Spiral magnetism, spin flop, and pressure induced ferromagnetism in the negative charge transfer gap insulator Sr2FeO4

Iron IV oxides are strongly correlated materials with negative charge transfer energy negative Delta , and exhibit peculiar electronic and magnetic properties such as topological helical spin structures in themetallic cubic perovskite SrFeO3. Here, the spin structure of the layered negative Delta insulator Sr2FeO4 was studied by powder neutron diffraction in zero field and magnetic fields up to 6.5 T. Below TN 56K, Sr2FeO4 adopts an elliptical cycloidal spin structure with modulated magnetic moments between 1.9 and 3.5 amp; 956;B and a propagation vector k amp; 964;, amp; 964;, 0 with amp; 964; 0.137. With increasing magnetic field the spin structure undergoes a spin flop transition near 5 T. Synchrotron 57Fe Mössbauer spectroscopy reveals that the spin spiral transforms to a ferromagnetic structure at pressures between 5 and 8 GPa, just in the pressure range where a Raman active phonon nonintrinsic to the K2NiF4 type crystal structure vanishes. These results indicate an insulating ground state which is stabilized by a hidden structural distortion and differs from the charge disproportionation in other Fe IV oxide

Optimizations in angular dispersive neutron powder diffraction using divergent beam geometries

Angular dispersive neutron powder diffractometers are usually built using beam divergencies defined by Soller type collimators. To account for the needs of resolution for crystal structure refinement a good in pile collimation 1, a high take off angle above 90 circ at the monochromator and a good collimation 3 in front of the detector bank are chosen whereas the valueof 2 for the collimation between monochromator and sample is less crucial. During the last years new strategies were developed at our institute using wide divergent beam geometries defined by fan collimators or slit type diaphragms which correlate ray direction and wavelength within the beam. Here we present the performance of a newly developed fan collimator, which enables one to adjust the opening of the collimator channels on both sides independently. This fan collimator is positioned in front of the monochromator at the instrument E6 at the Helmholtz Centre Berlin formerly Hahn Meitner Institut Berlin . It will be shown that control of the beam divergency allows optimization of the resolution in a large angular diffraction range. Hence the resolution and intensity can be adapted to the needs of powder diffraction. Monte Carlo simulations using McStas are used to check and prove the optimal setting of the instrument. We obtain a very good agreement between experimental and simulated data and demonstrate the superior outcome of the new instrument configuration with respect to Soller type instrument

On the form invariant volume transformation in phase space by focusing neutron guides An analytic treatment

Taperedguideswithsupermirrorcoatingarefrequentlyusedtofocusneutronbeamsonspecimens.The divergencedistributioninthefocusedbeamisofagreatimportanceforthequalityofneutron instrumentation. Usingananalyticapproachwederivethetaperingwhichisneededtoachieveaform invariantphasespacetransformationofarectangularphasevolume.Inadditionweconsidertheeffectof beam attenuationbythe finite reflectivityofsupermirror

Neutron guide geometries for homogeneous phase space volume transformation

We extend geometries for recently developed optical guide systems that perform homogeneous phase space volume transformations on neutron beams. These modules allow rotating beam directions and can simultaneously compress or expand the beam cross section. Guide systems combining these modules offer the possibility to optimize ballistic guides with and without direct view on the source and beam splitters. All systems are designed for monochromatic beams with a given divergence. The case of multispectral beams with wavelength dependent divergence distributions is addressed as wel

Non periodic multi slit masking for a single counter rotating 2 disc chopper and channeling guides for high resolution and high intensity neutron TOF spectroscopy

Energy resolution is an important design goal for time of flight instruments and neutron spectroscopy. For high resolution applications, it is required that the burst times of choppers be short, going down to the s range. To produce short pulses while maintaining high neutron flux, we propose beam masks with more than two slits on a counter rotating 2 disc chopper, behind specially adapted focusing multi channel guides. A novel non regular arrangement of the slits ensures that the beam opens only once per chopper cycle, when the masks are congruently aligned. Additionally, beam splitting and intensity focusing by guides before and after the chopper position provide high intensities even for small samples. Phase space analysis and Monte Carlo simulations on examples of four slit masks with adapted guide geometries show the potential of the proposed setu

The design of the inelastic neutron scattering mode for the Extreme Environment Diffractometer with the 26 T High Field Magnet

The Extreme Environment Diffractometer is a neutron time of flight instrument, designed to work with a constant field hybrid magnet capable of reaching fields over 26 T, unprecedented in neutron science; however, the presence of the magnet imposes both spatial and technical limitations on the surrounding instrument components. In addition to the existing diffraction and small angle neutron scattering modes, the instrument will operate also in an inelastic scattering mode, as a direct time of flight spectrometer. In this paper we present the Monte Carlo ray tracing simulations, the results of which illustrate the performance of the instrument in the inelastic scattering mode. We describe the focussing neutron guide and the chopper system of the existing instrument and the planned design for the instrument upgrade. The neutron flux, neutron spatial distribution, divergence distribution and energy resolution are calculated for standard instrument configuration

The magnetic spiral in the frustrated antiferromagnet RbCuCl3 studied by means of neutron diffraction

Neutron diffraction was performed to study the spiral spin arrangement of RbCuCl3 as a func tion of field and temperature. We clearly observe that the spiral lengthens with increasing temperature up to TN 19.3 K. A shortening of the spiral occurs when a magnetic field is ap plied within the plane of spin rotation. No change in the propagation vector is observed with the field applied perpendicular to the spin rotation plane. In addition we measured the tem perature dependence of the magnetization and close to TN and we determine a critical expo nent amp; 946; 0.24 1 and 0.26 1 at zero field and at 13.5 Tesla, respectivel