#
# arev:/usr/local/unicat/unireq/beamreq/req00056.txt
# UNICAT Member Beam Time Request  #56

# created Wed May 05 14:07:56 CDT 1999
#

days:
	5-7

description:
	We propose to study the antiferromagnetic phase transition in
	K2ReBr6, which occurs at about 15 K, using resonant magnetic
	x-ray diffraction.  K2ReBr6 crystallizes in the antiflourite structure
	(space group Fm3m with a=10.32A) and undergoes a type I antiferromagnetic
	phase transition below a transition temperature of 15 K with an ordered
	moment of 2.7 Bohr Mag. per Re^4+ ion.  Previous
	measurements performed at the NSLS on the related compound K2ReCl6
	have shown clear resonant enhancement in the vicinity of the LIII
	absorption edge of Re (10.5353 keV) [1].
	
	We propose to study this antiferromagnetic phase transition under
	the high-Q resolution conditions provided by the synchrotron source
	at temperatures in the immediate vicinity of the phase transition.
	In addition, neutron scattering [2,3,4] and ESR [5] measurements
	have indicated substantial delocalization of the magnetic electron
	density of up to 30% of the total magnetic moment onto the surrounding
	ligand ions (Br in this case).  Br has a K absorption edge at an
	energy of 13.4737 keV and by tuning the incident energy to this edge,
	the magnetism associated with these ions may be able to be measured
	directly.  This could provide a direct measurement of covalency
	effects in these 5d electron systems.
	
	We presently have several single crystals (approximate dimensions on
	the order of 1mm) which are suitable for magnetic x-ray diffraction.
	Because of the relatively low transition temperature we would prefer
	to perform the measurements in a heliplex.
	
	References
	
	[1] S.E. Nagler et. al. unpublished.
	[2] H.G. Smith and G.E. Bacon, J. Appl. Phys. 37, 979 (1966).
	[3] V.J. Minkiewicz et. al., J. Chem. Phys. Sol. 29, 881 (1968).
	[4] J.W. Lynn, G. Shirane and M. Blume, Phys. Rev. Lett. 37, 154 (1976).
	[5] K.W.H. Stevens, Proc. Roc. Soc. (London) A219, 532 (1953).
	
	

equipment+required:
	heliplex (must be able to get well below 15K)

experiment:
	Resonant Magnetic X-ray Scattering Study of K2ReBr6

hazards:
	

name:
	Stephen E. Nagler

nonmembers:
	The proposed experiment is being discussed with Ben Larson at ORNL -
	The investigators (Stephen E. Nagler and Mark D. Lumsden) are both from
	ORNL.
	

station:
	33ID-D

unacceptable+dates:
	July 2-11



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