Transcript Cadogan_169Tm MS of Tm alloys.ppt

169Tm
Mössbauer Spectroscopy
J.M. Cadogan
Department of Physics and Astronomy
University of Manitoba
Winnipeg, Manitoba, R3T 2N2
Canada
E-mail: [email protected]
Thulium
• Thulium is a Lanthanide metal (“Rare-Earth”) with an
atomic number of 69.
• Tm3+ has an outer electronic configuration of 4f12 and an
electronic ground-state 3H6 (J=6, L=5, S=1)
• The “free-ion” magnetic moment of Tm3+ is 7 mB.
Experimental
•
The 169Tm source is made by neutron irradiation of
168Er
168
68

Er  n  Er 169
69Tm
1
0
169
68
• The intrinsic 169Tm linewidth is about 25 times larger than
that of 57Fe.
• The low recoil energy of 169Tm allows measurements up to
~ 1000 K.
169Tm:
comparison with 57Fe
169Tm
57Fe
8.4
14.4
Excited state half-lifetime (ns)
4
98
Source half-life (d)
9
270
Internal conversion coefficient
220
8
Recoil energy (meV)
0.24
1.95
Isotopic abundance (%)
100
2.14
Magnetic moment (nuclear ground
state) (mN)
−0.231
+0.0906
Magnetic moment (nuclear excited
state) (mN)
+0.52
−0.155
Quadrupole moment (excited state) (b)
−1.2
+0.21
Eg (keV)
The 169Tm Mössbauer transition is a 3/2 → 1/2 transition, the same as that of 57Fe.
However, the energy splittings are 2 orders of magnitude larger.
Some examples of 169Tm Mössbauer studies
• Tm2Ge2O7 (5-fold symmetry ?)
• TmFe2 (crystal-field effects and magnetic order)
• Tm3Al2 & Tm2Al (exceptionally slow electronic
relaxation)
Tm2Ge2O7
•
•
Thulium pyrogermanate (TmPG) is tetragonal P41212
The Tm3+ site (8b) has triclinic symmetry and is coordinated by 7 O2− ions,
forming a distorted pentagonal bipyramid
•
The Tm3+ triclinic crystal-field hamiltonian contains 27 terms. A pentagonal
hamiltonian has only 5 terms (an obvious mathematical advantage) !
Can the Tm3+ magnetism be described using 5-fold symmetry ? (A triclinic
symmetry yields 13 non-magnetic singlets whereas a 5-fold symmetry permits
5 magnetic doublets and 3 non-magnetic singlets)
•
169Tm
Mössbauer spectra of TmPG
The 169Tm Mössbauer spectra of TmPG are broad
quadrupole-split doublets. The temperature dependence of
the quadrupole splitting can be fitted in terms of the Tm3+
crystal field Hamiltonian.
Triclinic symmetry model
Rel. Transmission (%)
QS
(mm/s)
5-fold model
T(K)
G. A. Stewart, J.M. Cadogan and A.V.J. Edge,
J. Phys. Condensed Matter, 4, 1849-58 (1992)
v (mm/s)
169Tm
Mössbauer spectra of TmFe2
TmFe2 is a cubic Laves phase compound. The 169Tm Mössbauer spectra
of TmFe2 are magnetically-split sextets corresponding to very large hyperfine fields
(720 T at 1.3 K).
Rel. Transmission (%)
Note the velocity scale: ± 700 mm/s. For
comparison, the magnetic splitting of a-Fe (a
standard calibration material for 57Fe work) is ± 5.3
mm/s – about the size of two dots on this picture !
v (mm/s)
B. Bleaney, G.J. Bowden, J.M. Cadogan, R.K.
Day and J.B. Dunlop.
J. Phys. F: Metal Physics, 12, 795-811 (1982)
169Tm
Mössbauer spectra of TmFe2
The temperature dependences of the magnetic hyperfine field and the electric
quadrupole splitting at the 169Tm nucleus can be fitted to yield the crystal-field and exchange
parameters describing the magnetism of the Tm3+ ion.
Bhf (T)
Reduced Quadrupole splitting
T(K)
B. Bleaney, G.J. Bowden, J.M. Cadogan, R.K. Day and J.B. Dunlop.
J. Phys. F: Metal Physics, 12, 795-811 (1982)
T(K)
169Tm
Mössbauer spectra of Tm3Al2
Slow electronic relaxation
Rel. Transmission (%)
Tm3Al2 is tetragonal (P42nm) with 3 Tm sites. The antiferromagnetic ordering
temperature is 6 K. 169Tm Mössbauer spectroscopy shows a fully magnetically split sextet even
up to 45 K, indicative of unusually slow electronic relaxation.
v (mm/s)
11.6 K
45 K
4.2 K
30 K
1.3 K
18 K
v (mm/s)
G.J. Bowden, J.M. Cadogan, R.K. Day and J.B. Dunlop. J. Phys. F: Metal Physics, 11, 503-510 (1981):
Hyp Int., 39, 359-67 (1988)