Experimental Report
Council:
14/02/2006
10/2004
Proposal:
5-41-362
Title:
Single Crystal Magnetic Structure Determination of the Frustrated Quantum Chain System CuBr2
This proposal is continuation of: 5-31-1441
Researh Area: Physics
Main proposer: BANKS Michael
Experimental Team: KREMER Reinhard K.
BANKS Michael
Local Contact: OULADDIAF Bachir
Samples:
CuBr2
Instrument
Req. Days All. Days
From
To
D10
9
25/04/2005
02/05/2005
7
Abstract:
Recently, a great deal of attention has been given to magnetic systems with strong magnetic frustration due to competing
interactions. An example, is the magnetic spin chain with nearest-neighbor (nn) and next nearest-neighbor interaction
which can also be considered as a two-chain lattice with ‘zigzag’ couplings. Recently, we have suggested that the
incommensurate magnetic ordering in the Cu2+ chain system LiCuVO4 detected by single crystal neutron diffraction on
D10 is due to frustration of nearest-neighbor Cu-O-Cu superexchange and next-nearest neighbor Cu-O-O-Cu supersuperexchange. Our search for new system with similar magnetic properties has focussed our attention to the binary Cu
dihalide CuBr2 which crystallizes with a chain structure.In a preceding experiment (5-31-1441, March 2004) we have
performed neutron diffraction on a polycrystalline sample of CuBr2 on D20 and found weak magnetic incommensurate
superstructure reflections. We propose to carry out a full magnetic structure determination on single crystal of CuBr2 using
the four-circle diffractometer D10.
________EXPERIMENTAL REPORT______________
EXPERIMENT N°
5-41-362
DATES OF EXPERIMENT
TITLE Single Crystal
INSTRUMENT
D10
24/04/2005 – 02/05/2005
Magnetic Structure Determination of the Frustrated Quantum Chain System
CuBr2
EXPERIMENTAL TEAM
(names and affiliation)
BANKS Michael MPI F. FESTKOERPERF., STUTTGART HEISENBERGSTR. 1 D-70569 STUTTGART
KREMER Reinhard K. MPI F. FESTKOERPERF., STUTTGART Germany
LOCAL CONTACT Dr Bachir OULADDIAF
Date of report
14/02/06
Our investigation into the one dimensional magnetism of binary copper halide
compounds focused on CuCl2 and CuBr2. The proposal (5-41-362) was to investigate the
magnetic structure of single crystals of CuBr2. However due to difficulties to get single
crystals of CuBr2 we instead succesfully determined the magnetic structure of CuCl2 with
our proposal. CuBr2 crystallizes in the same structure as that of CuCl2. However, growing
single crystals of CuBr2 large enough for single crystal neutron diffraction resulted in
crystals with inevitable high disorder in the ab plane which is the cleavage plane.
Therefore we chose to investigate the magnetic structure of CuCl2 in which we had well
defined single crystals large enough for neutron diffraction on a frustrated Cu2+ ion.
Using D10 with a wavelength RI Å, we collected a large number of nuclear
reflections at 300K and also at 30K. Searching for magnetic reflections resulting in
successfully detecting a magnetic reflection at 2K which was then not seen above the
Néel temperature (TN = 23.9K). The magnetic reflection was centered in reciprocal space
at (0.5, -0.224, -1) giving a propagation vector of W = (0.5, 0.224, 0). With this
propagation vector more than 20 unique magnetic reflections were measured. The
temperature dependence of the magnetic reflection at (0.5, -0.224, -1) proved it
unambiguously as a magnetic reflection. The temperature dependence is shown in fig 1.
Fitting it to a critical exponent power law gave a Néel temperature of 23.86K in perfect
agreement with thermodynamic properties of the specific heat and susceptibility. The best
fit of the critical exponent gave, = 0.22. A careful investigation of the crystallographic
reflections revealed many forbidden reflections which are not allowed in the accepted
space group for CuCl2, C 2/m. Further investigation by small crystal x-ray diffraction and
also Electron Spin Resonance (ESR) showed that the crystals are indeed twinned in the ac
plane. This gives a twin matrix of T
§0 0 1·
¨
¸
¨ 0 1 0¸ .
¨
¸
© 1 0 0¹
Using the SFTWIN routine in the
Cambridge Crystallographic Subroutine
Library (CCSL) allowed us to refine the
crystallographic structure. Using the
twin matrix above with SFTWIN, we
refined a domain population of 58% and
42%, in agreement with ESR and x-ray
investigations with the space group C
2/m. The lattice parameters were given
from a previous powder work (see
experimental report 5-31-1441) with a
final F2 = 14.
1250
integrated intensity
Experiment number:
CuCl2
1000
750
(0.5 -0.224 -1.00)
500
250
0
0
TN= 23.86K ± 0.21
E= 0.22 ± 0.05
5
10
15
T (K)
20
25
30
Fig 1. Temperature dependance of the reflection
at (0.5 -0.224 -1) showing it to be of magnetic
origin.
Refinement of the magnetic
structure was done using
Fullprof, with 15 unique
magnetic reflections. The best
refinement gave F2 = 7.2. and
is shown in fig 2. The refined
structure gives a magnetic
helix polarised in the ab plane.
The ordered moment was
refined as PCu = 0.63PB.
Fig 2. Proposed magnetic structure of CuCl2. Only the copper
atoms are shown which show the moment and direction.
Fig 2 Shows the proposed magnetic structure of CuCl2. The copper moments are shown
as the blue arrows. A full magnetic unit cell is shown. Our results show that the quasi
classical helicoidal ground state expected for frustrated systems with nearest and next
nearest neighbor interactions is realized in CuCl2. Although the ordered moment is quite
large for frustrated Cu2+, the results are quite similar to that of other frustrated low
dimensional systems. We have given here for the first time, a realization of a one
dimensional frustrated compound which is based on halides as the superexchange
pathway. The results are strikingly similar to that of cuprate based compounds.