon eld and the Dirac eld of the electrons. To our knowledge, a corresponding theory is missing. To investigate experimentally how the scattering cross-section behaves for higher photon energies, we measured magnetic Bragg reflections of MnF2 at energies up to 500 keV. Magnetic scattering cross-section for hard X-rays. { A calculation of the cross-section for X-ray scattering including the magnetic terms from a quasi-relativistic Hamiltonian for

We report on interlayer coupling in [Ern(Er)|Tbn(Tb)] superlattices. By means of neutron diffraction, helix-like and conical magnetic structures are observed in an [Er20|Tb5] sample. These structures are correlated across terbium layers at low temperatures, where bulk terbium is ferromagnetic. Moreover, resonant X-ray magnetic scattering gives clear evidence that the 5d conduction band of terbium is polarized according to the erbium magnetic structure.

We recently described an area in the ferret posterior suprasylvian (PSS) cortex characterized by a high proportion of direction selective neurons. To answer the question whether area PSS subserves functions similar to cat posteromediolateral suprasylvian area (PMLS) and monkey medial temporal area (MT) we investigated the contribution of area PSS to visual motion perception and optokinetic nystagmus. Ferrets were tested on global motion detection before and after bilateral lesions involving area PSS and control lesions of other extrastriate visual areas. Following PSS lesions motion coherence thresholds were significantly increased both in pigmented and albino ferrets, whereas control lesions sparing PSS did not affect visual motion perception. Optokinetic nystagmus was strongly reduced to absent after PSS lesions. These results indicate that area PSS is crucial for global motion processing in the ferret and in that sense may be functionally equivalent to PMLS in the cat and area MT in the monkey.

We report on the new synchrotron X-ray scattering results from the re-investigation of charge and orbital ordering in lightly doped La 1− x Sr x MnO 3 . The resonant X-ray scattering from forbidden Bragg reflections was observed not only in the ferromagnetic insulating phase, but also in strongly Jahn–Teller distorted regime. Some of characteristic superstructure reflections were also found to show a clear resonant behavior at the Mn K absorption edge. It can be argued that the exact pattern of charge and orbital ordering should be much more complicated.

Resonant magnetic x-ray scattering from Tb in the spiral phase is studied in great detail. Polarization analysis in the sigma-sigma' and sigma-pi' channels has been performed for magnetic (0, 0, l +/- tau) satellite reflections over the accessible wavevector range Q = 1.9-6.5 angstrom(-1). A characteristic splitting of the resonance signal as a function of energy has been observed at the L-II as well as at the L-III absorption edge. The shape of the resonance depends on Q and is different for the two edges. Up to three components were observed with a separation in energy between 3.3 and 4.9 eV, compared to a core hole lifetime broadening of 2.5 eV, which is in agreement with the density of states in Tb metal.

The triple-crystal diffractometer installed at HASYLAB beamline BW5 with a high-field wiggler of critical energy 27 keV for DORIS III, operated at 4.5 GeV electron energy, is described. Samples can be mounted in large cryostats or furnaces normally used in neutron scattering experiments. The instrument has been successfully applied to measure structure factors S(Q) in liquids and amorphous materials, to collect full data sets of highly accurate structure factors for charge-density work, to measure the spin component of the ground-state magnetization in transition-metal and rare-earth compounds, to study the diffuse scattering from stacking faults and dislocation loops in Si single crystals, and for the investigation of various aspects of structural phase transitions: critical scattering in SrTiO3, oxygen order and stripe order in high-To materials. A crossed-beam technique allows for local studies of texture, internal strain and phase changes in the bulk of materials.