Transcript Cadogan_166Er MS of Er3Ge4.ppt

Comparing erbium moments derived
from 166Er Mössbauer spectroscopy
and neutron diffraction
D.H. Ryan and J.M. Cadogan
Physics Department, McGill University, Montreal, QC, H3A 2T8, Canada
E-mail: [email protected]
Department of Physics and Astronomy, University of Manitoba,
Winnipeg, MB, R3T 2N2, Canada
E-mail: [email protected]
• Er3Ge4 adopts an orthorhombic structure (Cmcm #63) with
two crystallographically distinct Er sites (8f and 4c)
• Susceptibility measurements indicate a Néel temperature
of 7.2 K.
• Neutron diffraction data confirm this ordering and show
that Er atoms on the two distinct sites have significantly
different moments [1].
• The very different Er site populations will enable an
unambiguous assignment of the two subspectra in the 166Er
Mössbauer spectra so that a direct comparison between the
Mössbauer and neutron derived moments will be possible.
• We will be able to cross-validate moment determinations
by independent means.
[1] P. Schobinger-Papamantellos et al. J. Magn. Magn. Mater. 169 (1997), 253
• X-ray diffraction data
confirm that our sample
has the expected structure
with about 5 wt.% of
FeGe as an impurity.
• χac data show a cusp at
7.2±0.1 K, confirming the
expected Néel point.
• A Curie-Weiss fit to the
high temperature region of
the curve yields an
average paramagnetic
moment of 10.5±0.1 μB,
slightly larger than the
free-ion value, but
consistent with ErGe2
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166Er
Mössbauer spectroscopy
presents several problems.
No commercial sources exist. We
prepared Ho0.6Y0.4H2 which was then
activated by neutron irradiation in a
local SLOWPOKE reactor.
The source half-life is only 27 hours,
so you have to work quickly.
Dealing with the large γ energy, high
initial count rates and several nearby
x-rays demands a dedicated highresolution detector.
The low f-factor means that both the
source and sample must be cooled to
liquid helium temperatures and the
spectrometer is typically run
vertically.
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The spectrum taken at 1.5 K shows
two clearly resolved 5-line patterns
with an area ratio of 2:1 as
expected from the crystallography.
(The 2→0 transition yields a 5-line
pattern in the presence of a
magnetic field)
The distinct areas permit an
unambiguous site assignment and
we observe hyperfine fields of
654±1 T for the Er-8f site and
553±2 T for the Er-4c site at 1.5 K.
The field ratio is 1.183(5):1, fully
consistent with the 1.15(2):1
moment ratio derived from neutron
diffraction.
Our results also support the
reduced Er moments reported in the
neutron diffraction study as both
hyperfine fields are well below the
770 T expected for the full 9 μB
free-ion Er moment.
• To determine the Er moments
we need the scale factor
connecting the observed field to
the moment creating it.
• The free-ion hyperfine field for
9 μB on Er is 770.5±10.7 T [2].
• We expect an additional
14.1±2.1 T from conduction
electron polarisation for a total
field of 784.6±10.7 T and a
conversion factor of:
87.2±1.2 T/μB for 166Er
• The ratio between our average
hyperfine field and the average
Er moment at 1.5 K is: 88.3±0.5
T/μB.
The scaled moments are in perfect agreement
with our hyperfine fields at the Er-4c site all of
the way up to TN. However, the behaviour at the
8f site is quite different and is dominated by the
effects of slow paramagnetic relaxation.
[2] B. Bleaney, in ``Handbook on the Physics and Chemistry of Rare Earths'', Vol. 11, Chapter
77 (1988) K.A. Gschneidner Jr. and L. Eyring (eds.) Elsevier Science Publishers (Amsterdam).
Conclusions
• There is perfect agreement between Er moments derived from 166Er
Mössbauer spectroscopy and neutron diffraction.
• The two methods are completely independent and rely on quite
different physics.
• Moment determination using 166Er Mössbauer spectroscopy does not
require the presence of long-range order, nor is a knowledge of the
magnetic structure needed.
• The moments derived from 166Er Mössbauer spectroscopy are both
element and site specific so that contributions from other magnetic
species do not affect the results.
• Slow electronic relaxation [3] or short-range ordering [4] can interfere
with moment determinations by neutron diffraction, but may have
much less significant effects on the Mössbauer measurements.
[3] D.H. Ryan, J.M. Cadogan and R. Gagnon, Phys.Rev.B 68 (2003) 014413
[4] D.H. Ryan, J.M. Cadogan, R. Gagnon and I.P. Swainson, J.Phys.:CM 16 (2004) 3183