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Register a new userPolarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins
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We report on a polarized-neutron Laue diffraction experiment on a single crystal of neodynium doped lanthanum magnesium nitrate hydrate containing polarized proton spins. By using dynamic nuclear polarization to polarize the proton spins, we demonstrate that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.
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Polarised neutron diffraction measurements have been made on HoFeO$_3$ single crystals magnetised in both the [001] and [100] directions ($Pbnm$ setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of H = 9 T parallel to [001] at T = 70 K and with the lower field H = 0.5 T parallel to [100] at T = 5, 15, 25~K. A Fourier projection of magnetization induced parallel to [001], made using the $hk0$ reflections measured in 9~T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70~K, 9~T applied parallel to [001] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the $xmhyphen y$ plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature $Gamma_2$ structure with a lower field, 0.5 T $parallel$ [100] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the 180degree domain stabilised by the field were determined relative to the distorted perovskite structure,. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.
We describe a new sensitive method for the investigation of weakly magnetic films placed inside a tri-layer planar waveguide. Polarized neutrons tunnel into the waveguide through the surface, channel along the layers and are emitted from the end face as a narrow and slightly divergent microbeam. Polarization analysis permits to detect very small magnetization in the order of a few 10 Gauss. The magnetic film containing the rare-earth element Tb was investigated using both fixed wavelength and time-of-flight polarized neutron reflectometers. The experimental results are presented and discussed.
In-situ white light Laue diffraction has been successfully used to interrogate the structure of single crystal materials undergoing rapid (nanosecond) dynamic compression up to megabar pressures. However, information on strain state accessible via this technique is limited, reducing its applicability for a range of applications. We present an extension to the existing Laue diffraction platform in which we record the photon energy of a subset of diffraction peaks. This allows for a measurement of the longitudinal and transverse strains in-situ during compression. Consequently, we demonstrate measurement of volumetric compression of the unit cell, in addition to the limited aspect ratio information accessible in conventional white light Laue. We present preliminary results for silicon, where only an elastic strain is observed. VISAR measurements show the presence of a two wave structure and measurements show that material downstream of the second wave does not contribute to the observed diffraction peaks, supporting the idea that this material may be highly disordered, or has undergone large scale rotation.
This paper presents results of a recent study of multiferroic CCO by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T sub{N1}=24.2$ K and $T sub{N2}=23.6$ K. The low temperature magnetic structure below $T sub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T sub{N1}$ and $T sub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.
We present a combination of ab initio calculations, magnetic Compton scattering and polarized neutron experiments, which elucidate the density distribution of unpaired electrons in the kagome staircase system Co3V2O8. Ab initio wave functions were used to calculate the spin densities in real and momentum space, which show good agreement with the respective experiments. It has been found that the spin polarized orbitals are equally distributed between the t2g and the eg levels for the spine (s) Co ions, while the eg orbitals of the cross-tie (c) Co ions only represent 30% of the atomic spin density. Furthermore, the results reveal that the magnetic moments of the cross-tie Co ions, which are significantly smaller than those of the spine Co ions in the zero-field ferromagnetic structure, do not saturate by applying an external magnetic field of 2 T along the easy axis a, but that the increasing bulk magnetization originates from induced magnetic moments on the O and V sites. The refined individual magnetic moments are mu(Co_c)=1.54(4) mu_B, mu(Co_s)=2.87(3) mu_B, mu(V)=0.41(4) mu_B, mu(O1)=0.05(5) mu_B, mu(O2)=0.35(5) mu_B, and; mu(O3)=0.36(5) mu_B combining to the same macroscopic magnetization value, which was previously only attributed to the Co ions.
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