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[1] P. Schobinger-Papamantellos et al. J. Magn. Magn. Mater. 169 (1997), 253

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[1] P. Schobinger-Papamantellos et al. J. Magn. Magn. Mater. 169 (1997), 253

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  1. Comparing erbium moments derived from 166Er Mössbauer spectroscopy and neutron diffractionD.H. Ryan andJ.M. Cadogan Physics Department, McGill University, Montreal, QC, H3A 2T8, Canada E-mail: dhryan@physics.mcgill.caDepartment of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada E-mail: cadogan@physics.umanitoba.ca

  2. 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

  3. 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

  4. 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 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.

  5. 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.

  6. 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).

  7. 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

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