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Crystallography
 
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Magnetic Structures

Schematic representation of the high temperature lock-in magnetic structure of the orthorhombic Cmmm (low temperature modification) with the wave vector q1= (0 ½ 0) of the DyFe4Ge2 compound, at 46K when viewed along the (001) direction.
The magnetic cell is doubled along the b axis (a, 2b, c). The moments of all atoms change sign collectively when going from one cell to the next in the direction of the wave vector b. The Dy magnetic moments have a 2D-canted arrangement while the Fe moments a 3D.

Overview:

The study of crystal and magnetic structures is of fundamental importance for the understanding of physical properties and crystal chemistry of matter. Our field of research comprises the study of binary and ternary rare earth (R) compounds with magnetic and non- magnetic constituents as well as boracites. More specific we are interested on magnetic systems with competing interactions that try to resolve frustration by reorganizing the underlying degrees of freedom i.e. systems with two independent magnetic subsystems or systems with geometrical frustration with triangular or tetrahedral spin arrangements with antiferromagnetic interactions that cannot be satisfied due to the topology of the lattice. These systems emerge to new and complex magnetostructural phenomena.
From its nature this field involves interdisciplinary and international collaboration. Our main contribution is the collection and analysis of neutron and X-ray diffraction data and phenomenological description. The main steps of this investigation are:

Experimental:
(i) sample synthesis and characterization
(ii) magnetic-, specific heat- and electrical conductivity measurements as well as Moesbauer spectroscopy. This information is a prerequisite for tracing the strategy and carrying out diffraction experiments
(iii) Neutron and X-ray diffraction on powder and single crystals samples as a function of temperature, magnetic field or pressure but also in the data analysis. In several cases µSR (myon spin rotation/relaxation) is used to answer special questions on the local magnetic properties.

Data Analysis:
Neutron diffraction is the most powerful tool in the analysis of long range magnetic order. Magnetic ordering is related to a phase transition and in most cases to symmetry reduction characterized by the presence of one or more wave vector(s). The increasing complexity (see Fig. below) requires restricting the parameter space in order to derive a realistic model for further data analysis. We make use of a) all independent physical information, b) of symmetry analysis, for every special case (including incommensurately modulated systems) c) we combine various diffraction data sets (different instruments (x-ray or neutron) single crystal and powder samples) in the same calculation. In complex cases simulated annealing may be added to the list. Refinements of powder data use the Rietveld line-profile analysis method including stress and anisotropic strain analysis and quantitative sample analysis. Further effort is addressed to ab initio simulations (VASP) trying to check the uniqueness of a given model.

An example is given below.

Magnetic ordering of an incommensurate crystal structure: La2Co1.7

This compound represents one of the most complex situations. La2Co1.7 orders anti-ferromagnetically below TN=146K. It has a modulated crystal structure at room temperature and a modulated magnetic structure below TN. Its study comprises neutron powder diffraction, neutron single crystal Laue diffraction and X-ray single crystal diffraction. It is to note that a conventional single crystal neutron diffractometer even in a high flux reactor would need several weeks of beam time usually not available.

Neutron Laue diffraction (single crystal)
A preliminary analysis of neutron Laue diffraction single crystal data at temperatures between 295 and 15 K, recorded on neutron-sensitive image plates using a thermal neutron beam provided us very fast the most important information concerning the coexistence of incommensurate crystal and magnetic structures. Our data comprise three sets of reflections at low temperatures pertaining to distinct vectors (see figure).

LaueLaCo

Laue diagram of La2Co1.7 at 15 K. The main beam position is at the centre of the hole visible at the right-hand side of the image. Strong fundamental reflections and hexagonal groups of satellites are visible.

1. The modulated crystal structure is interpreted in terms of a charge (nuclear) density wave with a propagation vector qn = (qx, 0, qz) with qx=0.113(1)a* and qz=0.203(2)c* 293K.
2. The modulated magnetic structure has also been confirmed using a band-pass filter with a cold neutron beam that antiferromagnetic Bragg reflections which have a magnetic propagation vector qm = (1/3, 1/3, 0) exist below 144(1)K and correspond to a cobalt atom moment of 0.80(15) µB.
3. Magnetic satellite reflections with wave vectors corresponding to qn ± qm. have also been observed on the Laue diagrams below this temperature. The values of the propagation vector and magnetic moment have been confirmed from neutron powder diffraction diagrams taken at 150 and 1.5 K. The structural modulation has been confirmed [2] while the magnetic modulation of the reflection set qn ± qm is still under investigation.

[1] C. Wilkinson, P. Schobinger-Papamantellos, D. Myles, L.D. Tung, K.H.J. Buschow. J. Mag. Magn. Mat. 217 (2000) 55.
[2] M. Dusek, G. Chapuis, P. Schobinger-Papamantellos, C. Wilkinson, V. Petricek, L. D. Tung, K.H.J. Buschow. Acta Cryst. (2000). B56, 959.

Current research projects:

Magnetic Structures and magneto-elastic Phase Transitions in frustrated Rare Earth (R) Intermetallics and in Boracites. Data interpretation of multi-q magnetic structures.
This work is carried out with interdisciplinary and international collaboration (see publications).

Publications

 

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© 2016 ETH Zurich | Imprint | Disclaimer | 14 March 2006
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