169 Tm Mössbauer Spectroscopy J.M. Cadogan Department of Physics and Astronomy University of Manitoba Winnipeg, Manitoba, R3T 2N2 Canada

Thulium Thulium is a Lanthanide metal (Rare-Earth) with an atomic number of 69. Tm 3+ has an outer electronic configuration of 4f 12 and an electronic ground-state 3 H 6 (J=6, L=5, S=1) The free-ion magnetic moment of Tm 3+ is 7 B.

Experimental The 169 Tm source is made by neutron irradiation of 168 Er The intrinsic 169 Tm linewidth is about 25 times larger than that of 57 Fe. The low recoil energy of 169 Tm allows measurements up to ~ 1000 K.

169 Tm: comparison with 57 Fe 169 Tm 57 Fe E (keV) Excited state half-lifetime (ns) 498 Source half-life (d) 9270 Internal conversion coefficient 2208 Recoil energy (meV) Isotopic abundance (%) Magnetic moment (nuclear ground state) ( N ) Magnetic moment (nuclear excited state) ( N ) Quadrupole moment (excited state) (b)

The 169 Tm Mössbauer transition is a 3/2 1/2 transition, the same as that of 57 Fe. However, the energy splittings are 2 orders of magnitude larger.

Some examples of 169 Tm Mössbauer studies Tm 2 Ge 2 O 7 (5-fold symmetry ?) TmFe 2 (crystal-field effects and magnetic order) Tm 3 Al 2 & Tm 2 Al (exceptionally slow electronic relaxation)

Tm 2 Ge 2 O 7 Thulium pyrogermanate (TmPG) is tetragonal P The Tm 3+ site (8b) has triclinic symmetry and is coordinated by 7 O 2 ions, forming a distorted pentagonal bipyramid The Tm 3+ triclinic crystal-field hamiltonian contains 27 terms. A pentagonal hamiltonian has only 5 terms (an obvious mathematical advantage) ! Can the Tm 3+ 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)

v (mm/s) Rel. Transmission (%) T(K) 169 Tm Mössbauer spectra of TmPG G. A. Stewart, J.M. Cadogan and A.V.J. Edge, J. Phys. Condensed Matter, 4, (1992) The 169 Tm Mössbauer spectra of TmPG are broad quadrupole-split doublets. The temperature dependence of the quadrupole splitting can be fitted in terms of the Tm 3+ crystal field Hamiltonian. QS (mm/s) 5-fold model Triclinic symmetry model

Rel. Transmission (%) 169 Tm Mössbauer spectra of TmFe 2 B. Bleaney, G.J. Bowden, J.M. Cadogan, R.K. Day and J.B. Dunlop. J. Phys. F: Metal Physics, 12, (1982) TmFe 2 is a cubic Laves phase compound. The 169 Tm Mössbauer spectra of TmFe 2 are magnetically-split sextets corresponding to very large hyperfine fields (720 T at 1.3 K). v ( mm/s ) Note the velocity scale: ± 700 mm/s. For comparison, the magnetic splitting of -Fe (a standard calibration material for 57 Fe work) is ± 5.3 mm/s – about the size of two dots on this picture !

169 Tm Mössbauer spectra of TmFe 2 B. Bleaney, G.J. Bowden, J.M. Cadogan, R.K. Day and J.B. Dunlop. J. Phys. F: Metal Physics, 12, (1982) The temperature dependences of the magnetic hyperfine field and the electric quadrupole splitting at the 169 Tm nucleus can be fitted to yield the crystal-field and exchange parameters describing the magnetism of the Tm 3+ ion. B hf (T) T(K) Reduced Quadrupole splitting

Rel. Transmission (%) 169 Tm Mössbauer spectra of Tm 3 Al 2 Slow electronic relaxation G.J. Bowden, J.M. Cadogan, R.K. Day and J.B. Dunlop. J. Phys. F: Metal Physics, 11, (1981): Hyp Int., 39, (1988) Tm 3 Al 2 is tetragonal (P4 2 nm) with 3 Tm sites. The antiferromagnetic ordering temperature is 6 K. 169 Tm 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 4.2 K 1.3 K 45 K 30 K 18 K