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Submitted on 1 Jan 1978
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NEUTRON DIFFRACTION STUDY OF DAG AT
VERY LOW TEMPERATURES AND IN EXTERNAL
MAGNETIC FIELD
M. Steiner, N. Giordano
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-816
NEUTRON DIFFRACTION STUDY OF DAG AT VERY LOW TEMPERATURES AND IN EXTERNAL MAGNETIC FIELD
M. Steiner and N. Giordano
Hahn-Meitner -In@ti.tut, Glienisker Str. 100,D-1000 Berlin 39, West-Germany
Résumé.- Le diagramme des phases du DAG pour T < 1,2 K et H £ 2 T a été mesuré à l'aide de la diffu-sion élastique des neutrons. Les structures magnétiques proposées ont été confirmées. Il a été trouvé que les intensités magnétiques dépendent du chemin, dans le diagramme des phases, par lequel on atteint un point H, T déterminé. Les résultats seront expliqués et discutés.
Abstract.- By means of neutron diffraction the phase diagram for DAG at T < 1.2 K and H < 2 T has been measured. The proposed structure could be verified. It was found that the magnetic intensities depend on the way through the phase diagram used to reach a certain H-, T-point. The results are discussed and explained.
Dysprosium-Aluminium-Garnet (DAG) has been studied extensively in the past since it is regarded as an ideal model system to study tricritical points (see for example the review / l / ) . DAG has the cubic garnet structure and orders antiferromagnetically below T = 2.5 K. The magnetic structures (AFI) con-sist of three interpenetrating pairs of antiparallel sublattices whose moments point along the cube edges These three lattices are only loosely coupled. The magnetic phase diagram below 1.2 K in an external field has been studied and determined by magneto-thermal measurements /2/ for magnetic fields along
Q o o ] , D1 (D ' Q 3 Q - By the same authors new
magne-tic structures have been theoremagne-tically proposed : for H | | QOO]and T < 1.1 K a second antiferromagne-tic (AF2) and for H | | Q 10] and T < 0.5 K a ferro-magnetic (F) phase. In these cases the competition between the weak interaction between the three types of sublattices and the intrasublattice interaction lead to these structures and to verify the phase diagram.
In a neutron scattering experiment the dif-ferent phases show difdif-ferent sets of typical magne-tic Bragg-reflections. These sets have been measured for H || QooJ and T < 1 .2 K, and H || |jiq] and T < 0 . 5 K ; H E O and T < 2.7 K. For the measure-ments we used a continuous cycle 3He-cryostat with a base temperature of 0.37 K and a superconducting coil.
By measuring typical Bragg-reflections in dependence of H and T the phase diagrams were de-termined. It turned out that our phase boundaries
Permanent address : Yale University, New Haven, Conn., USA.
do agree very well with the results from the magne-to thermal /2/. In the course of the determination of the magnetic structures it turned out that the measured intensities were seriously affected by ex
-tinction. The crystallographic reflections could be corrected for extinction by the following formula :
J = S J . (1 - x J . ) meas theor theor with x = 0.00015.
In the case of the magnetic relections the situation was more complicated. It was found that the intensity of a reflection obtained after a cool down in zero field was only one half of the inten-sity at the same temperature at zero field after the application of an external field of 1 T.
These "quenched" intensities turned out to be consistent with the magnetic structures AF1 and with a moment of Dy of 11.0 yB. The magnetic re-flections of the AF2 phase for H | | Oo (0 gave the
same Dy-moment showing that the magnetic structures are correct. In order to explain the effect of quen-ching, we studied the time dependence of the (110) -reflections of the API- phase at 0.5 K. It turned out that the intensity dropped after the quench with a time constant of approximately 10 h.
The explanation is that at such low temperatu-res it is possible to achieve a domain configura-tion which doesn't show extincconfigura-tion, the domains being very small. But this non equilibrium configu-ration slowly changes to a final configuconfigu-ration with large domains, which cause extinction.
References
IM Wolf, W.P., Proc. Int. Conf. Magn. Amsterdam, Physica 86-88 B (1977) 550.
HI Keen, B.E., Landau, D.P. and Wolf, W.P., Phys. Lett._2JL (1966) 202.
