Comparing erbium moments derived from 166 Er Mössbauer spectroscopy and neutron diffraction D.H. Ryan and J.M. Cadogan Physics Department, McGill University, Montreal, QC, H3A 2T8, Canada Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada

Er 3 Ge 4 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 166 Er 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. χ ac 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 μ B, slightly larger than the free-ion value, but consistent with ErGe 2

166 Er Mössbauer spectroscopy presents several problems. No commercial sources exist. We prepared Ho 0.6 Y 0.4 H 2 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 high- resolution 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.

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 μ B 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 μ B 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/μ B for 166 Er The ratio between our average hyperfine field and the average Er moment at 1.5 K is: 88.3±0.5 T/μ B. [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). The scaled moments are in perfect agreement with our hyperfine fields at the Er-4c site all of the way up to T N. However, the behaviour at the 8f site is quite different and is dominated by the effects of slow paramagnetic relaxation.

Conclusions There is perfect agreement between Er moments derived from 166 Er Mössbauer spectroscopy and neutron diffraction. The two methods are completely independent and rely on quite different physics. Moment determination using 166 Er Mössbauer spectroscopy does not require the presence of long-range order, nor is a knowledge of the magnetic structure needed. The moments derived from 166 Er 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) [4] D.H. Ryan, J.M. Cadogan, R. Gagnon and I.P. Swainson, J.Phys.:CM 16 (2004) 3183